February 2018

EPRI: Analytics for predictive maintenance 23 • Thermally sensitive generator rotors 32

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Turbine Tech

Thermally sensitive generator rotors

A high efficiency coal-fired power technology with elevated and conventional turbine layout

By Weizhong Feng, Shanghai Waigaoqiao No.3 Power Generation Co., Ltd.

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EPRI: Analytics for predictive maintenance

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Vibration problems with a flexible shaft motor

By Patrick Smith, Air Products & Chemicals

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EDITOR’S NOTE

What are you looking forward to in 2018?

Welcome to Energy-Tech’s first quarterly print issue of 2018. Last year was certainly a year of changes and challenges world-wide and it’s looking like 2018 will be too.

In 2017, our authors were able to deliver excellent technical articles through Energy-Tech media sources. I have many good writers lined up again for 2018 and plan to continue to deliver the technical content you need each week in our Energy-Tech Current News e-mail newsletter and also in our quarterly print publications.

We’re starting off 2018 with a print issue filled with many great articles. Professor Weizhong Feng, winner of the 2016 ASME Prime Mover Award, presents a very intriguing article entitled “A high efficiency coal-fired power technology with elevated and conventional turbine layout” in this issue’s ASME article found on page 11. You’ll also find Patrick Smith’s Machine Doctor column on vibration problems, and TG Adviser’s Turbine Tech column on thermally sensitive generator rotors, worth the read.

Beyond just reading our magazine, I hope that you’ve had a chance to attend one of our online opportunities this past year whether it was a technical online training session on gas and steam turbine vibrations or a FREE webinar sponsored by one of our advertisers. Look for more opportunities in 2018.

We’ll be partnering with Environment One Corporation (E-One) again to co-host the biennial Generator Auxiliary Systems symposium to be held at the end of July in Saratoga Springs, NY. Make sure you get this in your budget and commit early. There is limited space available and this one will fill up fast! Watch for an email on this or call me for the details needed for your budget – 563-588-3857.

CALENDAR March 19-22, 2018 Electric Power Conference & Exhibition Gaylord Opryland Convention Center Nashville, TN 2018.electricpowerexpo.com June 24-28, 2018 ASME 2018 Power & Energy Conference & Exhibition Disney’s Contemporary Resort Lake Buena Vista, FL www.asme.org/events/power-energy July 30 – August 1, 2018 Generator Auxiliary Systems Symposium Hosted by Environment One Corporation (E/One) & Energy-Tech Magazine Saratoga Springs, NY www.Energy-Tech.com/Gen-Sym September 18-20, 2018 Turbomachinery & Pump Symposium George R. Brown Convention Center Houston, TX tps.tamu.edu December 4-6, 2018 Power-Gen International Orange County Convention Center, West Halls Orlando, Fla. www.power-gen.com Submit your events by emailing editorial@WoodwardBizMedia.com

Whatever changes or challenges you face in 2018, look to EnergyTech for the technical expertise needed in the power industry with our weekly enewsletter and our quarterly print publications. I’d love to hear your ideas about technical content or possible training. Email me at editorial@woodwardbizmedia.com with your ideas for topics that you’d like to see covered.

Thanks for reading,

Kathy Regan

4 ENERGY-TECH.com

February 2018

FEATURES

Machine Doctor: Vibration problems with a flexible shaft motor

By Patrick Smith, Air Products & Chemicals

The causes of high motor vibrations are not always obvious.

Although unbalance is a common cause, this would be unusual for a new motor that was shop tested. In this case, poor alignment, structural issues, or something external to the motor would be more likely. This case study has to do with a motor vibration problem that occurred with a new motor, but the problem only occurred after the coupling was installed. Although, both the coupling and motor were shop balanced separately prior to installation, the motor vibration problem could only be resolved by field balancing at the motor end coupling hub with the compressor/motor coupled together. This case study will cover the history and will explain why this problem occurred.

Introduction This case study pertains to a five stage integrally geared centrifugal compressor driven by a 2976 RPM, 6800 KW induction motor. The gearbox consists of a bullgear and three rotors. The gearbox utilizes tilting pad journal bearings for both pinions with “X” and “Y” non-contacting proximity type shaft vibration probes adjacent to each bearing (except for the free end of the cover rotor which has only one vibration probe.

The motor utilizes sleeve type radial bearings on both the drive end and non-drive end with “X” and “Y” non-contacting proximity type shaft vibration probes adjacent to each bearing.

There are no thrust bearings in the motor. The motor protection system includes high vibration alarm and shutdown protection.

The coupling is a disc pack type with an extended spacer. The spacer subassembly consists of coupling hubs at both ends that are shrunk fit onto the bullgear and motor shafts. Adapter plates are bolted to the hubs which are then bolted to the spacer through the disc packs. Because of the length of the spacer, it was made in two parts that are bolted together in the middle of the assembly. A picture of the coupling shown in Figure 1.

The compressor gearbox is mounted on a structural steel frame. The first and second stage intercooler are mounted on springs from the top of the structure steel frame. The third and fourth stage intercooler and aftercooler are mounted adjacent to the skid on spring supports. The motor is mounted on a concrete block. See Figure 2.

Commissioning When the motor was run uncoupled in the field for the first time, the DE shaft vibration levels were 51 and 39 microns, while the NDE vibration levels were 12 and 19 microns. These levels were below the manufacturer recommended alarm set point of 160 microns and trip set point of 240 microns. When the motor

Figure 1. Main drive coupling

Figure 2. Compressor installation

was coupled to the compressor, the DE vibration levels increased to 157 and 103 microns while the NDE vibration levels were essentially unchanged. The DE coupled vibration levels also increased about 10 to 20 microns when the load was increased.

The vibration could be felt on the motor foundation and loud noises were reported coming from the motor.

A spectrum analyzer was connected to the vibration transmitters to measure the spectrums and view the shaft orbits. This showed that the vibration was predominately at frequency of one times running speed. It also showed that a banana shaped DE orbit and a circular shaped NDE orbit.

A review of the motor rotor dynamics showed that the motor was a flexible shaft design that operated above the first bending critical speed of about 1650 CPM. It also showed no lateral or torsional natural frequencies near the operating speed.

Since the high vibration only occurred on the DE and only when the motor was coupled to the compressor, misalignment or coupling unbalance were considered likely causes of the high DE vibration.

February 2018 ENERGY-TECH.com

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FEATURES It was clear that the vibration levels were excessive and there was a real risk of a motor mechanical failure if the compressor were put into continuous operation. The author experienced a violent failure of a similar sized motor that ran at high vibration levels for an extended period. So, it was decided that the vibration issue had to be resolved before the compressor could be put into service.

Alignment As described above, the compressor gearbox is mounted on a large structural steel frame. The thermal growth of the compressor, frame and motor were reviewed based on measured data to determine if the original cold vertical offset used for the motor/ compressor alignment was correct. A hot alignment was also checked to validate this. Based on the analysis, it was determined that the motor needed to be lowered by approximately 0.78 mm.

Based on this, it was plausible that misalignment may have caused or contributed to the problem and so the motor was lowered to improve the hot alignment.

Balance The coupling manufacturer follows a multi-stage balancing program for flexible disc couplings in accordance with ISO 1940 G2.5. Motor-end and gearbox-end hubs each undergo single-plane balancing to G1 criteria, followed by a two-plane assembly balance within full G2.5 criteria. The coupling assembly is balanced on a standard hard-bearing rig, supported radially by rollers and driven by a belt. However, the coupling is only low speed balanced. The motor half coupling hub weighs 86 kg and the overall coupling weight is 446 kg.

The motor manufacturer performed a two-plane balance on the motor rotor in accordance with ISO 1940 G1.0. The motor rotor weighs 2250 kg. The motor bearing journal diameters are 160 mm with a diametral design clearance of 240 microns.

The DE vibration levels during the shop test were 26.7 and 25.7 microns and the NDE vibration levels were 16.8 and 20.0 microns.

The motor was supposedly shop tested with the motor half coupling hub installed. However, this was not consistent with the field data. It appeared that there could be some coupling unbalance.

Summarizing the vibration data:

Results of improved alignment After the re-alignment was completed, the motor was restarted.

Unfortunately, the motor vibration didn’t improve and the DE vibration increased slightly to 190 microns. This shifted the focus to the coupling.

6 ENERGY-TECH.com

Due to the need to put the compressor into service it was decided to field balance with the motor coupled to the compressor. Washers were used as balance weights and were added to the motor side coupling hub at the adapter plate bolted connection. There were some initial difficulties field balancing which will be explained in more detail later in this article. However, field balancing was successful in reducing the coupled DE “X” and “Y” motor vibration levels at load to 60 and 50 microns.

These levels were deemed low enough to put the compressor into operation.

However, the vibration levels were still higher than desirable and field balancing would be required in the future if the coupling was replaced. The suspected cause was coupling unbalance.

The successful field balance was accomplished by adding 84 grams at a radius of approximately 167.5 mm. These equates to a correction of 15,070 gram-mm, which is about 10 times the manufacturer’s maximum allowable unbalance at the coupling end. So, for coupling unbalance to be the cause, there would likely to have been a problem with the original coupling balance.

There was a spare coupling already being manufactured for this train and the coupling balance was witnessed to ensure there were no questions about the balance. Then, a plan was put together to exchange the active coupling with the spare while taking vibration data at key steps. The plan included: 1. After shutting down compressor, remove coupling spacer, leaving motor coupling hub installed. Start motor and record motor shaft vibration. Vibration data to include overall vibration, one times running speed vibration, spectrum and phase for all steps.

2. Shutdown motor and remove motor side coupling hub. Start motor and record motor shaft vibration.

3. Shutdown motor and remove compressor side coupling hub.

Install spare coupling motor side hub. Measure motor shaft vibration.

4. Shutdown motor, install spare compressor side coupling hub and spare coupling spacer.

5. Start coupled compressor/motor and measure motor shaft vibration.

Results after coupling replacement The results of the vibration testing during the various steps are shown in the table below.

While the DE vibration levels were improved, the vibration was still higher than desired. So, another field balance was planned. However, the technician performing the field balance struggled. After eight attempts, the motor vibration had not improved and was worse than the starting values. To better understand this an independent rotor dynamic analysis was performed.

February 2018

FEATURES Rotor dynamic analysis The damped unbalance response analysis is the principal tool used by API to evaluate relevant lateral rotor dynamics characteristics of a rotor. Due to nature of manufacturing of a rotor and its components, some unbalance always exists. Unbalance exists when geometric center and mass center do not coincide. This leaves behind what is known as “residual unbalance”. When performing an unbalance response analysis, unbalances are assumed at locations where there are large masses. The results will vary depending on the unbalance distribution and phase relationships.

The base model for the motor rotor is shown in Figure 3.

Unbalance response analyses were performed based on unbalances at the coupling, DE and NDE cooling fans and in the middle for the rotor core. Analyses were performed with and without the coupling to determine the influence of the coupling on the rotor dynamic behavior.

The unbalance response analysis with no coupling is shown in Figure 4. Unbalances at the DE and NDE fans and at the middle of the rotor core were included and these unbalances were all in phase with one another. As shown, there is a large response at about 1650 CPM, which corresponds with the first critical speed.

There is nothing near the design operating speed

Figure 4. Unbalance response; base model – no coupling

there is a larger response at all locations at the critical speed and at running speed. The response at the coupling at speeds above running speed is also higher. These analyses show the sensitivity to unbalance distribution and phase relationship. The calculated response at the DE bearing is about .08 mm, or 80 microns.

The mode shape is the deflected shape of a rotor calculated at the critical speed during a damped unbalance response analysis.

This can be used to evaluate the response along the entire length of the rotor. The mode shape for the last case is displayed in

Then, the analysis was run again with the addition of the coupling. Unbalances at the coupling, DE and NDE fans and at the middle of the rotor core were included and these unbalances were all in phase with one another. The coupling mass included the hub and 50% of the spacer mass. The results are displayed in Figure 5. The response at the first critical speed is a little higher at all locations, and the response at the coupling at running speed

Figure 5. Unbalance response: with coupling

Figure 8. As shown, there is a large response at the coupling, and a much smaller response at the DE bearing. When looking at this to evaluate vibrations measured in the field, it is important to

Figure 3. Motor rotor base model

is a little higher.

Recall that during the field balance, 84 grams of mass was added to the motor side coupling hub. Another analysis was then performed adding the equivalent amount of unbalance at the coupling. The results are displayed in Figure 6 and are similar to the previous analysis except that the response at the critical speed is a little higher and the response at the coupling is higher at speeds above the operating speed. The latter is likely a response to the second critical speed.

So, another analysis was performed, but this time the unbalance at the coupling and DE fan were 180° out of phase with one another. The results are displayed in Figure 7. As shown

Figure 6. Unbalance response: with coupling and 84 grams of unbalance; coupling and DE fan unbalance 180° out of phase

February 2018 ENERGY-TECH.com

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FEATURES understand where the vibration is being measured. In this case, the DE shaft vibration probe is on the coupling side of the bearing (left side of the DE bearing in this model). The response at the vibration probe location is much higher than at the bearing.

This should be considered when evaluating the overall vibration levels. In this case the measured vibration levels are not representative of the vibrations at the bearing. But, the high vibration may be an issue for the shaft and/or coupling.

The mode shape can also be used to determine possible balance planes and the phase relation with the balance weights. If balance weights are added at a location where there is little deflection, it will have little effect on the vibration. As shown in Figure 8, there is a large response at the coupling end and so it makes sense that this is a good balance plane.

Figure 8. Mode shape for case with coupling and DE fan unbalances 180째 out of phase

The rotor deflection at the vibration probe is in phase with the coupling. Had the vibration probe been on the other side of the bearing, the vibration at the coupling and at the probe would have been 180째 out of phase. The balance technician had trouble balancing the rotor because he incorrectly assumed that since this is a flexible shaft rotor that the balance weights needed to be added 180째 out of phase with the measured phase. Once this was recognized, there was no trouble field balancing. To balance the motor rotor with the spare coupling, a total of 31.5 grams of weight was added, which still equates to about 3.7 times the ISO 1940 G2.5 limits.

Figure 7. Unbalance response: coupling and DE fan unbalances 180째 out of phase

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February 2018

FEATURES The results of the rotor dynamic analysis show how sensitive this rotor is to coupling unbalance and unbalance distribution.

Even though the motor rotor and coupling were shop balanced separately, the coupling balance was not adequate to limit the DE motor shaft vibrations to acceptable levels. Unbalance response analyses with different bearing characteristics were run to see if a bearing change would help. But, it didn’t. There is no simple change that can be made to the motor. This issue is that the unbalance force with the current coupling is too high. A lighter coupling balanced to the same criteria or the same coupling balanced to tighter criteria are possible solutions. The other option is to field balance if the coupling ever needs to be replaced.

THE INFORMATION CONTAINED HEREIN IS BASED ON DATA BELIEVED TO BE ACCURATE AS OF THE DATE COMPILED. NO REPRESENTATION,WARRANTY, OR OTHER GUARANTEE, EXPRESS OR IMPLIED, IS MADE REGARDING THE MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, SUITABILITY, ACCURACY, RELIABILITY, OR COMPLETENESS OF THIS INFORMATION OR ANY PRODUCTS, MATERIALS, OR PROCESSES DESCRIBED. THE USER IS SOLELY RESPONSIBLE FOR ALL DETERMINATIONS REGARDING ANY USE OF INFORMATION, MATERIALS, PRODUCTS, OR PROCESSES IN ITS TERRITORIES OF INTEREST. AIR PRODUCTS EXPRESSLY DISCLAIMS LIABILITY FOR ANY LOSS, DAMAGE, OR INJURY RESULTING FROM OR RELATED TO THE USE OF OR RELIANCE ON ANY OF THE INFORMATION CONTAINED HEREIN.

Conclusions Motors that operate above the first lateral critical speed have some special challenges.

Although the rotor dynamics may be acceptable, the typical mode shape makes the rotor much more sensitive to overhung unbalance from the coupling. So, it is important to consider the coupling and coupling balance criteria. In this case, it appears the normal coupling balance was insufficient to prevent motor vibration problems in the field.

Specifying tighter shop coupling balance criteria or using a lighter coupling with the same balance criteria are possible solutions. ■ References API-684, “Tutorial on the API Standard Paragraphs Covering Rotor Dynamics and Balancing: An Introduction to Lateral Critical and Train Torsional Analysis and Rotor Balancing”, First Edition, API, Washington DC. ISO 1940/1, “Mechanical Vibration – Balance Quality Requirements of Rigid Rotors – Part 1: Determination of Residual Unbalance,” ISO, First Edition, Geneva, Switzerland Patrick J. Smith is lead machinery engineer at Air Products & Chemicals in Allentown, Pa., where he provides technical machinery support to the company’s operating air separation, hydrogen processing and cogeneration plants. You may contact him by emailing editorial@woodwardbizmedia.com. © Air Products and Chemicals, Inc. 2018. All rights reserved. This material may not be reproduced, displayed, modified or distributed without the express prior written consent of the copyright holder.

Those performing a risk assessment of any given hazardous scenario are responsible for validation of specific hazards and risk estimates used in making management decisions related to personnel safety.

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ASME FEATURE

A high efficiency coal-fired power technology with elevated and conventional turbine layout

By Weizhong Feng, Shanghai Waigaoqiao No.3 Power Generation Co., Ltd.

Abstract The bottlenecks which in developing high-efficiency Ultra Super Critical (USC) coal power technology is analyzed under the background of great pressure of reducing CO2 emission on coal power industry. The development of 700oC Advanced Ultra Super Critical (A-USC) technology has been much slower than expected mainly due to the material limitations. Double reheat systems increase the efficiency at the cost of significant increases in expense and complexity.

A cross compound unit with an elevated and conventional turbine layout greatly shorten the expensive hightemperature piping, significantly cutting the piping costs as well as reduce pressure drops and heat losses which increase the efficiency and the performance-price ratio of the power unit. Engineering study demonstrates the feasibility and advantages of this design.

Existing 600oC materials and equipment manufacturing capabilities were applied to the double reheat unit with the elevated and conventional turbine-generator layout, and adding other mature energysaving technologies which had succeed in Shanghai Waigaoqiao No.3 Power plant to achieve a net efficiency of 49.8% (6849Btu/kWh, Lower Heating Value (LHV)). Combined with a series of innovative technologies that can improve the operating efficiency and keep the efficiency from decreasing, the annual net efficiency can achieve 48.8% (LHV). This efficiency level is high enough to meet the strict CO2 emission standard (636g/kWh) issued by Environmental Protection Agency(EPA) of the USA, showing significant demonstration of reducing CO2 emission.

Key Words: emission reduction of carbon dioxide, elevated and conventional turbine layout, high efficiency coal power.

1. Introduction During the COP21 climate conference, 196 countries passed the Paris agreement, agreeing to control the temperature raise below 2 Celsius. Reducing the emission of green-house gas especially CO2 become the task shared by the whole world.

China is the largest developing country, as well as the largest coal producing and consuming country in the world. For a long period, coal power dominates in both electricity capacity and electricity production in China. Therefore, the coal power industry is faced with great pressure of reducing CO2 emission.

In view of the fact that coal accounts for about 70% of primary energy and 93% of fossil energy in China, it will February 2018 | ASME Power Division Special Section

remain as a major force for a long period. Meanwhile, a considerable number of countries, in particular developing countries, continues to rely on coal power to provide a stable, efficient and relatively inexpensive supply of electricity. On the other hand, the CCS (Carbon Capture and Sequestration) technology is still immature, expensive and costs a great deal of energy. Based on the above two points, research and development of high-efficiency coal power technologies is the inevitable choice for both China and the world.

At present, there are two main technological routes of highefficiency coal power: A-USC and Integrated Gasification Combined Cycle (IGCC). A-USC includes two developing directions: one is to continue to improve the level of initial parameters particularly temperature to 700oC or 760oC level; The other is double reheat technology which becomes a hot spot recently in China.

IGCC technology shrinks in the USA and Europe while develops relatively well in Japan. The proposed NAKOSO IGCC unit in Japan has a designed net efficiency of 48% (LHV). But the problems of high investment and maintenance cost and complex system still exist. Considering the worldwide developing routes of highefficiency coal power, IGCC technology can serve as a useful complement but it is very difficult to play a leading role.

A new coal power CO2 emission standard was released by the USA EPA in August 2015, requiring the emission not exceeding 636g/kWh based on the annual average operating gross efficiency. EPRI has proposed a research report about this standard, pointing out that it is impossible to meet this standard for conventional A-USC technological route even the steam temperature is raised up to 700 or 800oC[Ref.1].

On the other hand, in accordance with the current rate of development, the first commercial operation 700oC A-USC unit will not appear until about 2030. Considering Chinese Government’s commitment to CO2 emission peak in 2030 and to achieve that as soon as possible, the development of 700oC A-USC is too slow to help reducing CO2 emission for China as well as many other countries.

ENERGY-TECH.com

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ASME FEATURE As another option of A-USC, the conventional double reheat system is relatively mature. The Nordjylland Plant demonstration Unit 3 (411MW) in Denmark, a double reheat unit built at the end of the 20th century, was designed with a net efficiency of 47%, the highest design efficiency so far. Recently, China is experiencing a new construction wave of double reheat USC units, of which the capacities are 660MW or 1000MW. Take the Taizhou and Laiwu 1000MW USC double reheat units for instance, the unit net efficiency achieves 46%. However, the conventional double reheat technology has no advantages over the widely used single reheat USC technology due to its high complexity and cost. More importantly, the 46% net efficiency is too low to achieve the CO2 emission standard of the USA EPA. With the improvement of steam parameters, oxidation on the steam side and the solid particle erosion (SPE) problems are getting more serious, leading to the over-heating and explosion of boiler tubes and efficiency drop of the turbine. If it is not solved, the efficiency drop of the turbine can even counteract the benefit of higher parameters, which runs counter to the original intention with increased steam parameters.

Meanwhile, with the renewable energy’s development and an increasing proportion in the power grid, and the CO2 emission reduction requirements, the actual operating loads of coal-fired units get lower with larger gap between load peak and valley and stricter request for frequency control. All these factors make coal-fired unit’s operating efficiency decreases sharply.

Shanghai Waigaoqiao No.3 (WGQ3) power plant operates two 1000MW single reheat USC units. Through the research and application of the Energy-saving, Efficiency preservation, Environment protection and Ensuring safety technologies (4E of Feng’s 5E technologies), the annual average operating net efficiency of both unit achieves as high as 44.5%, equivalent to rated condition’s net efficiency of 46.5% [Ref.2], which is about 3% absolute value higher than that of the same type unit.

The high efficiency of WGQ3 units proves that under given steam parameters and system, it still has large room for the unit efficiency improving through system optimization, improvement and innovation.

Based on above background, to further improve efficiency and reduce CO2 emission of coal power especially USC technology, it is necessary to answer the following three questions: (i) How to significantly improve the design efficiency? (ii) How to significantly improve the operating efficiency (that is, how to narrow the efficiency gap between the design condition and operating condition)? (iii)How to maintain a persistent high efficiency (that is, how to prevent the efficiency from decreasing over time)?

The high-efficiency and low CO2 emission coal-fired power technologies with elevated and conventional turbine layout, is researched and developed to systemically address these questions, and is being applied in the Pingshan Phase 2 project in China.

12 ENERGY-TECH.com

ASME Power Division: Plant Operations

A message from the chair The international energy community is making tremendous strides and unprecedented impacts towards energy efficiency improvements, increased environmental protection and economic stability and growth.

We introduced the Revolution to End Energy Poverty (REEP) at the 2017 conference, which is an initiative that we hope will unite energy professionals, students, governments, media, NGOs, etc., towards eliminating poverty for the 1-2 billion that suffer under it. Most of these people don’t have access to a reliable source of electricity. We believe that by working together we can greatly increase the rate that we solve this problem.

A second area that our committee has been successful in is introducing more of the worldwide engineering community to the work of Professor Weizhong Feng. He is the winner of the 2016 ASME Prime Mover Award, the Power Divisions top honor, for work he presented through our committee. The ASME article in this issue is a paper he presented through our track in 2017. He has earned the title “China’s Thomas Edison” because his work has the potential to have profound implications for the international energy community as well as the US efforts to reduce CO2 from coal fired plants.

A third area that our committee has been successful in is providing an opportunity for engineering students and professionals from around the world to participate in ASME conferences, where their contributions immeasurably increase the technical knowledge shared by the power community worldwide.

Working with US and international ASME volunteers has been extremely rewarding. Our committee owes its success to these volunteer’s assistance. If you would like to volunteer and participate with ASME, I am sure you will also enjoy doing so.

Thank you, Christopher Marcella Chair, Plant Operations Note from the editor: The chair letter was shortened for print. See Chris's entire letter on our website - www.energy-tech.com.

2. Cross coumpoud unit with elevated and conventional turbinelayouts [REF.3] 2.1 PIPING PROBLEMS WITH THE CONVENTIONAL DOUBLE REHEAT SYSTEM DESIGN With the conventional double reheat system design, the addition of another reheat means that the steam has to shuttle twice between the boiler and the turbine house as shown in the traditional layout in Figure 1. Since the piping for a ASME Power Division Special Section | February 2018

ASME FEATURE 1000MW single reheat unit can be as long as 200 meters, the conventional double reheat design faces several problems related to the long pipelines. First, the high temperature resistance, thick walled, large diameter piping are very expensive. Second, the long pipe lines increase the flow resistance and the heat losses for the second reheat, which cut the efficiency increment.

Third, the huge steam volume in the piping system increases the inertia of load adjusting which slows the turbine load regulation. Fourth, simply arranging these large diameter piping in the buildings is hard to achieve. Thus, the conventional design needs to be altered.

2.2 ADVANTAGES OF A CROSS COMPOUND UNIT WITH THE ELEVATED AND CONVENTIONAL TURBINE LAYOUT Considering the design of a 1000MW double reheat unit with the steam parameters 31MPa/600oC/600oC/600oC. The four-flow turbine includes five cylinders with an HP turbine, an IP1 turbine, an IP2 turbine and two LP turbines. A cross compound design is used, so that the HP and IP1 turbines are coaxial with one generator as the front unit, while the rest of the cylinders are coaxial with another generator as the rear unit.

Unlike the conventional cross compound design, the front unit is located on top of a two-pass boiler or near the header outlet of a tower type boiler in an elevated position, while the rear unit is in the conventional location as shown in Figure 2.

The elevated and conventional turbine layout design minimizes the piping length between the boiler and the turbine. The main steam piping, both the cold and hot sections of the first reheat piping and the cold section of the second

reheat piping are mostly shortened, as shown in Figures 1 and 2. This design has two key benefits. The first is that the cost of the piping is significantly cut. The second is that the pressures drops and the heat losses of the piping system are significantly reduced.

In the conventional design, the pressure drop in the reheat system is usually 6-10% of the heat pressure (smaller pressure drops are more expensive) with this percentage usually divided equally between the reheat piping and the reheaters.

Eliminating most of the high temperature pipe lines brings several benefits. First, the pressure drops for the first reheat systemare cut in half due to the elimination of the first reheat pipes and the pressure drop in the second reheat system is reduced by 25%. These pressure drop reductions in the reheat system improve the efficiency.

Second, the elimination of the main steam piping means that the turbine inlet pressure can be increased, which also improves the efficiency. Third, the heat losses are greatly reduced by the shorter pipes. The total net efficiency increment will be about 0.5% from these improvements. Fourth, the amount of steam in the double reheat system will be about the same as for a single reheat system, so the system inertia for turbine load regulation will be about the same as for a conventional single reheat unit.

The cost would still be increased by the double reheat equipment, while the total cost of the two generators in the new design would be basically equal to that of one 1000MW generator.

Figure 1. Steam flows for a conventional double reheat unit

Figure 2. Steam flows in a cross compound double reheat unit with the elevated and conventional layout February 2018 | ASME Power Division Special Section

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ASME FEATURE

Figure 3. Spring support for the elevated turbine-generator platform

Figure 4. Welded turbine rotor

Apart from the efficiency increase and cost reductions, this design also allows designers to increase the unit capacity.

Existing boiler and turbine designs could possibly be increased to unit capacities of 1350-1500 MW. Despite these benefits of the elevated turbine layout, not all the casings can be placed on the high platform, mainly because the LP casings and their condensers are too large and heavy for the high platform and the huge power consumption needed by the circulating water pump will counteract most of the efficiency increment.

5. Connections of the generator power outputs, especially for the elevated unit, and the main transformer design.

6. Startup, synchronization and operating control of the unit, turbine over speed control, and a feasibility study of eliminating the IP1 control valve (the IP1 main steam valve would remain).

7. The design of the bypass and the first and second reheat safety valves.

2.3 RESEARCH PROJECTS RELATED TO THE ELEVATED DESIGN The elevated design brings benefits as well as several important subjects for further research.

1. Optimum equipment and piping arrangement, foundation and building design, and means to absorb the vibration energy of the elevated unit.

2. Seismic and wind design considerations, coordination of the elevated unit and boiler construction, prevention of excessive stresses in the short pipes connecting the elevated turbines and the boiler outlet headers.

3. The workshop design for the elevated unit, especially the facilities for lifting the turbine cylinders and generator of the elevated unit during maintenance.

4. Compensation for the thermal expansion of the short, large diameter, thick wall pipe.

These problems can be resolved by using existing technical knowledge. For instance, a spring support platform can be used to absorb the vibration energy from the elevated unit as shown in Figure 3.

Considering the cutting of the piping cost as this double reheat system with above special arrangement, the total cost increment of 1000MW unit is reduced to 40 million dollars compared with the conventional design of 1000MW single reheat unit. In addition, the net efficiency is increased by at least 2.5% compared with the single reheat system, resulting in a cost of about 16 million dollars for each 1% net efficiency increase.

Compared to the conventional design, the double reheat unit’s cost increment of 35 million dollars for each 1% net efficiency increase, the new design, has a much better performance-price ratio.

Figure 5. Modified design for 700oC with the IP2 also in the elevated layout

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ASME Power Division Special Section | February 2018

ASME FEATURE 2.4 APPLICATION OF THE ELEVATED DESIGN TO A-USC700oC UNITS The expensive nickel-based super alloys are now the only choice for steam temperatures of 700oC. For the boiler, only the steam in the final superheater and in the very high temperature part of the final reheater will reach that temperature; thus, the expensive alloys are only necessary in these heating surfaces. For the turbine, the use of welded rotors and inner cylinders also reduces the need for super alloys and the cost as shown in Figure 4.

In the conventional design, as indicated in Figure 1, the major need for the nickel-based super alloys is in the main steam piping and the high temperature segment of the double reheat steam piping. The conventional design does little to shorten these pipes while the elevated design significantly reduces the length of the highest temperature pipes. To further reduce the need for the super alloys, the IP2 turbine can also be moved to the front unit, as shown in Figure 5. Thus, the high temperature (higher than 700oC) segment of the second reheat piping is replace by the IP2 exhaust steam piping with much lower temperatures so carbon steel pipes can be used.

In view of the huge cost of the steam piping, this design should also be used with 700oC single reheat units. The advantages of this design are increasingly important as the steam parameters rise. In consideration of the performanceprice ratio, this design is very likely the only solution to control the investment costs for steam temperatures of 700oC.

3. The application of cross compound unit with elevated and conventional layout —improve the unit design efficiency Engineering studies on the new design began in 2009 with technical support from Siemens, Alstom and the East China Electric Power Design Institute. Existing materials and manufacturing methods were used with the target of maximizing the net efficiency of the unit. A double reheat system was designed with optimization of the steam parameters including the main steam, reheat steam and exhaust steam temperatures. The research needs related to the elevated and low turbine locations have also been analyzed and resolved.

The cost and efficiency of the first design of the cross compound unit with the elevated and low turbine locations for a 600oC system were acceptable with the HP and IP1 turbines elevated and the IP2 and LP turbines arranged as normal as shown in Figure 2. This arrangement takes into consideration both the risk and the cost of the expensive pipes with the number of high temperature pipes reduced from five to only one.

3.1 UNIT CAPACITY OPTIMIZATION Current equipment manufacturers can build the HP and IP1 turbines for unit capacities not larger than 1500MW, so the key point is the selection of the IP2 and LP turbines.

February 2018 | ASME Power Division Special Section

Table 1. Main turbine design parameters

For the selected second reheat pressure, the volumetric flow rate at the IP2 inlet is about 2.7 times that at the IP1 inlet, which limits the blade height of the first stage of the IP2 turbine. If the second reheat temperature is 620oC, the upper limit of the single IP2 cylinder capacity is about 700MW. Mature LP turbine designs can incorporate casings with 45 inch high steel blades in the last stage and 12.5m2 exhaust areas.

The optimum capacity was finally determined to be 1350MW with three LP casings after the condenser pressure was further optimized. With this capacity, the IP2 turbine needs to be divided into two casings, with each of the casing close to the optimum capacity of 700MW. The main design parameters of the turbines given by Siemens are listed in Table 1.

The net unit efficiency under rated conditions is 47.1% (including the desulfurization, denitrification and fly ash removal systems, a boiler efficiency of 94.1%, and house power load rate of 3.5%), which is almost equal to the expected net efficiency of 700oC single reheat A-USC units and significantly better than the net efficiency of existing 600oC single reheat USC units.

3.2 SELECTION OF MAJOR FACILITIES AND UNIT LAYOUT Both theoretical analyses and practical experience with SC/USC units has led to the conclusion that the turbine efficiencies are higher using the sliding pressure operating mode without a governing stage, compared to turbines with a governing stage. Thus, a turbine without a governing stage was selected.

A tower type boiler was preferred because it has smaller thermal variations, lower wear rates of the convective heating surfaces and better hydrodynamic characteristics than the twopass type boiler, especially for large capacity boilers.

Long-term operating experience with USC units has shown that large capacity series-connected bypass systems have Figure 5. Modified design for 700oC with the IP2 also in the elevated

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ASME FEATURE

Figure 6. General arrangement of the unit

layout shorter starting times and less solid particle erosion.

Hence, a three-stage series-connected bypass system was selected.

The general arrangement of the unit is shown in Figure 6.

Arrangement, the elevated platform is located between the final superheater and the final reheater. To shorten the high temperature piping, the elevated turbines are arranged as close as possible to the boiler heads by removing some of the steel frame front of the boiler. The HP turbine inlet is arranged close to the boiler outlet so that the piping between the turbines and the boiler can be short and symmetric, and the system can be further simplified while the thermal stresses are lower.

The elevated unit platform is placed on springs supported by a reinforced concrete frame with braces. The frame is connected to the main steel structure of the boiler by hinged girders to prevent excessive stresses in the short pipes between the platform and the boiler caused by asynchronous displacements during earthquakes or high winds.

The reinforced concrete frame is surrounded by a light steel frame to form the turbine workshop. The major equipment for the feed water system and the condensate system, including the high and low pressure heaters and the deaerator, are located inside the concrete structure that supports the platform. The central control room is also arranged inside the concrete structure so that the number of cables and its corridors, as well as the construction cost, are significantly reduced.

3.3 THE APPLICATION OF 4E TECHNOLOGIES The 4E technologies, that are the Feng’s 5E technologies except Elevated T-G technology have gained great success in WGQ3. The representative is the generalized regeneration theory and a series of related technologies.

According to thermodynamics, with the water and steam as working substance, the modern thermal power plant is theoretically based on the Rankine cycle. The thermodynamics 16 ENERGY-TECH.com

theories point out that basically there are two methods to improve the cycle efficiency: improving initial parameters (the average endothermic temperature), as well as reducing final parameters (reducing the average exothermic temperature).

However, raising the initial parameters is limited by materials technology development while reducing final parameters are restricted by the natural condition such as the ambient temperature. To further enhance the efficiency on the premise of limited parameters, the regenerative cycle and reheat cycle both based on the Rankine cycle are invented. The regenerative cycle uses the feedwater heated by the turbine extraction steam as the medium to return part of heat to the boiler and reduce the loss of turbine exhaust. However, even for the most advanced coal-fired power plants in the world, nearly 50% of the heat absorbed in the boiler is taken away by the turbine exhaust as the heat loss.

Despite the fact that the thermodynamic theories of modern power plants are relatively mature, there are still huge potential of efficiency improvement.Yet reducing the turbine exhaust losses is the primary way.

The existing regenerative cycle theory depends on the water and steam system singly. If the thermal cycle can be expanded to the whole unit even the whole plant, things will be quite different. It is known that the CHP (Combined Heat and Power) can improve the efficiency of power plants significantly because the turbine exhaust is utilized as external heating and thus the exhaust loss is reduced. The CHP actually expands the thermal cycle to the society.

The feedwater regenerative cycle, on the other hand, is a kind of CHP, for the turbine extractions heat carried by the feedwater return to the boiler to substitute the coal and reduce the exhaust losses. In fact, substances entering the boiler include not only feedwater but also air and coal. If the mediums of regenerative cycle are expanded from only water to water, air and coal, the utilization of the turbine extractions will undoubtedly be expanded and the exhaust losses will further reduce. This idea actually expands the regenerative cycle from single water and steam system to the whole unit.

The generalized regeneration theory that I have putt forward seven years ago, aiming at reducing the turbine exhaust losses and improving the efficiency, expands the regenerative cycle from single water and steam system to the whole unit, whole plant and even the society, as shown in Figure 7. According to this definition, the CHP is also the application of the generalized regeneration theory [Ref.4].

The elevated and conventional layout design technology, together with the 4E technologies including the comprehensive optimization of air pre-heater and flue gas heat recovery and so on, forms Feng’s 5E technologies. With the application of 5E technologies, the unit efficiency can be further improved from 47.1% to 48.92%.

ASME Power Division Special Section | February 2018

ASME FEATURE

Figure 7. The diagram of the generalized regeneration theory

3.4 FURTHER OPTIMIZATION OF PARAMETERS AND UPDATE OF 5E TECHNOLOGIES With the development of materials these years, on the basis of the 2010 version, and fully taking the advantages of elevated turbine arrangement, unit parameters are further promoted from 30MPa/600/610/620oC to 31.6MPa/610/630/630oC. Meanwhile, as the steam parameters improve, the degree of superheat of the steam increases, thus the application and effect of the generalized regeneration technologies expand.

The update of the 5E technologies, including the medium temperature economizer technology, can further enhance the unit net efficiency from 48.92% to 49.8%, close to the historic level of 50%.

2) Load changing conditions.

To respond faster to the request of power grid, the governor valve of the turbine is generally throttled to retain a certain degree of thermal storage capacity. But the throttle brings remarkable efficiency loss. Boiler steam pressure and temperature fluctuations caused by the frequent act of the turbine governor valve is bad for the unit safety. In addition, the overload valve of Siemens’s turbines suffers from efficiency drop caused by part of live steam bypassed.

3) Seasonal changes. Due to the consideration of annual average performance, the design back pressure of turbines usually bases on the average backpressure. Thus, the summer and winter conditions have to sacrifice a certain economy. For example, in summer conditions, high back pressure caused by high circulating water temperature will dramatically reduce the volume flow of LP exhaust steam, causing the exhaust speed losses to rise, especially under low load conditions. On the contrary, in winter conditions especially under high load, the LP exhaust area is inadequate, causing back-pressure congestion problems, resulting in insufficient effective enthalpy drop, thus affecting the unit economy.

4. Improve the unit operating efficiency

To greatly reduce the negative influence of the above factors, a series of innovation technologies are researched and applied, greatly narrowing the efficiency gap between the operating condition and design condition.

4.1 THE ANALYSIS OF EFFICIENCY GAP BETWEEN DESIGN AND OPERATING CONDITIONS

4.2 IMPROVE THE OPERATING EFFICIENCY UNDER LOW LOAD

Design efficiency represent the maximum efficiency that can be achieved under ideal conditions, while the actual operating efficiency was significantly lower than the efficiency of design, generally at least 2% lower in absolute value. From the perspective of unit operation, the reasons can be divided into the following three main factors: 1) Low load conditions. With hydropower, wind power and solar power and other renewable energy generating capacity continues to increase, coal power becomes a major force of frequency control of the power grid, thus low load and load changing operation is inevitable. Under low load conditions, significant deviation from the optimal operating point of the turbine will result in remarkable drop of the turbine efficiency. Due to the need of combustion stability ensuring the steam temperature, the oxygen content of the furnace will also greatly increase, leading to much higher flue gas losses.

Auxiliary facilities including pumps and fans also operate in low efficiency under low load conditions, resulting in higher power consumption rate. These factors will lead to significantly lower operating efficiency at low loads.

February 2018 | ASME Power Division Special Section

On the turbine side, the adjustable feedwater temperaturekeeping regeneration technique is designed specially to solve these problems, as shown in Figure 8.

This technique adds a higher-pressure extraction from the turbine HP as well as a supplementary heater to heating the feedwater. A valve is designed on the extraction pipe to keep the outlet pressure of the valve unchanged, thus the feedwater temperature can be remained at the rated level as the turbine load varies.

A distinguishing feature of this technique is that it has no influence on the regenerative cycle under the rated load, while as the turbine load decreases, the extraction increases along with the valve opening to keep the feedwater temperature unchanged.

On the boiler side, the low oxygen and low NOx combustion technology within the whole load range is developed. In fact, the application of air and coal powder regeneration improves the operation conditions of boiler greatly ENERGY-TECH.com

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ASME FEATURE

Figure 8. The adjustable feedwater temperature-keeping regeneration technique

within the whole load range. The significant improvement of drying capacity of the coal pulverizing system and the hot combustion air temperature greatly improves the burning velocity and burn-out rate of pulverized coal, resulting in a series of positive cycles.

Firstly, the excess air ratio can be reduced remarkably. The

an electricity generator at the same time. Big motors of the auxiliary facilities are linked to both the industry power bus and the frequency variable power bus. So, it can switch to either one as required. As the load gets lower, the speed of the baby turbine also get lower and the power frequency and the speed of all the motors which link to the frequency variable power bus reduces, saving remarkable energy of the auxiliary facilities under low load.

4.3 IMPROVE THE UNIT EFFICIENCY IN LOAD CHANGING CONDITIONS The turbine load changing by combination of condensate water frequency control and adjustable HP steam extraction can eliminate throttling losses of turbine control valves.

The condensate water frequency control technology, that adjust the mass flow of condensate water and related LP steam extractions by adjusting the exit valve of condensate pump to temporarily release or store part of the unit load.

Figure 9. The origin designed oxygen content and the optimized value

oxygen content of flue gas under rated load can be reduced from 3.5% to 1.8% with extreme low carbon content of fly ash. Better still, the oxygen content can keep the low level as the load gets lower, as shown in the Figure 9. Thus, the exhaust heat loss is greatly reduced and the boiler efficiency is significantly improved.

The reduction of the oxygen content brings new benefits.

Firstly, it is conducive to the low NOx combustion. Within the whole load range, the NOx contents downstream of the economizer can easily be controlled below 200 mg/Nm3 with extreme low carbon content of fly ash. Secondly, the reduction of flue gas mass flow increases the residence time in the SCR, FGD and ESP systems, which is beneficial to the efficiency of pollutant removal. In addition, the reduction of flue gas also reduces the power consumption of the fans and the heat losses of the boiler.

In view of auxiliary facilities, the generalized variable frequency power system is applied. This technology uses a speed changeable baby turbine to drive a feedwater pump and 18 ENERGY-TECH.com

During the adjustment, the extractions amount is balanced on the whole, so it has no impact on economics of the turbine regenerative system. In addition, since there is no throttle loss on the turbine control valves, there is no corresponding loss.

The adjustable HP steam extraction frequency control technology also by change the mass flow of additional HP steam extraction temporarily. This technology can instantly change the turbine output and reduce unit load response time.

4.4 IMPROVE THE UNIT EFFICIENCY IN SEASON CHANGING CONDITIONS The cutting off LP casing technology is specially developed to improve the unit efficiency in season change conditions, as shown in Figure 10.

LP1, LP2 and the front LP3 can consider enough exhaust area to meet high load and lower back pressure conditions during winter. As the LP turbine’s back pressure is higher and unit load is lower during summer, the total exhaust volume flow are very lower. Then the front LP casing can be cut off to improve the exhaust volume flow in the rest LP casings and reduce the exhaust speed loss.

ASME Power Division Special Section | February 2018

ASME FEATURE 5. Maintain a persistent high efficiency 5.1 THE DAMAGES OF SPE PROBLEMS

Figure 10. The principle of the cutting off LP casing technology

Meanwhile, since the front LP casing and the rest LP casings are arranged on both sides, the differential expansion between the cylinder and rotor can be smaller, making large unit multiple casing with single axis design feasible and more reasonable, and also can get higher cylinder efficiency.

In addition, since the exhaust of IP casing enters the front LP3 and the rest LPs separately via cross over pipe. Therefore, the resistance of cross over pipe is lower and the unit efficiency is higher.

Although the cutting off LP casing technology failed to be applied on Pingshan Phase 2 project mainly due to the R&D cycle, but related patent has been applied for. With the application of the technologies that mentioned above can improve operating efficiency, the unit annual average operating efficiency can be as high as 48.8%. The corresponding carbon dioxide gross emissions are 635g/kWh, succeeding to meet the EPA standard, showing enormous significance.

As ultra-supercritical units, with the increases of the steam parameters, especially the steam temperature, new problems and challenges are encountered, one of the outstanding problems is steam-side oxidization of the piping and subsequently SPE (solid particle erosion) of the turbine blades, which substantially threatens the safe and economic operation of the units. This issue often occurs during the unit startup and shutdown phase. The oxides in the boiler steam side peel off due to the heat shock to the boiler. In the different situations, it either deposited in the tubes or formed particles along with the steam movement. The tube burst will take place due to the serious deposit of the oxides. The turbine blading erosion can be caused by the particles carried by the moving steam, and its interior efficiency appears to be declined in a non-reversible manner.

Furthermore, the solid particles erode the sealing surface of the bypass valve plugs during the startup, and this causes the leakage and directly makes part of steam bypassed, thus the plant efficiency depressed. Recently the serious of SPE troubles have taken place in China’s super and ultra-supercritical units one after the other, the efficiency of some units has even reduced by 4% after 2 years’ operation.

For the proposed cross compound unit with elevated and conventional layout, the reheat temperature will be improved to unprecedented level as 630oC, and the SPE problems will be more serious.

Figure 11. The deposited oxides in boiler tubes, SPE to turbine blades and valves

February 2018 | ASME Power Division Special Section

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ASME FEATURE

Figure 12. Cut of SH3 and RH2 tubes of Waigaoqiao No.3 Power Plant

5.2 EFFICIENCY PRESERVATION TECHNOLOGIES It is significant for energy saving to prevent the efficiency from deteriorating by dramatically slowing the oxidization in the steam side of the tubes and SPE aroused thereof. This needs comprehensive prevention in all respects and whole process linking the main equipment selection, system design, installation and commissioning, operation mode and the control strategy.

After 15 years’ track and research, a whole set of preservations has been worked out based on series of breakthrough, they were implemented successfully in WGQ3.

Through the dry blowing out with higher steam superheat during the construction, the insufficient momentum of the blowing out in the both outlet area of water wall and inlet area of superheater has been solved, effectively enhanced the blowing out strength. To configure large capacity bypass system, to research and develop High Momentum Flushing Technology in the startup progress, and to implement high momentum flushing with load variation, parameters alteration and circuit changing prior to the turbine startup with special bypass operation mode, these oxides on the heating surface can be peeled off to greatest extent before each startup, and those deposited somewhere including the particles stay in the zero velocity zone would be sent to the condenser directly. The unit safety and efficiency can be ensured by fully cleaning of the oxides in the system and preventing oxides and particles from emerging in the high temperature and high load operation. The erosion to the bypass valve plugs has been eliminated effectively by working out its new configuration design and control

Figure 13. The First IP Blading after 30 Months Operation

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strategy. The startup time and the bypass operation process have been shortened. In addition, the steam heating startup and the low load stable combustion provide a hot environment for the boiler startup before its igniting by heating the whole boiler with the steam instead of oil and coal. This prevents the boiler convection surface from burning without steam after the ignition as its water in the water wall still under saturation, from speeding up the high temperature oxidation, and keeps the oxides from peeling off as the convection surface suddenly cooled down by the steam intake after evaporation.

After 30 months running, the tubes in the third superheater and the second reheater have been checked. There is no any indication of oxidation and deposits as shown in Figure 13, and the tubes are brand new.

Above SPE comprehensive prevention and control technologies will be applied on the new unit. In addition, the inner surface aluminum coating process will be adopted on the high temperature and large diameter pipes and headers.

This process can dramatically reduce the oxidation rate on the steam side, keeping the unit high efficiency for long time operation.

6. Conclusion 1) Studies have shown that the 700oC A-USC designs and the conventional double reheat layout both have poor performance-price ratios and are not efficient enough to meet the CO2 emission standard issued by the USA EPA. This paper describes a cross compound unit with an elevated and conventional turbine layout that substantially shorten the expensive high-temperature piping. Thus, this design greatly cut the piping costs, reduce the pressure drops and heat losses of the piping. This increases the efficiency of the unit and gives much better performance-price ratios.

2) Engineering studies of the elevated and conventional turbine layout design indicate that this design is feasible and economic. The design was then optimized using existing mature 600oC materials and equipment in a double reheat design to achieve a net efficiency of 47.1% (7244 Btu/kWh Figure 12. Cut of SH3 and RH2 tubes of Waigaoqiao No.3 Power Plant based on the LHV with FGD, SCR and ESP ASME Power Division Special Section | February 2018

ASME FEATURE systems included). If the series of energy-saving methods that have already been applied to WGQ3, are used in this double reheat unit, the net efficiency is expected to reach 49.8% (6849Btu/kWh, LHV). Once the 700oC materials becomes more mature, the unit net efficiency can be further improved to 53% (6435Btu/kWh, LHV).

Editor’s note: This paper PowerEnergy2017-3035 was printed with permission from ASME and was edited from its original format. To purchase this paper in its original format or find more information, visit the ASME Digital Store at www.asme.org

3) With the application of innovative technologies that can improve operating efficiency, the annual average operating efficiency can be as high as 48.8%. The corresponding gross CO2 emissions is 635g/kWh, which is lower than the EPA standard, showing enormous significance.

4) At present, this high efficiency and low CO2 emission unit with elevated and conventional layout has been approved as a national demonstration project.

This project is in the preparatory stage, planning to build a 1350MW demonstration unit in Pingshan, Anhui Province. This unit, as the most efficient and cleanest coal-fired unit in the world, will be put into operation by the end of 2019. ■

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Reference 1. Jeffrey Phillips, 2015, “Can Future Coal Power Plants Meet CO2 Emission Standards Without Carbon Capture & Storage?”, Low-Carbon Coal Technology Assessment.

2. A Feng, W., 2015, “DEVELOPING GREEN, HIGHLY EFFICIENT COAL-FIRED POWER TECHNOLOGIES”, PowerEnergy 2015-49551, ASME Power and Energy Conversion Conference.

3. A Feng, W., 2016, “CROSS COMPOUND TURBINE GENERATOR UNIT WITH ELEVATED AND CONVENTIONAL TURBINE LAYOUTS”, PowerEnergy 201659720, ASME Power and Energy Conversion Conference.

4. A Feng, W., 2015, “GENERALIZED REGENERATION THEORY AND ITS ENERGY SAVING AND EMISSION REDUCTION EFFECTS ON COAL-FIRED POWER GENERATION”, PowerEnergy 2016-59166, ASME Power and Energy Conversion Conference.

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MAINTENANCE MATTERS

EPRI: Analytics for predictive maintenance

By Rick Roberts and Steve Seachman, Electric Power Research Institute

Flexible operation of power plants has increased the risk of forced outages at plants and increased the need to maintain plant operations efficiently. One tool to help address these challenges is the use of novel data analytic techniques, which employ previously recorded data, as well as data gathered in real time, to provide guidance to plant operators regarding when maintenance of systems and equipment needs to occur.

However, when utilizing data analytic techniques in a plant environment, an understanding is needed of the technology and the complexities associated with applying analytics to improved plant maintenance.

The Electric Power Research Institute (EPRI) recently sponsored research to provide an introduction to data analytics as applied to plant maintenance in the power generation industry. The research team reviewed available literature on industrial analytics experience and compiled the results in a new report, Analytics for Predictive Maintenance in the Power Generation Industry (3002011017).

The report describes analytics concepts, challenges, workflow, and technology architecture. Details for each analytics component are provided, along with real-world applications where these tools can provide benefits to utilities. The concepts presented cut across all forms of power generation, including coal, wind, solar, hydro, and nuclear.

The literature review was supplemented by an industry survey, conducted to capture and evaluate input from industry engineers and managers who frequently work with analytics technologies and techniques. Other elements of the research included an on-site visit to a large-capacity coal plant and interviews with power industry professionals.

This article offers highlights of the research and a brief introduction to the key concepts of data analytics, particularly as they relate to power plant maintenance.

Advantages of predictive maintenance over preventive maintenance Before discussing data analytics, it is necessary to discuss the goal of data collection and analysis in power plants—predictive maintenance—and its advantages over preventive maintenance.

Preventive maintenance (PM), which has been traditionally applied in power plants, is based on manufacturer

recommendations pertaining to when to perform standard maintenance tasks, or repair or replace parts or components before they become costly problems. However, manufacturer guidelines may differ significantly from the realities of everyday operational usage. Each asset or component is subject to its own unique set of environment conditions and operational experiences, which are not fully considered by the average lifetime estimations used in PM strategies. As a result, PM guidelines are overly vague and err on the side of excessive maintenance.

Unlike PM, which relies on judgment and general guidelines, predictive maintenance (PdM) relies on data. The amount of data can be large and includes detailed usage, operations, and maintenance history, along with condition and environmental measurements that are specific to each asset. The information collected is automatically analyzed to determine a range of probabilities for the likelihood of failure over a series of time ranges. Data can also be used to determine the best plan for preventing or delaying the failure based on patterns of information from similar assets running under similar circumstances, correlated with prior prevention strategies and their outcomes.

By correctly estimating asset reliability, remaining useful life, and the probability of failure, power generation utilities can significantly reduce maintenance costs, unplanned downtime, and safety incidents. Computer systems can analyze thousands of data points, as opposed to the relatively few that humans can cognitively process. With effective implementation of PdM technologies and techniques that automatically evaluate asset health, the availability and reliability of operations can be improved, and the life of expensive, critical assets can be extended.

Meters and sensors in each plant can capture measurement readings, either directly from the devices through manual entry, or automated system integration with asset monitoring systems, test measurement devices, or other solutions. Threshold values are defined to independently trigger preventive or corrective maintenance work orders.

For example, a pressure reading near a filter may be below a predefined threshold value, indicating a clogged filter. A PdM system can then initiate a work order to have this filter changed and notify the appropriate personnel by email. Some equipment may even be set up to auto-generate work orders when a unit has been operating longer than a certain number

February 2018 ENERGY-TECH.com

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MAINTENANCE MATTERS of hours. Alternatively, corrective work orders can be issued by the PdM system based on certain detected behaviors, such as temperature fluctuations.

Challenges of data and the need for analytics While data collection and analysis hold great promise to support maintenance planning, a number of challenges exist in many power plants in the current fleet.

Most plant equipment and environment data is collected in a time-series format— that is, a sequence of data or events recorded along with corresponding timestamp values. Because of the sheer scale of power generation operations Figure 1. Path from raw data to actionable insight and data, humans are unable to monitor and correlate information in a data stream, a relational database management system, more commonly let alone multiple such streams, in order referred to as a relational database. This is a highly structured to understand patterns and derive value. For this reason, an data store that generally runs on a single, powerful computer, analytics ecosystem of technologies is required. known as a server, and that has a fast processor, plenty of RAM, and access to a lot of disk space.

Analytics pipeline What does an analytics ecosystem look like?

The path from raw data to actionable insight consists of several steps, as shown in Figure 1. It can be challenging to capture, analyze, and gain insights from the massive amounts of data that sensors and devices are capable of collecting.

To mitigate the flood of data, “edge” devices can connect to a gateway that filters and aggregates data before sending it downstream to a historian or staging area (that is, landing zone) for high-speed capture and retention.

In addition to sensors, data can come from enterprise applications, industry benchmark data, external feeds such as weather, or social media. But raw data by itself is not information. By structuring the data, it can be merged with other sources to give it context or meaning. This structuring occurs when loading data from the staging area to a data warehouse or “data lake”. Then the data can realize its true informational value as it is visualized using reports and graphs, analyzed, displayed on dashboards, or used for machine learning.

Finally, to really get the most out of the analytics process, decisions are made and action is taken. Such decisions and actions can be automated. For example, a certain combination of measurement values may trigger a work order to be automatically created and assigned in the enterprise work management system.

Data storage and processing To meet the needs of modern analytics, data is loaded and processed in file systems or databases, collectively known as data stores. For decades, the storage method of choice was 24 ENERGY-TECH.com

However, today’s predictive analytics workloads process large data sets that require a distributed architecture, in which the work is divided across multiple computers. Two data stores with distributed architectures are “not only SQL” (NoSQL) and Hadoop Data Lakes: • NoSQL databases use clustering technology to distribute data and workloads across multiple computers. This scale-out approach supports more rapid, flexible, and cost-effective increases in capacity, compared to the scale-up approach typically used by relational databases. The distributed architecture of NoSQL databases closely aligns with the elastic resource allocation capabilities offered by cloud computing methods.

• Hadoop Data Lakes is not a single technology, but rather, an ecosystem of related big data technologies originally invented and maintained by the open-source community. At the core of Hadoop is a distributed file system that allows large files to be split up and spread across multiple servers. One version of Hadoop, Apache Hadoop, has a familiar logo of a small elephant.

Data analysis tools and technologies The major categories of tools and technologies for data analysis are shown in Figure 2. They include query tools, reporting tools, data discovery tools, statistical analysis tools, and notebooks: • Query tools are used to query a database. They include SQuirreL SQL Client and Oracle SQL Developer. Big data

February 2018

MAINTENANCE MATTERS a component failure. Data governance consists of the people, processes, and systems required to ensure that data is valid, retained, and secure. It is usually a function of back-end enterprise systems.

Figure 2. Major categories of tools and technologies for data analysis

query and search tools include Apache Drill, Apache Impala, Redis, Sphinx, and Elasticsearch.

• Reporting tools are software packages, and more recently, cloud-based software services, that allow information consumers to see data in a structured manner, including standard tables and charts. Examples include Looker, Microsoft Power BI, QlikView, and Zoomdata.

• Data discovery tools take reporting a step further. Data discovery empowers engineers to follow up on new insights and ask new questions about their equipment and processes during real-time interaction with the software. These tools are good at enabling users to discover anomalies, outliers, and patterns in the data. Examples include Information Builders WebFOCUS InfoAssist+ and Oracle Big Data Discovery.

• Statistical analysis tools perform complex mathematical processing required for statistical, spatial, and predictive analysis. Examples include Alteryx and MATLAB. • Notebooks are web-based applications that are used for cleaning, exploring, and transforming data; visualizing results; and publishing or sharing those results. Examples include Jupyter Notebook and Apache Zeppelin.

Data governance Successful analytics efforts require governance considerations as well. For instance, the lineage of data from capture to final use, including any transformations in between, may be important to know when tracking down the root cause of

Data governance is about not only technology. It requires leadership and a designated team to plan necessary controls, prioritize them, and oversee their implementation. A major early step in implementing a governance program, especially for analytics, is to identify a data owner (or data custodian) for each major system or source of data. Data owners are responsible for ensuring that the data for which they are responsible is accurate, consistent, complete, up to date, and accessible, in accordance with the needs of the business.

Machine learning One important type of data analytics is machine learning.

Machine learning is a subset of artificial intelligence, and generally involves the development of computer programs that can access data and use it to learn for themselves.

A common use of machine learning is to classify entities into two or more categories. For instance, “cat” or “not cat” for images, “spam” or “not spam” for email, or, in the context of PdM, “OK,” “Low Alert,” or “High Alert.” This last example of classification is fundamental to predicting asset failure, and as such, is the most common use case for application of machine learning for PdM. In contrast to manually designing models based on engineering knowledge and principles of physics, a data-driven approach uses machine learning techniques to create models derived from curated operating and condition data. The datadriven approach supports the rapid, automated evaluation of numerous features and large volumes of data.

The machine learning process begins, first, with identifying the predictions that are going to be made. The input dataset contains numerous observations, with each observation containing feature (that is, attribute) values. The input training set, consisting of observations and features, is used to identify the set of possible predictive outputs, which are known as labels. Often data is not available in labeled form, and it needs to be transformed from raw data that is available. When the data is transformed, it needs to be converted to a format that is acceptable to the machine learning software and particular algorithms being applied. See Figure 3.

Figure 3. When the data is transformed, it needs to be converted to a format that is acceptable to the machine learning software and particular algorithms being applied February 2018 ENERGY-TECH.com

25

MAINTENANCE MATTERS Deployment of a machine learning model in a production setting is not the end of the model’s life cycle. For the model to retain its accuracy and predictive value, the distribution of new, incoming data must match that of the original training and testing sets. A drift in the distribution can lead to inaccurate predictions; this is a common issue that needs to be continuously evaluated. Monitoring the incoming dataset and quality of predictions can identify when retraining of the model is required. When the data profile changes or decreases in accuracy, the model can be automatically retrained. Another common approach is to retrain periodically (perhaps monthly

or quarterly), and automatically alert an analyst when the retrained model does not predict with sufficient accuracy.

Industry survey The EPRI Analytics Survey was conducted in March 2017.

Twenty professionals from across the power generation industry completed the survey. Respondents worked primarily in engineering, as well as management, maintenance, and IT. The most significant challenge related to data analytics, as rated by respondents, was the difficulty of analyzing information manually entered or scanned into text fields, such as operator notes and comments. It was also reported that the data flow process is hindered by too much manual data manipulation, in which performing useful analysis requires too many steps.

PERPETUAL MOTION

Another challenge respondents frequently faced was data integration. Silos of data are often stored in separate locations or managed by different business units, making it difficult to merge the data from different sources to realize the full value of an organization’s information assets.

Respondents were asked what decisions they would like to be able to make in an informed manner, sooner than they can now, so that faster action could be taken. A popular answer was remaining useful life of machines, including individual components, and decisions on when to repair, replace, or repurchase.

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Respondents were asked what additional data sources would be helpful inputs for PdM. The most popular answer was additional and better data, including maintenance history data. ■ Rick Roberts is the program manager of EPRI’s Maintenance Management and Technology Program.

Steve Seachman is a senior technical leader in EPRI’s Instrumentation, Controls, and Automation Program. You may email him at editorial@ woodwardbizmedia.com.

“A half-century of outstanding steam turbine experience” Skinner Power Systems A Division of Time Machine, Inc. 8214 Edinboro Road Erie, Pennsylvania 16509, U.S.A. Toll-free: (877) 868-8577 www.skinnerpowersystems.net

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February 2018

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MR. MEGAWATT

How do I know? -Tracking thermal performance -1 In our household Mrs. Megawatt is the holder of all things financial, if you have read many of these articles you probably know why. As the Megawatt household money monitor there are various “Key Performance Indicators” (KPI) that she uses to track everything from my latest RV blunder to the ongoing costs for the brewing operation. As such she can easily tell me the cost of that bottle of beer that I so proudly produce. Since the whole justification for my beer making endeavor was reducing costs this is an important indicator for her (I mean me). So, my modus operandi is to keep placing various things on my wish list so the cost bypasses the KPI via the generosity of the megawatt progeny.

This article is intended to be an introduction to the subject of how to track and trend plant performance indicators, future articles will dig into some of the details.

If you are involved in power plant thermal performance, the world of KPI’s is (or should be) in your wheelhouse. It may take on various forms depending on if you are a simple cycle, combined cycle, nuclear plant or fossil plant. A very common KPI for a power station is Capacity Factor which is essentially the ratio of MWhr’s Generated to MWhr’s possible based on the plant design value for generation. At some point someone wants to know how much product was made (MWe), how much it cost, and why we did not make what we thought we would make. Typically, the cost part of the equation is not something the thermal performance engineer will deal with. However, everything else ends up in their lap. It is very surprising how often we get so tied up in running a power plant that we forget why it is there in the first place. Sort of like a bartender who is frustrated that those people on the other side of that long skinny table keep interrupting his organizing the glasses. The KPI’s are a means of refocusing on what we are producing; electricity. The size and complexity of the power plant is often a function of how far removed we are from the product. At a simple cycle unit, it is pretty close but at a nuclear power plant running the reactor overshadows the production of electricity (which is why the reactor is there in the first place). Of course, another problem is that you focus so much on managing the KPI that you lose sight of what the KPI is for. I have been at power stations where plant management was concerned that the KPI was too low so we should move the target to make it look better which sort of defeats the purpose. Having said that it is important to understand what the target is. If the target is too high or not taking into account uncontrollable conditions then it will not be realistic. A thermal performance engineer wants to know

28 ENERGY-TECH.com

Figure 1. Fossil plant

how things are going on some periodic basis. This can be accomplished by considering the following things: 1.What should the power station be generating or what should the heat rate be 2. What is the power station generation or what is the heat rate 3. What is the difference 4. Why 5. How and when do we fix the problem Figure 1 shows an example of 1-3 for a fossil plant and Figure 2 for a nuclear plant. Figure 3 is another way to communicate the expected generation with respect to the actual generation. Figure 4 gets into the why we are not making our expected generation and Figure 5 is an example of the recovery effort.

Seems pretty simple, but often determining the expected heat rate or generation can be a difficult nut to crack.

Most power stations have some sort of design document or acceptance test which establishes expected generation at the design conditions. Of course, there is often a difference between what the design expected and what was actually installed. Also, depending on the age of the unit things may have changed, for instance new feedwater heaters, which could alter the expected heat rate or generation. Additionally, the design documentation and the testing are typically at or corrected to baseline conditions. The most important baseline values are thermal power and the condenser pressure

February 2018

MR. MEGAWATT

Figure 2. Nuclear plant

Figure 3. Another way to communicate the expected generation with respect to the actual generation

or condenser cooling water inlet temperature. There are other baseline conditions to be considered but these are the most important. A review of ASME PTC-6 provides a good explanation of turbine testing baseline conditions. As can be seen in Figure 4 the plant generation can vary significantly based on the temperature provided to the condenser. In establishing a baseline generation or heat rate you have to decide where to draw the box. If you just base your expected generation on condenser pressure then you will miss the effects of degraded condenser efficiency. If the plant is operating on a cooling tower then using condenser cooling water inlet temperature will miss the effects of a degraded cooling tower. Unfortunately, it is typically very difficult to evaluate the cooling tower due to the effects of weather, so most plants will use cooling water inlet temperature as the starting point for determining expected generation and develop another way to track cooling tower performance. Even when using the condenser cooling water temperature as the starting point, it can be difficult. The condenser design information can provide the expected condenser pressure for a given load at a given cooling water flow rate. This can be problematic if the condenser is not performing to design, not operated per design.

Additionally, some of the necessary parameters are not measured or the measurement is not accurate such as the condenser cooling water flow rate.

The condenser pressure correction curve provided with the turbine may or may not accurately reflect the variation in heat rate or generation. Often plants will determine the correction curves

Figure 4. Gets into the why we are not making our expected generation February 2018 ENERGY-TECH.com

29

MR. MEGAWATT

Figure 5. Example of the recovery effort

based on empirical or operational data. Another option is to use a thermodynamic model to characterize the expected condenser pressure and expected generation. Once again that pesky problem of obtaining an accurate condenser pressure measurement can foil the attempts at predicting expected generation. ASME provides guidance for test quality low pressure measurements that very few plants incorporate into their normal operational measurements.

Some plants will use their thermodynamic model to provide ongoing information on expected generation by providing boundary conditions to the model and having it determine the generation and other parameters which the actual plant parameters can be compared to. The ability to have reliable accurate measurements will also influence the outputs of these thermodynamic models. There are some newer calculational methodologies that can be incorporated into a thermal performance monitoring program that can deal with instrument errors. These newer methods incorporate the use of data reconciliation to provide corrections to the measured data.

30 ENERGY-TECH.com

As indicated above this article is intended as an introduction and future articles will discuss the details of how to develop a thermal performance monitoring program. Meanwhile, Mrs. Megawatt has informed me that she has produced another KPI to track the true value of the brew incorporating the progeny generosity correction factor. ■Mr. Megawatt is Frank Todd, manager of Thermal Performance for True North Consulting. True North serves the power industry in the areas of testing, training and plant analysis. Todd’s career, spanning more than 30 years in the power generation industry, has been centered on optimization, efficiency and overall Thermal Performance of power generation facilities. You may email him at editorial@woodwardbizmedia. com.

February 2018

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TURBINE TECH

Thermally sensitive generator rotors

By Brad Snyder, Senior Consultant, TG Advisers, Inc.

Thermally sensitive generator rotors tend to bow and increase bearing vibration. In some cases, a rotor rewind is required to restore the unit to acceptable operating vibration amplitudes. Thermal sensitivity is usually a symptom of generator rotor winding uneven heating or uneven growth. In this article, we discuss the causes and offer general advice for new equipment and repair specifications.

Turn to turn shorts can also lead to additional shorts, exacerbating the issue. Shorted areas are locally heated due to the I2R loss. This localized heating may accelerate thermal degradation of neighboring turn insulation causing even more shorted turns − informally known as the cascading short theory. Another issue is that a ground fault can occur if the localized heating burns through the ground wall insulation.

Uneven heating is typically caused by shorted turns of coils.

When coil turns are shorted, the field current does not take the designed path through all of the turns of the coil. Rather, the current takes a shorter electrical path and bypasses one or more turns. As a result, the total I2R ohmic heating loss of a coil slot containing short(s) will be less than coil slots containing non-shorted coils (adjacent slots and slots on the opposite side of the rotor). The I2R heating loss is less because the number of turns carrying the current has been reduced due to the shorted turn. Because the heating is a function of the square of the field current, this phenomenon will be more pronounced while operating the unit at higher levels of field current (i.e. the overexcited, rotor/field current limiting region of the capability curve). Typical causes of turn shorts include slipped or missing turn insulation, copper burrs, brazing spalls, uneven coil stack movement in the end-turns, and contamination.

Shorted turns are usually diagnosed by varying Vars via increasing and decreasing field current at a constant MW load and evaluating changes in generator rotor vibration. An stator-rotor air gap flux probe is another excellent tool for detection and diagnosis.Vibration issues, at least in the short term, can often be mitigated by balance moves reducing high load peak vibration with a trade-off of higher vibration at lower loads.

Not all rotors are equally susceptible to shorted turn related problems. For example, a rotor with a 5-turns per coil winding and high field current (e.g. 6,000+ amps) is more sensitive than one with a 15-turns per coil winding and lower field current (e.g. 1,500 amps). In a 5-turn per coil design, a full shorted turn reduces the ampere-turns in the slot by 20% versus 7% for a 15-turn coil design. These numbers are illustrative; not all field current follows the short.

Uneven growth of the generator rotor winding is a second contributor to thermal sensitivity. Uneven growth occurs when portions of the winding bind or stick with loading. The coefficient of thermal expansion of copper exceeds that of steel.

Accordingly, when the generator rotor is energized with excitation current, copper in the rotor slots will expand more than the steel rotor body. The winding must be able to freely move to accommodate this differential expansion. Otherwise, the non-uniform growth of the winding can create rotor unbalance and vibration issues.

Slip layers of the ground-wall insulation system (i.e. the slot liners/cells/armor and retaining ring insulation), blocking of the end-turn windings, and slot filling components are designed to allow the winding to expand and contract during operation.

If these systems do not function properly, binding and non-uniform expansion may result.

Heat runs after original manufacture or rewind are typically used to validate the winding system. The rotor is heated at speed to simulate copper expansion under CF loading.

Accordingly, the slip layers are tested to ensure proper materials and assembly. Although heating by energizing the rotor can mimic field operation, external heating along with windage and friction also works. The main objective is to know the rotor windings can expand and contract as designed without thermally induced vibration. Additionally, it is advisable to electrically test the rotor for shorted turns while the windings are in their hot/operational position and after cooling down.

Finally, blocked ventilation passages are another source of uneven heating and growth. If generator rotor coil ventilation passages are blocked, temperatures can easily exceed the operat-

Figure 1. Deteriorated end turn insulation causing shorted turns

32 ENERGY-TECH.com

February 2018

TURBINE TECH

Figure 2. Shorted turns in Pole 1 lead to thermally sensitive rotor bow

ing limits of turn and ground wall insulation. Typical blockages are caused by foreign debris, shifted turn insulation, or damaged end-turn blocking.

Brad Snyder is a Senior Consultant at TG Advisers, Inc. Brad and the TGA team provide engineering services to gas and steam turbine and generator users worldwide. You may email him at editorial@ woodwardbizmedia.com.

Thermal sensitivity is not just a concern for vintage designs.

Today, designers have excellent modeling tools to lower cost by reducing design margins or increasing capability for a given frame size. An example of the latter is 300 MVA air-cooled generators made possible by the use of computational fluid dynamics (CFD) to optimize cooling flow design and maximize heat transfer.

However, there is a limit to how accurate designers can approximate the rotational flows existing within a generator rotor. For example, since air is much heavier than hydrogen, it is sometimes problematic for the cooling air to access all the generator rotor coils, and depending on the direction of rotation and designed air flow management components, certain coils may run hotter and expand more than designers intended.

The net result of this will depend on which coil(s) are running hotter than expected. It is mentioned here as it relates to the possibility of bound expansion and contraction.

Specifications for new equipment or repair should include requirements to use proven technologies for slip layers, ground wall insulation, turn insulation, and adhesives. In-process, rotational, and expectations for final acceptance testing of the rotor should also be clarified. ■

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The Generator Auxiliary Systems Symposium will bring together plant operators, technical staff, risk managers and industry SMEs, all with an interest in generator auxiliary systems. Learn more about critical auxiliary systems for hydrogen cooled generators, with focus on gas cooling systems, associated safety standards, operating procedures and risk mitigation technologies. Attendees will earn continuing education credits and gain a wealth of knowledge while networking with colleagues from around the country during this multi-day symposium.

Join Environment One Corp. (E/One) and Energy-Tech University as they co-host this event.

The symposium begins with a meet & greet at 5:30 PM (EST) on Mon., July 30, continues all-day Tue., July 31 until noon on August 1.

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