August 2018

High steam turbine vibration due to coupling fit-up 18 I Instructional videos on plant practices 22 Cooling tower water conservation conundrums 26

August 2018

Instructional videos on plant practices

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FEATURE 5 Flexible operations – steam turbine high temperature casing cracking

By John Moreci, Consulting Engineer TG Advisers Inc.

Group Publisher Karen Ruden – kruden@WoodwardBizMedia.com General Manager Randy Rodgers – randy.rodgers@WoodwardBizMedia.com Editor Kathy Regan – editorial@WoodwardBizMedia.com Editorial views expressed within do not necessarily reflect those of Energy-Tech magazine or WoodwardBizMedia.

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COLUMNS

18 Machine Doctor

High steam turbine vibration due to coupling fit-up

By Patrick Smith, Sheekar S., Air Products & Chemicals

22 Maintenance Matters

Instructional videos on plant practices

By Dwayne Coffey, Electric Power Research Institute

26 Applied Tech

Cooling tower water conservation conundrums

By Brad Buecker, ChemTreat

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ASME FEATURE 9 Catalytic combustion in rotary regenerator type

combustors at different inlet velocities

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August 2018

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INDUSTRY CALENDAR

Turbomachinery and International Pump Users Symposia slated Sept. 18-20 Registration for the 47th Turbomachinery and 34th International Pump Users Symposia (TPS 2018) is now open, according to the Turbomachinery Laboratory at Texas A&M University.

TPS 2018 will feature a technical program selected by an advisory committee of industry leaders, combined with an international exhibition. The event will be held at the George R. Brown Convention Center in Houston, Texas, Sept. 18 ñ 20, with short courses offered on Monday, Sept. 17 in conjunction with the symposia.

The technical program includes short courses, lectures, tutorials, case studies and discussion groups led by experts in their fields. A preview of the technical program is available at tps.tamu.edu/manuscript-showcase/. See previous programs at turbolab.tamu.edu/ proceedings/.

The exhibition will feature booths from more than 300 companies from the oil and gas, petrochemical, power, pump and turbomachinery industries. Companies will display full-size equipment, cutting-edge technology and emerging industry trends. During a portion of the symposia, exhibit hall entry will be free to the public.

The annual TPS promotes professional development, technology transfer, peer networking and information exchange among industry professionals. TPS 2017, rescheduled to December due to Hurricane Harvey, attracted 4,620 delegates from 46 countries, and 359 exhibiting companies. ìI am excited to attend my first symposia as director of the Turbo Lab,î said Dr. Eric Petersen who assumed the post in March, following the retirement of former director Dr. Dara W. Childs. ìThis is where the best in the industry come to share their expertise. I am fortunate to be a part of this forum and look forward to supporting the advisory committees, leaders, staff and delegates as another successful symposia unfolds.î Petersen will deliver the welcome address at TPS on Tuesday, Sept. 18.

Industry events September 10-12, 2018 Condenser Life Cycle Seminar Royal Sonesta New Orleans New Oleans, La. www.condenserseminar.com September 18-20, 2018 Turbomachinery & Pump Symposium George R. Brown Convention Center Houston, TX www.tps.tamu.edu December 4-6, 2018 Power-Gen International Orange County Convention Center, West Halls Orlando, Fla. www.power-gen.com April 23-26, 2019 Electric Power Conference and Exhibition Mirage Events Center Las Vegas, NV 2019.electricpowerexpo.com

Registration is open online now and will be available onsite. For more information on TPS 2018, including the full technical program, event schedule, exhibiting company list, registration procedures and more, visit tps.tamu.edu.

The Turbomachinery & Pump Symposia is a conference for rotating equipment engineers and technicians worldwide, spanning oil and gas, petrochemical, chemical, power, aerospace, and water industries. TPS, founded in 1972, is organized by the Turbomachinery Laboratory, a center of the Texas A&M Engineering Experiment Station (TEES) and member of The Texas A&M University System.

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August 2018

FEATURES

Turbine Tech: Flexible operations – steam turbine high temperature casing cracking

By John Moreci, Consulting Engineer TG Advisers Inc.

Background Dispatching requirements for many combined cycle plants (CCP) have pushed design limits for fast starts, load and on-off cycling operations. Gas turbine technology incorporated in CCP arrangements typically has the design capability to cycle multiple times a day. But where does this leave the steam turbine? This article will focus on the impact of flexible operations on high temperature casings.

Cracking of high pressure, high temperature steam turbine inner and outer casings can result as a unit accumulates operating hours and cycles. Cracking is typically due to low cycle fatigue or fatigue in combination with high temperature creep.

Other factors such as design, casting quality, poor material properties, manufacturing issues and occurrence of operational excursions can be aggravating issues. In severe cases and if not addressed, cracks may propagate until they reach a critical size at which time rapid propagation and failure may occur. To prevent such a scenario, diligent inspection and maintenance is required.

stresses. In combination with geometric discontinuities, the local peak stress can exceed the material yield at temperature.

in high temperature steam leakage and unacceptably large deformations affecting alignment and hard part clearances.

Steady state conditions can also result in high thermal stresses. This is particularly true for inner casings since they are often subject to significant differences in steam temperature between inner and outer surfaces.

Diligent inspection and maintenance reduces the likelihood and impact of casing cracking. If cracking is significant, analysis provides insight into the risk of continued operation as well as to repair or replace options.

A further cause of cracking are operational excursions such as water induction. An introduction of water while casing surfaces are at a high temperature results in rapid surface chilling. The rapid chilling results in extremely high thermal stresses. A single and severe water induction event can result in casing distortion and greatly reduce the fatigue life of exposed surfaces.

Causes

Once cracks develop they may propagate with additional time and cycles. In extreme cases, cracks will propagate until they reach a critical crack length, at which time the crack propagates rapidly through the structure. Although catastrophic failures are rare, such crack propagation can result

Metals expand when heated based on the material coefficient of thermal expansion.

If a free casing of cylindrical geometry is heated gradually and uniformly, it will grow in all dimensions in a stress-free state.

However, if the casing is heated such that radial thermal and/or non-linear axial thermal gradients develop in the casing, the resulting thermal expansion and internal constraint of the cylinder geometry causes development of thermal stress. This stress is “selfequilibrating� in that the overall net stress is zero and any stresses that exceed yield are relieved due to plastic deformation. During transient and steady state operations of HP

Cracking will most often occur in and around inlet penetrations. These locations are exposed to high temperature differentials during both steady state and transient conditions. The casing around the inlet areas is usually thick walled with complex geometry. Other areas where cracking can occur are at transitions from the inlet to adjacent zones in the shell.

During transients, casing surfaces are exposed to rapid changes in steam temperature. Due to thermal capacitance, the inner core of the casing lags these changes. The result can be thermal gradients which cause high thermally induced surface August 2018

Photo1: Inner casing inlet inspection ENERGY-TECH.com

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FEATURES

Photo2: Example of cracking in a stress concentration of an inner cyclinder

and IP units, the turbine casings are subject to steam conditions which develop thermal gradients as described above.

In addition to thermally induced stresses, the casings are subject to stresses arising from primary loads such as pressure and loads transmitted from mating parts. The stresses from these loads are not relieved due to plastic deformation.

The combination of the above stresses in areas of stress concentration (e.g. sharp corners or edges) can result in very high localized stress. If the localized stress exceeds the yield, plastic deformation and residual stress result. Given enough cycles, cracks can initiate in such locations. This is typically due to low cycle fatigue given the relatively low number of cycles and high stresses involved.

Since these locations are subject to high temperatures at load, the combination of high temperatures and non-relieving stresses due to primary loads results in 6

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high temperature creep. The change of the material due to creep effectively reduces fatigue strength. Units that have operated over long periods of time (> 30 years) may experience cracking driven as much or more by creep as due to cyclic operation.

Once initiated, cracks may propagate with additional load cycles. The degree to which propagation occurs is driven by the stress field into which the crack propagates. In the case of purely low cycle fatigue driven by surface stresses, cracks may not propagate or propagate for a period of time and stop.

This happens when cracks are propagating away from a high surface stress environment into a lower stress field below the surface.

However, it is also possible a crack propagates into a stress field dominated by pressure or other primary loads. In such a case, the crack may continue to propagate.

In worse case scenarios, the crack may reach a length that equals the critical crack length based on material properties and

geometry. In such a case, the crack will propagate rapidly through the structure in the direction where the critical crack length is limiting. In this event, there may be severe consequences of steam leakage, gross deformation in the area of the crack, or a combination potentially leading to significant consequential damage and risk to personnel.

Several factors can accelerate the effects of cracking: severe transients that lead to sudden and rapid decreases in inlet temperature. Operation at temperatures above the normal operating limit (even for short durations). Cold start or shutdown transients more severe than predictable.

Reduced material properties due to manufacturing process (poor castings with high porosity) or machining (unintended stress risers). Poor design practices that do not properly or fully account for thermal effects have also been identified as a cause of cracking.

August 2018

FEATURES Due to the complexity of the geometry and the associate flow in the IP inlet, TGA utilized 3D finite element analysis for both a thermal and stress analysis. The analyses included a steady state case and thermal transient cases for a cold start and shut down. Since the analysis indicated the maximum stresses in the area where occurred exceeded the elastic limit of the material, additional finite element analysis for multiple start and stop cycles and accumulated creep damage were completed to evaluate inelastic effects and cumulative operating time. The results were incorporated into a crack growth model to evaluate the risk of continued operation.

In this case, the crack was predicted to continue to growth towards critical crack size. TGA provided a risk-based assessment of how many additional cycles and time the unit could operate before major repair or replacement was required. This provided the client sufficient time to evaluate weld repair options as well as determine cost and schedule for a replacement cylinder .■

Photo3: Finite element analysis of inner casing

Evaluation criteria It is important to non-destructively examine (NDE) casings for the presence of cracks whenever the unit is down for a major inspection or when the casings are available otherwise. If inspection confirms the presence of cracking, depths should be evaluated. Shallow cracks often can be removed by excavation, grinding or polishing as appropriate. For deeper cracks, further evaluation is required to determine severity and risk. In many cases, it is sufficient to note the crack depth for tracking and comparison at future outages. However, deep cracks should be analyzed to determine risk of continued operation.

To determine when analysis is merited, the critical criteria is the critical crack length and understanding of the nature of the underlying stresses. While under the critical crack length, crack propagation can be predicted and monitored. Once it exceeds the critical length it propagates quickly and in an unpredictable fashion. The other factor

August 2018

to consider is the stress field in which the cracks exists. Finite element analysis may be required depending on the situation.

John Moreci is a consulting engineer for TG Advisers Inc. Moreci and team provide engineering services to gas turbine, steam turbine and generator users worldwide. He holds a bachelor’s degree from North Carolina State University. You may contact him by emailing editorial@woodwardbizmedia.com.

Repair methodology If a crack is determined to be larger than what can be safely removed and likely to continue to propagate towards the critical crack size, weld repair may be an option.

Such repairs should only be performed by a well-qualified shop with demonstrated competency with complex repairs and stress relieving techniques. In extreme cases, component replacement may be the best option.

Case study TG Advisers (TGA), was recently engaged to develop recommendations for a combined cycle unit that relatively early in its design life revealed a number of cracks in the IP inner casing in and adjacent to the IP inlet.

The largest individual crack depth was approximately half of the local wall thickness.

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August 2018

ASME FEATURE

ASME: Catalytic combustion in rotary regenerator type combustors at different inlet velocities

By Zhenkun Sang, Zemin Bo, Xiaojing Lv, Yiwu Weng Key Laboratory for Power Machinery and Engineering, Shanghai Jiao Tong University

Abstract Ultra low calorific value gases (ULCVG) are hard to be realized by conventional combustion technology.

Most of them are discarded into the atmosphere directly, causing inadvertent waste and serous pollution.

Currently, a new type of gas turbine with catalytic combustion and rotary regenerator can be used to utilize these fuels and mitigate pollution.

is useful for the catalytic ignition.

The temperature distribution on the combustion side exhibits a smokingpipe-like shape, as well as the recuperative side. The results can provide data reference for RRTCC design.

Introduction

Differing from the conventional gas turbine, the chamber and regenerator of the new gas turbine is combined into one component, which is named rotary recuperative type catalytic chamber (RRTCC). The catalytic combustion is applied for RRTCC. The catalytic combustion characteristic of RRTCC is studied using the computational fluid dynamics (CFD). The results indicate that when the inlet velocity is 20 m/s, the methane conversion rate is 90%~95%, and the corresponding outlet gas temperature is 1030~1200K. When there is a variation of Âą25% in the inlet velocity, the variation of methane conversation rate is -15% and 5% respectively, and the variation of outlet gas temperature is - 6% and 2% respectively.

As a greenhouse gas, methane captures more than 20 times the amount of heat captured by carbon dioxide over a 100-year period [1].

Most ventilation air methane (VAM) and biomass gas, released during coal mine operations and agricultural activities respectively, belong to ULCVG, gas calorific value of less than 3 MJ / Nm3. It is estimated that by 2020 the world’s methane growth rate will be about 12-16% [2]. Usually ULCVG is directly discharged into the atmosphere, because the ignition and combustion of ULCVG can’t be realized by conventional methods. This action not only causes environmental pollution, but is also a waste of energy.

Therefore, recovery and utilization of methane in ULCVG, not only reduces methane impact on the greenhouse effect, but also generates available energy.

Additionally, it is found that the hotspot temperature of the combustor wall decreases with the increase of inlet velocity. The lowest value of hotspot temperature is about 1000K, which is higher than the ignition temperature of CH4. Therefore, the existence of hotspot temperature

Interest in this research appeared nearly 20 years ago. Through discussion and comparison of existing technologies for VAM mitigation and utilization, SU et al. believed that the Catalytic Monolith Combustor (CMR) is more suitable for power generation applications than fixed bed and fluidity

2018 9August ENERGY-TECH.com

bed reactors. This is due to its very low pressure drop at elevated mass throughputs, high geometrical area, high mechanical strength and high resistance to dust [3]. Catalytic flow reversal reactor is a common method to eliminate volatile organic compounds (VOCs), and relatively speaking, CMR is a more efficient, cost effective technology [4]. Catalytic combustion can be adapted to a wide range of concentrations and low temperature operation, and can achieve low or zero emissions of such gases as CO and NOx [5-8]. Australian Commonwealth Scientific and Research Institute had developed a novel ventilation air methane catalytic combustion gas turbine (VAMCAT) which included CMR and regenerator [9, 10], the recent experimental results shown that this system can produce 19 ~ 20Kwe power at very low methane concentration (vol.0.8%)[11]. Unfortunately the system volume and the metal regenerator are very large.

Rotary regenerators have more compact structure, smaller size and higher heat-transfer efficiency than conventional regenerators, because heat is not transferred across a wall separating the fluids but heat is stored and rejected alternately by each matrix element [12]. A rotary heat exchanger has a round disk matrix core making a large amount of heat-transfer surface.

As the matrix disk rotates, hot gas flows through and heats a portion of the disk, while cool gas flows in the opposite direction through the remaining preheated portion. MIT ENERGY-TECH.com 9 August 2018

ASME FEATURE effect of different inlet velocities on the catalytic combustion is investigated.

The results can provide a data reference for RRTCC design.

Description of the system

Figure1 Schematic process of catalytic regenerative gas turbine

Figure 2 Schematic of rotary recuperative type catalytic chamber

Professor David Gordon Wilson studied high temperature regenerator-type ceramic heat-transfer systems. The ceramic-honeycomb regenerator disk rotates incrementally through 90 degrees for each movement. In the tests the measured effectiveness was over 98%. The pressure drop of the hot and cold flows was under 2% [13].

Catalytic combustion is a complicated physical and chemical process. CFD can predict the complex flow field, even combined with heat and reaction transport, due to a state-of-the-art numerical algorithms and computer hardware. Numerical investigation of catalytic combustion is more and more prevalent [13-15], because it is very useful to understand the interactions between mass and heat transport and chemical reactions in catalytic reactors.

A cross-sectional channel shape had an effect on the ignition temperature, the conversion rate and catalyst costs [16]. Circular and square sections 10

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were investigated respectively by 3D CFD method. Results demonstrated that, due to the existence of hot spots the square cross-section channel was more resistant to extinction than the cylindrical channel [17]. Higher thermal conductivity material was useful to improve the inlet velocity [18], but will reduce the stability of the system.

However, most of these consequences were obtained under fixed wall temperature or heat flux densities conditions.

Due to the special combustor, the new gas turbine is different from traditional gas turbines. So this paper pays more attention to its rotary regenerator type catalytic combustion chamber. With the help of commercial CFD code, taking into account the special features such as periodic rotation, heat stored and released alternatively, catalytic combustion, based on unsteady conditions and multi-step chemical reactions, the

This new type gas turbine with coupled catalytic combustion and rotary regenerative, named catalytic regenerative gas turbine in brief (Fig.1), combined the function of combustion chamber and regenerator into one component. Compared to a conventional gas turbine, it has many outstanding advantages. First, ULCVG and air are compressed together in the compressor, and the main fuel is ULCVG. Second, pollution emission is low and combustion is easy to be controlled because of catalytic combustion application. Last, the composition of structure is more compact.

This gas turbine chamber is composed of a rotary regenerator type catalytic combustion reactor whose main part is a ceramic honeycomb monolith reactor (Fig.2). The catalyst is coated on the surface of the base structure. Heat exchange exists between the reactor wall and the gas.

In contrast to a metal regenerator, the ceramic core of this device can effectively allow more heat to be recovered and reused. In brief this device is not only a catalytic combustion chamber, but also a high-temperature heat exchanger. The rotary regenerator type of catalytic combustion reactor can be divided into two parts: the combustion and recuperative sides. There are both heat exchanger and combustion phenomena on the combustion side; there are only heat transfer phenomena on the recuperative side.

The system process can be seen clearly from Fig.1. The ULCVG and air mixture comes into the compressor from 1port. After the compressor the mixture gas comes into the combustion side from inlet 2a. The reactor wall which has stored enough power to heat the mixture gas to the August 2018

ASME FEATURE ignition temperature of methane, then catalytic combustion occurs rapidly, generating high temperature gas. The high temperature gas comes in the turbine from outlet 3 and drives turbine exporting machine power. Then the exhaust with sufficient energy enters the reactor from inlet 4. Because of the temperature difference between the exhaust and reactor wall, heat released from exhaust is stored in the reactor wall. In the end, exhaust is discharged into the ambient from outlet 5.

Due to the limit of heat storage capacity, after a period of time (5 ~ 20 s) the reactor wall in the combustion side can’t release enough heat to improve the gas temperature meeting the ignition temperature condition. The reactor should be rotated rapidly by certain angles (30 to 180 degrees).

After this action, the reactor wall on the combustion side can release heat to preheat the ULCVG to overcome the ignition temperature of methane again.

The pre-burner can be used as the startup chamber, and it is closed when this system is successfully launched. Also the pre-burner can be used as a compensate combustor when the methane concentration is lower than the design value.

Regenerative method provides the necessary ignition temperature for catalytic combustion without any extra preheating device. Rotating method can ensure the catalytic combustion occurring continuously, realizing stable operation of the reactor. This reactor can generate high temperature gas continuously which can be used in the boiler, turbine or SOFC service etc.

Figure 3 Schematic of the computational model

Figure 4 Verification of model: ignition temperature

Numerical models and simulation approach Model geometry and mesh The system simulation is hard to realize. We pay all the attention on the rotary recuperative type of catalytic chamber. The single channel hydraulic diameter of the catalytic chamber is 1~2 mm, and the length is between 5~10 cm, which is usually simulated AugustENERGY-TECH.com 2018 11

Table 1 Switch order of gas and waste-gas outlet/inlet

by one or several channels in the catalytic combustion condition [20].

Consider the rotary feature of this reactor, the 2D model with straight four

parallel passages is presented, Fig.3, combustion occurs in the adjacent two channels, while the heat recovery occurs in the other adjacent channels.

ENERGY-TECH.com 11 August 2018

ASME FEATURE For this simulation the parameters of each single channel are set as: h=1.2 mm height, s=0.4 mm wall thickness and L=508 mm length. Pt catalyst coated evenly on 74.2% L position, with a load density of 2.7063E-9 kg•mol/m2.

Table 2 Boundary condition setting

State I (solid line): 1-ULCVG inlet, 2-gas outlet, 3-exhaust gas inlet, 4-exhaust gas outlet.

State II (dotted line): 1-exhaust gas outlet, 2-exhaust gas inlet, 3-gas outlet, 4-ULCVG inlet.

Both solid and fluid field are generated by quadrilateral mesh.

Because fluid field is anisotropic, the node spacing is very dense about 0.01 mm in the vertical direction.

Because heat conduction in the solid field is isotropic, the node spacing in the reactor wall is sparse about 0.3 mm in the vertical direction. The mesh independent verification is considered by 30,000, 60,000 and 120,000 mesh cells respectively, at last the 60,000 mesh cells case is applied regarding the accuracy and calculation cost.

Figure 5 The effect of inlet velocity on outlet gas temperature

Reaction mechanism and mathematical model Due to small reaction space, low reaction temperature and short fuel residence time, in order to simplify the calculation, the gas phase reaction is ignored [21]. Also, the radiation heat transfer is ignored, because of the high aspect ratio of the channel (about 417) [22]. Ansys Fluent V12.0 software package combining methane on Pt 24-step reaction mechanism [23] is used to perform this simulation.

This mechanism contains the seven adsorption mechanisms of the reaction, the reaction surface 11 and 5 desorption reactions involving CH4, H20, CO2, CO, H2O, H, O, OH, N2Â component. The reaction rate is follows Arrheniu laws.

See Equations 1-7

Figure 6 Effect of inlet velocity on methane conversion rate.

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August 2018

ASME FEATURE Simulation methods and boundary conditions One rotation period of this reactor is 20 s, which includes two states, namely state I and state II respectively.

Stationary time of each state is 10 s, after 10 s this device is rotated rapidly by 180 degrees. Each 10 s exhaust and gas inlet/outlet should be exchanged according to Fig.3 and Tab1, the rotary function of this device is realized by this method.

The temperature drop in the turbine is constant by 220 K in this paper.

Because the outlet gas temperature is continuously variable in the period, the user defined function (UDF) is applied to decrease the outlet gas temperature by 220 K as exhaust inlet temperature after each calculated step. The mass flow of gas is equal to the mass flow of exhaust. The inlet and outlet boundary condition are set according to Tab 2.

The compressor outlet mixture gas temperature is about 427K after the compressor according to adiabatic compression process.

At the chamber wall, no-slip boundary condition and no species flux normal to the wall surface are applied.

Due to the low entrance Reynolds number, the flow is laminar within the channel. The fluid density is calculated using ideal gas law. The fluid mixture specific heat, viscosity, and thermal conductivity are calculated from a mass fraction with average of species properties. The boundary condition between gas and wall is coupled heat exchange boundary condition, the catalytic combustion occurs on the surface of the chamber wall. The honeycomb properties such as specific heat 970 J/(kg·K), density 1700 kg/m3, thermal conductivity 5 W/(m·K) is used.

The initial wall temperature distribution is linear variety from 420 K to 950 K which is realized by the user defined function.

EQ 1-7

Figure 7: Effect of inlet velocity on outlet flue temperature

AugustENERGY-TECH.com 2018 13

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ASME FEATURE Figure 4 shows that the catalytic ignition temperature vs C O2,in/ CCH4,in (a). The ignition temperature increases with increasing O2/CH4Â ratio.

From this finding, it is believed that the numerical model used in this study is reasonable and correct.

Results and discussion The effect on outlet temperature

Figure 8: Profile distribution along axis on the combustion side at t=10 s

It can be seen from Fig.5 that as the inlet velocity increases, the outlet gas temperature decreases monotonically.

At the low inlet velocity (15m/s), the outlet temperature in the state I (or II) is approximately linear variation. The faster the inlet velocity, the more the outlet gas temperature deviates from the linear variation. This is primarily because the higher inlet velocity can rapidly reduce the residence time in the reactor and heat transfer time between the gas and the reactor wall, at last resulting in the outlet gas temperature being reduced.

Figure 9: Profile of wall temperature along axis. v=20 m/s

EQ 8

Methane conversation rate is calculated by the formulation in EQ8.

Where C ch4,in  is the inlet methane concentration (by volume ). C ch4,out  is the outlet methane concentration (by volume ).

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Inlet velocity has a direct effect on the residence time which is a vital factor to the methane conversation.

Increasing the inlet velocity will lead to the residence time and methane conversation decrease. Decreasing the inlet velocity will lead to the residence time and methane conversation increase, but the volume of the system increases. A fixed composition (CH4 vol. 2%) of ULCVG is specified at the inlet boundary condition. Three cases under different inlet velocity, 15m / s, 20m / s and 25m / s respectively, are simulated and analyzed.

Model validation The simulation results based on this chemical mechanism and chemical kinetic data in this study had shown a quantitative agreement with the experimental data [22].

In the experiment, the catalytic combustion occurred on a platinum foil at atmosphere pressure.

The results shown in Fig.6 indicate that inlet velocity has an important influence on the methane conversion rate. At low speed (15 m/s) the methane conversation rate maintains at a high level (from 95% to 99%).

At middle speeds (20 m/s) methane conversion rate is relatively high from 90% to 95%. At high speed (25 m/s) the methane conversion rate has a relatively large range of change from 75% to 90%. With the increase of inlet velocity, the gas residence time in the August 2018

ASME FEATURE channel becomes short, resulting in the decrease of methane conversion rate and the outlet gas temperature.

When the inlet velocity (20 m/s) has increased by 25%, the minimum outlet gas temperature is changed from 1035 K to 981 K. The maximum outlet gas temperature is decreased from 1198 K to 1125 K. The variety is about 5-6% and 6-17% for gas temperature and methane conversation rate respectively. When the inlet velocity (20 m/s) reduced by 25%, the minimum outlet gas temperature is changed from approximately 1035 K to 1049 K. The maximum outlet gas temperature is changed from 1198 K to 1232 K. For this case the varieties of both outlet gas temperature and methane conversation rate are less than 3%. There is the same variety by 25%, but the outlet gas temperature and methane conversation rate have about 2-6 times differences. It means that there is a stable range for the inlet velocity. As long as the value of inlet velocity is between this range, the performance of RRTCC will be good.

The wall temperature along the axis increased slightly before ignition location, and decreased rapidly at the post-combustion zone. The maximum wall temperature is located at the ignition zone. With the increase of inlet velocity, the maximum wall temperature is decreased. Unfortunately at low inlet velocity (15 m/s) the maximum wall temperature is about 1700 K in the reactor. This value is close to the permissible limit of most materials, easily leading to catalyst deactivation [22]. Then the reactor should be rotated rapidly according to the operation period, to avoid heat accumulation in the reactor.

The broken lines in Fig.8 are the change of methane mole fraction along the axis at different inlet velocities. At the pre-combustion zone the content of methane is constant due to the only presence of heat transfer. At the catalytic ignition position, the content of methane is reduced dramatically, it

It can be obviously seen from Fig.7 that with the increase of inlet velocity, the outlet flue gas temperature in state I (or II) increases gradually. According to the mass conservation, the increase of inlet velocity will lead to the similar increase of exhaust, at last leading the decrease of heat exchange time between flue and the reactor wall.

Further the outlet flue temperature increases. There is only heat exchange but combustion on the recuperative side, due to the effect of thermal inertia of the wall, the outlet flue temperature is approximately linear variation.

The effect on the temperature profile along axis Because the maximum wall temperature is gradually increased in the state I (or II), this paper just analysis the wall temperature profile at the end time of each case. The solid lines shown in Fig.8 indicate the same trend for the profile of wall temperature even at different inlet velocities.

means that the reaction occurs rapidly.

Because of the different inlet velocities, the amount of methane is obviously different at the post-combustion zone.

It is clear that the lower the inlet velocity, the less methane content at the post-combustion zone, the higher the outlet gas temperature. This finding is in agreement with the previous studies.

The results shown in Fig. 9 demonstrate that reactor wall temperature at the same location gradually decreases at the precombustion zone, and gradually increases at the post-combustion zone. In the state I (or II) the outlet gas temperature and methane conversion rate gradually increased, but the wall temperature at the pre-combustion zone is reduced. This finding proves that the wall temperature at the postcombustion zone has an important effect on the characteristic of the reactor. In the ten seconds the hot

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ASME FEATURE spots temperature increases by about 500 K. There are high temperature zones in the vicinity of hot spots.

Even at starting time the hot spot temperature (about 1100 K) is much higher than the catalytic ignition temperature. This is very useful for the methane catalytic combustion. Thus it can be concluded that this reactor can burn the ULCVG, not only because reactor walls can store and release heat alternately, but also because the hot-spots temperature is higher than catalytic ignition temperature.

Conclusion The working process of the new gas turbine is introduced, especially the RRTCC. In this paper, the effect of different inlet velocities on the temperature distribution was studied. We found that this RRTCC which uses ULCVG as primary fuel can continuously generate high-temperature gas but a carefully chosen on inlet velocity is necessary for high efficient and stable combustion.

When the inlet velocity (20 m/s) increases by 25%, the outlet gas temperature is decreased by about 5%, and the methane conversation rate is decreased by about 16%. When the inlet velocity (20 m/s) is reduced by 25%, outlet gas temperature and methane conversation rate of change is less than 3%. It means that there is a range of inlet velocity for the high performance of RRCTT. There is the same trend temperature profile along an axis for different inlet velocities. The highest temperature is at the hotspot location, and the low temperature zone is located at the pre-combustion position.

When the inlet velocity is 20 m/s, the hotspot temperature is changed from 1600~1100K because of periodic rotation. The lowest hotspot temperature is still higher than the ignition temperature of methane, and it is very useful to catalytic combustion.

It can be seen that the profile of wall temperature exhibits a smoking-pipelike shape. Periodical rotation not only makes ULCVG continuously absorb heat from the recuperative side, but also avoids heat accumulation and catalyst deactivation.

In general, the inlet velocity not only determines the outlet gas temperature and methane conversion rate, but also affects the hotspot temperature and catalyst stability. Low velocity is easy to reduce the density of system energy output, causing the maximum wall temperature over the permissible limit temperature. On the contrary, extreme velocity is easy to reduce the outlet gas temperature and methane conversion rate. As a result, in order to stabilize the RRTCC performance there is only a narrow range of inlet velocity. â–

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ASME FEATURE Acknowledgements

References

The author(s) declare(s) that there is no conflict of interest regarding the publication of this article. This work was supported by the National Technology Research and Development Program of China (2014AA052803) and the National Natural Science Fund (51376123).

1. Warmuzinski, K., Harnessing methane emissions from coal mining. Process Safety and Environmental Protection, 2008. 86(5): p. 315-320.

2. Karakurt, I., G. Aydin, and K. Aydiner, Mine ventilation air methane as a sustainable energy source. Renewable and Sustainable Energy Reviews, 2011. 15(2): p. 1042-1049.

3. Su, S., et al., An assessment of mine methane mitigation and utilisation technologies.

Progress in Energy and Combustion Science, 2005. 31(2): p. 123-170.

4. Everaert, K. and J. Baeyens, Catalytic combustion of volatile organic compounds.

Journal of Hazardous Materials, 2004.

109(1–3): p. 113-139.

5. Sorab R., V., Low-emission gas turbines using catalytic combustion. Energy Conversion and Management, 1997. 38(10–13): p. 13271334.

6. Schlatter, J., Ultra-low NOx via catalytic combustion. Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy, 2000. 214(4): p. 377-383.

7. Ralph A. Dalla Betta, J.C.S., David K. Yee, et al. , Catalytic combustion technology to achieve ultra low NOx, emissions: Catalyst design and performance characteristics.

Catalysis Today, 1995. 26(3–4): p. 329-335.

8. Anson, D., M. Decorso, and W.P. Parks, Catalytic combustion for industrial gas turbines. International journal of energy research, 1996. 20(8): p. 693-711.

9. [Su, S. and J. Agnew, Catalytic combustion of coal mine ventilation air methane. Fuel, 2006.

85(9): p. 1201-1210.

10. Yin Juan, Weng Yiwu. Investigation of Combustion and Systematic Performance of a Gas Turbine SystemWith Lean Burn Catalytic Combustion Chamber. Proceedings of the CSEE, 2010. 30(11): p. 1-7. (in Chinese) 11. [Su, S. and X. Yu, A 25 kWe low concentration methane catalytic combustion gas turbine prototype unit. Energy, 2015. 79: p. 428-438.

12. Wilson, D.G. and J.M. Ballou. Design and performance of a high-temperature regenerator having very high effectiveness, low leakage and negligible seal wear. in ASME Turbo Expo 2006: Power for Land, Sea, and Air. 2006. American Society of Mechanical Engineers.

Nomenclature ULCVG Ultra low calorific value gas RRTCC rotary recuperative type catalytic chamber CFD computational fluid dynamics VAM ventilation air methane CMR Catalytic Monolith Combustor VOCs volatile organic compounds p density , [kg/m3] M molar mass D Diffusion coefficient h Enthalpy, [J/mol] R generation or consumption rate, [mol/ m3• s] H Heat of reaction, [J/mol] T Temperature, [K] p Pressure, [pa] u Velocity, [m/s] Y Molar mass μ Dynamic viscosity coefficient, [N•s/ m2]

Subscripts s s-th species w wall i, j x or y direction

AugustENERGY-TECH.com 2018 17

13. [Chou, C.-P., et al., Numerical studies of methane catalytic combustion inside a monolith honeycomb reactor using multi-step surface reactions. Combustion science and technology, 2000. 150(1-6): p. 27-57.

14. [Koop, J. and O. Deutschmann, Detailed surface reaction mechanism for Pt-catalyzed abatement of automotive exhaust gases.

Applied Catalysis B: Environmental, 2009.

91(1–2): p. 47-58.

15. [Cluff, D.L., et al., Capturing energy from ventilation air methane a preliminary design for a new approach. Applied Thermal Engineering, 2015.

16. Kolaczkowski, S., et al., Application of a CFD code (FLUENT) to formulate models of catalytic gas phase reactions in porous catalyst pellets. Chemical Engineering Research and Design, 2007. 85(11): p. 15391552.

17. Depcik, C.D. and A.J. Hausmann, Review and a Methodology to Investigate the Effects of Monolithic Channel Geometry. Journal of Engineering for Gas Turbines and PowerTransactions of the Asme, 2013. 135(3): p.

16.

18. [Di Benedetto, A., V. Di Sarli, and G. Russo, Effect of geometry on the thermal behavior of catalytic micro-combustors. Catalysis Today, 2010. 155(1–2): p. 116-122.

19. Hua, J., M. Wu, and K. Kumar, Numerical simulation of the combustion of hydrogen–air mixture in micro-scaled chambers. Part I: fundamental study. Chemical Engineering Science, 2005. 60(13): p. 3497-3506.

20. [Kaisare, N. and D. Vlachos. Optimal reactor dimensions for homogeneous combustion in small channels. Catalysis Today, 2007.

120(1): p. 96-106.

21. [RAN Jing-yu, HU Jian-hong. Characteristics of Generating Hydrogen from Methane-Wet Air Catalytic Reforming Reaction in the Microcombustor. Proceedings of the CSEE, 2007, 27 (8):42-48.

22. Sandip Mazumder, Michael Grimm, 2007, “Numerical investigation of radiation effects in catalytic combustion,” Proc. ASME-JSME Thermal engineering summer heat transfer conference, Vancouver, British Columbia, Canada. No. 2007- 32460 [23] Mazumder, S. Modeling full-scale monolithic catalytic converters: Challenges and possible solutions.

Journal of Heat Transfer-Transactions of the Asme, 2007. 129(4): p. 526-535.

ENERGY-TECH.com 17 August 2018

MACHINE DOCTOR

High steam turbine vibration due to coupling fit-up

By Patrick Smith, Sheekar S., Air Products & Chemicals

There are many causes of high steam turbine vibration. When the vibration is primarily at a frequency of one times running speed, unbalance or shaft alignment is typically suspected. However, rubs can cause similar conditions and rubs in steam turbines are more prevalent than in other turbomachinery due the high temperatures involved and resulting thermal expansions.

As described in “Steam Turbine Rotor Rubs”

by Smith, 2010, and “Steam Turbine Thermal Expansion Problem” by Smith, 2012, there are many causes of rubs. As a result, the cause of high vibrations in steam turbines can be difficult to diagnose with a high degree of confidence. Typically, more data and analysis is required.

When a new steam turbine is well referenced and being applied well within previous experience, a problem with coupling fit-up is usually not suspected. And even when identified, the corrective action is not always clear.

The purpose of this article is to present a case study of a high vibration problem due to an issue with coupling fit-up. The vibration behavior, investigation and corrective action will be discussed.

Introduction This case study pertains to a 21 stage, 5810 RPM, 46 MW condensing turbine that drives a generator through a gearbox.

The steam turbine to gearbox coupling is a flexible disc pack, spacer type. The arrangement is shown in Figure 1, and the turbine cross section is shown in Figure 2.

The turbine is fitted with tilting pad type journal bearings and tilting pad type thrust bearings, which are on the drive end (DE) of the steam turbine. The bearings are equipped with temperature probes and the turbine is also fitted with “X” and “Y” vibration probes at the inlet and exhaust ends and axial position probes at the inlet end. The drive gear is a double helical type. The pinion and bullgear bearings are equipped with temperature probes and there are accelerometers mounted on the gearbox for measuring case vibration. The machinery protections include high bearing temperature alarms, high high bearing temperature trips, high vibration alarms, high high vibration trips and high axial position alarms and high high vibration trips.

History Prior to the first start-up, the couplings between the turbine and gearbox, and gearbox and motor were installed. On the

first attempt to start the steam turbine the machine tripped on high non-drive (NDE) vibration. The vibration alarm set point was 90 microns p-p and the trip point was 118 microns p-p. These were defined by the supplier. By contrast, the API-612 limit would be 37 microns p-p. The supplier alarm set point was 1.7 times the API-612 limit and the trip set point was 3.2 times the API-612 limit. The supplier also defined a maximum normal operating vibration level of 63 microns p-p for a new machine.

A second start-up was attempted, but the train tripped on low oil pressure due to a bad pressure sensor. However, the speed reached 5253 RPM and the steam turbine NDE vibration was 90 microns p-p and climbing at the time of the trip. A third attempt was made, but was tripped manually by the operator when the NDE steam turbine vibration reached 99 microns at 5504 RPM and was climbing rapidly.

The steam turbine was then shut down and the steam turbine to gearbox alignment was checked. The results were reviewed and approved by the supplier specialists and no changes were made. The steam turbine DE and NDE journal bearings and the thrust bearings were removed and inspected. No issues were uncovered.

Figure 1: Train arrangement

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August 2018

MACHINE DOCTOR

Figure 2: Turbine cross section

Figure 3b: Waterfall Plot

Figure 3a: Trends of Vibration Excursions

The steam turbine was left uncoupled from the gearbox and a special fixture was made and installed to seal the DE steam turbine housing. Then, a fourth start-up attempt was made. This time the steam turbine reached the design speed of 5810 RPM with low vibration levels.

Under the direction of the supplier, the temporary fixture was removed and the coupling was re-installed with a different orientation. The coupling was rotated seven bolt holes relative to the hubs. A fifth start was then attempted without any change in the high NDE vibrations. The coupling was then field balanced, which brought down August 2018

the vibration levels to 98 microns p-p at running speed. Although this was below the trip set point, the vibration levels were still unacceptable. A summary of the vibration levels up to this point is shown in the table below.

It was then decided to install the spare coupling. The turbine to gearbox alignment was checked again and found to be slightly off, but within supplier limits. During the installation of the coupling, the technicians noticed that the coupling hub bolts were binding up during installation. Runout measurements showed that the coupling was being pulled off center. The coupling

was designed to be centered by a spigot fit that is within 0.02 mm of the shaft centerline and the hub bolts were supposed to be a loose fit. The coupling OEM designed the coupling to be centered by the hub flange using the body fit bolts. When the bolts were installed the coupling was being pulled off center to the hub bolt circle by 0.25 mm or more. To address this, the fit diameter of the hub bolts was machined down by 0.1 mm, which allowed the coupling to be centered by hand within 0.10 mm. The bolts and spigot fit were too loose a fit to center the coupling within the supplier requirements of 0.02 mm of the shaft centerline.

The steam turbine “X” and “Y” NDE vibration levels after the seventh start settled out at about 60/40 microns p-p. These levels were now low enough that the steam turbine was safe to operate and the machine was put into continuous operation. However, the replacement coupling with the modified hub bolts only resolved the short-term problem of excessive vibration and trips while trying to reach operating speed. There was an ENERGY-TECH.com

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MACHINE DOCTOR • Turbine rotor --imbalance, damage during assembly/operation, thermal instability, manufacturing/design errors.

• High speed coupling -- misalignment, manufacturing/design error, installation issue, and excessive runout.

• Gearbox -- manufacturing/design error and possible movement of gearbox gears during disassembly of high speed coupling • Turbine frame – stiffness, foundation bolts, control valve position and external forces The supplier eliminated all the possible contributors except for the excessive coupling runout. To understand this better, Figure 3 illustrates the hub interface issue.

On left is the spigot fit the steam turbine was designed for. It shows a close clearance fit at the OD of the hub and hub bolts with a loose fit in the holes. The drawing on the left is what the coupling manufacturer designed to. It shows a loose-fitting spigot and body fit bolts between the two flanges that are used to center the coupling.

Figure 4: Trends of Vibration Excursions

additional concern about long term operation and maintenance. If the hub to steam turbine flange joint is not properly centered, there will likely be installation and vibration problems in the future when the coupling is removed for maintenance. This could result in excessive downtime during outages and having to field balance the coupling again in the future.

The supplier was also requested to perform a root cause analysis to ensure that this issue was fully investigated with solutions that would address the long-term concerns discussed above.

Investigation The manufacturer created a fault tree with the following possible contributors of the high vibration issue: • Turbine casing --clearances, condensate drain function and thermal expansion

Let’s say the rotor weights 13,000 kg.

This is an estimate based on similar size steam turbines. The low speed rotor balance limits according to API-617 are based on the following equation:

• Turbine bearings -- lube oil parameters, manufacturing/design error, installation issue and/or damage during disassembly/ operation

U = 6350 * W/N where • U = maximum allowable residual unbalance in gm-mm

VIBRATION LIMITS ST Expected Max Operating Vibration Level, microns p-p

63

ST Alarm, microns p-p

90

• N = rotor operating speed in RPM

ST Trip, microns p-p

118

Gearbox Case Alarm, mms/second

7

Gearbox Case Trip, mms/second

11

In this case the static journal load would be half the rotor weight and so the maximum residual unbalance works out to 7079 gm-mm. The high-speed coupling weighs 403.5 kg. If the coupling runout is

• W = static journal load in pounds

LOCATION Run#

1

2

3

4

6

Yes

Yes

Yes

No

Yes

5639

5256

5504

5826

5810

ST DE “X”, microns p-p

77

58

71

28

98

ST NDE ‘X’, microns p-p

118

90

99

11

61

Gearbox Velocity (X), mm/sec

0.50

0.28

0.38

N/A

0.51

Gearbox Velocity (Y), mm/sec

2.14

1.34

1.88

N/A

2.00

Coupled to Gearbox Speed, RPM

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August 2018

MACHINE DOCTOR 0.28 mm that would mean the centerline is off by 0.14 mm. If half of the coupling weight is assumed to be supported by the steam turbine, then this results in an eccentricity of 0.14 mm and a possible unbalance equivalent to 28,245 gm-mm (403.5*1000/2*0.14). There are a lot of assumptions that go into this, but it shows that the possible unbalance due to coupling eccentricity could be several times higher than the steam turbine rotor balance criteria.

This supports the conclusion that coupling centerline offset could be a primary cause of the steam turbine high vibration.

API-671 is another reference which states that for couplings operating above 1800 RPM, the components of the coupling assembly shall be centered using piloted or rabbet fits, with a maximum eccentricity of 0.00008 mm/mm of diameter TIR (total indicated runout) or 0.013 mm TIR, whichever is greater. Based on the actual fit diameter of 403 mm, the TIR based on the first criteria is .032 mm. This is still almost ten times higher than the actual TIR. Another analysis that could have been done would have been to run an unbalance response analysis of the steam turbine rotor with an unbalance of 28,245 gm-mm at the coupling end to see if the actual rotor behavior matches the predicted response.

However, all signs pointed to coupling eccentricity as the cause of the high vibration.

Discussion To improve the vibration of the whole train, the supplier recommended a field balance.

They considered this as the final mitigation action because the balancing plane on the turbine rotor flange is designed for those purposes.

This was a concern because this corrective action may not be effective in reducing the vibration and may cause long term problems during maintenance after coupling removal. Prior field balancing yielded minimal reduction in vibration levels with two different couplings. In addition, this didn’t address the steam turbine flange to coupling hub alignment, which was concluded to be the primary cause of the high vibration. If the supplier still wanted to August 2018

pursue the field balance, they would have to demonstrate that there are no centering issues, and it was requested that they do this by removing the entire coupling after the test run post field balancing. If they could easily reinstall the coupling on-center and achieve the same runout and vibration readings from the post balancing test, then their solution would be acceptable. If this didn’t work, then further work would be necessary.

The supplier decided to modify the coupling by increasing the spigot fit and using new fitted bolts. Unfortunately, this didn’t work and the bolts had to be machined down again so that the coupling could be centered. The final eccentricity was 0.05 mm and the NDE vibration levels after start-up and loading the machine decreased to about 20 microns p-p. According to the measurements and observations during installation it appeared that the holes at the turbine rotor and coupling counterpart didn’t fully fit to each other, probably due to the manufacturing tolerances. See Figure 4 for the NDE “X” and “Y” vibration levels before and after the modifications.

Conclusion The cause(s) of high vibrations can be difficult to diagnose. In this case a communication problem between an experienced steam turbine supplier and an experienced coupling supplier created a coupling eccentricity that resulted in high steam turbine vibrations. This type of issue between two very experienced and reputable companies was a surprise.

Once the problem was identified it was critical to address the problem and not the symptoms. In this case the steam turbine supplier was recommending a field balance because it was quicker and easier to do, but it didn’t address the underlying cause of the problem. It is important to ensure that all the issues are addressed when suppliers are making recommendations to solve a problem. The final solution in this case study involved loosening the spigot fit and coupling hub bolt fit enough so that the coupling could be centered by hand. While not ideal, it was something that could be managed. ■

References 1. Smith, Patrick J., “Steam Turbine Rotor Rubs”, EnergyTech Magazine, October 2010 2. Smith, Patrick J., “Steam Turbine Thermal Expansion Problem”, Energy-Tech Magazine, September 2012 3. API-612,” Petroleum, Petrochemical, and Natural Gas Industries—Steam Turbines—Special-purpose Applications”, Seventh Edition, API, Washington, DC. 4. API-617, “Axial and Centrifugal Compressors and Expander-Compressors for Petroleum, Chemical and Gas Industry Services”, Seventh Edition, API, Washington, DC. 5. API-671, Special Purpose Couplings for Petroleum, Chemical and Gas Industry Services, Fourth Edition, August 2007, Reaffirmed, September 2010, API, Washington, DC. 6. © 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.

7. 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.

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.

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.

ENERGY-TECH.com

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MAINTENANCE MATTERS

EPRI: Instructional videos on plant practices

By Dwayne Coffey, Electric Power Research Institute

As electric utilities face the loss of a generation of long-time employees, what’s the best way of teaching inexperienced new-hires correct plant operating procedures?

Training methods might involve new-hires “shadowing” experienced personnel and observing practices in a kind of mentoring relationship. Or, they could include formal scheduled classes, computer-based training or informal on-the-job demonstrations. In other cases, first-time employees might be given technical manuals and procedural binders to read and absorb.

The challenge of training is not trivial.

Facilities can include hundreds of systems, each requiring unique step-by-step routines.

Procedures may not exist in written form but reside in the memory of veteran employees.

New employees may be overwhelmed by the number of procedures to learn. Different valves and switches can look the same to the untrained eye. Procedures incorrectly followed can have serious consequences.

Plant managers need ways of transferring operational information that is second nature in veteran employees to incoming new employees in a way that is clear, memorable, and not time consuming.

In recent years, the Electric Power Research (EPRI) has tried to address this need by developing a series of training videos. The series—called “What Does Good Look Like?”— demonstrates correct operating processes in a variety of operational areas. Each 5- to 10-minute video focuses on a specific process, with the process being visually demonstrated by actual utility personnel in a plant environment.

What does good look like?

EPRI’s Operations Management and Technology Program has long published reports to document sound operating principles. The reports were compiled by reviewing plant operating histories, incident reports, accident 22

ENERGY-TECH.com

Figure 1: Filming the “LockOut TagOut” video at South Carolina Electric & Gas, Cope Station.

reviews, equipment damage assessments, plant down-time reviews and personnel interactions.

These good operating procedures covered topics such as shift turnover, log keeping, etc.

Human performance

However, these reports did not provide visual examples of how these operating principles should be implemented or practiced.

• What Does Self Check Look Like?

Several years ago, an EPRI manager visited a plant to discuss best practices with personnel in a classroom setting and realized that many newer employees did not know what certain practices were supposed to look like. They had not had the opportunity to observe the practices being carried out on site. For someone who had never witnessed the procedures in person, describing them in a classroom or in a report did not fully convey how they would actually look.

• What Does Independent Verification Look Like?

With that realization, the idea for a series of training videos was born. The series began with several videos on human performance tools commonly used by power plant personnel to check and verify information. Over the years, more videos have been produced, now including 19 videos in four categories, as follows:

• What Does Questioning Attitude Look Like?

• What Does Three-Way Communication and Phonetic Alphabet Use Look Like?

• What Does Peer Check Look Like?

• What Does a Pre-Job Brief Look Like?

• Human Performance (Compilation Video) Lockout Tagout (LOTO) • Clearance and Tagging Process—Execution (Compilation Video) • Clearance Holder and Tagging Process (Introduction) • Clearance Writer and Approver • Clearance Hanger and Verifier • Clearance Holder and Zero Energy Checks Operations shift turnover • What Does Good Shift Supervisor Turnover Look Like?

• What Does Good Control Room Operator Turnover Look Like?

August 2018

MAINTENANCE MATTERS • What Does Good Outside Operator Turnover Look Like?

Operator rounds • What Does Good Outside Operator Paper Rounds Look Like?

• What Does Good Outside Operator Electronic Rounds Look Like?

• What Does Good Control Room Operator Paper Rounds Look Like?

• What Does Good Control Room Operator Electronic Rounds Look Like?

The topics for the videos were chosen based on the interests of EPRI member utilities. All videos are also available in separate Spanish language versions.

Figure 1: Filming the “Operator Rounds” video at East Kentucky Power Cooperative, Cooper Station.

Design of videos At the heart of each video is a demonstration of a process to show, as the title suggests, what a good execution of the process looks like. Over the action, a narrator describes what is being done, pointing out important features of what’s being portrayed. Crucially, the narrator also explains the rationale for the process—why it’s done that way. This explanation can help new employees, who may initially view the process as overly cautious or repetitive, to understand the need for the process. This need, which may involve plant safety or reliability, may not be selfevident unless explained.

Importantly, the videos are accurate and authentic. The videos adhere to the best practices described in EPRI’s written guidelines, and draw on the knowledge and insights from thousands of pages of reports. The scripts are written by power plant operations and maintenance personnel to ensure that the scenes and dialog are technically accurate and understandable.

The scenes are filmed on site at power plant locations, and those performing in the videos are not actors but actual plant employees. (See Figures 1 and 2.)

Each video also focuses on a single, defined topic, so that the amount of information presented can be understood and remembered.

Examples The initial series on human performance describes tools commonly used by utility personnel to check and verify information. These August 2018

tools can be used in a variety of plant situations.

As an introductory series, these videos help to instill good human performance habits.

The human performance video entitled “What Does Three-Way Communication and Phonetic Alphabet Use Look Like?” demonstrates two tools to aid in clear communication. Threeway communication, which is commonly used for giving and receiving instructions, involves one worker giving an instruction. A second worker repeats the instruction, and the first worker acknowledges that the second worker correctly stated the instruction. The phonetic alphabet assigns code words to the alphabet; the NATO phonetic alphabet is the most widely used version in the fossil power industry. A is alpha, B is bravo, etc. These tools are used whenever providing direction within the plant. In the video scene, a supervisor in a control room uses a walkie-talkie to instruct a technician in the field

to swap over the alpha pump. In a very noisy plant location, the technician hears the pump to swap as the bravo pump. When he repeats this, the supervisor corrects him, thus avoiding a mistaken action. The scene demonstrates what might appear to a trivial and unnecessary

ENERGY-TECH.com

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MAINTENANCE MATTERS routine is actually vital to successful plant operation.

The latest video series covers outside operator and control room operator rounds, when conducted with paper round sheets and mobile electronic devices. Good operator rounds help to determine plant status and configuration, observe operating equipment condition, identify any maintenance needs, and support reliable

and improved operational performance. They are one of the key foundations to high equipment reliability.

The video entitled “What Do Good Outside Operator Paper Rounds Look Like?” demonstrates best practice techniques, including: basic equipment needs of operator round personnel, sensory observations of equipment, recording of local plant equipment

information, use of applicable human performance techniques during operator rounds, use of a well-designed operator round sheet, performing temporary contingency response to plant equipment issue, generating work requests on equipment issues outside expected parameters, and the practice of observing general plant area conditions not found on round sheets.

Advantages of video Videos for training offer a number of advantages: • Watching them usually takes just a few minutes, thereby reducing training time.

PERPETUAL MOTION

• The videos can be viewed more than once, or paused, if a trainee is unclear on a concept.

• They do not depend on the availability of experienced personnel, and they do not take experienced personnel away from other duties.

• When the video is over, an on-site technical expert can discuss site-specific issues.

• The videos can be watched, on a variety of devices, at any time or any place.

• The videos can be archived, so that when the veteran employees are gone, their expertise is still available.

• Visual training retention is much higher than other training methods.

What every plant manager wants…

To achieve high productivity and financial success, today’s complex process operations require unyielding power that won’t quit. That’s why thousands of smart process operators have chosen custom-built Skinner Steam Turbine Generator packages to produce power & savings in such diverse applications as refineries, petrochemical plants, food processing and other facilities.

We configure the optimum system for your requirements up to 2.0 MW including a steam turbine generator along with other key accessories. Contact us for a reliable smaller-scale package that does the job you want it to do with a price to match.

Videos are also a good match for the new generation of utility workers, who are used to accessing digital information, doing Google searches, and watching short YouTube videos to gain new skills.

Utilities are using the videos in different ways.

Some are incorporating the videos into their plant training courses; others are posting the videos online to be viewed at the convenience of employees.

Future videos “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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A new series of videos will be shot in 2019 on a topic to be determined. ■ Dwayne Coffey is Program Manager of EPRI’s Operations Management and Technology Program.

You may contact him by emailing editorial@woodwardbizmedia.com.

August 2018

Specialty Machining and Welding Service Benefits An OEM that provides on-site contract services can represent a major cost and time saving alternative, to help ensure success for project managers.

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APPLIED TECH

Cooling tower water conservation conundrums

By Brad Buecker, ChemTreat

Water conservation at power and industrial plants continues to increase in importance, as fresh water supplies become more scarce. Also, industrial wastewater discharge quality has received increased scrutiny due to environmental impacts.

These two issues can cause a conundrum at plants with cooling towers, if water is conserved by increasing the cooling tower cycles of concentration to reduce blowdown volume, the concentration of impurities rises in the cooling water, which may cause the blowdown quality, and perhaps also air emissions particulate levels, to exceed limits in the plant’s discharge permits. This article examines methods, and sometimes drawbacks, of methods to avoid this conundrum.

Some cooling tower basics Once-through cooling was a common design feature for large power plants in the last century, as the process could effectively supply the high volumes of water needed for turbine exhaust steam condensation.

However, environmental concerns with regard to protection of aquatic creatures at both the intake and outfall of these systems

have essentially eliminated once-through cooling for modern plants. Now, cooling towers, or some variation thereof such as wet-surface air coolers (WSAC®) or even air-cooled condensers, are the norm. For purposes of this discussion, we will focus on the most common industrial cooling tower, as illustrated below.

Some key components of this design will be highlighted later, but for the moment the discussion centers upon heat transfer in a tower. Figure 2 illustrates actual conditions that might be seen in a cooling tower operating on a mild spring day.

Notice that the relative humidity (RH) of the inlet air is 50 percent, while the RH of tower exhaust is nearly 100 percent. This data helps to illustrate that the primary method of heat transfer in a cooling tower is via evaporation of what is typically a small fraction (2 to 3 percent) of the recirculating water. While the mathematics of cooling tower flow design can be somewhat complex, several simple equations have been developed to straightforwardly approximate the evaporation, blowdown and makeup flows to a cooling tower.

The standard formula for evaporation is, (Eq. 1) E = (f * R * ΔT)/1000, where E = Evaporation in gpm R = Recirculation rate in gpm ΔT = Temperature difference (range) between the warm and cooled circulating water (°F) f = A correction factor that helps to account for sensible heat transfer, where f typically ranges between 0.65 to 0.90, and which rises in summer and declines in winter The factor of 1,000 is a good approximation of the latent heat of vaporization (Btu/lb) of water at ambient conditions. From some previous work done by the author, ¦ for the example in Figure 2 calculates to 0.78. So, the evaporation rate for this example, with a recirculation flow of 150,000 gpm and a range of 27°F, is 3,159 gpm.

Cycles of concentration and water quality impacts Evaporation causes dissolved and suspended solids in the cooling water to increase in concentration. This concentration factor is (logically) termed the cycles of concentration (C, or COC). C, or perhaps more accurately, allowable C, varies from tower to tower depending upon several factors including makeup (MU) water chemistry, effectiveness of chemical treatment programs, and (the primary focus of this article) potential restrictions on makeup or discharge quantities. The algebraic equation for calculating the cycles of concentration is: (Eq. 2) C = MU/BD

Figure 1 Schematic of one cell of an induced-draft, counter-flow cooling tower. Source: Reference 1.

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Comparison of the concentrations of a common ion such as chloride or magnesium in the makeup and recirculating water will determine cycles of concentration, but August 2018

APPLIED TECH common in the field to calculate C is on-line measurement of the blowdown (BD) and makeup (MU) specific conductivities. The measurements allow for instantaneous blowdown adjustment to maintain the desired C value. In all cases, the cycles of concentration has a cutoff point, where further increases can lead to scaling or corrosion issues in the cooling system, even with good chemical treatment.

the other hand, convert significant quantities of water to vapor. This represents water lost from the supply. In some locations now, and California has been a noticeable example, an annual makeup limit appears in the plant permit. Often, the limit is expressed as acre-feet of water, where an acre-foot equals 325,851 gallons. The previous example suggested an evaporation rate of 3,159

gpm. This rate, projected over one day, calculates to a volume of 4,549,000 gallons, or 14.0 af. Thus, if the tower were at full load over a year’s time, the evaporation would be 5,095 af/yr. This author has seen permits in which the annual allowed makeup volume is near or sometimes even lower than what would be the annual full-load evaporation quantity. Of course, power plants typically do

The ratio of blowdown to evaporation is outlined by the following equation: (Eq. 3) BD = E/(C – 1) Besides blowdown, some water also escapes the process as fine moisture droplets in the cooling tower fan exhaust.

This water loss is known as drift (D). Modern mist eliminators can reduce drift to 0.0005% of the recirculation rate, and Brentwood Industries has come out with a design that reaches 0.00025% drift rate. Leaks in the cooling system are referred to as losses (L).

The following equation show the relationship between makeup and evaporation, blowdown, drift and any other losses. (Eq. 4) MU = E + BD + D + L With a well-designed and operated tower, the last two terms are negligible, so the water requirements of the tower are basically functions of evaporation and blowdown.

Returning to equation 3, the figure below illustrates the relationship in blowdown rate vs. cycles of concentration for the tower illustrated in Figure 2.

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As is clearly evident, the curve is asymptotic, and the reduction in blowdown at higher cycles drops off dramatically with increasing C. By the same token, the concentration of impurities increases, and it is these two issues that have led to the conundrum observed at a number of new facilities.

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The conundrum and potential solutions While once-through cooling has mostly been regulated out of consideration for new plants due to environmental impacts, one advantage of the process is a small loss of water, with virtually all of the intake volume returned to the source. Cooling towers, on August 2018

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APPLIED TECH But, this is where the pinch often occurs.

If the plant is restricted on makeup volume, the issue may be addressed via the following techniques: • Select a supply that may not have a quantity limit. In some places, alternatives to fresh water are being mandated. A common alternative is effluent from a Publicly Owned Treatment Works (POTW), aka municipal wastewater treatment plant effluent.

• Especially in the design phase, it is important to select makeup pre-treatment processes that clean such waters before introduction to the cooling system and other processes.

Figure 2 Example of a real-world set of conditions for a cooling tower. Source: Reference 2.

not operate at full load for an entire year, but such restrictions may still put plant personnel in a pinch, and especially at co-generation or other industrial plants that are often baseloaded. Now add the water lost to blowdown, and water supply issues may become acute.

A very common range for C in cooling towers

is 4 to 8, for two primary reasons. One, as Figure 3 indicates, the savings in water by operating at high cycles of concentration becomes markedly less as C reaches high values. Second, cooling water chemistry is much easier to control at moderate cycles of concentration.

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• Work with a reputable cooling water chemical vendor to select the proper chemical treatment program to maintain tower and heat exchanger cleanliness, and avoid extra loss of water due to efficiency degradation.

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August 2018

APPLIED TECH • Critical to this effort are accurate and comprehensive makeup water chemistry analyses. This issue is too often overlooked in initial project design, where some owners and owners’ engineers have the mindset that “water is water” without realizing the significance of the many impurities in all raw waters.

• Increase the cycles of concentration to reduce blowdown.

In locations where government and/ or industry officials are eager to conserve water, selecting an alternate source may be a good option with regard to quantity issues.

Wastewater plants need to get rid of their treated water, and do not normally have a problem with supplying the needs of an industrial facility. However, alternate supplies such as this can introduce significant complications to plant operations. Consider the following case history, which ties in the “water is water” comment from bullet #2.

Figure 3 Blowdown vs. cycles of concentration

The author was recently involved with a project in which secondary-treated POTW effluent was chosen for makeup to a combined-cycle power plant, most of it to

supply the plant cooling tower. The design engineers originally selected just basic clarification (for suspended solids removal) as pre-treatment for the plant makeup.

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APPLIED TECH Their thought was that a more sophisticated pre-treatment scheme would make the overall project bid non-competitive. When the anticipated tower makeup water quality was given to a major cooling tower manufacturer, the company’s personnel quickly replied that film fill could not be utilized in the tower due to the fouling potential, and that the tower would have to be designed with splash fill.

The increase in tower size would have raised the cost by nearly eight figures. This was a real eye-opener for the engineers, who then realized that their water group co-workers had been correct in insisting on more robust treatment. More robust methods include biological treatment, in this case a movingbed bioreactor (MBBR), to remove organic compounds and ammonia from the stream.

For more information on this subject, please see reference 3.

Beyond makeup pre-treatment, selection of proper cooling water chemical treatment programs for scale, corrosion and microbiological fouling is critical. Even with fresh water supplies, corrosion and deposition, and especially microbiological fouling, can occur very rapidly in cooling systems. The author addressed these topics in an Energy-Tech article earlier this year. [4] Lastly, at some plants where usage is near the withdrawal limit, plant personnel have decided to increase the cooling tower cycles of concentration to reduce blowdown. But, for each cycle of concentration increase, the impurity levels in the recirculating water grow accordingly. The increase places the cooling system under more stress, and requires additional chemical treatment.

Another issue that the author has observed more than once is a call for high COC, 10 or even greater, in the initial specs of projects where the supply water is plentiful and no makeup quantity limit is in place. It seems as if the owners wish to be “green” without realizing that water savings at these high cycles are very minimal, and essentially only increase the difficulty of chemical treatment and cooling system operation. Also, the higher concentration of dissolved solids can potentially cause the cooling tower plume to be in violation of the plant air permit. Modern cooling tower mist eliminators have helped in reducing particulate loading in tower plumes. ■

References

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1. Post, R. and B. Buecker, “Power Plant Cooling Water Fundamentals”; pre-conference seminar to the 37th Annual Electric Utility Chemistry Workshop, June 6-8, 2017, Champaign, Illinois. To learn about future EUCWs, please go to the web site, www.conferences.illinois. edu/eucw 2. Potter, M.C. and C.W. Somerton, Schaum’s Outlines Thermodynamics for Engineers; McGraw-Hill, New York, NY, 1993.

3. B. Buecker, “Gray Water for Industrial Plant Makeup”, Industrial WaterWorld, Mar/Apr 2018.

4. B. Buecker, “Industry Spotlight: Keeping your cooling system clean”, Energy-Tech, May 2018.

August 2018

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balances, and advanced inorganic chemistry. He is a member of the American Chemical Society, American Institute of Chemical Engineers, American Society of Mechanical Engineers, Association of Iron and Steel Technology, Cooling Technology Institute (via corporate membership), National Association of Corrosion Engineers, the Electric Utility Chemistry Workshop planning committee, and the Power-Gen International planning committee. Buecker has authored many articles and three books on power plant and water/ steam chemistry topics.

You may contact him by emailing editorial@woodwardbizmedia.com.

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Brad Buecker is Senior Technical Publicist with ChemTreat. He has 35 years of experience in or affiliated with the power industry, much of it in steam generation chemistry, water treatment, air quality control, and results engineering positions with City Water, Light & Power (Springfield, Illinois) and Kansas City Power & Light Company’s La Cygne, Kansas station. He also spent two years as acting water/wastewater supervisor at a chemical plant.

Most recently he was a technical specialist with Kiewit Engineering Group Inc. Buecker has a B.S. in chemistry from Iowa State University with additional course work in fluid mechanics, energy and materials

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