18 Degassing
Degassing: futureproofing, or eliminating installation problems? Dave Cannon, Technical Manager, UK, for Reflex Winkelmann, points out some of the issues that can be solved by degassing in a pumping system, and offers some advice for selecting a suitable solution.
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any within our industry would think of the installation of degassers, in both atmospheric and vacuum formats, as being methods by which issues such as air locking were cleared. Other than that, they are seen as a long-term solution to corrosion issues. We have all probably seen these problems encountered by both installers and manufacturers, but over the years we’ve gained an understanding of those issues, and the solution. With experience, our views on these issues have probably changed significantly. Some years ago, when most pumps had a fixed speed drive, it was my experience that service engineers would often be called to sites to be told ‘your pumps are down on duty’. This caused delays in practical completion, as systems could not be signed off. After a while, a pattern tended to emerge. Generally, the flow was down to 80-90% of duty, and it was often on chilled water circuits. In each case, it would be found that the pumped medium was highly micro-bubbled, compromising the hydraulic performance. Had a vacuum degasser been installed, the system would have been clear of any air/gas and the pumps’ performance would have followed the published performance curve. In each case, the project experienced unnecessary and often costly delays. These days, variable speed drives are the norm, but under fixed speed conditions, where a pump selection would have been made with an envisaged operating frequency of 40Hz, it was found that the pumps needed to run at nearer 50Hz to achieve the required duty. It’s worth remembering that because the hydraulic properties of the pumped medium are compromised, the efficiency of devices across the system will be severely reduced. By simply restoring the hydraulic properties to the correct values, the energy savings across the entire system can be considerable. Let’s look at a real-life example at Heathrow Terminal 2. It’s a chilled water system, with a content of some 1.3 million litres. I had spent some considerable time on site, wiring and setting up a dual Reflex Giga system and had always attributed the shaking of the building’s structure to sympathetic vibration, generated by the jets outside. Although ten years ago, I can still remember on a Monday morning, the commissioning manager asking how long it would be before the equipment would be operational, and how long it would take to degas
Quarter 3 2020
the pumped medium atmospherically. He went on to explain, they were unable to balance the system hydraulically, and it was proving impossible to achieve repeatable readings across the metering stations. I gave an estimate of about seven days, before I went off to complete the commissioning, Dave Cannon, Technical Manager, UK, and set the units up to Reflex Winkelmann. operate in a continuous degassing mode. It was only four days later when I returned to site that the commissioning manager was delighted to tell me that the system readings were now stable; however, that was not what impressed me the most about this installation. With the now smooth, laminar flow through the pump volutes and impellers, the structure of the building was no longer shaking, and the plant rooms were now relatively quiet. The vibration had been due to the 200kW pumps shuddering as small pockets of air entered the impeller vanes. The mechanical stresses on the pump shaft bearings must have been considerable. It is these conditions under which you will see spreading of the impeller keyways, rolling of the key steels and in extreme cases, failure of the impeller hubs. At that time, it also occurred to me that the amount of energy being dissipated must be huge to shake a structure of that size. Both from the manufacturers and installers points of view, one of the biggest issues is the repeated failure of mechanical seals on some of the larger pumps. Not only can this ruin the finish on an otherwise pristine plant room floor, but the installer will also ask if substandard seals are being used. An engineer will attend site, remove the mechanical seal, see that the rotating assembly is covered in deposits of material, the rotating and stationary seats are scored and conclude that the system has not been fully cleaned.
“The energy savings across the entire system can be considerable.”
www.bpma.org.uk
