Back to basics
HYDRAULIC SHOCK
By Andrew Perks
ANDREW PERKS
Andrew Perks is a subject expert in ammonia refrigeration . Since undertaking his apprenticeship in Glasgow in the 1960s he has held positions of contracts engineer , project engineer , refrigeration design engineer , company director for a refrigeration contracting company and eventually owning his own contracting company and low temperature cold store . He is now involved in adding skills to the ammonia industry , is merSETA accredited and has written a variety of unit standards for SAQA that define the levels to be achieved in training in our industry .
Where do the months go ? What a year 2020 was , coronavirus , USA elections , lock downs and all the usual life-issues . 2021 is going to likely be a struggle year but surely it holds more promise than the one we have just endured .
have mentioned before some of the advantages of lockdown for
I me . One of these is the technical webinars that are now available . Being an IIAR member I have access to loads of information . I watched an interesting webinar on hydraulic shock in ammonia systems which taught me a lot . It has always been my contention that we need to listen to our plants because when they start making weird noises we need to ask why .
There was an incident in August 2010 at the Millard facility in Alabama where the system suffered from hydraulic shock releasing 14 500kg of ammonia . I had already looked at the incident but the IIAR webinar went a lot deeper . The piping at an end cap of a 300mm suction header catastrophically failed .
Now , I thought how does that happen when it is not in the flow path ? I have seen end caps come off on hot gas headers due to condensed hot gas forming liquid in the header where due to velocity pressure acts like a liquid slug . That ’ s all pretty understandable and easy to design out of a system , but this header failure was something different .
Ammonia has an expansion rate of 850 to 1 . That is 1l of liquid will form 850l of vapour - pretty massive and is why we don ’ t want to lock liquid up in a system . I have heard recently of two condensers that have been locked up full of liquid and left to stand . They didn ’ t do too well , Both failed and it was estimated that they developed internal pressures of between 80 to 100 Bar prior to failing . As I said , that is understandable .
On the other hand , we need to consider the condensation of vapour . That said , 850l of vapour when condensed will result in 1l of liquid . This creates a vacuum . I never really appreciated the forces at play when this happens .
At Millard , similar to a hot gas header with a liquid slug being driven by a hot gas stream , the mentioned suction header had a slug of liquid driven by a vacuum when the plant conditions permitted the suction gas to condense . Same difference , but the pipework was sitting at -40 º C . That ’ s a different set of circumstances . Now we are looking at brittle fracture , and did it fracture !
You might ask how did this happen ? We have plants all over South Africa and this doesn ’ t happen to us . Well , I beg to differ . I was recently at a plant where the end cap of a blast freezer coil ruptured releasing 4 000kg , and it ' s not the first . These events were also covered in the IIAR webinar . Sometimes we just don ’ t really appreciate the forces at play in our systems . Quickly pushing pressure and temperature up in a cooling coil then rapidly reducing the pressure and temperature has its issues .
As I said , a well-designed ammonia plant is not supposed to moan and complain , especially not after or during a defrost cycle . The event at Millar is reputed to have been caused by a plant operator interrupting a defrost cycle where a whole load of different forces was put into play resulting in a hydraulic shock incident .
There are some interesting circumstances around this event . The plant had a power outage for 7 hours and on start up a particular room went onto defrost . The plant operator interrupted the defrost cycle to get the room back onto cooling , and was probably told to get that room on temperature and how did he do that ? You got it . He went into the PLC programme and did a few modifications with disastrous results . Operator error , lack of risk assessment , bad instruction from someone who should have been aware of the possible consequences and a whole lot more .
Again , the lack of training and appreciation of actions , and their consequences .
As we are still using old-world technology in South Africa most of our defrost systems are hard wired and follow a specific sequence so this type of incident is highly unlikely but not impossible . It ' s up to us as an industry to ensure that plant operators and supervisors fully understand the consequences of their actions and that before we institute any changes or new procedures , a full risk assessment is carried out to quantify the risks and avoid rushing into responses .
Funny how there is never enough time to get it right the first time but there is always enough time to fix something .
I have recently been involved with an MHI report on a plant in Cape Town and I have been looking at the hazard analysis – some interesting issues to discuss next time . So be safe until then . RACA
www . hvacronline . co . za RACA Journal I January 2021 63