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Strange knocks and occurrences

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A portion of an ammonia plant at a meat processing facility. Image credit: Cold Link Africa
A portion of an ammonia plant at a meat processing facility. Image credit: Cold Link Africa

By Andrew Perks

I have been thinking for some time about plant operators’ response to these factors in their refrigeration systems.I know as an apprentice I used to think, what was that – to which I was told, “oh that’s just the plant having a tummy ache.” Now as I have gotten older, not necessarily wiser, I have come to realise that those “tummy aches” are in fact the plant telling us that it needs attention as something is just not right.

We can look at the 2010 incident at Millard Cold Store where a large cold store in Alabama, USA, suffered a major release of some 1300kgs of ammonia from the system after a hydraulic shock incident resulted in the main suction header rupturing along with some of the cooling coils in the system failing at the same time.

There were 800 people working about 400 metres away in the open and across a major river when the leak occurred, 152 of them were affected with 32 being taken to hospital. 4 people were admitted to intensive care – no doubt the latter were asthmatic as they are seriously at risk with any chemical spill.

No doubt, the question you are asking yourself now is, “what is hydraulic shock?” For many years I have been aware of strange things happening in ammonia systems which, when you delve into them, there is always a logical answer. Let’s have a look then at hydraulic shock.

Hydraulic shock, liquid hammer or surge, can be caused by vapour-propelled liquid where the movement of the liquid refrigerant propelled at high velocity by high pressure vapour in hot gas and suction lines causes the liquid to impact on pipework in the system. This was not the case in the Millard incident though, as we will allude to shortly.

It is an acknowledged fact that of the majority of incidents in the industry, up to 80% are caused by operator error, the remainder being corrosion or lack of maintenance which we could say is also attributed to operator error, or just poor housekeeping. This particular incident, from the reports of the incident’s investigation team was in fact caused by operator error.

The background to the incident was that the cold storage facility had been part of a major power outage for some seven hours (we are not alone, power outages do happen elsewhere too). Obviously when the plant came back online, there was a lot of sorting out to do. During this period there was a cold room on defrost that a plant operator for whatever reason interrupted the defrost cycle mid process to get the cold room back onto cooling. Why he did this is open to conjecture, but probably he was under pressure to get some product down to temperature so he adjusted the controls via a programmable logic controller (PLC) to get the cooling going again and that’s where it all went pear-shaped.

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Lots of things happened. Firstly, the operator did not realise the possible implications of what he was doing. It’s the old story – for every action there is an equal and opposite reaction. What happened in the cooling coils was that when the cold liquid entered the cooling coil the internal hot gas condensed creating a vacuum accelerating the liquid forming a surge which impacted on the cooling coils distribution header, rupturing it.

From here it just got worse. The main return suction header was impacted by the system drop in pressure, causing suction gas in the return header on the roof to also condense creating a vacuum. (1 litre of liquid evaporates to form 85o litre of Ammonia vapour and vice versa). The net result was a surge of high velocity liquid being propelled by a differential gas pressure to strike the header end cap, rupturing the pipe and releasing the cloud of low-pressure liquid ammonia.

I specifically mention the low-pressure liquid as it tends to stay intact with less flashing off than say a high-pressure liquid release. Now I don’t really think that this was the first time that the system underwent these stresses, but it certainly was the last.

So, the question is: what have we learned from the event? It all comes down to competent people when working with hazardous materials. Sure, the operator got the blame but just how many of us would fail in the same set of circumstances? This is specialist training, not your normal plant operator. Not sure how many of us would fall foul of this incident, you can just be grateful it wasn’t you!

I am a great advocate of risk assessments before we do anything. Four simple questions – what am I doing, what could go wrong, who will it affect, and if it all goes pear shaped is there a plan to get it under control? There is never enough time to get it right but there is always enough time to fix it.

Till next month, stay safe.

About Andrew Perks

Image credit: 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.

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