By Andrew Perks
When do you know that the insulation has failed and there is corrosion going on at the pipework?
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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. |
I recently visited a customer in the Northern Cape who previously had an ammonia leak in one of his cold rooms.
Well, the plant had been around for some time, so the question asked by the client was what I thought about the condition of the rest of the site’s pipework. Do we do a patch job or go the ‘Full Monty’? The first reaction is “replace it all”. However, there is cost consideration, not to mention down time. Nobody wants to take off all the insulation as they will need to replace it and who’s to say that the pipework needs to be condemned – but the insulation most certainly would be.
While debating this, I was notified that IIAR were running a webinar on that very subject called ‘Clues and confirmation that your pipework is in jeopardy’. Now for you guys out there that are members of IIAR you will know that they have jacked up their presentations and have some interesting information available. This webinar was right on the money, and I would like to share some of the notable information with you.
The issue of corrosion on refrigeration pipework encompasses the entire plant. I know most people think it only happens under insulation, well not so. Some of the worst corrosion I have seen is on the high- pressure side of the system around the condenser area. Corrosion does not exist in every instance of trapped moisture. From statistics raised in the webinar it is evident there is a level of predictability, as I will highlight later. Far too often corrosion will thrive with no external visual indications.
It was stated that an average of 25% of pipework insulation is waterlogged, with levels fluctuating between 5% to 89%. No system pipework indicated the full quantity of moisture. During the IIAR study only 42% of waterlogged insulation showed signs of the problem. So just because there is no moisture or ice on the surface of the insulation doesn’t mean there is none beneath.
Pie chart ‘wet insulation’
The common cause of ‘wet insulation’ is a breakdown of the vapour barrier. This can be caused with poor consideration of expansion and contraction of pipework bearing in mind that the insulation is applied while the piping is hot, it will contract when cooled down in operation, mechanical damage caused by people walking on the pipework, poor bracketing etc. Vulnerable areas where pipework penetrates through walls and concrete roof slabs will always be suspect, the piping needs to be in a sleeve, and changes in direction at tees and elbow are seriously affected by expansion and contraction on long pipe lengths.
I suppose I can understand the undetected corrosion under insulation but what really does upset me is when I walk onto a site and the evident corrosion around condensers and other pipework that can only be classed as poor maintenance.
We are dealing with a hazardous medium: it’s quite easy to keep it in the pipes, all it needs is a proper maintenance plan and to monitor that it is being done. That’s why we do the three-yearly SANS 10147 plant audits. A plant that is tidy and clean always seems to work better.
The preparation of pipework prior to the application of the insulation is paramount to the longevity of the material. I once decommissioned an abattoir in Windhoek and the freezer pipe mains in one of the corridors was totally enclosed in ice. Well, there were two suction mains, one on each side of the corridor. Both circuits were decommissioned while one that had been painted with red oxide was totally corroded, the other was painted with bitumen and once the ice had defrosted it was as good as new, and we could have reused it. This is all about the preparation of the pipework when it’s new.
The type of insulation being installed should be done in double layers where the joints are all covered by the second layer. The pie chart indicates where wet insulation is more susceptible with elbows, tees and valves being the more problematic, accounting for 85% of moisture found trapped in insulation. With corrosion comes a wall thickness degradation. Overall, 51% of systems investigated for the presentation exhibited pipe thickness wall loss of more than 30%. Not all corrosion will lead to the material being condemned. Whether to repair or replace will depend on the actual corrosion, is it now below an acceptable limit. What are the limits? There are different thoughts on this, best get an NDT thickness test and an AIA’s opinion.
At the end of the day, it comes down to introducing a rigorously controlled and monitored corrosion protection programme.
