Intermittent Faults & A Valuable Lesson Learned
A recent post by Samanth Isin related to cavitation really hit home with me. Early in my refinery career I was trying to solve a problem where a really expensive pump and gearbox was getting wrecked, and replaced in-kind, approximately every 6 weeks. While the refinery employed a very sophisticated Honeywell TDC Distributive Control System, a common cause as to what created or lead up to these events could not seem to be captured. The NDT and Inspection crew would increase their frequencies of vibration readings on this machine and from their analysis could never really capture anything other "normal" or "critical". In between, production operators would report noises coming from this machine at various, but infrequent times. The service of this machine was in transferring aromatic hydrocarbons from an intermediate storage tank back into one or more of the refinery process units.
After 3 months of manually configuring and trapping various conditions around this pump, storage tank and other upstream and downstream inputs into the TDC, I found that because of the varying vapor pressures of the hydrocarbons being stored and transferred, this pump could exhibit degrees of cavitation, from slight to significant, depending upon the level of the tank. Needless to say, this pump was not properly spec'd (NPSH) for the range of hydrocarbon properties for which the tank was being utilized. The quick fix was establishing a minimum tank level permissive for the pump to operate, albeit limiting the desired function of the tank. The real fix was specifying the correct pump for the range of service (at 25% of the capital cost as well). This problem had been going on for over 3 years and being the new guy, I was thrown to the wolves.
Although this was years ago, the opportunity remains the same. Had we the luxury of continuous monitoring, this issue could've been solved quickly and without all the blame and emotion that consumed everyone in Plant Engineering, Inspection and Operations & Planning, not to mention the financial and service expense.
While there is much energy placed in the more mechanical causes of machine failure, always keep in mind the realities of process and methods eating your best mechanicals for lunch!
Comments
-
That was a great discovery. I am having an issue with loosing pump seals. I am currently installing endpoints for continuous monitoring to help locate and eliminate the problem.
4
