DA Tank overfill

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Posted by AKP on July 29, 2001 at 08:21:27:


Here is the situation. The plant consists of 3 Johnston 400hp dryback firetube boilers, and the standard associated equipment for this type of plant. The original design for our DA tank/Condensate tank system was this (again, industry standard operation), the Condensate tank received condensate and make-up water, the make-up through a level regulator actuating an automatic fill valve. This tank operated as a condensate receriver and a reservoir for the DA tank. When the DA tank called for water a valve opened and two small pumps took suction from the condensate tank and filled the DA tank to the proper level, simple standard operation. Problem. O2 pitting was found on some boiler tubes (likely a very old problem from when the plant was built, the contractor who built it steamed the plant with no chemical treatment, and tubes had to be replaced then as a result). A management decision was made to re-route the make-up water from the condensate tank to the DA tank, utilizing the emergency overflow valve as a fill to the condensate tank. Now, here is how this circus operates. The condensate tank calls for water and opens the fill valve, which now goes to the DA tank, the DA tank goes into a high water situation and the emergency overflow valve opens and fills the condensate tank. When the DA tank calls for water, it fills normally from the condensate tank pumps. So, the condensate tank fills the DA tank, and the DA tank fills the condensate tank! The idea was to deaerate the make-up prior to entering the system, however, teh condensate tank is not pressurized and is vented and drained open to the atmosphere! Tracing a drop of water, the condensate tank calls for water, our drop goes into the DA tank, heated and deareated, and overfilled into the condensatet tank, where it cools and is REareated! Then, when the DA tank calls for water it then moves back to the DA tank to be reheated and redeareated, then on to the boilers. We have many many problems with the emergency overflow valve, using it as a constant use fill valve, and the manufacturer stats that this is an abuse of this type of valve. Management says "it ain't so". The position of the operators is that this is not only a uslelss exercise in attempting to combat possible O2 contamination, but is an abuse of both the DA tank and overflow valve, but a high water situation in the DA tank can be dangerous. Your opinions? PLEASE resopond via email to Anthony.Pritchard@med.va.gov

Thanks all!


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