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Posted by Bob D. on October 03, 2006 at 09:43:28:
In Reply to: Re: pump cavitation posted by Tony Conner on September 30, 2006 at 17:38:43:
Mr. Connor is correct, in that the intention of the deaerator is (1) remove dissolved oxygen from the feedwater by heating and increasing watr surface area by spray or trays, and (2) heat combined condensate and make-up to an appropriate temperature for the plant. "Typically", many commercial and smaller industrial plants opearte their DAs at 3-5 PSIG at the associated saturation pressure of 221 F to 227 F. Some specialty plants, like utilities, operate their DAs at 20 PSIG and up. If you have a tank that is operating at only 180 F, then you have a "feedwater heater", not really a deaerator. It is really important that the feed pumps that take suction from the DA be properly selected based on the total net positive suction head (NPSH) available. Like Mr. Connor said, there's a lot of ways to screw this up. A while ago, some pumps were designed to cavitate in order to control volume flow under certain conditions (submergence control) such as condenser hotwell level control. I haven't seen any like that recently. By and large, cavitation is bad, and leads to shortened impeller/seal/pump life. You'd really have to get into a little more of your plant's history to figure out all the "whys and wherefores". The fluctuating discharge pressure is a pretty good indicator that something isn't right. What your pressure should be is usually dictated by how the feedwater control valve was specified, i.e., what pressure drop across the valve lets the valve control best. Depending on what the designer of your plant was trying to do, and how old he was, the feedwater header could be anywhere from 15 PSI above the boiler pressure to 150 PSI above the boiler operating pressure. On the other hand, it could just be some equipment vendor's wild guess at what you should have, or what he had in stock at the time.
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