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Posted by Eric on March 23, 2004 at 13:57:43:
In Reply to: BOILER FEEDWATER PUMPS posted by HARISH on March 18, 2004 at 02:12:16:
The answer to your question is not easy. It requires more information than what you gave.
I assume that the boiler operating conditions are going to be fairly constant pressure with a variable flow.
First, the pump design point needs to be analyzed compared to the boiler operating point. If the pump was, for some reason, overdesigned, then a variable frequecy drive can be used to run the pump closer to the operating conditions of the boiler. However, the pump will run on its curve, meaning that when you lower the rpm, not only the flow but also the TDH will go down. So you will need to analyze the pump curve. Some pumps have fairly flat curves making the variable frequency operation more attractive, however, I would not try it on a pump with a rather steep curve.
The above still assumes that you use a secondary device (feedwater control valve) for the level control.
Replacing the feedwater valve with a VFD is much more complicated. It has been tried on some cyclic HRSG units to get around problems with the feedwater valves but I see it only possible with a very flat curve pump that is slightly overdesigned for you application.
If you want help determining what to do, then email me a pump spec and pump purve and a 24 hr. boiler load diagram.
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