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Posted by Todd on October 27, 2004 at 11:46:43:
In Reply to: water column and water level controller posted by MDysert on October 17, 2004 at 07:31:10:
: Recently replaced controller (McDonnell-Miller
: 150s) and seems to be extremely sensitive
: to water oscillation when feed water pump
: comes on causing numerous low water
: shutdowns. Have increased blowdown in that
: suspected TDS problems but indications are
: that has not significantly increased. Should
: I suspect my water column equalization lines?
: Any other obvious things I am overlooking?
Notice to all:
After reading all the follow-up threads to this post, it seams like everyong interested is suffering from the same problem. When the boiler water level suddenly "drops" 3 to 4 inches, this is an indication that the feedwater delivery system is not responding fast enough and allowing the boiler to flash off alot of steam very fast. Usually what happends, is that the water will swell just enough to throddle back on the feedwater input, but as soon as the boiler starts putting out steam in massive amounts, it calls for the feedwater at a rate faster then the pumps can deliver it and thus, the boiler trips out on a LWCO switch. Any boiler situation where the feedwater pumps are turned on with a column mounted level switch is a poor design to start with. Although the McDonnald switches are relitivly reliable, they leave very little room for proper tuneing. The best way to correct this situation is to do the following:
1) Determine the total safety valve capacity of the boiler in question. If the feedwater pump(s) are rated for this amount or less, then the feedwater pumps are TOO SMALL. - Replace the pumps with one(s)at can deliver the proper feedwater, including line friction loss and by-pass, to the boiler.
2) If the pumps are ok from a size standpoint, then a small redesign of the feedwater system is in order. This involves doing one of a few things.
A - run your feedwater pumps all the time and install a by-pass line to prevent the pump from reaching its' break horsepower, re-checking the pump capacity to verify that it can still deliver the proper amout of water. In the by-pass line, install a back pressure regulator instead of an oriface plate. This will maintain the proper feedwater pressure in the system and utilize the full capacity of the pump.
B - Replace the McDonnald switch with a 4-20ma transmitter and install a PRV to control your feedwater flow into the boiler. Connect the transmitter to the PRV. ( You an also use the McDonnald switch to turn on the feedwater pump, but use the transmitter to control your water level, not the McDonnald switch.)IF you are installing a PRV, insure that the Cv across the valve does not decrease the DELIVERED feedwater capacity to the boiler - Remember....you still need to deliver to the boiler the MINUMUM amout of feedwater that the safety valves release.
I hope this helps...:)
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