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Posted by bgb on February 16, 2007 at 03:13:28:
In Reply to: McDonald-Miller for level controll on DA tank posted by SUNY Cortland Heating Plant on December 13, 2006 at 23:36:02:
Have you considered eliminating switches altogether and going to a continuous modulating system? For example, if you visit www.isemagtech.com, you'll find magnetic level gauges with optional transmitters (magnetostrictive) on the side. The gauges provide you with visual indication via magnetic flippers, and the external transmitter can provide you with a 4-20 mA modulating output that can be fed to a simple controller that adjusts the feed valve. In addition, you can add magnetic switches to the side of the gauge should you wish a back-up to the transmitter. This solution is budget dependent, but could likely be implemented in a cost effective manner as well. Note - maintenance is gone, as the transmitters don't "wear out" or require calibration.
In addition, if you did increase the deadband between existing switches (i.e. to 1" from 1/2" control), you'd possibly extend the life of the switches to 2x current based on twice the volume to fill and therefore twice the time, and half the cycling. This of course depends on the flexibility of the system to accomodate a larger swing in level.
: We use a McDonald-Miller for level controll on our DA tank. This holds a tight and constant level for us (about 1/2 inch), but the repeated cycling of the switch wears it out in about six months. Does anyone have any sugestions of what else can be used for our level controll. I thought about stagering two MM's about five inches apart, the lower one to turn condensate pumps on and the upper one to turn it off. but i fear that this will add water to fast and drop the DA tank temp and thus limit the oxygen removal. Thanks.