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Posted by Boiler Guy on March 11, 2011 at 15:28:54:
In Reply to: Feewd water controller posted by Mark on March 11, 2011 at 14:55:07:
Two different things in your question:
It appears as if you are referring to a modulating feedwater valve opposed to an on/off type of feedwater supply. There are three fundimental control schemes: single element, two element, and three element. A single element scheme controls the valve solely as a function of drum level, two-element incorporates a steam flow meter to the drum level, three element adds a water flow meter.
Once you determine your control scheme you can move to the actual devices. The basic level control is a float such as a McDonnell Miller 150/157 or simililar for the specific application. You can also step up to dp transmitters and loop controllers such as Hays Cleveland or Preferred Utilities. That leads to the final question of how you intend to actuate your control valve, typically pneumatic actuated with a positioner but on smaller ones I've seen mod motor driven too. I've had great success with Fisher valves, model 667 if I remember off top of my head.
Boiler Guy