R
Compared to wet leg hydrostatic level applications with the vessel near atmospheric pressure, vessels with low absolute (vacuum) seem extremely critical when it comes to adjusting impulse line purge water flow. Why?
I have been involved in applications with hot purge water that would flash in the wet leg and make the level signal unusable. It is easy to see why you must keep the wet leg full for the system to work. What I am refering to are applications using low flow rotometers (<5GPM) in which balancing the flows with the rotometers can give 50% errors in level.(With cold purge water.) What we end up doing is checking the actual level in a sight glass, and then carefully adjusting the flow until the output of the D/P transmitter is equal to the actual level.
I have been involved in applications with hot purge water that would flash in the wet leg and make the level signal unusable. It is easy to see why you must keep the wet leg full for the system to work. What I am refering to are applications using low flow rotometers (<5GPM) in which balancing the flows with the rotometers can give 50% errors in level.(With cold purge water.) What we end up doing is checking the actual level in a sight glass, and then carefully adjusting the flow until the output of the D/P transmitter is equal to the actual level.
