I have a customer that has put two back pressure regulators in parallel in a fluid application. They are using one as a primary and one as a back up in case the first one fails. They were not meant to run concurrently. So after a year of service with no problems they have decided to turn both BPR's on at that same time. The setpoint pressure on one is 45psi on the other is 55psi. When one is on you get 45 psi, when the other is on you get 55psi when they are both on you get 25psi. I have no idea why this is the case. I would think that it would take the path of least resistance and still be 45psi. Does anyone know why this would be? This is weird, and I think you will find that they are using a remote sensing line on the high-pressure side of a partially-closed balancing valve or some such strange piping effect. Verify that the sensing point is actually at the point of interest. Without knowing the type of valve, there may be one other possibility: If it's a balanced valve installed with the flow direction reversed, the balancing would be defeated and the hydrostatic forces can be forcing the plug open when it should be closed. Check flow arrows on the valve bodies or tags.

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