Back Pressure Regulating Valves

[vicarsb]

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?

 

[insult2injury]

Two parallel flow paths = less resistance than 1 flow path = lower pressure

I2I

 

[BigInch]

Apparently they are both set to begin closing when pressures reach their set points. When only one is turned on, for example, the 45 psig regulator, it adjusts to some percent open to maintain the back pressure at or below its setting of 45 psig. When only the 55 psig requlator is on, it tries to adjust its position to maintain its backpressure at or below 55 psig. When only one of these regulators is on, there isn't enough flow through it to keep the backpressure from building, so it builds to the setpoint and the controller kicks in and varies the flow to hold that pressure.

When both valves are on, the large flow through both will not allow any backpressure buildup, so neither controller ever starts to reach its set point and both valves stay 100% open for which condition 26 psig is the discharge pressure you wind up with.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[JLSeagull]

Consider the regulators like using proportional only control. The setpoint and results occur at one load condition. The result of a load change will be an offset.

 

[JimCasey]

Generally the set pressure with a BPR is the cracking pressure.
Pressure experienced in the system should not much exceed 45 psi. JLSeagull is correct that they should act with proportional only control but the proportional band should start at 45 psi. Typically the proportional band is about 10% of the range, so if these are adjustable to 60, the PB would be about 6, so cracking at 45, full open by 51 on the first one. (actual numbers may vary, but you get the idea)

In a first-ever event: I apologize for being blunt but I think insult 2 injury is wrong. These are not like parallel resistors,if you want an electrical analogy, but more like parallel zener diodes. The breakdown voltage is analogous to the setpressure, and there should be no flow until the pressure reaches the magic number. UNLESS THE VALVES ARE STUCK OPEN.

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.

 

[zdas04]

I think this is a broken valve or the wrong valve.

A BP valve is intended to maintain a constant backpressure. In the above example, both the 45 psig valve and the 55 psig valve should be shut at any value below 45 psig. With slightly increasing pressure, the 45 psig valve would begin opening to try to maintain 45 psig. At 55 psig the second valve would begin opening (and the 45 psig should be fully open).

The only way that you could get 26 psig is if a valve is broken or you've installed a PR valve instead of a BP valve. A PR valve (like a suction controller on a compressor) will be fully open until downstream pressure approaches its setpoint then it will begin throttling towards shut. If one or both of the valves is a PR then it (they) would be fully open anytime the pressure is less than 45 psig (or 55 psig).

David

 

[BigInch]

I think (above) its like Insult2 said and that he's got working backpressure valves (they work separately), but instead of having NC = +1, its -1, causing them to hold <= SP rather than >= SP, or as Zdas says, turning them into "PR" valves.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Scotsinst]

zdas04 is correct.