Back Pressure Regulator Sizing 2

[PuffJr]

A process to which I am pumping liquid has maximum allowable inlet pressure associated with it. I am being asked to limit the pressure of the incoming liquid using a back pressure at the tank from which I am pumping.

I would appreciate some help in mastering back pressure regulator terminology and a few concepts. In the past I have successfully sized PRVS, which are similar, so I should be able to size my regulators as well, once I clear this initial hurdle.

First, I know that a viscosity correction factor enters into this, and I understand why, but where do I obtain this number?

Second, I have seen references to "desired high flow rate" and "max allowable high flow rate." What is the difference?

Finally (for now), what pressures are used to determine delta P in the flow coefficient?

This will probably be the first of several questions on back pressure regulators.

 

[MortenA]

"Back pressure" regulators (or valves) are like any other control valve - noting magic there.

If you know how to size a valve then you know how to do this.

You 2. quetsion: Max allowable flow rate is the flowrate at valve full open

Desired high flow rate is the max normal flow rate (best control point=60-70% open)

You must know what pressure you want to maintain. But downstream it will usually be low == atm+whatever loss in downstream piping

 

[JimCasey]

>>>A process to which I am pumping liquid has maximum allowable inlet pressure associated with it. I am being asked to limit the pressure of the incoming liquid using a back pressure at the tank from which I am pumping.<<

I have read that 7 or 8 times, but I am still not seeing how you will accomplish your goal reliably or safely.

A pressure reducing valve (regulator) will measure the downstream pressure and close the valve when the pressure exceeds setpoint. Thus it will protect the tank. It will open progressively when the downstream tank pressure gets lower, but if you're not pumping, there will be no flow-just an open valve. Additionally there should be a code safety-relief device, but that's a different discussion at this point.

A backpressure valve only looks at the pressure upstream, and opens if the upstream pressure exceeds setpoint. If the downstream tank is bursting at the seams it won't care.

 

[dcasto]

Me too JimC.

I believe a reducing valve setup is what is needed.

 

[PuffJr]

Perhaps I have inadequately described the proposed installation.

The regulator would be installed on a leg between the pump discharge and the storage tank. When the regulator sees pressure greater than the set point, it would dump back into the tank, thus regulating pressure at the process.

 

[pleckner]

May I recommend you copy and paste this link into your browser.

http://www.fisherregulators.com/technical/tableofcontents/#Regulator Components

 

[dcasto]

The regulator you want is controlling the upstream pressure. You might want to draw what you are looking for. The optimum control system will be dependant on the pump curve and the expected process "curvr" or the process flow and pressure highs, lows, and normal values.

 

[JLSeagull]

The back pressure or pressure relieving regulator is like a pressure relief valve. As the pressure at the INLET exceeds setpoint the valve begins opening - thus reduces the inlet pressure. If the inlet pressure is below the setpoint the valve closes. However, these are not suitable for use as a pressure relief valve to comply with ASME or API standards.

A pressure reducing regulator monitors the OUTLET pressure and opens to permit more flow if the pressure is too low downstream, or closes if the pressure exceeds setpoint.

These are both regulating valves but not necessarily linear to any value. In small valves these are self-contained. For large flow rates these may be air-operated valves.