Using a backpressure regulator to maintain a minimum pressure

[jwpyr8]

I'm designing a pumping system where I have a large variance in flow and the starting Elevaiton is higher than my end elevation. So at low flow, I would not stay on the pump curve. However, at the peak flow, I am on the curve due to line losses. I'm considering installing a backpressure valve on the line to maintain a minimum pressure in the line so the pump is always on the curve. Does this backpressure valve act as a static pressure loss that is always present in the line? As an example:

Lets say my pump needs to have at least 10 psi of backpressure to stay on the curve at a good spot. So I set my backpressure regulator at 10 psi. During peak flow, my line loss (assume 12 psi) downstream of the backpressure will exceed my backpressure regulator setpoint. Ignoring elevation head, would my pump see a velocity head loss of 22 psi?

Thanks in advance.

 

[1503-44]

The valve will have a head loss as flow passes through it according to the valve's Cv at the time. The loss will be constant, if the flow, upstream and downstream pressures remain constant. Of course, if you open or lose the valve a bit, then Cv changes, then flow changes and the system must reach a new pressure equilibrium with that new flow passing through the valve.

It's not clear how you intend to use the valve. Your symbol indicates a downstream pressure controller.

If you set your valve to give a constant pressure drop of 10psi at all flow rates, then the pump will see an initial resisting pressure of 10psi, because at no flow, you will not yet have a 12psi pressure loss in your downstream pipe length. As flow increases in the pipe, pipe pressure loss will rise and the pump will see a total resistance of 10psi + whatever the pressure loss of the pipe is at the current flow rate.

In any case, no matter how the valve is controlled, the pump will see a resistance of the sum of the valve loss + line loss at any given flow rate.

 

[jhnblgr]

What is the purpose of this?
Why not just let gravity do the work?
If you need variable flow rates use a vfd.
So far this sounds like a really bad idea and not fully developed or understood

 

[Snickster]

My understanding of how a backpressure regulator works is that you set it for a given pressure to be maintained on the upstream side. So if you set it for 10 psig it will try to maintain this pressure on the upstream side. If the pressure downstream is lower than 10 psi say 5 psi at a given flow, it will close just enough to produce a pressure differential of 5 psi so that the gauge pressure on the upstream side is the set pressure 10 psig. If the pressure rises on the upstream side (due to upstream head increasing or downstream friction loss increasing) then the valve will open more to reduce the pressure loss through the valve to maintain 10 psig on the inlet. If the pressure keeps rising above 10 psig the valve will go wide open which at that point the only pressure drop through the valve is based on the wide open Cv.

The system you have is confusing. If you have a constant speed centrifugal pump, it will be pumping on a given pump curve. You cannot control the flow. At a low flowrate it will produce a higher discharge pressure and at a higher flow it will produce a lower discharge pressure. The flow would be controlled by the system say if you have users downstream that open or close more valves creating more or less of a demand. If this is the case the pump will automatically adjust to produce the flow at a given discharge pressure. With such a system the pump will run out towards the end of curve (low pressure at high flow) as the flow increases not decreases.

Please explain how your system operates in more detail.

 

[LittleInch]

My understanding of how a backpressure regulator works is that you set it for a given pressure to be maintained on the upstream side. So if you set it for 10 psig it will try to maintain this pressure on the upstream side. If the pressure downstream is lower than 10 psi say 5 psi at a given flow, it will close just enough to produce a pressure differential of 5 psi so that the gauge pressure on the upstream side is the set pressure 10 psig. If the pressure rises on the upstream side (due to upstream head increasing or downstream friction loss increasing) then the valve will open more to reduce the pressure loss through the valve to maintain 10 psig on the inlet. If the pressure keeps rising above 10 psig the valve will go wide open which at that point the only pressure drop through the valve is based on the wide open Cv.

The system you have is confusing. If you have a constant speed centrifugal pump, it will be pumping on a given pump curve. You cannot control the flow. At a low flowrate it will produce a higher discharge pressure and at a higher flow it will produce a lower discharge pressure. The flow would be controlled by the system say if you have users downstream that open or close more valves creating more or less of a demand. If this is the case the pump will automatically adjust to produce the flow at a given discharge pressure. With such a system the pump will run out towards the end of curve (low pressure at high flow) as the flow increases not decreases.

Please explain how your system operates in more detail.

I'm with snickster here.

It's not clear what is controlling flow?

But if the inlet side of your back pressure valve goes above 10psi it will just go full open and then you have the pressure drop across a fully open valve to deal with

 

[goutam_freelance]

So at low flow, I would not stay on the pump curve

How are you creating the low flow? Is it by throttling some downstream valve(s)? If so, at the upstream of those valves, high pump pressure at low flow will automatically exist. The shortfall in piping losses will be taken care of by valve throttling the flow and the system will remain on pump curve.

The concept is not clear. Please give complete flow diagram.

 

[georgeverghese]

For a pump to "stay at a good operating point", it must be operating at constant differential pressure. This then gives you constant flow. Presume this is what you are aiming for ?
Given that you have a varying level in the source tank, the simplest way to enable constant diff pressure is to measure dP across the pump, and use the dp to operate a dPIC loop, which will modulate a dPCV on the pump exit. Dial up the required dP as setpoint at this dPIC. If you have trouble picturing this, talk to your process controls engineer - its a relatively simple loop to configure.

 

[pierreick]

A better description is needed.
Why not considering a VFD to modulate the rotation speed of the pump to acknowledge the flow rate variations +LIC.
LSL and LSH are needed to protect the pump from cavitation and cut the feed to the source tank.
In any case back pressure valve will not help (review its principle)
Pierre