Back Pressure Regulator Behavior 2

[PuffJr]

A fluid is stored in a tank, and is fed to a particular process through a few hundred feet of pipe using a PD pump. A v-port ball valve at the process controls incoming flow, and the inlet pressure at the ball valve is limited by a back pressure regulator at the storage tank. The set point of the back pressure regulator is equal to the allowable inlet pressure for the ball valve plus anticipated frictional pressure loss in the pipeline. If I close down the ball valve to reduce flow by 98%, will the back pressure regulator be capable of maintaining the same inlet pressure at the ball valve?

 

[davefitz]

It depends.

I assume the backpressure regulator inlet pipe is also a few hundred feet long, so it will have a lot of frictional pressure drop for the case of 98% of fluid bypassing the ball valve and flowing thru the backpressure regulator. If the backpressure regulator is controlled by a PT located at the ball valve or if teh backpressure inlet pipe is suitably large enough, then I guess it might work OK. If the backpressure valve is controlled by a local pressure sensing tube and its inlet pipe has a large frictional press drop at the max flow, then I guess not.

If the PD pump is constant speed, then one might expect presure and flow pulsations due to the PD pump characteristics, unless you add a compressible chamber or bladder to the piping to snub the pulsations.Alternatively, you can use a VS controller on the PD pump to reduce the performance demands on the backpressure valve, and reduce the size of this valev and its piping.

 

[PuffJr]

No, the back pressure regulator is located immediately downstream from the pump, so the inlet and outlet are both very short.

 

[btrueblood]

Then you should see a magnification of the regulator "droop" in the pressure at the inlet of the valve. I.e. a typical bp regulator will tend to show a decreasing curve of pressure as the flow increases, the dp across the long line also increases as flow increases, so there will be higher pressure at the valve for low flows, and lower pressure at the valve for high flows.

 

[BigInch]

So this is it?

Tank]--[P]----[BPR]-----------> ------------[V]----

No it will not hold pressure at the V-ball constant. The backpressure regulator will only try to maintain the BPR's inlet pressure equal to its set point. That forms a boundary condition for all downstream pipe flow and that pressure will (should) be constant. Subtract all downstream differential pressures from that. As flow drops with the closure of the v-ball, the frictional pipeline pressure loss will decrease, thus all downstream pressures will tend to increase up to the point where the reduced flow is being imposed (presumedly at the V-ball). The increase will go over the V-ball's allowable inlet pressure, (unless your V-ball 2% open is enough to keep the pressure from building up to that level... something I would NOT count on, so ...

I would call that a safety violation!

Not to mention this is not a good arrangement for a PD pump, unless you want to test its relief valve and/or its drive's strength. I hope it has something that opens to relieve hi discharge pressure back to the tank (or to suction?), or a VSD starts ramping its speed down as a discharge pressure signal goes hi when your regulator begins to close too much.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[PuffJr]

To be clear: the BPR is on the branch of tee just past the pump; the BPR dumps back to the tank when pressure exceeds the setpoint.

 

[roydm]

I assume that the PD pump is already suppling an excess of fluid and the remainder is returning to the tank via the back-pressure reg. As you close off the process valve more fluid must return through the regulator. If it's just a simple spring loaded regulator the pressure will go up with flow. A pilot operated regulator will give you much better control (more gain/less droop)
Regards
Roy

 

[BigInch]

But then its not exactly a "back pressure regulator". In fact, its really not regulating anything.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[PuffJr]

Big Inch, I disagree. When the pressure at the inlet to the back pressure regulator exceeds the set point, the device opens to divert some flow back to the tank. Thus the inlet pressure is regulated, and consequently so is the pressure at the control valve, downstream from where the back pressure regulator tees off the pipeline.

 

[BigInch]

Sorry about my confusion. You know best what kind of valve you have, so no argument from me about that. That action itself is typical of a relief valve. One must be very specific about the action of the device you're using. Its possible to see a lot of things (including as named by their manufacturers) that don't actually do what the name implies.

http://www.plastomatic.com/bprvspr.html

Now take this example by Fairchid, where they call the device a backpressure regulator, but in the technical description they say action is via a "RELIEF VALVE". So unless you give the model number of the specific piece of equipment, things can get confusing.

When system pressure increases, the force
on the bottom of the Diaphragm Assembly
increases until it reaches the set point.
When system pressure increases beyond the
set point, the assembly moves upward, lifting
the Relief Valve from its seat and vents the
downstream air.

http://www.finecontrols.co.uk/pdfs/10bpdata.pdf

And this example by Fisher

"Type 63EG Relief Valve or]/color] Backpressure Regulator"

http://www.fisherregulators.com/products/literature/63eg/

In any case, if you don't set the relief pressure for the inlet pressure of the V-ball (or whatever the lowest downstram MAOP is), you still have a problem.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain