"Backpressure" Definition 1

[RJB32482]

Hello,
My question is exactly what is "backpressure"? If I have a pumping system that has a discharge pressure of 100 psi and add a backpressure valve to the line that reduces the pressure to the outlet of the valve to 20 psi, will I have 80 psi backpressure on the pump?

Also, would increasing the amount the backpressure valve is closed increase the discharge pressure of the pump?

Thanks in Advance

 

[quark]

There was a great discussion on back pressure in these forums long time back, unfortunately I am unable to locate it now. Finally there was no agreement.

Your case is a bit different. First of all, if your pump discharge pressure is 100 psi then it need not develop 100psi pressure upon hook up to a system if the resistance of the system is not 100psi. If it is a centrifugal pump then system resistance below 100psi tends to increase the flow output of the pump. So to keep the required flowrate, you have to create excess pressure drop by restricting the flow. So discharge pressure of the pump will not change.

If it is a PD pump then you need not worry about excess flowrates.

Regards,

 

[TrevorP]

To my way of thinking, the backpressure on a piece of equipment is the pressure immediately downstream of that equipment. So if we consider a pump developing 100 psi, with a downstream valve just after it dropping it to 20 psi such that it has 80 psi pressure drop across it (ignoring other pipework etc. for simplification), the backpressure on the pump is 100 psi, and the backpressure on the valve is 20 psi.
The same is true with relief valves - the backpressure is the pressure immediately after the relief valve. When the valve is not relieving, this is static backpressure. When the valve is relieving, this is relieving backpressure or total backpressure.

On the second question, the answer is generally yes. With a centrifugal pump, the pressure will increase and the flowrate will drop to the point where the new system curve meets the pump curve. With a PD pump, the discharge pressure will increase, and the flowrate will stay pretty much the same. It might back off a little if there is increased slippage at the higher discharge pressure.

 

[RJB32482]

Thanks for the answers guys, the pump is a PD pump that is known to have inconsistent flow rates due to the check valves on the pump. Increasing back pressure might help them to close. The flow rate is low (0.8 gal/hr). Does anyone have knowledge with this low flow rates and how to keep it consistent?

 

[sshep]

I think the best installations have a pulsation dampener on a vertical pipe run just after the pump discharge. If this is done a control valve and flow measuring device downstream can then be used to control the flow. Otherwise injection type flowrates are normally achieved by controlling pump speed (frequency), or stroke.

To help insure the pump internal check valves seat properly, a strainer on the pump suction can be used to keep rust, scale, trash out of the pump. In my experience most of these check valves are closed by springs so constant back pressure should not be neccessary for closure.

The other consideration with PD injection pumps is the spillback or relief valve. Most pumps have an internal relief valve, but many companies (including mine) prefer not to trust to these and install something external. I am not sure if there is a standard on this. This valve must be a modulating device for hydraulic service- a conventional psv will fail quickly if called on to cycle with the pump (i.e. when pump capacity exceeds controlled injection flow) and start to leak through making control unreliable.

best wishes, sshep

 

[StoneCold]

RJB32482
We have back pressure regulators on the discharg of all our PD pumps (Pulsafeeders). We have found that it makes the pumps operate much more consistantly and it helps the internal check valves to seat. Let me claify that we use them on batch pumping operations with very little pipe downstream for pressure drop.

Regards
StoneCold

 

[JOM]

quark.

hi - long time no chat.

that thread was in the Boiler and Pressure Vessel forum:-

http://www.eng-tips.com/viewthread.cfm?qid=33900&page=2

i still don't know the technical definition of backpressure, if there is one.

i was told i should get out more

John.

J.

 

[quark]

JOM,

That was the thread. Let me know if you come up with anything positive.

Thanks,

 

[JOM]

Hi Quark,

I think all I concluded was that there is no text book definition of the term, but people who use it understand what they mean. And it isn't very important.



J.