Pump theory and residual pressure 1

[Basel Zaidia]

Hallow Their,
It is known that the pump doesn't create pressure but flow against pressure.
The operation point of the pump will move along its performance curve according to the pressure ahead the pump ; static and frictions
Now , How the residual pressure is created? If we need 1 bar at the exit of the discharge pipe so if you add this pressure to the head of the pump ( during the calculation ) so the pump will give another position to its operation point?
Could any one explain how the residual pressure is created?

Basel

 

[JJPellin]

I do not agree with your statement about how a pump works. A centrifugal pump is a machine that produces differential head. That head can be used to drive flow at the point where the pump curve intersects the system curve. I am not aware of anything that could be called residual pressure.

If, on the other hand, you are referring to a positive displacement pump, the principles are different. A PD pump produces a flow. That flow will enter the system at whatever pressure the system requires. You could use the same concept as the centrifugal pump. For a PD pump, the pump curve is basically a vertical line. The pump will run at the intersection of that line with the system curve. The pump produces a nearly constant flow (depending on the particular pump design). The pressure is produced by the system by the combination of the system static pressure and the flow losses at the specific flow rate.


Johnny Pellin

 

[Basel Zaidia]

Thank You JJPellin for your appreciated reply
Let us talk about Centrifugal pumps , non positive displacement
Suppose a pump it's Head is 3 bar and Q1 , if the head is increased ( by closing valve for example) the head will increased and the flow will decreased , the pressure at the outlet pipe will be 0 gauge pressure.
If I take this pump to another system and the losses at this system is 2 bar so the flow of the pump will increased ( in general ) because the operation point will move along the performance curve of the pump to the right side.
Now if I need a residual pressure ( for example fire fighting pump ) how can I obtain ( theoretically )1 bar - gauge pressure at the sprinkler or at the outlet of the discharge pipe?
If I calculated the total head of the system is H1=3 bar then I added a residual pressure for irrigation pump or fire fighting pump another 1 bar then the total head is H2=4 bar for example but the selected pump will operate for just 3 bar so it will produce extra flow and the gauge pressure is 0
Hope I highlighted my point very well

 

[BrianPetersen]

No, you have not.

Draw a schematic of the system that you are proposing and the performance characteristics chart of the pump that you are proposing to use, and how much flow is happening where. Keep it simple so that the rest of us can understand what you are talking about.

Your statement "... the pressure at the outlet pipe will be 0 gauge pressure." is incorrect. If the gauge is at the pump outlet and before the closed valve that is forcing the pump to deliver zero flow, that gauge will read whatever the pressure is that corresponds to zero flow for the performance curve of whatever RPM the pump is running at, less the static head pressure difference (if the gauge is at a higher elevation than wherever the zero-reference pressure is for the pump inlet).

Your statement "residual pressure" makes no sense.

 

[Basel Zaidia]

OK , I'll do that
But I mean by the "0" gauge pressure mot at the outlet of pump but at the outlet of the pipe at the end of the system ( the inlet of the received Tank)
In general I meant the increasing of the head of the pump will affect the performance of the pump

 

[BrianPetersen]

Don't use "residual pressure". Just say "pressure".

A centrifugal pump that is only rated at 3 bar output pressure at a given pump RPM when deadheaded (flow shut off = 0 flow) is not going to produce 4 bar of pressure ... unless you change something.

You could spin the pump at a higher RPM. Or you could use a different pump that has a higher delivery pressure.

I don't understand what the problem is ... aside from that perhaps you have a pump that doesn't meet your actual requirements.

I should add that running a pump under deadheaded conditions may not be good for it and it is certainly a waste of power.

 

[Basel Zaidia]

OK , I'll use "pressure" only
WE have a Project , the pump is around 100 m3/hr and 30m head according to the design , later on some modifications happened to the project so the system changed , after the new calculation the head become 15 m only so the Contractor said the head of the pump is more than the new required head so its ok and we have extra 15 m at the end of the delivery pipe
I replied no it isn't , I instructed him to re-select another pump to match with the new head because the operation point of the pump will move along the performance curve to the right side and the pump will be operated at low efficiency or it will be out of its curve
He replied no because the fire fighting system requires a pressure at the nozzle so if the pump pressure is higher than the system pressure so the end of the discharge pipe will be ( pressure of the pump- losses of the system )

 

[hydtools]

You need to add the nozzle flow and pressure charastics to the system characteristics then see when the pump curve crosses the total system curve for the operating point. Will the flow rate at that operating point meet the required nozzle delivery flow rate? If the flow rate is as required through the nozzle, then the pressure at the nozzle will be as required.

Ted

 

[Artisi]

Maybe contract a qualified engineer who understands and can apply pumps to systems.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[3DDave]

What calculation did you do before deciding to change the pump because the head requirement changed?
What is the entire system requirement?
What is the link to the pump curve that you used?

 

[Basel Zaidia]

ِArtisi
Good Note
3DDAVE
The pump was designed to deliver water to a certain tank and the head was around 30 m but later on and now during the Construction , some modifications had been done so the pump will deliver water to another reservoir which located near to the pumps so before selection the pump and supply it we instructed the Contractor to re-calculate the required head and after the new calculation I instructed him to select another pump

 

[Basel Zaidia]

Hydtools
I think so, adding Nozzle pressure
Thank you

 

[3DDave]

Artisi is right.