piping , parallel pump, system curve

[mojista]

thread407-393952
hi every one
I have a question about system curve and parallel pumps.
assume we have 2 parallel pumps , when we turn of one of them the pressure should be decrease, but in some system the pressure increase , I think the reason of it is changes in the system curve but I don't know how it is possible .
thank you very much

 

[1503-44]

This is the diagram from the thread you reference. It shows two similar pumps operating in parallel. Note that it is for similar pumps.

I think you are seeing the effects of trying to run two or more pumps in parallel, but the pumps are not similar.

If pump A is not similar to B and A's curve is everywhere higher than pump B's curve, when you turn off pump B, pressure may rise to the higher head of Pump A.

It is difficult to say exactly what is happening, if you do not post the data for the curves of each pump.

The line B-B is horizontal in the diagram below only because the pumps are the same.

 

[LittleInch]

Well if you don't know how it's possible how are we going to know??

Something else must be changing as well as turning off one pump. Find out what that thing is.

Are these pumps VFD?

No details provided of your system so we can't see what you can see.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Artisi]

LI, flat batteries in your crystal ball?

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.)

 

[LittleInch]

In serious need of some polishing....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Agree. Even with mismatched pumps, pressure should drop at least a little, or there is a strange system curve.
Maybe the lower flow going to the pump is increasing the suction pressure.
My crystal ball just cracked.

 

[bimr]

Agree. Even with mismatched pumps, pressure should drop at least a little, or there is a strange system curve.
Maybe the lower flow going to the pump is increasing the suction pressure.
My crystal ball just cracked.

Do you mean the higher flow on a single pump is increasing the suction pressure?

 

[mojista]

thank you every one for answering my question
1- Unfortunately I don't have exact information because theses pumps are not in the company I work, But it was again my knowledge. because depend on the formula I ATTACH when we increase the number of the pumps(Q should increase) so dynamic head of the system curve should increase. I wanted to know have you ever seen increase of pressure in single pump .
2- Mr bimr , my purpose is increase of pump pressure in single pump than parallel .
thanks

 

[mojista]

also anyone can introduce me book, document, white paper or handbook about system curves ?

 

[1503-44]

I have not seen that in a basic system with parallel pumps flow into free flowing pipe, however pump output depends on the system curve which depends on flow input and output to the system, how that is controlled or not, and how the pump and any attached valves are controlled. The system has as much affect on the pumps as the pumps have on the system, so it is impossible to say what might be happening based only on a description of one pump stops, pressure rises. Given so many unknown variables of your system, it is not possible to say what you might see in your particular system. I would think it is unusual, but perhaps not totally impossible.

System curves is nothing more than calculating the sum of all friction from all pipe, fittings and valves at each of the flow rates you want to run and plotting the results. Start with pipe.

https://www.youtube.com/watch?v=vTUj4F-EFQw

 

[bimr]

Pumps don't create pressure, pumps create flow. The pressure created in a piping network is due to the fluid resistance to flow. Therefore, it would not be a very likely occurrence that in a 2 pump system, the flow would increase when the 2nd pump was shut down.

also anyone can introduce me book, document, white paper or handbook about system curves ?

Pumping Station Design has a description.

Link

The condition you describe may occur if your system is working as a siphon.

The figure shows the pipeline operating as a siphon. This may work if z is very small. In areas where the pipe is above the hydraulic grade line (HGL), the pressures are negative; something that is not allowed in potable water distribution piping. But there are some problems with this design. First, is the flow sufficiently large and the downhill pipe slope sufficiently flat that the pipe will flow full? If the downhill slope is small or the flows low, the pipe will never reach full flow and siphon will not be established. Second, if z is greater than 32 ft (9.2 m), the liquid water will be below the vapor pressure of water and the siphon will never form. Third, even if the siphon works, eventually gases will collect at the high point and reduce the pipe capacity.

If the pipeline is not flowing full, then a single pump would have to have to lift the fluid to a pressure equal to the highest point.

 

[LittleInch]

Increase in system pressure when flow rate reduces can happen, but only when pumping non newtonian fluids or fluids which have a very high viscosity.

Then the action of fluid flow can increase temperature and reduce viscosity which reduces system resistance.

SO yes it is possible to happen, but you need to understand your fluid and what happens to it, especially viscosity, as the flow rate changes.

The vast majority of the time, if you have two parallel pumps pumping and you turn one off without changing anything else, the flow rate will go down, but so will the pressure at the pump discharge of the pump as it changes position on its pump curve to match the system curve ( your first attachment)

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bimr]

Some fluids become thinner as the rate of flow (shear) increases. These fluids are referred to as shear-thinning or thixotropic. Fluids that experience shear-thinning include: ketchup, lotions, and blood, for example. Some thixotropic fluids return to a gel state almost instantly, such as ketchup, and are called pseudoplastic fluids.