The Pump Curve

It depends. Can the process handle a larger flow at a smaller pressure? What happens to the flow when the valve is fully open?

we did not try to open it full, but there is a VFD ( variable feed driver) for the motor when we try to work it in 90% of the speed the flow decrised and.

In that case, it would be best to run at a reduced speed using the VFD, in which case, no, the pump is not incorrectly sized.

However, the question about whether the process can handle the flow with the valve fully open remains interesting, if unanswered.

You can have either 315L/s or 615m, but not both.

Than's the point of a pump curve. At a given flow, you'll have xxx pressure.

But at no point can the pump produce both.

If you need 315L/s at 615m, you'll need to find a pump that can produce these figures at the "knee" of the curve.

Please give us the make & model (if it is a standard range) and the impeller diameter of the pump you have.

Are the discharge side , piping lengths and diameters , the static and working pressures as designed?

I have seen cases where the customer mixed and matched parts of different tenders resulting in total chaos and confusion.

It is also possible that an over-sized control vale does not show enough effect at openings of more than 20%.

What is the effect at different openings?

Hi,

Any pump (without VFD) will move fluid at duty point on the curve (or near enough, depending on efficiencies etc). The pressure developed is due to the frictional losses in the downstream piping system (the higher the fluid velocity the higher the frictional losses). So the flowrate and pressure produced by the pump are interrelated by the system pressure loss characteristic.

Without restricting your valve, the system pressure isn't high enough (and the pump is running off the curve - ie too much flow). So you have restricted the valve (to create system pressure, and this reduces the flowrate of the pump according to the pump curve characteristic.

Right or wrong - there is no answer to this. The question really is ... Does this give you the flow or pressure you require?

Actually can you define what it is you require? Do you need the flowrate at 315L/sec or the pressure at 615m? Normally it is only one of the two, can't think of a situation where both flow and pressure need to be specified. Please advise?

You mention in a followup post, that the system has a VFD, in this case you should be able to (within limits) tailor the system to give you the flow or pressure you desire without closing the control valve at all (or very minimally).

Hop this helps,

Anthony

Are there any pump rules which say the "discharge valve should be fully open" when the pump is in operation ?. In saudi arabia a client,an arabic speaking engineer, told me he won't accept our pump if we throttle the discharge valve to give the required pressure.

He is right; if you throttle the discharge valve to achieve higher pressure adjacent to the pump, then the pressure after the valve will be even less, which may not meet the design requirements.

The Fluid being handled in this discussion is Water only. To the best of my knowledge no such Rule Exist for Water. In Saudi Arabia, if you are handling an inflammable fluid then the case may be different. Pls refer to applicable API codes for such guidance. Development of Static Charge has to be prevented when handling hazardous fluids, apart from other safety aspects.

Its a clear case of OVER ESTIMATION of Head Losses in the Piping System and the Over design. Running the pump with the discharge valve only 20% open IS NOT ADVISABLE. It not only is ENERGY EFFICIENT, but also HARMFUL to the Pump.

The possible Alternatives are :

- Reduce the Pump RPM with VFD. This will reduce the motor efficiency and also since Turn Down Ratio is quite High, you have check whether it is possible on not by referring to Pump curves and VFD Manual.

- Replace the Pump with a suitably designed Pump. Now you have the ACTUAL Field data and selection can be Exact.

- The Third option though not a bit unconventional but very effective in some cases is Trimming the Diameter of the Pump. Again you have to refer the Pump CUrves and analyse the Pros and Cons.

Thanks for reading through.

IN our project we have pumps with this characteristic (315L/s - 615m) but to make it work in its curve and this characteristic (315L/s - 615m) we have to close the discharge valve about 80%( only opininng is 20% ), is it wrong or right .

It is correct. When you see the curve go to point where starting motor power is and you will see head and flow you wrote.

Be confidence it it right.

What is the rated pump flow. If the design flow is 315 L/s and your discharge piping is all installed and you have to close the diascharge valve to 20% opening, then you have a pump which has been overspecified in terms of discharge head.

Since the actual system resiatance is much lower than the estimated, the pump is operating at higher flow and you may note at lower head than 615m. To bring it back to its design point, you are closing the discharge valve to provide additional resistance.

Fortunately you have a VSD that would do the job without having to close the discharge valve.

With regard to throttling a centrifugal pump:

Let's assume the pump is driven by a 10 HP motor. You are dissipating 7.35Kw, regardless of output flow.

By throttling the pump, the fluid that is retained in the pump cavity absorbs the heat that would have been carried away if the valve was fully open.

You certainly can destroy a centrifugal pump that is pumping "just water" by throttling it back too much.

From your description that pump is seriously overszed. You can get by by throttling it with the discharge valve but by doing that you will pay for that pump over and over in wasted energy pumping costs. Here are several options:

1. First and best, replace the existing pump with one properly sized and motored.

2. Install a variable fequdency starter and slow the existing pump to the proper speed. If the pump is too far oversized you may not be able to slow it sufficiently.

3. Trim the impeller of the existing pump. You will need the perfrmance curve of the pump in order to know the necessary diameter to obtain. Same caveat as in #2.

Good luck

Lou Bindner

The pump curve is based of a specified rpm and fluid viscosity, if either your rpm is moving away from that which the curve specifies or you are pumping a different viscosity fluid, the flow/head will not match the curve.

Perhaps the motor you are using is not adequate or you are pumping a different viscosity fluid......

You are basically stating that in order to operate at a certain point on the pump's operating curve you have to throttle the outlet flow with the discharge valve to back the TDH back to a given point. We have to assume of course that this point would exist on the given pumps operating curve (capacity vs. head).

There is nothing unusual about this. A process engineer states a given flow, a required delta pressure or head based on piping, valve and vessel losses and the specific gravity of the fluid involved. This is generally based on the design conditions or output of the process. There is nothing unusual about operating at different flow conditions based on process throughput, reduction due to less feedstock or product need. That is the function of control valves that is to throttle output and require the pump to climb back on it's curve to pump less fluid at that TDH point on it's operating curve. If you look at the BPHPr KW required at that point you will see that it also decreases. An electric motor drive will only put out what is necessary and if the BHP/KW requirement is less then the current draw will be less.

If you were told to meet a different set of operating conditions you would have to either open or close the discharge valve accordingly. You would only cause serious harm if you were operate with a totally closed discharge valve for an excessive amount of time. When you have parallel pumps you always start the standby pump with the discharge closed if there is no check valve in the discharge line of each. When the pump is up to speed and it's discharge pressure is up to line pressure then one opens it's discharge valve.

No pump/driver draws anymore power than is required and this is basically why the pump manufacturer gives the BHP/KW curve. The major problem there often comes when the S.G. of the fluid is changed especially made heavier. Then the driver can be in trouble.

Have you tested this pump on actual duty of pressure and flow or you are reporting status of bench test. If pump is able to achive required pressure with 80% closed discharge valve, that means pump is working well, once it is connected to system, you can achive the flow and pressure as per curves.