Pump Curve Has a Maximum Head
Pump Curve Has a Maximum Head
(OP)
This is an abnormal curve. When the flow increases from zero flow, the head gets bigger till to a maximum point and then the head get lower like a normal curve. I know this kind of curve is not good but do not know what may happen or what will be the impact in the operation.
Need your help.
Need your help.





RE: Pump Curve Has a Maximum Head
Pump curves and fan curves are similar, they only pump different fluids. Fan curves are like that you described, however what manufacturers usually do is to publish the curve that is either the most efficient or where stable flow is probable. (like fan curves)
You will probably find that the curve is pretty normal, just dont use the 'bad part' of it.
The pump manufacturer (if they are any good), will have the power absorbed curves and efficiency curves for your pump. If they can't give these to you, you might be dealing with a low budget pump manufacturer who doesn't have the required test facilities.
Generally, use the RHS of the curve and dont go above about 75% up it. This will give you reasonable operating conditions without causing problems.
go to the mcnally institute for further info.
www.mcnallyinstitute.com
Friar Tuck of Sherwood
RE: Pump Curve Has a Maximum Head
Usually too may vanes cause a drooping curve. There are some things that can be done to correct or offset the droop: put a 15 degree or higher bias on the trim of the impeller (i.e. trimming is performed so that it is tapered at 15 or greater degrees from front to back, so that the rear shround is smaller than the front). Also, scalloping the impeller (removing material between the vanes at the perophery of the impeller) but will effect efficieny by about 3-5 points.
RE: Pump Curve Has a Maximum Head
Regards,
RE: Pump Curve Has a Maximum Head
However, when, for example, the mains frequency drops, from, say, 50 Hz to 47.5 Hz, the system curve may cut the pump's curve at two points bringing about unstable operation. And, as you know, stable characteristics are a basic necessity for the successful automatic control of centrifugal pumps.
RE: Pump Curve Has a Maximum Head
RE: Pump Curve Has a Maximum Head
RE: Pump Curve Has a Maximum Head
RE: Pump Curve Has a Maximum Head
I have occasionally operated pumps in the "grey" area above the shutoff head; more for curiosity than to do anything useful. Never had a surge problem (unlike when I tried it with a squirrel cage fan), although the friction component was high in relation to the lift. You will normally find that the peak pump curve head is well to the left of the BEP, and you have no business trying to run a pump there anyway.
Cheers
Steve
RE: Pump Curve Has a Maximum Head
Another piece of typicaly artwork on pump curves is the NPSHr curve. Generally speaking an accurate NPSHr curve will rise as you approach shut-off, but again most companies draw it horizontal at zero flow.
Be careful what you reject because of a curve. Is one telling the truth and the other not? Put in the spec to have a non-drooping curve and get a performance test.
RE: Pump Curve Has a Maximum Head
RE: Pump Curve Has a Maximum Head
Let me explain, at unstable points on the curve, the flow can become pulsing and cause vibration etc. but the fluid is still moves. also at some point on the curve, there can be 2 operating points and the pump/fan will hunt between curve points.
If the fan is used for emergency purposes, i.e. for only intermitent or one off conditions like fire extract, then its not really a problem since the fan is a one off use anyway.
Operating something long tern obviously isn't recommended.
There are pro's and con's for publishibg the whole curve. If you grey out the curve, then the user is not tempted to use the greyed out section. (But some people assume that the curve simply carries on up. (Fatal mistake).
If the manufacturers shows the whole curve, there is the risk that the eselctor can try and use the curve at an inappropriate condition.
Try to use the curve no more than 65-75% from the base to keep well in and create stable operating conditions. People quite often clip the underside of the curve and get away with it, but its not worth the risk.
Friar Tuck of Sherwood
RE: Pump Curve Has a Maximum Head
RE: Pump Curve Has a Maximum Head
It all depends. It is best not to operate the pump above shutoff head. However if you have to make sure the intersection angle between the system curve and the pump curve is as small as possible.
Pumps can be operated in this area as long as the system inertia is high enough to prevent the flow from shifting.
I do not see how you can refuse the pump curve if a rizing curve to shutoff was not specified. Is per API 610 not only the operating point on the curve guaranteed?
By the way you can always install a discharge orrifice to make the curve continuously rizing to shutoff and if you do that you will simply have to pay the wasted energy.
Best Regards.
Scalleke
RE: Pump Curve Has a Maximum Head
Drooping curves are not inherently bad, but they are tough for control purposes in many but not all applications.
Stepanoff points out that the most efficient design of any scroll pump ALWAYS has a droop near shut off. Many pump manufacturers sacrifice a little efficiency in order to eliminate that droop.
Some manufacturers are not concerned and many or most of their pumps have the droop, Berkeley is one example of a company where many of their pumps in a certain range have droop.
At least one company we know of hides the droop by providing the performance curve starting at 40 gpm.
The systems we have seen where the droop is not a control problem are where the system is very simple, they start the pump, run at BEP or higher, and never try to really control the pump, just let'er rip. No pressure controls, no safeties, nothing, just start 'er up and let 'er run. Ag is the place for those pumps.
For those of us who must control the pump properly droop is a disaster.
PUMPDESIGNER
RE: Pump Curve Has a Maximum Head
Where you must be careful is if you wish to operate the pump in parallel with another. Here you may get problems due to surging and pressure fluctuations if the pumps don't share the load or you may find the assist pump cannot even start because the head at zero flow is lower than the line pressure.
So I would not operate drooping head pumps in parallel or where significant turn down / control needed but otherwise go for it.
jonelv
RE: Pump Curve Has a Maximum Head
I have got an assignment in which I've gotta test a centrifugal pump.
its a pedal driven(manually operated pump).
I determined discharge head values alone.
wil it do or should i consider the suction too(actually we conducted tests by suctionfrom a barrel at ground level)
hope someone will help out
RE: Pump Curve Has a Maximum Head
Naresuan University
Phitsanulok
Thailand