Pump Performance Curve Clarification
Pump Performance Curve Clarification
(OP)
So I'm looking at a Pump Performance curve.
RPM is stated as constant. Pump casing is a constant. You've got multiple impeller curves that descend in head as you get to higher flow rates.
My question is, how is flow rate subject to change if the impeller speed is constant and your only looking at the curve of the same impeller? If the curve represents a single impeller and it's spinning at a constant RPM, wouldn't the amount of fluid it's displacing be constant, ie, what it can yield as a flow rate?
I must be missing something here...
RPM is stated as constant. Pump casing is a constant. You've got multiple impeller curves that descend in head as you get to higher flow rates.
My question is, how is flow rate subject to change if the impeller speed is constant and your only looking at the curve of the same impeller? If the curve represents a single impeller and it's spinning at a constant RPM, wouldn't the amount of fluid it's displacing be constant, ie, what it can yield as a flow rate?
I must be missing something here...





RE: Pump Performance Curve Clarification
Johnny Pellin
RE: Pump Performance Curve Clarification
RE: Pump Performance Curve Clarification
Some are more flat than others, but they all have same curve for a given speed.
The flow will only be constant for a constant head at a constant speed
RE: Pump Performance Curve Clarification
you must get smarter than the software you're using.
RE: Pump Performance Curve Clarification
Centrifugal pumps are essentially (~ 15%) a constant pressure unit within their normal flow range. As the flow increases the losses in the pump increase and hence the head will tend to fall, and power requirement increases until the head of the pump output matches the head required for a certain flow. You need to match these quite closely to get a good design.
As BI and others above say, to work out your flow, you need to know the flow / head curve for your downstream equipment.
you can think of this type of unit like an electrical circuit - your pump in this instance is your supply voltage - change the transformer settings (impellor diameter) and you have a different voltage. How much current you draw is dependant on your circuit. E.g. a supply rated for 100 light bulbs, One light bulb is very low current, but 200 lightbulbs will drop the voltage and possibly blow up your transformer / trip the circuit on overload.
You can take this analogy a little bit far, but it sometimes helps to understand the basic principles.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Pump Performance Curve Clarification
So another question is, you'd determine the minimum flow rate you can run your pump at given a set RPM/impellor diameter by making sure it's head won't exceed the MAOP and you'd determine your maximum flow rate based on how low you'd allow your head to get, right?
So for booster pumps where the flow going into the suction side is far less than the flow going out of the discharge side, are flow control valves basically the only thing that ensures you get the flow rates/heads you want, given a set impeller size/RPM?
RE: Pump Performance Curve Clarification
Same flow in and out, conservation of mass.
The control valve is how you get the flow you want, the head is what it is. Note that there is a pressure drop across the control valve, so the pump creates a higher head than you would see measured after the valve. This is important when trying to locate where you're at on the pump curve.
RE: Pump Performance Curve Clarification
From a pump manufacturer's standpoint, below some flow rate, the fluid flowing through the pump is not able to move heat of mechanical friction out the pump faster than it builds up. That flow is specified as Minimum Flow. That figure may be adjusted higher in certain pumps due to other considerations such as discharge recirculation, radial shaft loads, or even process conditions, but never lower.
The pump will physically run below minimum flow, down to shutoff head, its just a matter of how long. The right side of the curve is different however. With the valve fully open, the flow will increase to the point at which it loses prime due to lack of NPSH. That is as far as I will offer on that can of worms.
The shape and slope of the flow/head curve is determined by impeller geometry.
RE: Pump Performance Curve Clarification
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.)
RE: Pump Performance Curve Clarification
From your second comment I think these pages are not going to answer all your many questions. I would try looking up some basic diagrams and docuemtns such as http://www.pumps.org/content_detail.aspx?id=5022 or a pump suppliers design guide.
You normally try and design a pumped system looking at the duty the pump needs to do matched to it's downstream system as a complete package.
Normally yes, you try and make sure that at low / no flow your pump won't exceed the MAOP of the system, but it's not the main criteria. Maximum flow should be within the pump and motors normal range, but rated flow should be close to the pumps best efficiency point (BEP). If you go beyond this you either start to get high vibrations or you exceed the motor power and it trips on high temperature or excess amps.
your last para defies the law of constant mass - mass in to a pump = mass out of a pump. Sometimes yes, yu need to insert a control valve downstream of a pump in order to match pump flow/head with that of a varying or lower system curve. To do this on a "normal" basis is just throwing away money as you've created the head, now you're lowering it and turing that energy into heat. However for startup and for a varying demand / head requirement downstream such a valve is useful to stop the pump pumping too much flow and going "off the end of the curve", i.e. going too far to the right hand side of the pump curve.
Hope this helps, but I think you need more than this.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Pump Performance Curve Clarification
RE: Pump Performance Curve Clarification
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.)
RE: Pump Performance Curve Clarification
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Pump Performance Curve Clarification
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.)
RE: Pump Performance Curve Clarification
RE: Pump Performance Curve Clarification
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.)
RE: Pump Performance Curve Clarification
I've never seen one of these pumps where the flow going in is far less than the flow going out. Does anyone have a picture or datasheet? I can think of several places they'd come in handy...
RE: Pump Performance Curve Clarification
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.)