Characteristic Curve of a Pump
One of the most complicated and also misunderstood concepts in our industry is “pump performance”. That is a pump is built to do a specific job and it’s ability to do this job is a measure of its performance. Like a car’s performance that is measured in horsepower, torque and speed, a pump too has a number of different factors that determine how it works. The main measurements of a pump’s performance are motor power (kilowatts or horsepower), motor speed (2 pole, 4 pole or multispeed), pressure (meters, kpa or psi), volume (L/min, L/sec.or m3/hr) and NPSHR or net positive suction head required.
These are all quite technical engineering terms and it is very easy to misunderstand them. It is very common in the shop to have customers who are having issues with a pump setup and have either mismeasured something or not quite got the exact performance of their pump matched properly. The other common thing we hear is variations of “I just want more pressure out of the pump as I want to run twice as many sprinklers”. The customer doesn’t quite understand what exactly they want the pump to achieve, other than more!
What is pressure?
Pressure is one of the basic scientific measurements. It is the measure of force on a perpendicular surface, ie how much force is being applied by a gas or fluid to its vessel’s walls. The more pressure that a fluid has, the more potential energy it has to lift to a height,
The characteristic curve of a pump is the interaction of two variables that describe its performance:
- HEAD: the energy by unit of mass that the pump can supply to the fluid
- FLOW RATE: the quantity of fluid that goes through a section in a certain period of time
See below for an example.
The head is plotted on the vertical axis and the flow rate on the horizontal axis.
Two more curves can be plotted, such as the performance curve:
which shows the relation between the rated power of the pump and the absorbed power, in relation to the volumetric flow rate and the absorbed power curve:
that shows the electric power used in relation to the flow rate.
It must be noted that the centrifugal pump is a mechanical element that is driven by an electric motor, and every characteristic curve refers to a precise rotation of the motor which depends, in the most common asynchronous motors, on the power supply frequency and the number of poles of the motor (e.g. 50 hertz, 2-pole motor: 2900 RPM).
On certain models of centrifugal pumps we can find indications of the NPSH (Net Positive Suction Head), that is the maximum suction head allowed, before triggering cavitations that can damage the mechanical parts of the pump.
TROUBLESHOOTING:
If my pump was working, why is it suddenly not working?
There are a number of problems that can cause a pump to either not work well or completely stop working altogether.
The main faults in pumps are:
- The motor not working. This can mean it doesn’t start, or runs really noisily or hasn’t got the power to run at full speed.
- There is a leak in one of the pump parts and water is coming out. This is normally through the mechanical shaft seal wearing out but could also be an o ring seal or a part of the pump has a hole or crack in it.
- The pump can’t hold prime. When you go to start the pump it can’t draw water in to the pump to start the pumping process, it sucks in a heap of air and stops working. This can be caused by a number of different problems both inside the pump and out.
- Everything appears to be working normally but no water comes out. There is either something wrong internally with the pump or a blockage somewhere.
- The controller on the pump won’t let it start or turn off. When this happens it may just be a pressure or float switch that is faulty or the pressure controller unit on top of the pump. Replacing the faulty part is usually the cure. Make sure you use a qualifies and experienced repairer for the job as there is mains electricity involved.
Do you need more information or assistance in choosing the correct pumps for your application – “CONTACT US“
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This article is referenced from Debem Srl. (www.debem.com)
Fine Flow Technologies LLP does not claim ownership of any part of the technical details.
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