My pump is not working properly…. An explanation of pressure and flow

My pump is not working properly…. An explanation of pressure and flow

Pump performance.

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.

Water flow

Water flowing at a high rate from a lay flat hose.

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,

Pressure gauge reading pipe pressure

Pressure gauge reading pipe pressure

spray over a distance or move something like a hydraulic ram. As pressure is a measure of force there a two common modern measurement scales of it kilopascals (kpa) and pounds per square inch (psi). PSI is an easy term as it is pounds of weight (lb) applied to a single square inch of surface. Kpa is a different term as it refers to 1000 pascals (like kilograms and kilometres). A pascal is defined as one newton of force per square meter. The imperial scale for pressure is psi and the metric scale is kpa and 1 psi equals 6.89kpa.

There is also another measure of pressure that is limited mainly to pumps, that is the term “head”. Head is measured in meters and is a system for measuring the different dynamic parts of a pump and pipe system to determine a system performance. One meter of head is the pressure required to lift a column of water 1m above the pressure source, usually a pump. As gravity is not a precise number and this is the force that the pressure is acting against to lift that 1 meter, then 1m of pressure is equal to 9.87kpa. As the water source for your pump may be either above or below the pump (a tank or a bore), then the “static head” that is present is a very important part of the system. The static head is the height above or below the pump, positive head for above and negative head for below.

Pressure is measured by a gauge or sensor that reads the amount of pressure in a system and displays this on a dial or converts it into a signal for a reader.

What is flow rate (volume)?

A magflow meter for measuring flow rate

A magflow meter for measuring flow rate

Flow rate or volume is the measure of the amount of a fluid that passes a particular point in a system for a particular time period. This is also a basic scientific measurement. One of the main functions of a pump is to move an amount of water from point A to point B, usually via pipes. While the pressure that a pump produces is very important, the volume of water that is moves while producing that pressure is even more so as the volume of water is the thing that irrigates a crop, fills a tank or puts out a fire.

There are a couple of ways in which people express the volume that something produces. The main scales are litres per minute (domestic pumps), litres per second (dewatering and sewerage pumps) and cubic meters per hour (commercial pumps). They are indicated by L/min, L/sec and m3/hr. A liter is one of the main volume measures and is defined as the amount of space inside a cube 10cm x 10cm x 10cm. A cubic meter (the other main measure of volume) is the space inside a cube 1m x 1m x1m, therefore 1000L = 1m3 . Volume is measured by a couple of different methods, either via a meter, filling a tank or dam or via a weir measurement. Depending on how accurate you want to be, what the quality of the water is and what equipment you have determines how you measure a flow rate.

Flow rate is very important in a pump for a number of reasons:

  1. The flow rate provided by the pump needs to be suited to the application. If you only need 30L/min for a house to run taps, then a 300L/min transfer pump will damage itself and possibly the pipework too.
  2. If there are only certain sized pipes in place, then if you try and pump too much volume through the pipe all the pressure will be consumed in friction loss with the pipe walls.
  3. If there is insufficient volume for what is required, then the pump will run really low pressure to maximise its volume. This will cause the pump to cavitate and damage the pump and cause problems like losing prime and insufficient pressure.

How do you work out what a pump should be doing?

All pumps are made to do a specific job, they could pump from a tank to transfer water, pump up a big hill to gain pressure, suck up out of a bore or well or recirculate water in a pool or hot water system. To ensure that they achieve what they are required to do, a pump is designed to move water at a specific flow rate and at a specific pressure. The pump may also be constructed in a certain way too, like being submersible in the water its pumping or having a pump mechanism that moves water to very high pressures.

So when you are looking at what a pump should be doing yes you look at what volume it pumps but also make sure that you know what pressure it can produce. The volume a pump moves also varies with the amount of pressure it produces, this is commonly known as the “pump curve”. A specific pressure and flow on a pump curve is called a duty point.

To look at how this works, this is a pump curve for a Grundfos CMBSP 3-46. This is a normal domestic house pump and works best by placing it next to the water source (usually a tank) and pumping the water out for a house or gardens. This pump can do a couple of main duties, the notable ones are:

Shut head of 450kpa (46m), that is the pressure when there is zero flow.

Most efficient duty of 3m3/hr (3000L/hr) operating at 31m or 300kpa.

Open flow of approx. 70L/min. That is when all the outlets are open and there is no restrictions, what volume is the pump capable of moving.

It also has the ability to suck water up from 7m down and is what is known as “self priming”.

Performance curve for a Grundfos CMB pump.

Performance curve for a Grundfos CMB pump.

Now pumps are not designed to operate near their dead head or open flow duties, prolonged use at these points promotes cavitation, wastes power and will damage your pump. Pumps are designed to be operated at a range of duty points either side of the most efficient duty. This is the point on the curve where you are moving the most water volume, at the highest pressure for the lowest amount of power requirement.

One of the first tests we carry out when repairing a pump is to place it on our test bench away from its normal pipework etc and carry out a wet test. This involves sucking water out of a small tank and then recirculating it through a flow meter. When we apply different amounts of back pressure by shutting off a valve, then we are able to determine a pump performance at a number of duty points. We then compare this to what the pump was supposed to do from the factory and if there is a large difference that is not down to wear and tear, then we have to work out what is wrong.

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:

  1. 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.
  2. 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.
  3. 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.
  4. Everything appears to be working normally but no water comes out. There is either something wrong internally with the pump or a blockage somewhere.
  5. 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.

Once you have isolated where the problem is occurring and what is happening, then you need to determine what needs to be done to make the pump return to its fully performance again. Sometimes this is a cheap and easy fix, sometimes with the price or parts and labour, it is cheaper to purchase a new unit with a new warranty.

It is also very important when there is a problem with a pumping system to jump to conclusions. One of the very first steps should always be to remove the pump from its setting and ensure it is tested on a reliable test apparatus. If a fault is found with the pump, then carry out the repairs and return it to operation. If there is no fault found with the pump, then you need to keep going with the detective work to find out if the fault is a pipework, a valve fault or maybe a water supply fault. Sometimes it’s as simple as there is no water in the tank but there may be a small crack in a pipe or a hard to find blockage and this is causing the problem.

In this article, we have discussed the concepts of flow rate and pressure and how they relate to each other. We also discussed the concept of pump performance and how this can be measured and compared. If you would like to discuss issues with how your pump is working or how your system can be improved then give our experienced team a call on 08 97213577 or send us an email.



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