1. Flow and pressure relationship of a centrifugal pump
When the flow increases, the discharge pressure of the pump decreases, and when the flow decreases the discharge pressure increases.
2. Do not let a pump run at zero flow
Do not let a centrifugal pump operate for long periods of time at zero flow. It will cause excessive heat generated inside the pump casing, and the pump will be running noisy and vibrate excessively.
3. Use pressure gauges
Make sure your pump has a pressure gauge on the discharge side close to the outlet of the pump this will help you diagnose pump system problems. It is also useful to have a pressure gauge on the suction side; the difference in pressure is proportional to the total head. The pressure gauge reading will have to be corrected for elevation since the reference plane for total head calculation is the centerline of the pump.
4. Do not let a pump run dry, use a check valve
Most centrifugal pumps cannot run dry. Ensure that the pump is always full of liquid by using a check valve (also called a foot valve) at the water source end of the suction line.
Valves must be suitable for the pressure involved and may require independent support for the large unit. When an isolation valve is needed at the pump suction and discharge side, gate valves should be used as these offer low resistance to flow and can provide a tight shut-off. Butterfly valves are often used but they do provide some resistance and their presence in the flow stream can potentially be a source of hang-ups which would be critical at the suction. They do close faster than gate valves but are not as leak proof. If flow must be regulated at pump discharge side during service, a globe valve is preferable for its smooth throttling.
6. Eccentric reducer
Always use an eccentric reducer at the pump suction when a pipe size transition is required. Put the flat on top when the fluid is coming from below or straight (see figure below) and the flat on the bottom when the fluid is coming from the top. This will avoid an air pocket at the pump suction and allow air to be evacuated.
7. Flow control
If you need to control the flow, use a valve on the discharge side of the pump. Never use a valve on the suction side for this purpose.
8. Avoid pockets and high points
Avoid pockets or high point where air can accumulate in the discharge piping. An ideal pipe run is one where the piping gradually slopes up from the pump to the outlet. This will ensure that any air in the discharge side of the pump can be evacuated to the outlet.
9. Location of control valves
Position control valves closer to the pump discharge outlet than the system outlet. This will ensure positive pressure at the valve inlet and therefore reduce the risk of cavitation.
When the valve must be located at the outlet such as the feed to a tank, bring the end of the pipe to the bottom of the tank and put the valve close to that point to provide some pressure on the discharge side of the valve making it easier to size the valve, extending it's life and reducing the possibility of cavitation.
10. The right pipe size
The right pipe size is a compromise between cost (bigger pipes are more expensive) and excessive friction loss (small pipes cause high friction loss and will affect the pump performance). Generally speaking, the discharge pipe size can be the same size as the pump discharge connection, you can see if this is reasonable by calculating the friction loss of the whole system. For the suction side, you can also use the same size pipe as the pump suction connection, often one size bigger is used. A typical velocity range used for sizing pipes on the discharge side of the pump is 2.5 - 3.5 m/s and for the suction side 1 - 2 m/s.
11. Inadequate pump suction submersion
There is a minimum height to be respected between the free surface of the pump suction tank and the pump suction. If this height is not maintained a vortex will form at the surface and cause air to be entrained in the pump reducing the pump capacity. Figure below shows the recommendation of minimum pump suction submersion.
12. Pump selection
Select your pump based on total head (not discharge pressure) and flow rate. The flow rate will depend on your maximum requirement. Total head is the amount of energy that the pump needs to deliver to account for the elevation difference and friction loss in your system.
Depending on the industry or plant that you work in, you will either select a certain type of pump or manufacturer or both. Manufacturers are normally a very good source of information for final pump selection and you should always consult with them, do your own selection first and confirm it with the manufacturer. They can help you select the right type, model, and speed if you have all the operating conditions.
In the selection process, try to match the flow rate with the B.E.P. (best efficiency point) of the pump. If it is not possible to match the flow rate with the B.E.P., try to remain in the range of 80% to 110% of the B.E.P.. Operating outside this range will lead to excessive vibration, recirculation and cavitation (see figure below).
13. Air in pump reduces capacity
When air enters a pump it sometimes gets trapped in the volute, this reduces the capacity, creates vibration and noise. To remedy, shut the pump down and open the vent valve to remove the air. If the pump is excessively noisy do not automatically assume that the problem is cavitation as air in the pump will create vibration and noise as well.
14. Effect of viscosity on pump performance
Viscosity is the main criteria which determine whether the application requires a centrifugal pump or a positive displacement pump. Centrifugal pumps can pump viscous fluids up to a certain degree, however the performance would be adversely affected. If your fluid is over 400 cSt (centistokes) in viscosity, consider using a positive displacement pump.
15. Avoid running pump in reverse direction
Avoid running a pump in reverse direction. Pump shafts have been broken this way especially if the pump is started while running backwards. The simplest solution is to install a check valve on the discharge line.
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This is a collection of tips and recommendations compiled by Kewpump’s engineers accumulated over years of manufacturing pumps for your useful reference.
These are by no means comprehensive but are some of industry best practices that can help you get the best out of your pumping solutions. However, please be advised that not all of the tips and recommendations given are applicable under all circumstances. Knowledge and good judgement is necessary to avoid property damage or personal injury.
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