How to maximize pump performance
Pumps used in factories are used for a variety of purposes, and there are many types depending on the purpose and fluid used.
However, all pumps share the same indicators to measure performance, so it is important to understand the indicators and maintain performance in proper use for any pump.
This time, we will explain the indicators that show pump performance, how to make the most of performance, and points to note when piping.
1. What is a pump?
A pump is a device that applies external power to apply mechanical work to fluids such as liquids and gases.
Mechanical force applied to a fluid is converted into velocity, pressure, and potential energy, and is widely used for transporting, pumping, and stirring liquids.
In order to select and use the pump that best suits your purpose, you need to understand the indicators that show the pump's performance and how to utilize it. We will explain this in detail in the next chapter.
2. Pump performance index
Pump performance can be expressed mainly by four indicators: flow rate, total head, output (shaft power), and pump efficiency. Let's take a look at each of these indicators.
■ ①． flow rate
Flow rate, also known as discharge volume or lift volume, refers to the amount of liquid that can be discharged in a certain period of time when a pump is operating. The following units are generally used:
L/min: The amount of fluid that can be discharged in liters per minute
m³/min: The amount of fluid that can be discharged in one minute expressed in cubic meters
m³/h: The amount of fluid that can be discharged in one hour, expressed in cubic metres
■ ②． Total head
Total head is the energy imparted to a liquid converted into the height (water column) that the water can be lifted to. External forces include velocity, pressure, and potential energy, and there are various types of fluids involved, but when expressing total head, it is always expressed in terms of water.
The unit is "m", but it is sometimes written as "mAq" to emphasize that it is a water equivalent. When converting height to pressure (MPa), 10m is approximately 0.1MPa.
The total head when pumping water can be calculated using the following formula:
Total lifting head (H)
= Total suction head + total discharge head
= (actual suction head [Hsa] + suction piping resistance [Hsf]) + (actual discharge head [Hds] + discharge piping resistance [Hdf])
■ ③． Output (shaft power)
The energy that rotates the motor is called output or shaft power, and this output can rotate the pump and pump water.
The unit used is kW.
■ ④. Pump efficiency
Pump efficiency is a value that indicates the rate at which output can be converted into hydraulic power. Pump efficiency can be calculated using the following formula.
The unit of pump efficiency is "η (eta)," but even for the same pump, efficiency can fluctuate depending on collisions and friction between fluids and operating conditions.
Eta [η] = water power [Pw]/output [P] x 100%
* Hydraulic power (Pw): The energy that a pump gives to water over a certain period of time
3. Key points for making the most of pump performance
What should you pay attention to in order to use the pump without compromising its performance? Let's take a closer look at the key points to making the most of the pump's performance.
■ Fill the casing and suction pipe with fluid
Non-positive displacement pumps are designed to be used with the pump casing and suction piping filled with liquid. If they are operated while empty, they may run dry and burn out. This is called dry running.
If dry running occurs, the pump will not be able to function properly. To prevent dry running, install the pump shaft below the liquid level in the tank and take measures to allow the liquid to flow in naturally.
If the pump shaft must be installed higher than the liquid level, measures such as priming and bleeding the air, or using a valve to prevent backflow of the liquid, must be taken.
■ Arrange the piping so that air does not enter the piping.
If air gets into the piping, the conveying efficiency will decrease accordingly. To obtain the maximum possible discharge pressure and flow rate, install the piping so that air does not get into it.
Air mixing can be minimized by avoiding Torii piping and downward slope piping, which make it difficult for air to escape, on the suction side piping, and by installing an air vent valve on the discharge side piping.
■ Prevent pressure drop in the suction pipe
When a pressure difference occurs inside a pipe, bubbles can repeatedly appear and disappear in a short period of time. This phenomenon is called cavitation, and when it occurs, the inside of the pipe is subjected to extremely large shock pressures, which can cause damage to the inside of the pipe.
Naturally, if the pump is damaged, performance will decrease, so in order to maintain performance and continue using it, it is effective to take measures to prevent cavitation by preventing pressure drops in the suction piping.
Possible solutions include installing the pump at a lower position, using fittings and valves with a low resistance coefficient, and running the liquid at a lower temperature.
■ Start and stop the pump slowly
Like cavitation, water hammer can also damage pipes. Water hammer is a phenomenon caused by sudden changes in pressure within pipes, and can significantly reduce pump performance.
Water hammer is likely to occur when pumps or valves are operated suddenly, so effective countermeasures include using valves that can be opened and closed slowly, such as globe valves or gate valves, and attaching a flywheel (inertia wheel) to the pump to stop the pump as slowly as possible.
■ Select a sealless pump depending on the liquid
Due to its structural role, pumps transmit the power of the motor to the impeller inside the casing, which creates a gap between the shaft and the casing. Fluid leaking from this gap is called shaft seal leakage.
A leaking shaft seal can cause the pump to corrode, so a sealless pump is used when pumping flammable, explosive, or toxic liquids, as well as strong acids and alkalis, or other liquids that must not be released into environment.
Sealless pumps include magnetic pumps and canned motor pumps, and are designed to prevent liquid from leaking out of the casing.
4. Precautions for piping around the pump
Daily maintenance is essential to maximize pump performance and ensure stable operation. For this reason, piping must be installed with sufficient space for maintenance.
Pumps in particular are large in size, so you need to leave a large space for maintenance. You should also consider the overhead space required for lifting and the working space for lifters, etc.
５．まとめ
To get the most out of a performance, it is necessary to understand indicators such as flow rate, total head, output, and pump efficiency, and to operate the pump in a way that maintains performance.
To prevent dry running, cavitation, and water hammer, it is effective to fill the pump with liquid and operate the pump slowly.
Daily maintenance is also essential to maintaining pump performance, so be sure to leave enough space around the pump for maintenance when installing piping.
