When we speak about any machine's efficiency, we refer to how well it can convert one form of energy into another. To give an example, let's assume that we supply one unit of energy to a machine. If the output is one unit of energy in the same measure, its efficiency is 100 percent. On the other hand, if the output is just half-a-unit of energy, then the machine's efficiency is considered 50 percent.
In the pump industry, much of the work is carried by two major components:
The AC induction motor converts electrical energy into mechanical energy.
The centrifugal pump converts mechanical energy into hydraulic energy.
Coming to centrifugal pumps, the efficiency refers to how effectively the pump can convert one form of energy (mechanical energy) going into the pump into another form of energy (hydraulic energy) coming out of the pump. In simple terms, it can be defined as the difference in horsepower going into the pump and coming out of it.
The overall efficiency of a centrifugal pump can be calculated using the formula:
Ef = Pw/Ps
Ef – refers to the efficiency of the centrifugal pump.
Pw – refers to water power.
Ps – refers to the shaft power.
If pumps were to operate in an ideal environment, then the water power exiting the pump should be equal to the shaft power entering the pump, thereby producing 100% efficiency. Unfortunately, it's not practically possible to operate pumps in an ideal environment. Due to friction, energy losses, and leakage, the pump's shaft power is always higher than the water power exiting the pump.
Why worry about Pump Efficiency?
The pump efficiency plays a crucial factor in helping you choose the right pump for the job. A pump that isn't efficient cannot handle the job at hand. Also, comparing the pump's previous efficiencies to the current efficiency will help you understand if your pump is facing any troubles.
How to calculate the Water Power (Output of the Pump)?
To determine the pump's efficiency, you need to convert the head units and capacity of the pump into horsepower output. Here capacity refers to the total water output, measured in the number of gallons per minute. Head units are measured as the distance between the source water and the pump's output. It's usually calculated in feet.
Here’s an illustration to help you understand efficiency calculation hypothetically. Let’s take the case of a centrifugal pump that pumps fresh water.
Water flow rate is 350 gallons/minute
Head units: 100 feet
Water power = head * capacity * specific gravity of water * 8.333
Notice that there are more components to the equation besides the head and capacity. Here, 8.333 is the weight of one gallon of water, and the specific gravity of water is 1. With these factors included, the final output,
Water power = 100 * 350 * 1 * 8.333 = 8.84 horsepower out.
Now that we have calculated the output horsepower, the next step is to calculate the input horsepower. This can be easily found by referring to the pump manufacturer's manual. Let's assume that it is 20 horsepower.
Now, we can calculate the efficiency of this centrifugal pump:
Pump Efficiency = Water horsepower/mechanical horsepower = 8.84/20 * 100 = 44%
In this hypothetical example, our centrifugal pump has a peak efficiency of 44%. Which makes it a relatively poor design.
Broadly speaking, large and medium centrifugal pumps have efficiencies ranging from 75% to 93%. Smaller centrifugal pumps have an efficiency ranging from 50% to 70%. Depending on the pump design, use, and operating conditions, centrifugal pumps have an efficiency of up to 90%. Regular maintenance, proper operating conditions, and timely repairs can help you keep your pumps operating at maximum efficiency. Stay tuned for future articles on how to improve pump efficiency.
For any other questions on your centrifugal pump, get in touch with our experts at Sintech, India’s No.1 pump manufacturer and exporter.