Pumps are ubiquitous and essential in industrial processes involving liquids. They come in all shapes and sizes, with some injecting a few drops per day and others pumping tens of thousands of gallons per minute.
Pumps are one of the most energy-intensive components of industrial operations. Pump motors, in particular, consume a lot of energy. As a result, operators should naturally strive to maximize energy efficiency, in order to get the most bang for their buck. Even a 1% increase in efficiency can save tens of thousands, in annual pumping costs.
Let’s see the effectiveness of pumps in WWTPs and their calculations for pump power.
Equation for the efficiency of motor pump
The overall efficiency of a centrifugal pump is simply the ratio of water (output) power, to shaft (input) power, as shown by the following equation:
Ef = PW / PS
Pw= the water power
Ps= the shaft power
Ps: It is the power given to the pump shaft in brake horsepower.
Pw = (Q x H) / 3960
Q= Flow (gallons per minute—GPM)
H= Head (feet)
The constant (3,960) converts the flow and head product (GPM-feet) into BHP.
According to these equations, a pump producing 100 GPM at 30 feet of head and requiring 1 BHP, will have an overall efficiency of 75.7 percent, at that flow point.
If we know the hydraulic efficiency of a pump, we can use an extension of the second equation to calculate the BHP required at any point, on its performance curve.
How to calculate pump efficiency in WWTP?
The pump's mechanical power, or input power is a property of the pump. The output power or hydraulic power, is calculated by multiplying the liquid flow rate by the system's "total head."
Total Head = Discharge Head + Elevation Head
Let us take one example:
Consider a 1,000 HP pump that pumps water at a rate of 5,000 BPD at 10,000 PSI from a pond 20 feet below.
Efficiency = Output Power / Input Power
Efficiency = Output Power / 1,000 HP
We already know the input power, so all that remains is to calculate the output power using the unit conversions mentioned above.
Output Power = Flow Rate * Total Head
Output Power = 5,000 BPD * Total Head
We know the flow rate, but we need to calculate the total head by taking elevation changes into account, when calculating the discharge pressure.
Total Head = Discharge Pressure + Elevation Head
Total Head = 10,000 PSI + Elevation Head
Because, every foot of water adds 0.434 PSI of pressure, we'll calculate the elevation head by converting the change in elevation in feet, to the suction pressure created by the water.
Elevation Head = Elevation Change in (ft) * 0.434 PSI / ft H2O= 8.67 PSI
We can calculate the total head by adding the elevation head to the discharge pressure.
Total Head = 10,000 PSI + 8.67 PSI
Total Head = 10,009 PSI
You'll notice that the elevation head is small in comparison to the discharge pressure, and has little effect on the pump's efficiency. However, as the elevation change or discharge pressure decreases, the elevation change has a greater impact on the total head.
Now, that we know the total head, we can calculate the output power using unit conversions.
Output Power = 5,000 BPD x 10,009
5,000 BPD = 0.325 ft3/s
10,009 PSI = 1,441,296 lb/ft2
Output Power = 0.325 ft3/s x 1,441,296 lb/ft2
Output Power = 468421 lb-ft/s
468421 lb-ft/s = 851.67 HP
We can easily calculate pump efficiency now that we have the output power in HP!
Efficiency = 851.67 HP / 1,000 HP
Efficiency = 85.2%
It is important to monitor pump efficiency to minimize energy losses.
If you have any questions about wastewater treatment plants and the calculation of pump power in WWTPs, please contact the Netsol Water team right away. Our team is an expert in water treatment and can help you tailor a solution to your specific requirements.
We collaborate with our clients to design a customized wastewater treatment solution that meets your specific requirements, as well as an on-going service plan to completely maintain your system.