Achieving High Pump Efficiency: Best Efficiency Point and Fluids Viscosity
Originally published in Pumps & Systems
By George Taber – Applications Engineer-Technical Services Supervisor, Taco, Inc.
Continuing our discussion on factors affecting pump efficiency, let’s discuss pumps and their BEP or Best Efficiency Point as well as fluids viscosity.
Centrifugal Pumps and BEP
A centrifugal pump is designed for best performance at a head and flow at a certain speed. This is called the Best Efficiency Point (BEP). A pump should be selected so that it will always operate near its BEP. Operating a pump at less than or more than the BEP will lower the operational efficiency and place
additional stress on the pump shaft and bearing due to increased thrust and radial load. Higher flows will increase the NPSH required and erosion due to cavitation could result along with an increase in noise and vibration.
Pumps are variable torque machines that follow the Affinity Laws. These laws explain the change in performance of a pump when the speed is changed or the impeller diameter is changed. These laws can be used to predict the performance of a pump at a reduced speed or smaller diameter impeller. The energy saving can be calculated. If a pump has excess performance, a greater energy savings can be achieved by using a variable speed drive or correcting the impeller trim to match the system resistance. Throttling the pump adds additional resistance to the system to control the pump and is not as efficient as reducing the speed or diameter of the impeller.
The Affinity Laws are RPM2 / RPM1 ) X GPM1 = GPM2, ( RPM2 / RPM1 )2 X H1 = H2, ( RPM2 /RPM1 )3 X BHP1 = BHP2
You can see from the Brake Horse Power formula that theBHP changes with the cube of ratios of the speeds, which is a big energy savings for a small change in speed. Replacing the RPMs with the impeller diameter will follow the same rules. Decreasing the diameter of the impeller from full size does reduce the head, flow and BHP. The further you get away from full size diameter, there will be a drop in efficiency, but the reduction in horsepower due to a lower head should offset this efficiency drop.
The BHP can be calculated from the formulaBHP= Q X H X Sp Gr. / 3960 X pump efficiency. This formula can also be used to predict the operating cost. The electric motor driving the pump also has an efficiency factor, so to determine the operating cost we would factor in the motor by BHPX .746 / efficiency of the motor = Pump KW. (Note: With fluids other than water the specific gravity of the fluid has an effect on the BHP.)
As a heating-cooling system may operate at full load for only a small portion of a given day, if the pump speed can be changed, more of an energy savings can be achieved than worrying about a few +/- points on pump efficiency. As mentioned earlier, proper impeller trim, pump size, and operating point are all important to best operational efficiency. It may be economical to stage different size pumps to carry the load instead of a single large pump.
Viscosity of Fluids
The viscosity of fluids being pumped has a big influence in the performance and efficiency of a pump. Viscosity also has a big effect on the friction loss of all the components of the system and the heat transfer rate of heat exchangers in the system. It is the responsibility of the system designer to supply to the pump manufacturers the true flow and head requirements of the system that is operating with fluids other than water.
There are many engineering handbooks with tables and charts to facilitate calculating the friction loss in pipes and fittings with different fluids. The Hydraulic Institute has an engineering data book available with much information on fluids and methods for calculating losses.
Once flow, resistance (head), fluid temperature and the type of fluid – and if it is a mixture of water and fluid the concentration – are calculated, the pump manufacturer can select the proper sized pump, materials of construction, and motor. The viscosity of the pumped liquid is a critical factor to take into account, as it has a big effect on pump performance and horsepower required. Centrifugal pumps normally use pumping liquids with viscosities below 3,000 SSU (660 centistokes, CST). They may be used up to at least 15,000 SSU (3,300 CST. The higher the viscosity the more significant the reduction in capacity, head and efficiency.
The effects of viscosity on performance of a centrifugal pump operating at the Best Efficiency Point can be seen in the following table.
Over the years The Hydraulic Institute published a chart that was used to determine the performance of pumps pumping different viscosity fluids. Since its initial publication in the 1960s much data has been collected from pump manufacturers, and a new guideline (ANSI/HI 9.6.7-2004) “The Effects of Liquid
Viscosity on Rotodynamic (Centrifugal and Vertical) Pump Performance” has been published. The new guideline allows the engineer to calculate the performance of a pump more accurately.
Next Up: Variable Speed Pumping
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