Strategies to Increase Energy Efficiency of Centrifugal Pumps and General Pumps
Pumps left fluids to higher elevations and/or pressure and consume energy in the process:
This relationship is applicable to any pump. However, the following will be developed
only for centrifugal pumps.
Numerous factors influence the efficient selection and operation of centrifugal pumps, in particular:
- Fluid flow rate.
- Pressure rise from pump suction to pump discharge.
- Pump efficiency and rotating speed.
- Fluid characteristics (e.g., specific gravity and viscosity)
- Type of flow control (e.g., discharge throttling or speed control)
A centrifugal pump is capable of operating over a wide range of conditions. Unfortunately, there is a limited operating range around the pump’s best efficiency point (bep) before the pump efficiency rapidly declines. Prolonged time’ pump operation remote from the best efficiency point results in energy wasted.
If the original design condition change significantly or a pump requires replacement for maintenance reasons, the pump and its driver should be reevaluated at its current head and flow requirements.
Centrifugal pump horsepower can be calculated directly from:
Q = actual flow, gal/min
PD = pump discharge pressure, psig
PS = pump suction pressure, psig
E = pump efficiency, decimal %
SG = specific gravity of the pump fluid
The pumping horsepower can be minimized by:
- Reducing flow rate, Q
- Reducing pressure differential, (PD-PS) or head, H
- Increasing efficiency, E
Consider a continuously operating motor driven 200 GPM, 100 psi, 70% efficient, 16.7 horsepower pump and an electrical value of 0.05 $/KWH. A 10% reduction of either capacity or head would result in approximate 1.7 HP reduction or 540 $/YR savings. Similarly, savings due a 5% efficiency improvements is 1.1 HP or 360 $/YR.
Before investing money to improve pump system efficiency, a review of operation should be made to determine if any pumps could be shut-down. Eliminating or minimizing pumping time or flowrate can contribute significantly to overall energy savings. Services which are suspect to over-utilization include, but should not be limited to:
– Rerun or recycle pumps.
– Cooling water circulation pumps.
– Pumps operating in parallel.
- Automatic control is preferable.
- Mechanical / maintenance
- Speed control
- Reducing impeller diameter
- Worn wear rings
- Minimum flow bypass control valve
A centrifugal pump operates at the intersection of the head/capacity H-Q curve, and the system head curve. The system head curve is the summation of pressure, static and dynamic heads of the system. The static head is the elevation through which the fluid is lifted, the pressure head is the equivalent of downstream vessel or tank pressure and dynamic head is the fraction loss due to flow through piping, valves and fittings.
A typical system head curve can be developed by taking a pressure and elevation traverse of the system. The dynamic head or variable system resistance head increases in relation to the square of the fluid flow rate. For a constant –speed pump, operation at flow less than that at the intersection of the H-Q curve and system head curve requires a control valve drop equal to the head difference between the curves. Operation at flows much less than rated capacity results in excessively high pressure drop across the flow control valve which wasted energy.
Pump impeller diameter varies, speed is constant.
D = diameter
Q (GPM) = actual flowrate at pumping temperature.
S (RPM) = pump speed.
H = head
BHP = horsepower
Where: n is new or actual conditions.
b is original or base performance condition.
Example – rerated by change in impeller diameter
Performance difference between two different diameter impellers can be calculated by using the “Affinity laws”. Assume the constant-speed motor-driven centrifugal pump with an 8.5 in diameter impeller.
Calculate the new performance for the same pump but with an 8 inch impeller.
BHP savings = 22.3 – 18.9 = 3.4 HP
A new H-Q curve can be drawn through the newly calculated point parallel to the original curve.
Where diameter changes are small the efficiency correction factor can be ignored.
Using the same equations and trial and error calculations a new impeller diameter could be calculated to match a new H-Q point required by the process.