Whether operating under harsh or mild external conditions, environmental surroundings can put a great deal of stress on the pumping equipment that is required to run 365 days a year. To achieve optimum performance and reliability in a centrifugal pump, it must operate close to its best efficiency point, or BEP – the point at which the hydrodynamic unbalanced load of the centrifugal pump is at its minimum.
When a pump operates at a point some distance from the actual BEP, the result is an overall increase in hydrodynamic unbalanced load. This, in turn, affects the performance, reliability, and efficiency of the centrifugal pump. (Based on experience and experiments, the unbalanced load is at its peak at the shutoff point.)
In any operational atmosphere, a routine maintenance program will extend the life of a pump since well-maintained equipment lasts longer and requires fewer and less-expensive repairs. This article outlines a basic checklist for the preventive maintenance of centrifugal pumps.
Overview of pump efficiency
A machine’s efficiency indicates its ability to convert one form of energy to another. If one unit of energy is input to a machine and its output is one-half unit, the machine’s efficiency is 50%.
For a centrifugal pump, much of the work involves two very efficient machines: the pump itself and the AC induction motor that drives it. The centrifugal pump converts mechanical energy into hydraulic energy (flow, velocity, and pressure), and the AC motor converts electrical energy into mechanical energy.
Medium and large centrifugal pumps generally operate with efficiencies above 75%, and smaller ones usually fall into the 50% to 70% range. Large AC motors, on the other hand, approach an efficiency of 97%.
The overall efficiency of centrifugal pumps is the ratio of the water (output) power to the shaft (input) power, and it is described by the following equation:
Ef = PW / PS
Ef = efficiency
PW = the water power
PS = the shaft power provided to the pump shaft in brake horsepower (BHP)
PW = (Q x H) / 3,960
Q = Flow (gallons per minute, gpm)
H = Head (feet)
3,960 = converts to BHP
For example, a pump that produces 100 gpm at 30 feet of head and requires 1 BHP will have an overall efficiency of 75.7% at that flow point.
The total efficiency of a centrifugal pump is the product of mechanical, volumetric, and hydraulic efficiencies. Mechanical efficiency accounts for losses in the bearing frame, stuffing box, and mechanical seals. Volumetric pump efficiency comprises losses due to wear ring leakage, balancing holes and vane clearances (in the case of semi-open impellers). Hydraulic efficiency includes liquid friction and other losses in the volute and impeller.
While mechanical and volumetric losses are important contributors to total efficiency, hydraulic efficiency is the largest factor. There are various conditions that decrease the efficiency of a centrifugal pump:
- Heat generated due to packing
- Rubbing between wear rings and maintaining impeller clearances
- Recirculation using a bypass line from the discharge of the pump to the suction
- Double volute design
- Throttled discharge valve
- Corroded internal pump passages that cause fluid turbulence
- Obstacles, hindrances, or any sort of restrictions inside the piping passages that might include foreign particles or dirt
- Overlubricated bearings
A detailed record of preventive maintenance performed and required repairs should be kept to aid in diagnosing problems and to eliminate or minimize any future equipment downtime. Operators who adhere to a preventive maintenance program will reap the benefits of a facility that functions without encountering breakdowns and out-of-service situations.
Pump manufacturers supply a manual with recommended maintenance procedures for their centrifugal pumps. However, routine preventive and protective maintenance practices should, at a minimum, include the monitoring of the following:
- Bearing and lubricant condition. Monitor bearing temperatures, lubricant level, and vibration. The lubricant should be clear with no signs of frothing. Excessive vibration and an increase in bearing temperature may indicate imminent failure.
- Shaft seal condition. The mechanical seals should show no signs of visible leakage. Some packing leaking is normal, but this should not exceed a rate of about 40 to 60 drops per minute.
- Overall pump vibration. Imminent bearing failure can be preceded by a change in bearing vibration. Excessive vibration can result from a change in pump alignment or cavitation resonances between the pump, its foundation, or the valves located in the suction and/or discharge lines.
- Pump discharge pressure. A gradual decrease in the developed head pressure of the pump may indicate that the impeller clearance has widened. An impeller clearance adjustment may be required to restore the pump to its intended design performance.