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By Thomas A. Westerkamp
October 2012 -
HVAC Article Use Policy
Managers and technicians have a number of tools and tactics at their disposal when it comes to gathering essential data on pump performance. Technicians can track pressure gain across the pump by attaching pressure gauges to 1/4-inch pipe nipples and valves installed on suction and discharge piping. It is good practice to remove the gauges after taking readings so they do not get blocked or damaged by vibration.
Technicians can compare baseline readings to subsequent readings to check for wear of the impeller, casing wear plates, and wear rings. Low or erratic pressure differences can indicate loose belts. As pumps age, the space between the rotating and stationary parts will widen, causing a loss of pressure.
Technicians can track pump-motor loads by using a digital multimeter and check insulation with an ohmmeter when the motor is shut down. A clamp-on ammeter, or tong tester, checks running amps by clamping onto the leads at the starter and comparing this reading with the nameplate's full-load amps. This on-the-run check can help technicians identify overload conditions before they cause a breakdown.
Packing also can be a source of pump problems, so technicians should check them weekly. Slight leaks are common, but a steady flow indicates the packing gland should be snugged up a little at a time. Technicians should replace the mechanical seal if leaks become more severe.
Some pumps use a lantern ring in the stuffing box and pipe water into it to seal and lubricate the shaft wear sleeve. In this case, it is important to insert the right number of packing rings before and after the lantern ring so it lines up with the cooling-water piping. If the packing wears too much before replacement, the tendency by technicians is to tighten the packing gland too far in an attempt to stop leaks, which leads to the misalignment of the lantern ring. This mistake can result in early sleeve wear and more leaks.
While a pump is down for preventive maintenance, technicians can identify the cause for lost capacity by checking impeller clearances with a feeler gage and comparing the readings with the original specification. Technicians also can conduct vibration analysis monthly to identify such conditions as slipping belts, bent shafts, worn bearings, imbalance, cavitation, worn couplings, and misalignment of the motor and pump.
Managers monitoring pump performance to track energy efficiency also must be aware of the impact of flow-control measures. In an effort to control pump flow, technicians often throttle the pump discharge to adjust the flow for lower demand. But this tactic can change the performance curve and cause cavitation and wear on impeller vane tips, reducing capacity while still using the same amount of energy.
Also, when operators throttle a flow that is greater than the demand, the excess energy this tactic produces is converted into noise and vibration, which can cause stress on pipes, weld joints, and pipe hangers and lead to early failure.
A better method of flow control for varying demand is a variable-frequency drive (VFD). VFDs have a drive-control system that allows the technician to preset operating variables, log operating performance continuously, perform self-diagnosis, and alert a supervisor at a remote location in case of a problem. Another advantage of VFDs is lower startup current. An across-the-line starter can draw five times the normal motor current at startup, while VFDs typically draw only one-and-a-half times normal current by setting startup mode to gradually ramp up to speed.
Still another advantage of VFDs is that they enable pumps to operate at lower speeds, lessening bearing loads, shaft deflection, and cavitation. The results include extended time between repairs and lower maintenance costs.
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