Maintenance and Application Guide for Centrifugal Pump

EPRI Licensed Material 

Deep Draft Vertical Centrifugal Pump Guide

Flow may be determined by installing a venturi, nozzle, or plate orifice in the discharge piping A weir may also be used downstream of the condenser in the return channel to the river or lake source. These types of flow measurement devices are stable over long periods of time provided the throat area is not subjec ted to erosion or corrosion. The use of stainless steel will normally aid to resist attack. From time to time the diameter of the throat area should be accurately measured using a micrometer and the dimension compared to the asnew measurement. Wear to the throat can be detected before test readings are significantly affected. The original supplier of the device may be able to supply a modified constant for a slightly worn throat area which will allow continued use of the device. Once again, it is important to locate the flow measuring device so it will not be adversely affected by non-uniform flow conditions preceding and/or following the meter. It is recommended that these type of meters be located m accordance with Hydraulic Institute Standards to achieve accurate results. For a venturi the distances ahead of the mete r vary between one (1) and twelve (1 2) pipe diameters while for nozzles and orifices the range is from six(6) to forty (40) pipe diameters. Distances after the meter vary from zero (0) to six (6) pipe diameters. When the flow is measured using a weir, Hydraulic Institute Standards recommend a rectangular sharp crested weir with smooth vertical crest walls, complete with end contraction suppressed. Values for the proper coefficients may be found in the latest Hydraulic Institute Standards. Other configurations may be used for weirs provided there is a way to determine the proper coefficient. Note that a weir can only be used in an open channel return. Weirs cannot be used for cooling tower arrangements. Liquid temperature near the pump must be determined so that the proper specific gravity can be used in calculating performance. Temperature is usually checked m the sump as close to the pump as practical using a standard thermometer. Temperature may also be determined using a thermocouple well in the discharge pipe or elbow. The horsepower draw of the motor must also be determined. The most accurate method to accomplish this is to secure kilowatt readings with a polyphase wa ttmeter connected to the motor leads at the motor conduit box. Connecting at this point prevents inclusion of  erroneous data (including lights, heaters or other electrical loads) that are not related to the motor load. A curve of brake horsepower vs. input kilowatts must be prepared using information from the motor manufacturer concerning the motor efficiency at 1/2 load, 3/4 load, full load and 1.15 load if the motor furnished has a service factor greater than 1.0. Note that actual test data should be used in lieu of calculated data when available.

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