Machinery's Handbook, 31st Edition
2806 PUMP EFFICIENCY Volumetric efficiency is calculated using the following equation: η V 1 ∆ p β – = where b is the compressibility factor of the fluid, C / D is the pumping chamber clearance to displacement ratio, and L V is the fluid loss to slippage back through the pump valves before they can seal. D p = p d - p s where p d is pressure at discharge and p s is pressure at suction. Centrifugal pumps are intended for use near their best efficiency. When operated below peak efficiency, a phenomenon called “recirculation” can occur, which causes flow rever sal at the inlet or discharge tips of the impeller. This can be very detrimental to perfor mance and damaging to the pump. Specific Speed: Centrifugal pumps are characterized in terms of specific speed. Specific speed is the speed of an ideal pump needed to raise a unit of volume through a unit of head in a unit of time. It assumes the ideal pump is geometrically similar to the pump being evaluated. Specific speed is used to determine the geometry (type of impeller) needed for a given set of input and output requirements. It is calculated using the following equations where n is rotation in RPM: h s = n Q h t 0.75 ------ where Q is in gal/min, and h t is total dynamic head in feet. h s = n V · h t 0.75 ------ where V · is in m 3 /sec, and h t is in meters. Low specific speeds produce the largest heads. In US units, specific speeds range from approximately 500 to 1500 RPM for radial vane pumps, 1500–4000 for Francis vane, 4000–9000 for mixed flow impeller designs, and above 9000 for axial flow impellers. When specific speed is low for a given style, efficiency suffers, and a multi-stage pump should be used. Once specific speed has been calculated from a given head, drive speed, and flow rate, the appropriate type of impeller can be selected. In addition, pump effi ciency, NPSHR , maximum speed, and suction lift can be determined using specific speed. It is good practice to select a centrifugal pump that has the highest specific speed, since that will yield the smallest pump. Critical Speed: Critical speed is related to static deflection of the shaft in the horizontal position. When operated within about 20 percent of critical speed, the pump will vibrate excessively and damage can occur. The wet critical speed must be well above the operat- ing speed of the pump in order to ensure smooth running. Centrifugal pumps have a “dry” critical speed and a “wet” critical speed. Wet critical speed is normally higher (smaller deflections experienced) than dry critical speed. As the pump shaft support components (bearings or bushings) wear, the wet critical speed drops. When a pump is operated away from its best efficiency, higher deflections are likely, causing critical speed to drop. Dry critical speed can be determined using the following equations: n c 187.7 y = ------- where y is shaft deflection in inches, and n c is critical speed in RPM. Shaft deflection is typically limited to 0.006 inches in commercial pumps. This leads of a critical speed value of 2423 RPM. n c 946 y = ----- where y is shaft deflection in mm. A maximum shaft deflection of 0.15mm leads to a criti cal speed of 2442 RPM. C D -- L V +
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online