Pump Horsepower Calculations

A rapid method to estimate pump horsepower is by the following formula  

GPM = Gallons per minute of liquid being pumped AP = Delivered pressure (discharge minus suction), psig T) = Pump efficiency 

Note that the expression can be rearranged to obtain AP in terms of other variables. The expression can therefore be used to generate a family of curves of HP versus AP with GPM as a parametric variable. 

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Table III shows the additional horsepower that would be added to the calculated pump horsepower that has been adjusted by two or three points for the change in speed.

The increase in horsepower required should be applied to the operating pump horsepower to calculate the resulting efficiency. This differential is greater when pump horsepower has been stepped from one speed to another by using the affinity laws (rather than by actually measuring efficiency at the speed in question). 

Efficiency and Brake Horsepower Calculations for the 6-inch Centrifugal Pump Program Inputs and Outputs... 

THIS PROGRAM CALCULATES ALL PUMP HEAD CALCULATIONS REQUIRED TO WRITE A PUMP SPECIFICATION SUITABLE FOR A VENDOR INQUIRY OR PURCHASE ORDER. THE PROGRAM FURTHER CALCULATES THE CENTRIFUGAL PUMP EFFICIENCY AND PUMP HORSEPOWER USING AN EQUATION DEVELOPED BY BY C. BRANAN (The Process Engineer s Pocket Handbook, Gulf Publishing Company, Houston 1976.)... 

C THIS PROGRAM CALCULATES EFFICIENCY AND PUMP HORSEPOWER USING... 

Power. There are two main ways to measure the power deUvered by the driver to the pump. The first method is to install a torque meter between the pump and the driver. A torque meter is a rotating bat having a strain gauge to measure shear deformation of a torqued shaft. Discussion of the principle of torque meter operation is available (16). The benefit of this method is direct and accurate measurements. The power deUveted to the pump from the driver is calculated from torque, T, and speed (tpm) in units of brake horsepower, ie, BHP (eq. 4a) when Tis in lbs-ft, and kW (eq. 4b) when T is N-m. 

When arriving at the performance of a pump, it is customary to calculate its power output, which is the product of (1) the total dynamic head and (2) the mass of liquid pumped in a given time. In SI units power is expressed in kilowatts horsepower is the conventional unit used in the United States. 

Involved in producing the curves for Figs. 29-53 and 29-55 is a calculation of the so-called balance point at which the flow and revolutions per minute required by the recovery unit match those provided by the pump. If the recovery turbine is the sole driver (as for the lean pump of Fig. 29-54), both the speed and the brake horsepower of the recoveiy turbine and its driven pump must be the same at the so-called balance point. If there is a makeup driver and the recovery unit has available to it just the flow from the pump that it is driving, as for the pump of Fig. 29-56, then the speea ana capacity must match at the balance point. 

FIG. 29-61 Horsepower-r/min balance for a lean pump tandem-connected with a power-recovery turbine operating as the sole driver. Horsepower differences are calculated from excess head requirements as typically shown in Fig. 29-60. To convert gallons per minute to cubic meters per hour, multiply by 0.2271 to convert horsepower to kilowatts, multiply by 0.746. 

The driver horsepower must be greater than the calculated (or value read from curves) input BHP to the shaft of the pump. The mechanical losses in the coupling, V-belt, gear-box, or other drive plus the losses in the driver must be accounted for in order that the driver rated power output will be sufficient to handle the pump. 

Mdien dscous liquids are handled in centrifugal pumps, the brake horsepower is increased, the head is reduced, and the capacity is reduced as compared to the performance with water. The corrections may be negligible for viscosities in the same order of magnitude as water, but become significant above 10 centistokes (10 centipoise for SpGr = 1.0) for heavy materials. While the calculation m.ethods are accepta oly good, for exact performance charts test must be run using the pump in the service. 

In the example the manufacturer has been specified from available performance curves, and the details of construction must be obtained. The pump is selected to operate at 22 GPM and 196 to 200 feet head of fluid, and must also perform at good efficiency at 18 GPM and a head which has not been calculated, but w hich will be close to 196 to 200 feet, say about 185 feet. Ordinarily the pump is rated as shown on the specification sheet. This insures adequate capacity and head at conditions somewhat in excess of normal. In this case the design GPM w as determined by adding 10 percent to the capacity and allowing for operation at 90 percent of the rated efficiency. Often this latter condition is not considered, although factors of safety of 20 percent are not unusual. However, the efficiency must be noted and the increase in horsepower recognized as factors w hich are mounted onto normal operating conditions. 

The required pump hydraulic horsepower (PHHP) can be calculated as... 

Brake horsepower, centrifugal pumps, 200 Driver horsepower, 201 Burst pressure, 405, 456 Cartridge filters, 274-278 Capture mechanism, 279 Edge filler, 278 Filter media, table, 278 Micron ratings, 277 Reusable elements, 281 Sintered metal, 280 Types, 276, 277, 279 Wound vs. pleated, 276, 277 Centrifugal pumps, operating characteristics, 177-180 Calculations, see hydraulic performance Capacity, 180... 

Overall coefficients, 332 Vertical plate coil, 331 Hindered settling velocities, 231, 236 Horsepower, centrifugal pump driver, 201 Hydraulic performance, calculations, 180-188... 

Some designers suggest adding 10% to the calculated liquid horsepower to allow for changes in the process. For reflux pumps the LHP should be increased 35% beyond that calculated.26 This is because the amount of reflux is a determinant in the amount of separation possible and the pump should not limit the options available to the plant operator. 

Such performance curves are normally determined by the manufacturer from operating data using water at 60°F. Note from Eq. (8-6) that the head is independent of fluid properties, although from Eq. (8-4) the power is proportional to the fluid density (as is the developed pressure). The horsepower curves in Fig. 8-2 indicate the motor horsepower required to pump water at 60° F and must be corrected for density when operating with other fluids and/or at other temperatures. Actually, it is better to use Eq. (8-4) to calculate the required motor horsepower from the values of the head, flow rate, and efficiency at the operating point. The curves on Fig. 8-2 labeled minimum NPSH refer to the cavitation characteristics of the pump, which will be discussed later. 

Step 6. Multiply the horsepower per pound of steam value calculated in step 5 by the turbine steam flow, in pounds per hour. This is the total shaft work that appears at the turbine s coupling. This is the amount of horsepower that is available to spin a centrifugal pump. 

For purposes of example, assume a flow of 8.71 mVmin (2300 gal/min) through the tower The maximum head available to the recovery turbine was calculated to be 604 m (1982 ft) this value will be slightly in error when part of the flow is bypassed since frictional losses into and out of the recovery unit will change. First, assume the lean pump to be at 3.03 mVmin (800 gahmin) running at 3900 r/min with the semilean pump at 5.68 mVmin (1500 gal/min) to get the total flow of 8.71 mVmin (2300 gal/min). At 3.03 mVmin (800 gal/min) and 3900 r/min the available head of the lean pump is read from the curve. This must be greater than the required head, and the excess is plotted as in Fig. 29-60. The brake horsepower of the lean pump is also read. 

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