Horsepower curve, pump

Figures 3-36A, 3-36B, and 3-36C represent typical and actual performance curves showing discharge total head head pressure at pump outlet connection for any fluid), required minimum water horsepower (for pumping water), and capacity or pumping volume of the pump (for any fluid) for several impeller diameters that would fit the same case (housing). In addition the important NPSHR (net positive suction head required by the pump) charac-...

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. 

Using the affinity law (Equation 6-76), the corresponding values for the head at varying flow rates are shown in Table 6-8. The computer program PROG63 calculates both the brake horsepower and pump efficiencies of the 6- and 8-inch pumps. Tables 6-9 and 6-10 show the input data and the computer results of these pumps. The characteristic curves are shown in Figure 6-6. 

Figure 18.4 contains three curves, each depicting the pump performance with a different impeller diameter. The punp casing can accept any of these three inpellers and each impeller has a unique pump curve. Punp curves often display efficiency and horsepower curves in addition to the curves shown in Figure 18.4. 

If the pump manufacturer uses motors for pump power measurement, these motors ate caUbrated to determine the horsepower from the electric power reading and caUbration curves. Such test motors ate recaUbrated periodically, ensuring the same degree of accuracy as shown by the torque meters. 

FIG. 10-28 Characteristic curve of a centrifugal pump operating at a constant speed of. 3450 r/min. To convert gallons per minute to cubic meters per hour, multiply hy 0.2271 to convert feet to meters, multiply hy 0..3048 to convert horsepower to kilowatts, multiply hy 0.746 and to convert inches to centimeters, multiply hy 2.54. 

Change process to adjust specific gravity to design value, or throttle pump to reduce horsepower requirements. This will not correct problem with some vertical turbine pumps that have a flat horsepower-required curve. 

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. 

In reality, the performance curve is easy to understand. It isn t rocket. science. The performance curve indicates that the pump will discharge a certain volume or flow (gpm) of a liquid, at a certain pressure or head (H), at an indicated velocity or speed, while consuming a specific quantity of horsepower (BHP). The performance curve is actually four curves relating with each other on a common graph. These four curves are ... 

Next, let s eonsider the eiierg) eurve, the brake horsepower (BHp), ree]uired by the pump. This curve is probably the easiest to interpret because it is practically a straight line. Con.sider the following the pump consumes a certain e]uantity of energy just to maintain shut-off head. Then, as flow begins and increases, the horsepower consumption normally increases. (On certain specific duty pumps, the BHp may remain mostly flat or even fall with an increa.se in flow.) The BHp eurve is normally seen this way (Figure 7-5). 

Actually, everything we said about bearings, mechanical seals, piping, TDH, system curves and mating the pump curve to the system curve, the affinity laws, cavitation, horsepower and efficiency arc as applicable to PD pumps as centrifugal pumps. 

When a pump has a motor drive, the process engineer must verify that the motor will not overload from extreme process changes. The horsepower for a centrifugal pump increases with flow. If the control valve in the discharge line fully opens or an operator opens the control valve bypass, the pump will tend to run out on its curve, giving more flow and requiring more... 

For the rising type characteristic curve, the maximum brake horsepower required to drive the pump over the entire pumping range is expressed as a function of the... 

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. 

Figure 3-55 illustrates the application of these performance laws to the 1750 rpm curves (capacity, brake horsepower, and efficiency) of a particular pump to arrive at the 1450 rpm and 1150 rpm curves. Note that the key value is the constant efficiency of points (1) and (2). When the speed drops to 1450 rpm, capacity drops ... 

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. 

Figure 5-5T. Standard six-blade vertical curved blade turbine impeller. Gives good efficiency per unit of horsepower for suspensions, mixing fibrous materials. Gives high pumping capacity. Courtesy of Philadelphia Gear Corp.

A typical performance range of capacides of rotary lobe vacuum pumps is shown in Figures 6-48A, 6-48B and 6-48C. Another set of curves for rotary lobe pumps, shown in Figure 6-49, provides the brake horsepower, airflow at inlet (CFM) (referred to as their standard volume at 70°F and 29.92 inch Hg abs discharge pressure—essentially atmosphere), and the temperature rise through a non-cooled (no internal or external cooling) vacuum. All data... 

Figure 5-5S. Four-blade, vertical Figure 5-5T. Standard six-blade fiat blade turbine Impeller. Very vertical curved blade turbine versatile, one of the most used impeller. Gives good efficiency in wide application range. Cour- per unit of horsepower for sus-tesy of Philadelphia Gear Corp. pensions, mixing fibrous materials. Gives high pumping capacity. Courtesy of Philadelphia Gear Corp.

Effects of performance changes, 201-203 Head curve for single pump, 198 Relations between head, horsepower, capacity and speed, 200 Temperature rise 207-209 Viscosity corrections, 203-207 Purging, flare stack systems, 535 Reciprocating pumps, 215—219 Flow patterns, 219 Specification form, 219 Relief areas, 437 External fires, 451, 453 Sizing, 434, 436... 

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