Document ymrjQNoGqdVjj5JNxra3Zjw86

864 CHAPTER 56 . 1965 Guide And Data Boot fig. 2.... Specific Gravity of Water as a '.Function of Temperature Fig. 3 .... Typical Performance, and BHP Requirements .for a Single-Suction, Long-Coupled .Centrifugal Pump at Various Speeds tion 2 is multiplied by the specific gravity of the fluid. Pump efficiency decreases somewhat with other fluids. This must be taken in account when computing pump horsepower. Pump presures are expressed in either pounds per square inch or in feet of liquid. A column of 2.3 ft of water at 62 F exerts a pressure of 1 psi. Pump selection charts usually give pressures or heads in feet. To convert readings given in pounds per square inch into feet of water they must be multiplied by 2.3. Dividing by 2.3 reduces readings in feet of water into pounds per square inch. The unit for measuring head is the foot. Therefore: , ' , psi X 2.31 X 62.3 . psi X 2:31 head in ft --------------- ------------- ---------- -------- (4) where W ~ specific weight in pounds per cubic foot. S ** specific gravity of fluid. Pump capacity or volume is expressed in gallons per minute (gpm), cu ft per second (cfs), or gallons per 24rhr day (gpd), delivered against a specified head or pressure. Specific gravity is the ratio of the weight of a substance to that of an equal volume of water at 39.2 F (the temperature of maximum density of water). The specific gravity of water at various temperatures is given in Fig. 2. For brines, refer to Chapter 21. Pump speed is the speed at which the shaft rotates the impeller, and is given in revolutions per minute (rpm). Tip speed refers to the speed at the impeller periphery, and is given in feet per second. Specific speed is a type number or index used by the design engineer to select the most applicable speed for a pump designed to operate in a certain range. However, tip speed and specific speed will both be within satisfactory limits if pumps are selected from the data shown ' in manufacturer's catalogs. ' When pump speed is changed, capacity varies directly as the speed, pressure (head) varies directly as the' square of the speed, and power requirements vary directly as the cube of tiie speed. (See Fig. 3). Performance Curves A pump's' performance is most commonly shown by means of plotted curves, as in Fig. 1, which relate the flow (gallons per minute), the pressure produced (head' is feet of fluid), the power required (brake horsepower), the hydraulic efficiency (water hp output +bhp input), the specific gravity, and vis cosity of the fluid, the shaft speed (revolutions-per minute), and the net positive suction head (feet of fluid) required for normal operation, of the pump. See section System Charac teristics for net positive suction head (NPSH).' For many amah pumps, some of this information is omitted. - Pump curves present the average results obtained from testing several pumps of the same design under standardized test conditions. Manufacturers should be consulted for pump applications that differ considerably from ordinary practice. ' There is a'common but arbitrary classification of centrifugal pumps into steep-curve pumps and fiat-curve pumps, -Which refers to the shape of the. head capacity curve as shown in Fig. 4. Pumps with flat-curves'often preferred' for closed circuit systems,' because large changes in pump capacity can be achieved with a, small change 'in head: This is valuable when balancing1'several circuits' in a system. Steep-curve pumps are frequently selected for*open circuit systems, .such as for cooling towers. They have the advantage tKat'a change in piping resistance,*resulting from corrosion or scale accumu lation, has a minimum throttling effect on the flow. > The maximum efficiency characteristic of a pump` is not always the most important feature in making a selection. A selection' must be made in consideration ofthe system -re sistance if the pump'is'to operateat'its peak efficiency. Sys tems in which there is no corrosion and systems that are pro tected against corrosion* should;not be designed from pipe fig. 4 .... Performance Curves for Steep-Curve*' Pumpi . and fiat^Gnve Pumps ^ Pumps .. ! 865 Table;;!:..Performance Characteristics of Various Types of Pumps - ........... - Characteristic Flow. ... i- Pump Response to Throttling of Discharge . >;.'*./' ' j Resulting Effect of Raising Head Pressure ' , Power Capacity i - - Power ;; * i ' Centrifugal : Even Increase Decrease Decrease Decrease fypo of fuap Rotary rorbtao- Rodprocof<a0 Even Even Pulsating With no Relief Valve---Increase to Destruction - Increases to limits of Power None* *. Slight - None Increase Increase Increase Resulting Effect ..of- Lowering.',* .'Capacity Head ' Power' - . Increase ' * Increase None- . Decrease - Slight Decrease , Slight to- None Decrease friction loss tables containing high corrosion allowance. Use of such tables results in excessive heads and causes selection of a larger pump thanactiially is required, with consequent higher initial costs and increased.power consumption. .When deter mining friction loss-for. closed , and .corrosion-free: systems, tables.for resistance of clean pipeishould always be used.* It should also be noted that the pump user is always interested in performance at the.actual pumping-conditions-of the sys? tern,' rather than.in performance:at .some other operating point on its performance-curve.. .' - It is a general rule that pumps for systems using liquids for heating and cooling should.be hydraulically and mechanically designed,for quiet operation, durability, .simple,Bervice, mini mum maintimn-nfa, , ftnH - minimum niftt.mn - .raquirpmpntft rather than for.minimum cost.or eize. , -, j.,- >.. . - ' HYDRAUUC'; SYST^ ^; Particular attention must be'given to the condition of the fluid as it enters the suction'flange of-the pump. If-the abso lute pressure on the fluid at the suction flange approaches the vapor pressure of the fluid too closely, vapor pockets will form in the impeller passages. This condition will interfere with pump performance, and-the collapse of the vapor pockets will be noisy and even destructive. The amount of pressure in excess of the 'vapor pressure required to prevent the formation of vapor pockets, is known as the required net posi tive suction .head (NPSH). The required NPSH varies con siderably with pump capacity and increases rapidly at'.very high capacities, as' shown in Fig. 1. This rapid increase of the NPSH explains the noisy and often unsatisfactory performance of centrifugal pumps at extreme capacities: -A* common rule is to operate a pump at no more than-J.of its maximum capacity. Particular attention must be given to this when operating, with hot fluids or with fluids pumped from vacuum conditions. Then it is often necessary to calculate the available net positive suction head. This can be done simply by redrnlnting the absolute static pressure'at the suction,flange,- adding the velocity head of the fluid at the suction flange, and subtracting the vapor pressure of the fluid' at the'suction flange. The result will be the arotZaNe-NPSH. If the available NPSH is smaller than the pump manufacturer's recommended required NPSH; cavitation noise, inadequate pumping, and mechanical problems are to be expected. If the required NPSH is not given in the performance table, the pump manufacturer must be consulted. A centrifugal pump responds to the hydraulic characteris tics of the system to which it is applied and produces fluid flow and pressure related to the pressure conditions of the sys tem for that .flow (see Table 1). This .condition makes it necessary to carefully predict the system characteristics for proper pump selection. Pumps are applied to two general types of piping systems: closed systems and open systems.. For a closed system, since the hydraulic circuit is dosed' the system pressure can be arbitrarily changed. These systems usually have a compres sion tank to control the system pressure. A system having an expansion tank open to the atmosphere as described in Chapter 10,1954 Guide And Data Book, is actually a dosed hydraulic dreuit.' An open system has a break in .the hydraulic circuit and therefore requires an analysis of the static pressure difference between the suction and discharge conditions. In both types of- systems there is resistance to fluid flow because of pipe friction. This resistance varies with the amount of flow approximately in proportion to the square of the change in velocity of the fluid. This relation can be shown graphically and is called a system curve, Fig. 5. ^ Hv/H,_-(Q,/Qip (5) where Hi -- pressure drop at design flow. Qi. Hi = system pressure drop at flow Qi. When the'pump capacity curve is'diperimposed-oh .the system curve, the intersection of the two curves, as shown on .