Document 2jj3xmQQzmjEMQy6bx4Zd2n4a

866 CHAPTER 56 1965 Guide And Data Book Fig. 6 .... Relation Between Pump Performance Curve and System Characteristic Curve fig. 6, indicates the flow and pressure of the pump when coupled to the system. In the case of the open system, the discontinuity of the' hydraulic , system requires adjustment of the system curve to compensate for the static pressure difference at the break in: the. hydraulic circuit. Thus a system with more static pressure on the suction flange of the pump than on the dis charge flange has static pressure available to increase the sys tem flow. This available static pressure is graphically shown by shifting the system curve down by an amount equal to the static pressure A in fig. 7. *. . Conversely, a system that, has less static pressure ou the suction flange thanon the discharge flange has Static pressure available to decrease the'system flow. .This is. graphically shown by shifting the system curve up by an amount equal to the static pressure difference between suction and discharge. (See A in fig. 8.) - Too often, originally accurate estimates of friction losses in circuits are arbitrarily adjusted and cause prediction of pump operation at unrealistically high heads. When the actual system losses are less than the adjusted estimate, the actual system curve is lower than predicted, and, consequently, the actual pump operating point will shift as shown in fig. 9, and power requirements will be increased. This shift in pump operation also increases the net positive suction head requirements, and can cause noise m<w,hnnWt damage to the pumping equipment." PUMP LOCATION IN THE HYDRAULIC SYSTEM In an open system,' the pump should be placed so that the suction is flooded and flow is not restricted at pump entrance. Condensate,pumps are located at the source or reservoir and so.connected that the pump inlet is flooded. The pump can ..then handle condensate as close to the flash point as possible. - In this case there is no appreciable pressure drop or friction toss from the reservoir to the impeller eye. In most cases, the pump is attached to, or is part of, the condensate reservoir. Knowledge of the NPSH requirement is desirable, and some times essential/ for the proper selection of any pump operating with fluids at elevated temperatures. - The selection of the pump location when applied to cooling towers is influenced by the available space, by the necessary protection from the -weather, and by the need to prevent freezing. When the pump is placed close to the tower pan or sump, long runs of costly suction piping areeliminated. Where a prime condition'must-be maintained, the suction line from the tower must be horizontal or run at a slope to make it ' self-venting, and the pump must' be placed below the fluid level in the tower. (See fig: 10.) ' Only -the discharge line may be looped' above the' liquid level. If the suction line is looped above the liquid level, it may collect air; restrict full coolant flow and lower system capacity. The suction line must go through, around, or below tire'ob struction,-even if.this makes it necessary,to relocate or raise tiie tower/Hie best position for the pumpis at the tower. Closed systems work best if the pump is located so'that it pumps away from the expansion tank ami ia located dose to it* The .pump head!is then dissipated through, the piping. Rg. 7 ..Shift in System Curve Caused by Higher Static Pressure on Suction Side of Pump . Rg. 8 .... Shift in System Curve Caused by lower Static / Pressure on Suction Side of Pump r&iTr.- -V . i-.. Pumps 867 splash-proof, totally-enclosed, and explosion-proof motors are furnished by all manufacturers on request. ' Gasoline- or diesel engine-driven centrifugal pumps- are called dose-coupled, when the impellers and volutes are mounted directly on the engine-shaft and housing; and longcoupled, when toe pump shaft is connected to toe motor by a flexible coupling, or is belt driven. Conversion kits are avail able for most engines to convert them for use of either natural gas or liquefied petroleum gases (butane or propane) instead of Due to speed variations possible with this type of drive, a governor is essential for satisfactory operation, in order to regulate the speed and delivery desired. Steam is usually available to drive centrifugal boiler feed-pumps and booster pumps in power stations by means of turbines. Re ciprocating boiler-feed pumps are also usually steam-driven. Fig;' 9 .... Effect ' on Pump Performance of Difference Between Predicted and Actual System Curve (See Chapter 10, 1664 Guide And Data Book cm effect of pump location on hot water heating systems.) , . ... . In open and dosed systems, the quality of the fluid reaching - the pump has a direct bearing on overall performance and life expectancy. Vortexing-and air pick-up at toe sump in.open systems, and circulation of entrained, air or gases in dosed systems, must be eliminated.- Adequate sump depth and rise, prevent vortering, and also keep.the suction-pipe entrance velocity below 3 fpal. Furthermore, the low-velodty areas in closed systems should be used to eliminate air. It is especially helpful if the velocity in the separating area does not exceed one-half feet per second. Commercial dip..tubes and low- velocity separators are furnished by some manufacturers as packaged equipment. MOTIVE POWER Electrically-driven centrifugal pumps are called" closecoupled when toe impeller is mounted directly on the motor shaft, and long-coupled when the pump shaft is connected to the motor by a flexible coupling, or is belt driven. Due to the amount of starting torque required on centrifugal pumps driven by squirrel-cage induction motors, acrosa-theline starting is usually employed with proper phase protection and switching. This applies especially to boiler feed-pumps, condensate pumps, circulating pumps, and tower pumps. For hazardous pump locations, such as wet pits, pits subject to flooding, or the accumulation of combustible gases, special motors are available. Unless otherwise specified, open, dripproof motors are supplied by the pump manufacturer, but BLOW DOWN PUMP. SELECTION AND INSTALLATION Selection. Where noise or vibration generation must be kept at a minimum, it is essential that the pump be suited to the hydraulic characteristics of the system and be accurately se lected in order to operate without cavitation. Vibrations and unbalance become more critical with increasing speeds. There fore, a low-speed pump will operate in most cases more quietly than a pump of the bead and capacity at higher speeds. Sleeve-bearing motors are considered quieter in operation than ball-bearing motors. Close-coupled pumps are generally not used when sound control is important because the motor noise cannot be isolated and consequently, any noise'gener ated will pass directly into the system. In toe case of long- coupled pumps, where a coupling separates the motor from the pump shaft, the motor can be isolated from the building structure by mounting it on resilient supports. Some motor manufacturers supply resilient mounting for motors, up to 10 hp. Specially designed quiet motors are available above that limit. The selection of the proper pump, speed, and motor combi nation is more effective in eliminating system noise than the installation of pipe isolators which are useful, however, in some cases. Sleeve-bearing motors and pumps generally are re stricted to a speed of 1740 rpm. In systems having highly varying leads, use of pumps with nonoverloading motors is recommended. Nonoverloading motors are sized to provide adequate power at any point of the pump curve. If operating conditions can be accurately predicted, a motor can be selected specifically for the power required. In this case the selected motor will often be smaller than the motor originally provided. Pumps for handling liquids of medium temperatures have special seals and bearing lubricants. High-temperature pumps for liquids above 325 F have special pump construction and materials. The bearings and seal areas are water-cooled. Medium-temperature installations sometimes include a high-temperature pump. Packing gland or stuffing box pumps / are also used for high-temperature liquids, but must then also be water-cooled. However, in order to keep the high-temper ature installation as free of water losses as possible, special mechanical seal pumps are somewhat more popular. (See Chapter 12,1964 Guide And Data Book.) Installation. Pump suction and discharge piping connec tions are not necessarily of the same size as the system leading to or from the pump, but changes in system piping to fit the pump connections should be as close to the pump as possible. Suction and discharge piping must be supported, so that when the flange bolts are tightened, no strain will be transmitted to the pump. Close-coupled centrifugal pumps, because of the nature of their derign, are properly aligned throughout their life with