Regulation Of Flow Rate

Flow rate variation ordinarily is accomplished by a change in pump head, speed, or both simultaneously. The flow rate and power input of pumps with specific speeds up to about 4000 (1.464) double suction increase with decreasing head, so the drivers of such pumps may be overloaded if the head falls below a safe minimum value. Increasing the head of high-specific-speed pumps decreases the flow rate but increases the power input. The drivers of these pumps should either be able to meet possible load increases or be equipped with suitable overload protection. Flow rate regulation by the various methods given below may be manual or automatic (see also References 1, 7, 12, 34 and 49).

FLOW RATE Q, GPM

FIGURE 39 Power requirements of two double-suction pumps in series operated at constant head and variable flow rate. Total Hn = 382 ft (116 m) for both pumps at 1800 rpm (gpm X 0.06309 = l/s hp X 0.7457 = kW) (Reference 50). Curve AA: constant speed with discharge throttling. Curve BB: synchronous motor with variable-speed hydraulic coupling on each pump. Curve cc: variable-speed wound-rotor induction motor. Curve DD: dc motor with rectifier and shunt field control. Curve EE: synchronous motor with variable-speed constant-efficiency mechanical speed reducer

FLOW RATE Q, GPM

FIGURE 39 Power requirements of two double-suction pumps in series operated at constant head and variable flow rate. Total Hn = 382 ft (116 m) for both pumps at 1800 rpm (gpm X 0.06309 = l/s hp X 0.7457 = kW) (Reference 50). Curve AA: constant speed with discharge throttling. Curve BB: synchronous motor with variable-speed hydraulic coupling on each pump. Curve cc: variable-speed wound-rotor induction motor. Curve DD: dc motor with rectifier and shunt field control. Curve EE: synchronous motor with variable-speed constant-efficiency mechanical speed reducer

Discharge Throttling This is the cheapest and most common method of flow rate modulation for low- and medium- specific-speed pumps. Usually its use is restricted to such pumps. Partial closure of any type of valve in the discharge line will increase the system head so the system-head curve will intersect the pump head curve at a smaller flow rate, as shown in Figure 40. Discharge throttling moves the operating point to one of lower efficiency, and power is lost at the throttle valve. This may be important in large installations, where more costly methods of modulation may be economically attractive. Throttling to the point of cutoff may cause excessive heating of the liquid in the pump. This may require a bypass to maintain the necessary minimum flow or use of different method of modulation. This is particularly important with pumps handling hot water or volatile liquids, as previously mentioned. Refer to Section 8.2 for information regarding the sizing of a pump bypass.

Suction Throttling If sufficient NPSH is available, some power can be saved by throttling in the suction line. Jet engine fuel pumps frequently are suction throttled5 because discharge throttling may cause overheating and vaporization of the liquid. At very low flow rate, the impellers of these pumps are only partly filled with liquid, so the power input and temperature rise are about one-third the values for impellers running full with discharge throttling. The capacity of condensate pumps frequently is submergence-controlled,7 which is equivalent to suction throttling. Special design reduces cavitation damage of these pumps to a negligible amount, the energy level (Section 2.1) being quite low.

Bypass Regulation All or part of the pump flow may be diverted from the discharge line to the pump suction or other suitable point through a bypass line. The bypass may contain one or more metering orifices and suitable control valves. Metered bypasses are commonly used with boiler-feed pumps for reduced-flow operation, mainly to prevent overheating. There is a considerable power saving if excess capacity of propeller pumps is bypassed instead of using discharge throttling.

Speed Regulation This can be used to minimize power requirements and eliminate overheating during flow rate modulation. Steam turbines and internal combustion engines are readily adaptable to speed regulation at small extra cost. A wide variety of variable-speed mechanical, magnetic, and hydraulic drives are available, as well as both ac and dc variable-speed motors. Usually variable-speed motors are so expensive that they can be justified only by an economic study of a particular case. Figure 39 shows a study by Richardson50 of power requirements with various drivers wherein substantial economies in power may be obtained from variable-speed drives.

Regulation by Adjustable Vanes Adjustable guide vanes ahead of the impeller have been investigated and found effective with a pump of specific speed ns = 5700 (2.086). The vanes produced a positive prewhirl that reduced the head, flow rate, and efficiency. Relatively little regulation was obtained from the vanes with pumps having ns = 3920 (1.204) and 1060 (0.39). Adjustable outlet diffusion vanes have been used with good success on several large European storage pumps for hydroelectric developments. Propeller pumps with adjustable-pitch blades have been investigated with good success. Wide flow rate variation was obtained at constant head and with relatively little loss in efficiency. These methods are so complicated and expensive that they have very limited application in practice. Reference 34 may be consulted for further discussion and bibliography.

Air Admission Admitting air into the pump suction has been demonstrated as a means of flow-rate regulation, with some savings in power over discharge throttling. Usually air in the pumped liquid is undesirable, and there is always the danger that too much air will cause the pump to lose its prime. The method has rarely been used in practice but might be applicable to isolated cases.

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Survival Treasure

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