materials Handling
Jul~ 1650
Some basic facts relating to the proper selection and succeseful operation of centrifugal pumps bg RoQertE. Wright
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have an important place in materials handling for chemical plants because so much production is handled in liquid form. Of the various types of pumps in common use, the centrifugal pump is the most widely used in the chemical industry. Its widespread application results from its adaptability to high speed motor and turbine drive, its few moving parts, its small size and low cost for the amount of liquid moved, and its availability in almost all materials of construction. Unlike positive displacement pumps, the discharge of a centrifugal pump can be throttled-a fact of importance in chemical processes which require flow control valves. I n addition, the centrifugal pump can handle, with fair efficiency, liquids carrying suspended solids. A centrifugal pump consists of a rotating impeller enclosed in a stationary casing. The rotating impeller adds energy in the form of velocity to an already flowing liquid. The casing, usually in the form of a volute, guides the liquid to the impeller from a suction connection, converts the velocity of the liquid into pressure, and conducts liquid away from the impeller to the discharge connection. The impeller is keyed to a shaft, and the shaft is supported on bearings. A stuffing box prevents leakage from the pump through the space between the shaft and the casing. The main parts of a centrifugal pump are thus comparatively few and simple. The centrifugal pump is used in a wide range of pressures and capacities. I n the chemical industry it has now almost completely replaced the reciprocating pump except for a small range where there is low capacity combined with high head. Centrifugal pumps are commercially available for pressures up to 5000 pounds per square inch and in capacities as high as 607,000 gallons per minute. According to Gerbereux (2), the centrifugal pump should not be used for viscosities over 1500 Saybolt Universal seconds unless the capacity is over 2000 gallons per minute. The reason for this limitation is the decrease in efficiency with higher viscosities. Head and capacity drop when viscosity rises and the power required increases. At least fifty major companies in the United States manufacture centrifugal pumps in a variety of forms and with numerous features for special applications. UMPS
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Suetton condttions and cavitatbn If a centrifugal pump is operating under a suction lift, that is, above the surface of the liquid being pumped-it is necessary to prime the pump before it can operate. This can be done by an ejector, a vacuum pump, or automatic priming devices. I n some cases the use of self-priming centrifugal pumps may be justified. There is no problem in handling many ordinary liquids a t atmospheric temperatures where there is a low suction lift. Rut there may be serious problems if there is a high suction lift or if volatile or high temperature liquids are being handled. Any of these conditions, unless proper provisions are made, may result in loss of capacity, reduction in efficiency, cavitation, or even failure to operate. Cavitation is a phenomenon which occurs in a centrifugal 81 A
pump when the pressure of the liquid in the pump passages is reduced to the vapor pressure of the liquid. Under certain conditions, as the liquid flows through the impeller there may bo a slight reduction of pressure in the passages, causing formation of vapor bubbles. Further along in the impeller passages the pressure of the liquid may increase, resulting in a sudden collapse of the vapor bubbles. This collapse occurs with great violence, similar to the action of water hammer, and exerts a force of thousands of pounds per square inch in a small area of the impeller surface. These enormous pressures actually remove metal from pump parts, leaving a pitting of the surface. The final results of a cavitation failure may often be confused with a corrosion failure, particularly if a cavitating pump is handling a corrosive liquid. The external evidences of cavitation are noise and vibration. There are other explanations for cavitation, but it usually can be avoided by proper pump suction conditions and by keeping the specific speed of a centrifugal pump below certain limits.
SpecZPtc speed and charactertsttc curves The specific speed of a centrifugal pump is determined by the formula N , = N d / & / H 3 I 4 where , N , is specific speed, N is revolutions per minute, Q' is capacity in gallons per minute, and H is total dynamic head per stage in feet. One of the uses of this formula is that specific speed has been found to have a direct bearing on the maximum suction lift that is possible without cavitation. Experience has shown that pumps having lower specific speeds will operate safely a t greater suction lifts than higher speed pumps, but this means that the pumps are bigger, slower, and more costly. Economic considerations require the selection of a pump with the highest possible specific speed, because, for a given capacity and head, the dimensions of the pump will be smaller a t the higher speeds. The Hydraulic Institute has developed specific speed curves (4) which define the safe upper limits of specific speeds under various suction conditions. These curves are useful in deciding whether a proposed pump is likely to cavitate. Characteristic curves are usually furnished by the pump manufacturer showing head, power, and efficiency plotted against capacity. Such curves are useful both in the selection and operation of centrifugal pumps. When punip quotations have been received from several vendors, an examination of the characteristic curves will help determine which pump is best suited for the conditions. Each pump has a designed optimum operating condition. If the proposed operating condition is near the pump's optimum condition, the pump will suit the requirements especially well. Sometimes the better efficiency of one pump, over a given range, may allow the use of a smaller size motor. Several operating troubles caused by centrifugal pumps (Contznued on page 82 A ) can be avoided by a correct
original calculation of the head conditions of the system in which the pump is t o operate and by then matching characteristic curves to this condition. A few examples \Till show the importance of selecting a punip with the right headcapacity curve to meet the operating conditions. If the head developed by the pump is loner than the system pressure, the pump may not even be able t o get on the line. Or. if the head is higher than the system pressure, the pump may go to the outer extremitv of the head capacity curve. pumping more liquid than it should, and consequently requiring evcessive horsepower, causing the motor to overload. Finally, if the system head can vary within certain limits and the pump is poorly selected to meet these conditions, the purnp operation may be erratic or the pump will “hunt.” Characteristic curves are also useful in determining the head and capacity of pumps operated in series or parallel. In general, the performance of two or more pumps. whether similar or dissimilar and whether in series or in parallel, may be determined by combining the characteristic curves of each of the pumps. In series operation the combined hesdcapacity curve of two or more pumps is obtained by adding the heads a t different capacities. I n parallel operation, the combined head-capacity curve is obtained by adding the capacities a t different heads. Khen a new centrifugal pump is operating poorly one good n a y to start an investigation is to compare the actual operating conditions with the characteristic curves of the pump. Discharge, head, and power relationships
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Knowledge of the following pump laws showing the effect of speed, n, is useful both in the selection and operation of centrifugal pumps: Discharge varies as n; head varies as n2; and power varies as n3. There are also relationships for determining the effect of limited variations of diameters of impeller, d, in a particular pump running a t a given speed: Discharge varies as d2: head varies a5 dZ; and power varies as d4. These relations, both for speed and diameter variations, are approximations. Fl’hen viscosities other than water are involved, the relationships are less accurate. However. knowledge of these relationships may permit the shifting of a pump from one service to another. -4t times a pump may be adapted to new conditions by using an 1800 r.p.m. instead of R 3600-r.p.m. motor, or vice versa. It may also be possible to machine down the impeller or, in some cases, to order a larger impeller from the pump manufacturer to suit different conditions than those for which the pump vias originally selected. ?\fore detailed information on subjects briefly discussed here can be secured from numerous sources (1-3, S-7). LITERATURE CITED
(1) Allis Chalmers Mfg. Co., Milwaukee, Wis., “Handbook for Cai e
of Centrifugal Pumps.” ( 2 ) Gerbereux, V. deP., “Kent’s Mechanical Engineers’ Handbook,” 2-83, New York, John U‘iley & Sons, Inc., 1941. 4th ed., p p ~ (3) Hydraulic Institute, 90 West St., Kew York 6, N.Y., “Standaids of Hydraulic Institute, Centrifugal Pump Section.” (4) Ibid.,pp. 14 and 15. ( 5 ) Ingersoll-Rand Co., 11 Broadway, New Yolk 4,K,Y., “Cameron Pump Operator’s Handbook.” (6) Kristal, I?. A,, and Annett, F. A.. “Pumps,” New Yolk, McGrawHill Book Co., 1940. (7) Stepanoff, A. J., “Centrifugal and A x i a l Flow Pumps,” S e w York, John Wiley & Sons, Inc., 1948.
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