Centrifugal Pump User’s Guidebook

The energy transfer in a centrifugal pump is accomplished by changing the state of motion of the liquid. The magnitude of this change depends upon the dimensions and shape of the waterways and on the operating speed of the impeller. For a given pump geometry and operating speed, the velocities of the liquid vary with the flow rate. Consequently, centrifugal pumps develop different heads at different flow rates. Similarly, efficiency, power requirements, and suction capability also vary with flow rate.

Centrifugal pumps are used in a variety of applications, including the following: water supply and irrigation, power-generating utilities, flood control, sewage handling and treatment, food industries, chemical and petrochemical industries, process industries (e.g., textiles and leather), domestic appliances, mining and ore processing, transporting liquid-solid mixtures, environmental control, spaceships, airplanes, and motor vehicles. The list can go on and on.

In general, the presence of free air (or any other gas) in the pumped liquid always adversely affects centrifugal pump performance. One negative aspect of the presence of free air has been discussed earlier in the section on priming (Chapter 1). However, even when a pump has been primed properly, free air can appear during operation and disrupt the performance of the pump.

It is difficult to draw the distinction between cavitation and boiling. The closest definition might be “local boiling within a liquid that is otherwise in a nonboiling state.” This can be best illustrated by the example of the teapot, when heating water, simmering noises, and cracking are heard long before the water boils. These noises occur when the water in immediate contact with the bottom of the teapot starts to boil while the rest of the water is still well below the boiling point. The vapor bubbles generated at the bottom of the teapot, being lighter than the liquid, move upward through the nonboiling water. The moment they reach a cooler zone of liquid they collapse vigorously and create cracking noises.

The suction characteristics of a pump are determined by a series of tests performed at nearly constant speed. Often, however, a pump is not used at its test speed. Other pumps may be too big to test on any available rig, and all tests must be performed on a scale model. In both cases, one must resort to using model laws to estimate whether the pump will perform satisfactorily under actual working conditions.

When a pump must deliver a certain amount of liquid to an elevation H _e , it must develop a head H that is higher than H _e because part of the head developed by the pump is used up by the resistance of the pipelines to flow. This resistance is caused by friction between the liquid and the wetted surfaces of the pipes (or ducts), and by changes in the direction of flow. It is also caused by the changes in the diameter of the pipes and the resistance rendered by the various fittings.

Problems related to elevated temperatures can be subdivided into two classes: overheating caused by mechanical faults and problems related to pumping hot liquids.

For many years, recirculation was regarded as some sort of mysterious factor responsible for every problem that could not be explained. As such, it had no exact definition suitable for analytical discussion. This sometimes led to inaccurate or misleading conclusions, as shown by the case history presented here.

When a liquid is acted on by an impeller, its head increases by an amount equal to H₂.This head consists of the static pressure head H_p2 and the velocity head C ₂ ² /2g, where C₂ is the absolute velocity of the liquid at the impeller outlet.

At present, two major causes of a drooping curve are recognized. One was discussed in Chapter 9 in connection with Figs. 9–17 and 9–18 and supported by the test results presented in Figs. 9–15 and 9–16. The second factor that can cause a drooping curve usually appears when a pump is designed for a high head coefficient and is due to the hydraulic losses associated with the flow of the pumped liquid through the waterways of the pump.

In Chapter 2 we discussed the performance characteristics of centrifugal pumps and their importance for selecting the proper pump for a given duty. These characteristics are usually determined on the basis of tests performed by the manufacturer during the development stage of a given unit. However, to be of real value to a pump user, such tests must be carried out correctly. Otherwise, the pump user my be confronted with very unpleasant surprises.

To choose the right pump for a given low-NPSH application, more information about the suction performance of a centrifugal pump than a single NPSH value at a given flow rate is needed. Rarely does a pump operate under constant conditions. In practice, it usually operates over a wide spectrum of heads, flow rates, and available NPSH values. This may impose more requirements on a pump than simply operating satisfactorily under a given NPSH.

The size and geometry of a sump, tank, or container from which liquid enters the suction line has enormous effects on pump performance. In particular, the choice of these parameters may be critical when the dimensions of the source where the pump gets its liquid is compared to the handled flow rate.

When a mixture of gas and liquid is acted on by a rotating impeller, the heavier liquid is thrown outward by centrifugal force, displacing the gas toward the center. Depending on the amount of gas contained in the liquid, this displaced gas fully or partially blocks the inlet to the impeller vanes. In the first case, the pump is not able to deliver any liquid. In the second case, the pump may start to deliver a reduced amount of liquid, provided that it can develop the head required to overcome the resistance of the system. In this case, one of the following may occur: The flow rate increases gradually until it is restored to the expected value; the flow rate decreases gradually until the pump ceases to deliver any liquid; or the pump operates at the same, reduced, flow rate.

Bearings support the rotating elements of the pump and keep them in proper position relative to other parts and the driver. Failure of a bearing may change the position of the rotating elements, which will cause interference between the stationary and moving parts of the pump, and may lead to fractures or seizing of parts.

One of the first problems that arises with packed stuffing boxes is the choice of proper packing material. Until recently, the most popular kinds of packing were asbestos-filled. Even today, these packings are still regarded by many as the most versatile and economical. However, because the use of these packings might constitute a health hazard under certain circumstances, substitutes are being used with increasing frequency.

Noise is a sequence of rapid variations in the pressure of the air. When these pulsations reach the human ear, we encounter it as sound or noise. The source of these pulsations is vibrations of some object or medium that is in direct or indirect contact with the atmosphere.

A semi-open impeller has only one shroud. The second shroud is replaced by a stationary wear face cast integrally with the casing or by a stationary wear plate firmly attached to the casing. Experience has shown that the clearance C (Fig. 19–1) between the impeller vanes and the wear face has a profound influence on the performance of the pump.

All discussions about cavitation in centrifugal pumps lead to one firm conclusion: cavitation is a direct result of an available net positive suction head (NPSH) that is too low. This implies that when the available NPSH is lowered, the intensity of cavitation increases and that an increase in the available NPSH reduces the intensity of cavitation. However, there are cases in which the opposite appears to be true. In these cases, the pump developed intense cavitation when the level of the liquid in the suction tank was very high. As that level was lowered, the noise was reduced. Finally, when the liquid neared its lowest level, cavitation disappeared completely.

Upon learning that a pump does not operate properly, the first step is to ask as many questions as possible. The source of many problems can often be located before an on-site inspection is undertaken. Several kinds of questions should be asked.

Sometimes, damaged or broken parts provide clues to the cause of a pump’s failure. Small parts, can be shipped to the pump specialist, thus allowing him to solve the problem without an on-site inspection. When the failed parts are too bulky or cannot be removed from the site for any other reason, an on-site inspection may reveal the source of the failure at an early stage of the inspection.

In most cases, complaints about unsatisfactory pump performance come from its user, who is usually familiar with what the malfunction of the pump does to its performance. However, he may know very little about the pump proper. Because of this, reports about unsatisfactory pump performance are often distorted and are intermingled with the personal opinions of the user. This, in turn, inaccurately represent the problem.

It is often very costly and time-consuming to determine the cause of a malfunction directly. Often, it is faster and less expensive to try a simple remedy and see whether it works. In other cases, performance of a pump needs to be modified for one or more of the following reasons:

One of the most common ways to alter the performance of a centrifugal pump is to let it run at a different speed or to machine the impeller rim to a different diameter. In fact, it has become a standard of the pump industry to publish the performance of any pump at several different speeds (usually the standard speeds of electrical motors) and for different impeller cut-downs (see Fig. 25–1).

One of the means used in industry for altering the performance of a pump, is to reduce the width of the impeller. In the case of a semi-open impeller, the procedure is simple. Machine the open face of the blades until the impeller width is reduced to the desired magnitude.

One common means of increasing the heads developed by a centrifugal pump is to underfile the outlet tips of the impeller blades, as shown in Figs. 28–1 and 28–2. However, as can be concluded from the performance charts presented in these figures, the effects of under filing vary in pumps of different geometries. Even in the same pump, the percentage of increase in head achieved by underfiling varies with flow rate.

The contents of this chapter are primarily of interest to the pump designer. However, as will be demonstrated later, the knowledge of these findings may help the pump user to solve problems that, without that information, were beyond the user’s reach.

This chapter discusses a few of the future applications of the theory presented in Chapter 29. It is expected that, with time, the pump user will be able to make use of it in many additional ways.