Can Pump Intakes

A can pump is defined as one that has a can, or barrel, surrounding the pumping unit. This can acts as a "sump" or intake structure for the pump suction impeller. The can may be either closed bottom, and contain the pump suction nozzle, or open bottom and connect directly to a piping header. The can design must provide uniform, stable flow distribution to the suction impeller inlet.

Vertical turbine pumps (Figure 19) require uniform inflow to the suction bell to avoid swirling and submerged vortices that may result in cavitation, vibration, and accelerated pump wear. When a pump with an open bottom can design is connected to a horizontal header (Figure 20a), the velocity in the header should be no more than 6 ft/s (1.8 m/s) to allow the liquid to turn and flow upward to the pump suction bell. The velocity in the can rising to the pump should not exceed 5 ft/s (1.5 m/s), and the suction bell inlet should be located at least three diameters above the top of the horizontal header. For velocities approaching the maximum recommended levels, vortex-suppressing vanes may be added to the suction bell area to break up swirling and nonsymmetrical flow patterns as they approach the impeller inlet.

If a 90 degree turning vane elbow on the can assembly surrounding the pumping element is used (Figure 20b), the velocity in the horizontal header can be as high as 8 ft/s (2.4 m/s). The turning vane elbow should be sized for a maximum velocity of 5 ft/s (1.5 m/s).

Most can pumps are of the closed bottom design (Figure 21). In this arrangement, the pump suction nozzle is located either in the can or in the pump nozzle head that contains both the suction and discharge nozzles. The pumping element (assembly) must be centered in the can to avoid non-uniform flow to the suction impeller. Flow straightening vanes are suggested for all can intakes. Because of the limited volume of liquid in the can, surging of the liquid level within the can—or barrel—cannot be tolerated; the can should be

FIGURE 21 Closed bottom can (American National Standard for Pump Intake Design, ANSI/HI 9.8-1998, Reference 1)

arranged so it is always full. The velocity of the flow between the pumping element and the inside of the can should not exceed 5 ft/s (1.5 m/s). For a suction nozzle velocity of 4 ft/s (1.2 m/s) maximum, the centerline of the nozzle should be at least two diameters above the suction bell inlet. To ensure uniform flow distribution into the can, the suction nozzle should be connected to at least five pipe diameters of straight pipe before elbows or other flow-disturbing fittings are installed.

Pumps with submersible well-type motors require flow around the motor for cooling. A shroud is typically used to direct flow across the motor as it goes into the pump. The top of such a cooling shroud is covered to restrict downward fluid flow and still allow venting of air from the shroud. This arrangement is described in detail in ANSI/HI 9.8-1998 (Reference 1). It is recommended that the inlet piping be sized to limit draw-down of the liquid below the minimum required level during the startup to a period of less than 3 seconds.

Single Pumps Piping to the suction of a dry-pit centrifugal pump (Figure 22) must be carefully planned to provide uniform, straight-line flow to the impeller, and adequate pressure and sealing against leakage, in or out. Air pockets just prior to entrance to the pump should be avoided, as well as down flow lines subject to sudden pressure changes. Air pock-

Pneumatic Long Radius Elbow

FIGURE 22 Suction piping faults for dry-pit centrifugal pump. (a) Air pockets should be avoided. (b) Suction elbow should not be in a plane parallel to pump shaft. (c) Valve location may be critical to pump nozzle loading. (d), (e), Suction elbow should be one size larger than pump's, long radius and three large pipe diameters distant. (d), (f) Concentric reducer should be installed distant from pump.

FIGURE 22 Suction piping faults for dry-pit centrifugal pump. (a) Air pockets should be avoided. (b) Suction elbow should not be in a plane parallel to pump shaft. (c) Valve location may be critical to pump nozzle loading. (d), (e), Suction elbow should be one size larger than pump's, long radius and three large pipe diameters distant. (d), (f) Concentric reducer should be installed distant from pump.

ets can be prevented by proper elevations (Figure 22a). Pressure surges can be controlled by surge tanks, air tubes, and so on, which may require a system pulsation study to determine possible need and solution.

To provide an optimum flow pattern to avoid impeller disturbance, it may be necessary to have a straight run of pipe of as much as eight pipe diameters immediately prior to the pump suction (for example, following a short radius elbow or tee). Following a long radius elbow or a concentric reducer, a straight run of at least three pipe diameters is recommended (Figure 22c to f). Eccentric reducers should not be used next to the pump suction nozzle. Although installing eccentric reducers with the flat side on top will eliminate a potential air pocket, large changes in diameter could result in a disturbed flow pattern to the impeller and cause vibration and rapid wear. Pipe venting, in conjunction with a concentric reducer, may be preferable to the use of an eccentric reducer.

Ideally, a suction pipe should approach a double suction pump perpendicular to the shaft centerline. If there is an elbow in the suction piping upstream of the suction flange, it should be bringing flow from either overhead or below, not from the side of the pump. If there is a short radius elbow or other flow-disturbing device in the suction piping upstream of the suction flange, there should be at least five pipe diameters between the device and the suction flange. If a short radius elbow is in the same plane as the impeller shaft, there should be at least eight pipe diameters between the elbow and the suction flange. An incorrect installation could result in an uneven flow to both sides of the double suction impeller. This could cause a reduction in capacity and efficiency, an increase in thrust on the bearing, noise, and possible cavitation damage to the impeller.

A dry-pit pump (Figure 1g) may operate with a suction lift and therefore will be located above the liquid source. All losses in piping and fittings will reduce the available suction pressure. Suction piping should be kept as simple and straightforward as possible. Any pipe flange joint or threaded connection on the suction line should be gasketed or sealed to prevent air in-leakage, which would upset the vacuum and keep the pump from operating properly.

If expansion joints are required at the suction of a pump, an anchor should be interposed between the pump nozzle and the expansion joint to prevent additional forces from being transmitted to the pump case and disturbing rotating clearances. The same requirement applies to a sleeve coupling used to facilitate installation alignment.

Reciprocating pumps must have additional consideration because of the pulsating nature of their flow. Suction piping should be as short as possible and have as few turns as possible. Elbows should be long-radius. Pipe should be large enough to keep the velocity between 1 and 2 ft/s (0.3 and 0.6 m/s). This will generally result in pipe one to two sizes larger than the pump nozzle. High points that may collect vapor are to be avoided or, if necessary, properly vented. A pulsation dampener or suction bottle should be installed next to the pump inlet. Available NPSH should be sufficient to cover not only reciprocating pump requirements and frictional losses but also acceleration head (see "Surge and Vibration").

Manifold Systems All comments relative to single pumps apply to manifold-pump systems, as well as some additional points.

In a suction manifold, the main-line flow should not be more than 3 ft/s (0.9 m/s). Branch outlets should be at 30 to 45 degrees relative to main-line flow rather than 90 degrees, and velocity can increase to 5 ft/s (1.5 m/s) through a reducer (Figure 23). With such velocities, branch outlets can be spaced to suit pump dimensions in order to avoid crowding. Also, if the angled manifold outlet is used, pumps can be set as close to the manifold as the elbow, valve, and tapered reducer will allow.

Manifold sections beyond each branch takeoff should be reduced to such a size that the velocity remains constant. One exception to this scheme is a tunnel suction (Figure 24). The flow through the tunnel may operate independent of the pumps, which are suspended in boreholes drilled into the roof of the tunnel. Boreholes at least one-third of the tunnel diameter should be horizontally spaced at least 12 borehole diameters apart. Smaller ratios can be closer to a minimum of six diameters. Pump suction bells should be at least two borehole diameters above the tunnel roof. Velocities in the tunnel should be kept below 8 ft/s (2.4 m/s) for best pump performance.

FIGURE 23 Suction pipe header recommendations for dry-pit centrifugal pump

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