Attività

The pump casing refers to the outside shell of the pump. It has to seal off the inside of the unit to the outside with respect to pressure and fluids. The structure of the casing differs depending on the type of pump. The casing of a centrifugal pump is visibly determined by the infeed and delivery of the flow to the impeller. Special importance is also attributed to the casing for these pumps because the gap between casing and impeller plays a major role in determining the efficiency of the pump. The Pump Casing Casting of a positive displacement pump, on the other hand, encloses the displacing elements such as the piston. In practice, all pumps already provide an indication of the pump type by the shape of their casing.

Depending on the location, a pump casing can be made of cast or wrought iron, of chromium or duplex steel. Also some pumps can have a casing made of plastic. Casing type can have a large impact on pump reliability (the mean time between repairs) and, to a lesser extent, pump efficiency when pump flow rates are above or below the best efficiency point (BEP). Although the objective of many pump applications may be to operate the pump at its BEP.

Each of these casing types has its advantages and disadvantages regarding bearing and shaft seal life, pump efficiency and pump cost. Although pump users do not always have a choice of the casing type, there are applications where at least some vendors may provide the pump user with this decision. It is, therefore, advantageous for pump users to be knowledgeable regarding the impact of these potential casing selections.

Single Volute Casing

Single volute pumps have been in existence for several years. With the exception of vertical pumps of the turbine type, the majority of single-stage pumps built in the United States are of the single volute type. The main advantage of single volute casing is its simplicity. It is less difficult to cast, and therefore less expensive to produce given the open areas surrounding the impeller periphery. At flow rates near the pump BEP, single volute casings are more efficient than double volute casings. Further, the inherent greater throat areas can also handle larger solids, as found in wastewater.

The volute scroll is designed for constant velocity near the best efficiency flow rate, which yields a uniform static pressure around the periphery of the impeller but only at the BEP. This pressure equilibrium is, however, destroyed when the pump is operating at flow rates below or above the BEP. This off-BEP, non-uniform pressure distribution around the impeller results in increased net radial loads on the rotor. Likewise, this increased load deflects the pump shaft and can result in excessive deterioration at the wearing rings, seals or bearings. In extreme cases, shaft breakage due to fatigue can result.

The primary load on the pump bearings typically derives from this pressure imbalance around the impeller (at least in a single volute), especially at off-design flow conditions. The impact of this unbalanced pressure force on the bearing L10 fatigue life is further amplified by the fact that the antifriction (ball or roller) bearing life is a function of the cube of the radial load. This means that the bearing life quickly diminishes away from the BEP. The statistical (L10) ball bearing life ratings are based on metal fatigue failure, with 90 percent of the bearings surviving for the rated time period.

The only way to improve single volute pump bearing life is to operate the pumps as close to the pump BEP as practical for the application, possibly by adding a bypass line from the discharge header to the suction pipe. This can, however, also reduce the overall pump efficiency for the resulting net flow rate.

Double volute casings

Double volute casings were introduced to eliminate the radial thrust problems inherent in the single volute design. Test measurements do, however, indicate that while the radial forces in a double volute are greatly reduced, they are not completely eliminated.

In a double volute casing, the flow is divided into two equal streams with two cut-waters set 180 degrees apart. The total throat area of the two volutes is identical to that which would be used on a comparable single volute design. As such, a double volute casing design is actually a single volute with a splitter vane extending 180 degrees from what would be the single volute throat welling into the diffuser section, creating two equal scrolls, each with constant velocity at the pump BEP. The two volute sections still generate non-uniform pressures around the impeller at off-BEP flow rates but tend to cancel each other out to a large extent, as shown by the resulting lower radial loads. This is the main reason for using a double volute casing.

The hydraulic performance of double volute pumps is nearly as good as that of single volute pumps. Tests indicate that a double volute pump will be approximately one to one-and-a-half points less efficient at the BEP but will be approximately two points more efficient on either side of the BEP than with a comparable single volute pump. This means that a double volute casing will produce a slightly higher efficiency over the full range of the head-capacity curve than a single volute.

Double volutes are most common with larger-axial, split-case, double-suction impeller pumps, which also have lower-axial impeller loads. Double volute casings should not be used in smaller, low-flow pumps (below about 400 gpm). Since larger pumps are better candidates for double volutes, they should normally be selected for such service when available and if the application permits. The splitter vane in a double volute can cause blockage problems when handling solids, such as in wastewater applications, which would preclude their use in these applications.