Pump casings serve to seal off the intside of the pump to atmosphere to prevent leakage and retain pressure. In the case of centrifugal pumps, they surround the pump rotor which transmits energy to the fluid handled via the impeller(s) mounted on the rotating shaft.
In the case of positive displacement pumps, they surround the rotary or reciprocating displacement elements (e. g. one or more pistons).
The inlet and outlet nozzles serve to direct the fluid handled into and out of the pump and are often classified (by their function) as inlet or suction nozzle and discharge nozzle. They are either attached to the piping (e. g. using flanges, pipe unions) or:
- The suction-side nozzle is immersed in the open liquid tank, i.e. on vertical tubular casing pumps.
See Fig. 1 Pump casing
- Both nozzles are immersed in the fluid together with the complete pump casing, i.e. in submersible motor pumps.
See Fig. 2 Pump casing
If the pump design requires the drive shaft to pass through the pump casing, shaft seals are provided to prevent excessive leakage of fluid from or ingress of air into the pump casing. These portions of the pump casing are designated seal housing or stuffing box housing.
Almost every pump type has a different casing type by which it can be recognised. With increasing specific speeds, the following casing types are used:
- Volute casing, which is sometimes fitted with a double volute consisting of two volutes offset by 180° to balance the radial thrust. See Fig. 3 Pump casing and Fig. 6 Volute casing
- Vortex volute whose volute cross-section is markedly asymmetrical when viewed in the meridional section. See Fig. 4 Pump casing
- Circular (or annular) casing which has a constant cross-section around the periphery. See Figs. 13, 15 Pump casing
- Tubular casing which guides the flow of the pump from the diffuseraxially downstream. See Fig. 1 Pump casing
- Elbow casing pump whose diffuser discharges into an elbow (casing).
See Fig. 2 Propeller pump
The pressure range also has an influence on the casing types. Low-pressure pumps require different design solutions than those suitable for high and ultra-high pressure pumps. Increasing pressure levels require that the wall thickness of discharge casings be increased. The pump casing's dimensions must however remain in compliance with national and international codes and standards.
In the case of volute casing and multistage high-pressure pumps, the external geometry is designed to produce cylindrical (see Barrel pull-out pump), conical or spherical (see Circulating pump) pump casings. See Figs. 5, 6 Pump casing
These designs minimise stresses on the pump casing despite high internal pressure and are well-suited to manufacturing methods used when producing thick-walled (forged) components.
A pump type which should not be confused with barrel pull-out pumps or barrel casing pumps is the vertical can-type pump whose suction characteristics have made it popular for use as condensate or refinery pump. Its can encloses the pump's suction side.
An additional characteristic design feature of casings is the fact that they may have to be split for mounting reasons, with the split running either radially or axially relative to the shaft. See Figs. 7, 8 Pump casing
Finally, the location of the pump nozzles also influences the shape of the pump casing. The axial inlet nozzle on volute casing pumps, for instance, is a characteristic feature of this pump type in contrast to in-line pumps which have nozzles arranged opposite to each other, or in contrast to refinery pumps with "top-top" nozzles (both nozzles pointing vertically upwards).
See Figs. 9, 10 Pump casing
Even the method of supporting the bearings of overhung pump shafts in a bearing bracket or use of a bearing pedestal (as on volute casing pumps) represent design classification characteristics of the pump casing.
The casing components of radially split ring-section pumps are named according to their function: suction casing, stage casing (usually several of these are arranged in sequence) and discharge casing. When assembled, pressure-tight connection of the casing is ensured by tie bolts.
The individual casing components are mostly sealed against the internal pressure by gaskets, O-rings or via direct metal-to-metal sealing which is the sealing method used for ultra-high pressure pumps whose plane-parallel stage casing faces are pressed together.
If the casing is split axially along the shaft centreline, it will consist, in the case of a horizontal pump, of a bottom half which accommodates the two nozzles for connection to the piping and also the pump feet, and of a top half of relatively simple design. See Fig. 11 Pump casing
The casing joint consists of a flange (see Flange type) on both casing halves which extends around the entire pump casing including the two stuffing box housings, and which seals the casing in a leak- and pressure-tight manner by means of a number of bolts. As the pump rotor of a single-stage or multistage pump with vertical shaft is frequently supported by a product-lubricated bearing, a second shaft seal is not required.
See Fig. 14 Pump casing
Multistage pumps with product-lubricated bearings can be extremely compact in design and run very smoothly due to their extremely rigid pump rotors which result from the short distances between the bearings. Moreover, pump configurations that exclusively employ product-lubricated bearings save on materials and only require low-pressure mechanical seals. See Fig. 14 Pump casing
As well as the casing components already described, the pump casing sometimes also incorporates a heat barrier and a cooling casing which is often closed by a special cooling cover. Both of these items are designed to reduce the heat flow from the inside of a pump handling hot fluids to the pump bearing(s) and to the shaft seal (if any) or the wet rotor motor.
Conversely, a pump heating jacket is designed to keep the contents of the pump casing of a pump which is not in operation at operating temperature via an uninterrupted supply of heat in order to prevent any undesired sedimentation or crystal growth, or even solidification of the fluid handled.
See Fig. 12 Pump casing
Pump casings are primarily cast, but are also forged, welded, pressed or drawn. See Figs. 3, 4, 5 Boiler feed pump
As the operational safety of the machine depends to a large extent on the durability of the pump casing, many different codes and standards relating to specific industries stipulate the casing materials to be used, and in some instances the wall thicknesses as well. Cast metal materials frequently include cast iron, nodular cast iron, cast steel, ferritic or austenitic chrome steels and austenitic cast iron, but also duplex steels for applications requiring highly corrosion-resistant materials such as seawater desalination (see Materials). See Fig. 13 Pump casing
The pump casings of singe-stage vertical pumps for large flow rates are often made of concrete. This applies above all to tubular casing pumps, see Fig. 2 Pump for use in low-liftpumping station, but also for very large volute casing pumps. See Fig. 6 Cooling water pump