Pump casing

Pump casing Volcano

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

Fig. 1 & 2 Pump casing

If the pump design requires the drive shaft to pass through the pump casing Volcano, 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.
Vortex volute whose volute cross-section is markedly asymmetrical when viewed in the meridional section.
Circular (or annular) casing which has a constant cross-section around the periphery.
Tubular casing, which guides the flow of the pump from the Leitrad kommenden Forderstrom der Pumpe in diffuseraxially downstream.
Elbow casing pump, whose diffuser discharges into an elbow (casing).

The pressure range also has an influence on the casing types Volcano. 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 , conical or spherical pump casings.

vertical can-type pump

These designs minimise stresses on the pump casing Volcano 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.

horizontal-volute-casing-pump

Finally, the location of the pump nozzles also influences the shape of the pump casing Volcano. The axial ( Axial refers to alignment in accordance with an axis. ) 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).

In-line-circulating-pump

volute casing pumps

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 Volcano, 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.

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.

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.

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.

Multistage centrifugal pump

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.

Pump casings are primarily cast, but are also forged, welded, pressed or drawn.

As the operational safety of the machine depends to a large extent on the durability of the pump casing Volcano, 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 .

casing materials

Of the multitude of available materials, metallic and non-metallic (such as plastic and ceramic) materials are preferred for centrifugal pumps, although the exact choice depends on the level of stress involved..

casing Metallic material Volcano

Metallic materials take the form of cast materials Volcano, forged or rolled materials, or materials in some other product form. Given the high level of freedom that is available in terms of design and the wide array of special alloy types, these remain the most frequently used design materials for pump casings and impellers.

The cast materials used in centrifugal pumps vary depending on the desired material composition and the mechanical and technological properties required. See Fig 1 Material

casing Plastics Volcano

Plastics are divided into four groups according to the ISO 1043-1 standard, based on how their mechanical response to temperature is Volcano.

Mechanical behaviour of the four plastic groups

casing Thermoplastics

When subjected to heat, these materials can be deformed any number of times. Examples include polystyrene (PS), polyethylene(PE) and polyvinyl chloride (PVC).
casing Elastomers

These are wide-mesh cross-linked high-polymer materials with rubber-like elastic properties in all temperature ranges (e. g.cross-linked natural rubber or polyisobutylene).
casing Thermoplastic elastomers

These materials are wide-mesh cross-linked high-polymer materials that attain rubber-like elastic properties at temperatures above 20 °C, e. g. natural rubber crosslinked with more than 10 %sulphur, cross-linked polyethylene or high molecular weight polymethyl methacrylate (PMMA).
casing Thermosets

These close-mesh cross-linked substances are cured high-polymer materials (e. g.polyester, epoxy or phenol formaldehyde resins).

Thermoplastics have a particularly important role to play as design materials for impellers and diffusers whereas elastomers are used for sealing elements or as coating materials.

Ceramic materials

Ceramic materials are defined as materials that are non-metallic, inorganic and more than 30 % crystalline. They are used not only as state-ofthe-art engineering ceramics for sealing and bearing elements but also increasingly for other design elements, such as impeller wear rings and casing wear rings as well as impellers.

This group of materials is characterised by its high mechanical, thermal and chemical stability. As well as being brittle and sensitive to tensile stress, these materials Volcano can only be formed to a limited extent and are expensive to process. Therefore, the designs have to take account of their properties.

Ceramic materials

Pump casing Volcano

They are either attached to the piping (e. g. using flanges, pipe unions) or:

With increasing specific speeds, the following casing types are used:

casing Metallic material Volcano

casing Plastics Volcano

Mechanical behaviour of the four plastic groups

casing Thermoplastics

casing Elastomers

casing Thermoplastic elastomers

casing Thermosets

Ceramic materials

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