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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

[url=http://www.wxuniversalcasting.com/pump-parts-investment-

casting/pump-casing-casting/]Pump Casing Casting[/url] 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.

[b]Single Volute Casing[/b]

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.

[b]Double volute casings[/b]

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.

Except that, there are other types of Pump Casing like

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casting/]Valve Parts Investment Casting[/url],

[url=http://www.wxuniversalcasting.com/pump-parts-investment-

casting/]Pump Parts Investment Casting[/url],

[url=http://www.wxuniversalcasting.com/pump-parts-investment-

casting/pump-cover-casting/]Pump Cover Casting[/url],

[url=http://www.wxuniversalcasting.com/pump-parts-investment-

casting/pump-impeller-casting/]Pump Impeller Casting[/url],

[url=http://www.wxuniversalcasting.com/machinery-parts-investment-

casting/]Machinery Parts Investment Casting[/url] and so on used

widely in the industrial production.
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