Diffuser Versus Volute Casing - pumps

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Diffuser versus voluteDiffuser versus voluteDiffuser versus voluteDiffuser versus volute

The stator section of a centrifugal pump, after flow exits the impeller, is usually either a diffuser or a volute. The

purpose of each of these two stator types is to efficiently diffuse velocity energy into pressure. Diffusers are

characterized by a plurality of radially symmetric diffusing passageways surrounding the impeller. Either a volute-

shaped or annular collector is used in tandem with the diffuser. Volutes are most often characterized by one or two

scroll-shaped diffusing passageways, depending on the pump configuration. Some multi-port volute stators might

just as well be called a diffuser. Occasionally there are overlapping characteristics and designation of stator type is

arbitrary.

volute

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Most of the time the specifying engineer or end user does not decide on the type of stator to be used. By default it is

determined by what the pump manufacturers offer. The principal deciding issues are manufacturability and cost,

suitability for the application, modularity of design and efficiency.

For high pressure between bearings multi-stage pumps, diffuser designs are more compact compared to volute

designs. Compactness generally translates into a smaller pump casing size and lower cost of materials and

manufacturing.

Diffusers are normally designed as a one-piece or a two-piece ring assembly secured into the pump casing. Diffusers

are modular components. For a given pump casing, variations of the diffuser passages can be designed to cover a

range of operating conditions.

Diffuser

For a single stage centrifugal pump, a diffuser type design is usually more costly to produce because the diffuser

ring is an extra part plus some incremental added machining for the casing. The casing must still function as a

collector to convey the flow from the diffuser to the discharge nozzle. No matter how this is done, the diffuser can

offer little comparative advantage in the size of a single-stage pump.

Diffuser designs are often more efficient at the best efficiency rate of flow, compared to that of a volute. Also, a

custom diffuser can be made for each application in order to maximize the efficiency for a specific duty point.

A volute proponent might argue that the diffuser is less efficient at off-peak flow rates where the pump will operate

a good portion of the time. The efficiency differences may not be significant and unless large amounts of power are

involved, these debates seldom carry much weight in relation to the competing prices of the pumps offered, or user

preference for either volute or diffuser.

Radial thrust acting on the impeller develops from a non-uniform circumferential pressure distribution. The stator

design plays an important role in this. For some applications, especially with a single-stage overhung impeller type

pump that will operate continuously at flows substantially away from its Best Efficiency Point flow, a

diffuser/collector arrangement can provide a lower magnitude of radial thrust.



One of the two basic stator types may be particularly suited for specific applications. For instance, most API users of

axial split case type BB3 between bearings pumps prefer opposed impellers in a double volute casing, which offers

some assembly and maintainability advantages. Volute type casings are the norm for solids handling pumps that

require wide open passageways. A few specialty high pressure single casing pumps utilize the structural support that

the vaned diffuser can provide for the collector scroll. Vertical turbine and vertical bowl type casings are mostly of

the vaned diffuser type.

Manufacturers have generally rationalized the choice of pump stator based on market needs, application

requirements and production costs. Any evaluation regarding the selection of a diffuser or a volute should be

considered in the context of specific pump types, specific applications and manufacturers product offerings.

For an independent evaluation of function and performance, contact an experienced consulting engineer who can

help with your specific applications. See the complete list of Ekwestrels engineeringservices.

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TAGGED: TAGGED: TAGGED: TAGGED: CENTRIFUGAL PUMPCENTRIFUGAL PUMPCENTRIFUGAL PUMPCENTRIFUGAL PUMP, , , , COLLECTORCOLLECTORCOLLECTORCOLLECTOR, , , , DIFFUSERDIFFUSERDIFFUSERDIFFUSER, , , , ENGINEERING CONSULTINGENGINEERING CONSULTINGENGINEERING CONSULTINGENGINEERING CONSULTING, , , , IMPELLERIMPELLERIMPELLERIMPELLER, , , , PUMPPUMPPUMPPUMP, , , , PUMP ENGINEERING CONSULTINGPUMP ENGINEERING CONSULTINGPUMP ENGINEERING CONSULTINGPUMP ENGINEERING CONSULTING, , , , RADIAL THRUSTRADIAL THRUSTRADIAL THRUSTRADIAL THRUST, , , ,

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About Randal FermanAbout Randal FermanAbout Randal FermanAbout Randal Ferman

I have more than 35 years experience in the pump industry. During my 32 years with Flowserve, a global pump

manufacturer, I was responsible for assignments in order engineering, field technical services, hydraulic design,

training and product development involving a broad range of pump types and applications. In 2009, I became a full

-time principal of Ekwestrel Corp in Los Angeles, providing independent, objective engineering consulting services on new and

existing pumping equipment and systems. Connect with me on LinkedIn here.

CommentsCommentsCommentsComments

Manan Arora says:

July 16, 2012 at 1:41 am

Can a double volute casing design serve better than a diffuser casing ?..In a double volute casing the resultant

unbalanced loads on the impeller can be balanced .This will increase the cost but can it serve the purpose???

MANAN ARORA

B.E.(CHEMICAL),

ENGINEERING TRAINEE ,WOOD GROUP INDIA.

Reply

Randal Ferman says:



July 16, 2012 at 2:03 pm

Manan Arora, I would not say that double volute designs are better than diffuser designs for a given pump type, with

respect to radial load balancing. Pump manufacturers and users most often have other reasons, such as cost or ease of

assembly, for choosing either volute or diffuser designs. Randal

Reply

Sabrina Maceda says:

May 29, 2013 at 4:30 pm

Hi Randal,

I found this article more than helpful since finally I could get insight on the difference between these two designs for a

translation into Spanish of a centrifugal slurry pump that I am working on. Thank you.

Reply

Mostafa Sokkar says:

September 1, 2013 at 11:08 pm

I was looking to some information to enhance my power point

looking to the rush of information in the net though out Google

came to this page.

I found some good aspects which help me to accomplish my

work, so I am thanking who delivered this hints to us and

appeciated their effort.

Reply

mayank says:

September 23, 2013 at 11:24 am

sir, i have a ques that what will happen if we do not provide any volute or we provide a constant area volute?

Reply

Randal Ferman says:

September 23, 2013 at 11:34 pm

Mayank, There are applications where a volute is not needed such as when an impeller discharges into a

relatively large collector volume. But the usual pump through-flow arrangement requires a volute for both velocity-to-

pressure conversion and guidance towards the discharge nozzle. The volute also tends to equalize radial forces acting on

the impeller, at the best efficiency rate of flow. While it is possible to omit the volute, or to use an annular (constant area)

collector, the optimal velocity conversion and guidance to the pump discharge, as well as the effects on radial thrust,

should all be considered. I hope this helps. Randal

Reply

Nahee says:

August 24, 2014 at 8:56 am

Sir,

For a engine Pump( open impeller), there is a requirement of an compact inducer. Very less info on Inducer design available in



the net. What are the basic design guidelines required for inducer to match with impeller. I can visualize the subject is

exhaustive.

Reply

Randal Ferman says:

August 24, 2014 at 10:43 pm

Dear Nahee,

Chapter 7 of the book Centrifugal Pumps Second Centrifugal Pumps Second Centrifugal Pumps Second Centrifugal Pumps Second EditionEditionEditionEdition by Johann Friedrich Glich contains a nice monograph of inducer

design including the interface to an impeller. Another reference I recommend is the Cambridge University Press 2011

edition of Hydrodynamics of PumpsHydrodynamics of PumpsHydrodynamics of PumpsHydrodynamics of Pumps by Christopher E. Brennen.

Best of luck with this! Randal

Reply

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Diffuser Versus Volute Casing - pumps