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.
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.
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 Ekwestrel’s engineering services.
Manan Arora says
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…???
ENGINEERING TRAINEE ,WOOD GROUP INDIA.
Randal Ferman says
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
Sabrina Maceda says
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.
Mostafa Sokkar says
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.
sir, i have a ques that what will happen if we do not provide any volute or we provide a constant area volute?
Randal Ferman says
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
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.
Randal Ferman says
Chapter 7 of the book Centrifugal Pumps Second Edition by Johann Friedrich Gülich 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 Pumps by Christopher E. Brennen.
Best of luck with this! Randal
Please tell which is the best software for impeller and Volute Design. I’ll be very thankful to you??
Randal Ferman says
First of all, I do not personally use any of the available hydraulic design and CFD software packages. So by commenting on some of the products I’ve seen, I risk leaving out others that are worthy of mention.
CD-adapco offers CAE products and engineering services for a broad array of industries. This is a company to consider if you think you may need advanced engineering support in addition to proven CFD or FEA software.
NUMECA offers highly advanced CFD software and design services for a wide range of turbomachinery, fluid flow design and analysis projects.
Concepts NREC has been in the turbomachinery analysis, prototyping and software business for many years and offers a full suite of CAE and CAM software products. They have highly skilled technical staff that can help you through the most demanding turbomachinery design challenges.
TURBOdesign from Advanced Design Technology is a well-developed, user-friendly suite of turbomachinery CAE products. ADT is an international joint venture between the University College London and Ebara Research Co. Ltd.
Simerics offers attractive, fast and easy-to-use pump simulation CAE software.
ANSYS offers a broad range of CAE products for engineering analysis, including turbomachinery. If you’re looking for integrated CAE and simulation software that goes beyond pump hydraulic design and CFD, ANSYS is certainly worth looking into.
The above mentioned are among the best pump design CAE products out there. Prepare to have a robust budget. There are other products out there to consider and, of course, there are free software options. In your search and evaluation you will want to consider your individual or organization’s engineering requirements, product training, service support, access to design and analysis experts, validation of software results, software integration and budget.
I trust this has been of some help and I wish you the best of luck in finding the design package you’re looking for.
Tom Wittebrood says
Dear Mr. Ferman,
Me and my projectgroup are working on a schoolproject wich include researching different centrifugal pumps with in particular the centifugal pumps used by firefighting teams. Do you know any calculations or formulas we can use in our research? We are especially curious about the difference between calculations used for diffusor pumps and pumps without diffusor.
We hope that you will get back to us.
Kind regards, Tom Wittebrood
Randal Ferman says
Dear Tom W.,
I suggest choosing one or both of these two books: 1) Centrifugal Pumps, Second Edition by Johannn Friedrich Gülich and/or 2) The Interaction between Geometry and Performance of a Centrifugal Pump by B. Neumann.
Good luck on your project, Randal
Lovepreet singh says
thank you very much this information is very helpful for me .plz….how much balancing done by balancing line in multistage pump and thrust bearing.
Lovepreet singh trainee HMEL india
Randal Ferman says
I’m happy to hear this information is helpful to you.
I assume your question concerns the balance device itself (which leaks into the balance line returning to suction).
A disk type balancing device can achieve 100% balance, but there is still need for a startup and shut down thrust bearing device. API 610 disallows the use of a close axial clearance for axial thrust balance. This exclusion effectively disallows disk type balancing devices.
A balance drum device will have some residual unbalance. The pump manufacturer must size the drum to compensate for as-new as well as worn clearances, across a range of head-flow conditions. The residual axial hydraulic unbalance is taken up by a separate thrust bearing arrangement.
Combination disk and drum balancing devices are sometimes used.
Best regards, Randal
Dear Mr Randal,
I find Your article interesting , and easily understandable.
I am offering consultancy on Mechanical seals for Rotating Equipment, and present technical presentation on Basics of Rotating Equipment and Sealing systems for Rotating Equipment.
Your article gives value input for seminars, and thank you very much.
Randal Ferman says
I see you posted your comment over a year ago. Not sure how I missed it. I do make an effort to keep the material readily comprehendable to those seeking general understanding of the topic.
Mathew Abraham says
Is there any design advantage of volute style BB5 over the diffuser style BB5 from a maintenance perspective? I understand it is easier to take the rotor out and put it back on an axially split volute compared to a radially split diffuser stack along with the ease for measuring the running clearances with a feeler gauge. However, is the volute style more forgiving to the running clearances being out of spec while putting back the pump compared to a diffuser style? Is that going to cause more axial thrust issue or performance issue on a diffuser than a volute style? Also is the entrance area from an impeller to diffuser much tighter compared to a volute? If so, does its become a hassle while reassembling a diffuser style pump compared to volute style?
Randal Ferman says
It’s easy to understand why some users prefer an inner volute casing for a type BB5 versus a stacked diffuser arrangement. An important relative advantage of the volute arrangement you did not mention is reduction of unbalanced axial thrust. This is because the volute style is normally an opposed impeller arrangement. Opposed impeller arrangements are less prone to catastrophic failure due to a pump system upset. An opposed impeller arrangement for a stacked diffuser design adds complexity and cost to the assembly, is quite rare and is reserved only for the highest head applications such as for oil production water injection service.
I’ve never heard about the volute style being more forgiving of running clearances being out of spec. It may be easier to “sag bore” an inner volute casing to compensate for rotor sag, if that is deemed necessary.
Concerning the entrance area that you mention: axial tolerances on the stacked diffuser elements are easier to control. It is an inherent advantage with the diffuser design and it is possible to allow less internal clearance end float.
My personal inclination would be to evaluate the manufacturer’s installed based of the same model and size of pump, in similar conditions of service, as the main selection criterion. Offhand, I don’t think you should pay more than a 10 or 15 percent premium to go for the volute style, based on preference.
I trust this helps. Randal
Emir Martins says
I found this article very useful.
I found this article very useful. Actually, I’m experiencing a problem of excessive cavitation in a water pump (CW-Hydro, Inc., 1780 rpm, 510 m3/h, AMT: 180 mca, Shutoff Press: 21 kgf/cm2).
The pump has an excessive level of cavitation and vibration (2x, 3x rpm + high frequency) and has also a lot of wear on the volute. We already did some repair where put belzona but the problem continuous.
During these repairs, we machined (a little) the face of the scroll because excessive wear. So today we do not know if the clearance between the rotor and the faces (front and back) of the volute are as the manufacturer recommends. Unfortunately, we also did not get this information in the manufacturer’s manual.
Please, I would like your help, according to your experience in the subject, how much these clearances affect the performance of the pump and in cavitation. Do you would know, as a good practice, what these clearances (before, back and top rotor)?