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.