What is the advantage of double volute casing?

A pump casing is often referred to as a volute. A volute is a spiral-like geometry with an increasing through-flow area, reducing the velocity of the fluid and increasing the static pressure. The fluid exiting the impeller is then diffused towards the casing discharge nozzle. These volute casings come in two different types: single volute and double volute.

The casing is the major stationary component of the pump and mainly provides two functions:

Single Volute Casing

In a single volute casing, the impeller discharges into one volute that wraps completely around the impeller. This type of casing features one cutwater that directs the flow of the liquid towards the discharge of the pump.

Double Volute Casing

Double volute casings have two cutwaters located 180 degrees apart from each other. Normally a double volute pump can be identified simply by looking down the discharge flange: a noticeable vane is located inside the nozzle that divides the inner diameter of the discharge nozzle.

The main advantage of a double volute over a single volute is the balancing of radial loads on the impeller, as the double cutwater construction leads to a more equal pressure distribution in the volute. Minimizing the radial load on the bearings over the full operating range can have a significant impact on the lifetime of a pump, since bearing failures are the second most common reason for pump failures. However, a double volute adds additional hydraulic resistance.

In addition to single and double volute casings, another type of pump casing exists: the diffuser casing.

By Allan Budris

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, this is often not achieved due to:

• The current pump system friction head not being accurately identified when the pump is purchased
• Excessive safety factors added to the pump selection to plan for future pump wear
• Wear of the pump over time
• Changes and/or increases in the system friction head over time
• Changes in the system static and/or friction head during normal operation

The three basic casing types primarily used for centrifugal pumps are the single volute, double volute and vaned diffuser. Of these, the most common - at least for low- to medium-flow, single-stage pumps - is the single volute, with double volutes used more for larger, single-stage pumps handling liquids without solids, and vaned diffusers primarily applied to multistage pumps.

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.

Single Volute Casing

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 (see Fig. 1). This pressure equilibrium is, however, destroyed when the pump is operating at flow rates below (see Fig. 2) or above the BEP. Figure 2 shows the pressure distribution around the impeller in a single volute at zero flow rate, where A = total reaction, B = reaction on side shrouds, C = reaction on impeller periphery, and D = non-uniform fluid momentum. This off-BEP, non-uniform pressure distribution around the impeller results in increased net radial loads on the rotor (see Fig. 3). 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 casing is the major stationary component of the pump and mainly provides two functions:

Single Volute Casing

In a single volute casing, the impeller discharges into one volute that wraps completely around the impeller. This type of casing features one cutwater that directs the flow of the liquid towards the discharge of the pump.

Double Volute Casing

Double volute casings have two cutwaters located 180 degrees apart from each other. Normally a double volute pump can be identified simply by looking down the discharge flange: a noticeable vane is located inside the nozzle that divides the inner diameter of the discharge nozzle.

The main advantage of a double volute over a single volute is the balancing of radial loads on the impeller, as the double cutwater construction leads to a more equal pressure distribution in the volute. Minimizing the radial load on the bearings over the full operating range can have a significant impact on the lifetime of a pump, since bearing failures are the second most common reason for pump failures. However, a double volute adds additional hydraulic resistance.

In addition to single and double volute casings, another type of pump casing exists: the diffuser casing.

Based on the most recent updates in the field, a double volute casing is increasingly favored in applications requiring high reliability and longevity. With advancements in hydraulic modeling and simulation, it's now possible to design double volute casings that minimize the hydraulic resistance previously noted as a disadvantage. This ensures that pumps can maintain better efficiency across a wider range of operating conditions. Additional improvements in material technology also contribute to enhanced durability and reduced wear in modern double volute designs, making them more cost-effective over the lifespan of the pump.