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A volute pump is one of the most common types of industrial centrifugal pump. A volute pump uses a cutwater and a volute built into the pump casing to direct fluid to the discharge port, or to the next stage impeller.
A diffuser is a separate part that is situated around the rotating pump impeller, within the pump case.
A volute is built into the pump case. An important part of the volute is the cutwater, which separates the fluid from the impeller cavity from the fluid going to the discharge port or to the next stage impeller.
Although it is common in a single stage centrifugal pump to have only one volute and cutwater, multistage centrifugal pumps often have two volutes and cutwaters located approximately 180 degrees from each other.
This is an example of a typical multistage centrifugal pump. This pump has 2 cutwaters and 2 volutes for each of the 6 impellers.
Coupling Hub – A coupling hub adapts the pump shaft to the coupling. Usually this is a disc coupling or a gear coupling rated for high speeds.
Input Shaft – The pump shaft runs the length of the entire pump and is what all the impellers are attached to, as well as the coupling hub. The entire rotating assembly is suspended between the drive end and the non-drive end bearings.
Radial Bearings – Radial bearings support the pump shaft radially. In this case, the bearings are axially split sleeve bearings.
Thrust Bearing – Thrust bearings support the pump shaft axially. Although this pump is primarily hydraulically balanced, there are still axial forces as the pump operates at different speeds and pressures. In this case, the thrust bearing is a set of ball bearings. In some cases, a pivot shoe bearing (sometimes referred to as a tilt shoe bearing), such as a Kingsbury Thrust Bearing, is used.
Drive End Bearing Housing – This bearing housing supports the radial bearing closest to the coupling hub on the drive end of the pump.
Non-Drive End Bearing Housing – This bearing housing supports the radial bearing and thrust bearing on the thrust end (non-drive end) of the pump. This is often referred to as the thrust end bearing housing.
Mechanical Seal – The mechanical seal isolates the pressurized fluid from the atmosphere. The design consists of two very smooth surfaces gliding on each other which are held together with springs. One surface rotates with the shaft, and the other is stationary. In some cases, when leakage is not an issue, (such as when pumping potable water) rope packing is used instead of a mechanical seal.
Impellers – Fluid is drawn in the center of the impeller in an axial direction. Then, as the impeller rotates, the fluid is forced outward, in the radial direction, due to centrifugal force.
Casing Eye Wear Rings – Each impeller has a casing eye ring that is built to have a very tight clearance (0.001” to 0.020” depending on the fluid being pumped) with the outside of the impeller eye. This is because there is a pressure differential between the eye of the impeller and the outside of the impeller, and the casing ring limits the leakage of the fluid back to a lower pressure zone.
Casing Hub Wear Rings – Like the Casing Eye Wear Ring, the Casing Hub Wear Ring serves the same purpose, but on the hub side of the impeller.
Flow Diverters – These help direct the fluid into the eye of the impeller. Often, these are built into the Casing Eye Wear Ring.
Throat Bushings – The throat bushings are made to maintain positive pressure in the seal chamber and keep contaminants away from the mechanical seal surfaces.
Throttle Bushing – The throttle bushing is only required on the higher-pressure side of the pump. Its purpose is to isolate the mechanical seal from the pressure of the mid stage impeller. A balance line is usually used between the two seal chambers to make both mechanical seals operate at the same pressure, even though the drive end seal is on stage 1 and the non-drive end seal is on stage 4 (in the example above). The throttle bushing is also designed to help hydraulically balance the axial thrust load of the pump.
Center Bushing – The center bushing is positioned between stage 3 impeller and stage 6 impeller. This means that the pressure differential between one side of the center bushing and the other can be quite high. The center bushing is designed to help hydraulically balance the axial thrust load of the rotating assembly.
Pump Case – What is shown in this photo is only the bottom half of the pump case. An axially split centrifugal pump has a bottom half and a top half, clamped together by large studs and nuts with a gasket in the middle.
Clamping Studs – Attached to the bottom half and the top half of the pump case are clamping studs. These studs and nuts are torqued up to 10,000 ft lbs on large pumps to contain the pressure in the pump case.
Suction Port & Discharge Port – In a multistage, horizontal, axially split volute pump, the suction connection will always be in one of the four corners of the pump, and the discharge flange will always be in the middle (on either side).
Volute pumps are used in a wide variety of applications. The applications where Power Zone uses volute pumps the most are Mine Dewatering, Oil & Gas Refineries, Midstream and Pipelines, Water Injection, Power Generation Boiler Feedwater, Manufacturing and general Water Transfer. Volute pumps are very common in every industry.
In general, a diffuser pump can be made slightly more efficient than a volute pump. As a tradeoff, though, the diffuser pump is more complicated to manufacture and repair than a volute pump is. A diffuser is a complex item (much like an impeller) that surrounds the impeller and makes the impeller harder to access. Because a volute pump does not have a diffuser, inspection and repair is generally easier.
Overall, there are not large advantages or disadvantages between either diffuser pumps or volute pumps. In most applications, either pump type will work very well, if the pump is sized correctly and has the correct material. Most major pump manufacturers, such as Flowserve, Sulzer, Goulds etc. make both volute pumps and diffuser pumps.
When selecting a single volute pump vs a double volute pump, it should be noted that a double volute pump is generally better for several reasons: (a) a double volute pumps and a single volute pump can both have high efficiencies at the Best Efficiency Point (BEP), but a double volute pump will maintain high efficiencies when the pump operates above and below the BEP. (b) a double volute pump causes less radial load on the rotating assembly than a single volute pump does.
When selecting a volute pump or a diffuser pump, the main considerations apply to both. It must be determined that the material is compatible with the fluid being pumped. In some cases, such as when the fluid is corrosive or contains particulates, exotic materials must be used for all wetted parts inside the pump. Mechanical seals especially must be selected carefully, along with the correct API 682 seal flush plan to ensure reliable operation. Bearing types and bearing arrangements should be considered carefully according to the application and whether a forced lubrication system is available or not.
All centrifugal pumps, whether diffuser pumps or volute pumps, should have regular maintenance and preventative maintenance procedures in place to ensure maximum reliability and longevity. Here are a few important maintenance and troubleshooting tips:
Monitor Vibration – Most failures within a pump, even in very early stages, will result in increased vibration. For this reason, monitoring vibration closely can be useful in identifying early warning signs that something is failing. For troubleshooting, advanced vibration analysis can be quite useful. This generally requires portable vibration instruments and a professional vibration analyst but can be essential in identifying what part of the pump is failing.
Seal Leak Detection – In most cases, a small amount of leakage from a mechanical seal is not an issue, but a large amount of leakage is. A properly designed leak detection system can detect the difference between normal leakage and excessive leakage.
Seal Support System – Seals are the most problematic part of any centrifugal pump, but a seal can be made much more reliable with a proper support system. These can range from a simple seal flush system to an elaborate system with filters and coolers. For more information on seal support systems, refer to the API 682 standard. These seal support systems are relevant to all centrifugal pumps, both volute pumps and diffuser pumps.
Suction Conditions – Often, a lot of the focus is put into pump selection, material selection, elaborate control systems, etc. and the suction conditions to the pump is secondary and somewhat overlooked. It should be noted that poor suction conditions is one of the leading causes of pump failure. A centrifugal pump should have lots of charge pressure. As a rule of thumb, the pump will be happiest with 2 – 3 times higher than what the pump curve states is needed for NPSHR. Keep elbows, valves and other piping restrictions as far back from the pump as possible and keep the suction piping large diameter.
Add an Oil Cooler – The temperature of the oil should be kept low if possible. Depending on the specific oil being used, most oils start to break down at temperatures above 140 degrees. For this reason, if a cooler can be installed to lower the temperature of the oil, oil change procedures can occur less often. Coolers can be separate air cooler or shell and tube heat exchangers, or, some pumps have a built in oil cooling loop built into the bearing housing. Overall, though, if the oil is kept clean and fresh, most bearings can safely run up to 180 degrees and higher.
Volute – The passageway that directs fluid from the impeller to the discharge port or the next stage impeller.
Cutwater – The beginning edge of the volute. The cutwater is often sharpened and angled to reduce vane pass vibration. Overall, the exact shape and location of the cutwater can have large impacts on the efficiency and performance of the pump.
Axially Split – An axially split pump means that it opens axially, parallel to the pump shaft, such as the one shown in the example above.
Radially Split – A radially split pump means that it opens radially, perpendicular to the pump shaft. Generally, all multistage radial pumps are diffuser pumps, not volute pumps. Volute pumps can be radially split, but only in Overhung designs, such as a common single stage pump.
Crossover – The crossover is a passageway within the pump case that transfers the fluid between the two middle stages. In the example above the pump case has a crossover between the 3rd and 4th stages (although it cannot be seen in the photo). The purpose of this is so that impellers 1, 2 and 3 can face towards the drive end and impellers 4, 5 and 6 can face towards the non-drive end. This keeps the pump hydraulically balanced, greatly reducing the size and load of the thrust bearing. A hydraulically balanced pump is far more reliable and more efficient than a pump that relies solely on the thrust bearing to support the axial thrust.
BEP – The Best Efficiency Point of a pump is the flow and pressure at which the pump operates the most efficiently at any given speed. This point (sometimes shown as a line across multiple speeds) is usually depicted on a centrifugal pump curve.
NPSHR – Net Positive Suction Head Required is the pressure that the pump needs at the eye of the stage 1 impeller to prevent the fluid from cavitating during pump operation. It should be noted that to obtain this value, the pump manufacture “starves” the pump until cavitation is present. For this reason, actual suction pressure should be considerably higher than what the pump curve states is required.
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