There are a few components that virtually every centrifugal pump has in common. These components include:
- An impeller
- A volute or diffuser style casing
- A shaft
- Shaft sleeves
- A sealing arrangement
These parts can be subdivided into the wet end and the mechanical end.
The wet end of the pump includes those parts that dictate the hydraulic performance of pump.
The mechanical end includes those parts that support the impeller within the casing, seal the casing where the shaft passes through it and enable rotation – the means by which the wet end creates flow and pressure.
We’ve already talked about the two primary parts of a pump: the casing and impeller. So we won’t spend much time on those here.
The short explanation is that the impeller rotates at a high speed and imparts velocity to the water. The impeller is positioned within the casing, and the casing converts the velocity generated by the rotation of the impeller into pressure. For a more detailed look at this process take a look at the article on this topic.
The mechanical portion of the pump includes the pump shaft, shaft sleeve, sealing arrangement, and the bearings.
The impeller is mounted on a shaft. The shaft is usually made of steel or stainless steel and is sized to support the impeller. Shafts have to be sized carefully. An undersized shaft can result in increased pump vibration, shorter bearing life, the potential for shaft breakage, and an overall reduced pump life. However, an oversize shaft can increase the cost of the pump unnecessarily.
In most pumps, the portion of the shaft that is under the sealing arrangement is covered with a shaft sleeve. The shaft sleeve is a sleeve of metal, usually bronze or stainless steel, that is designed to either slide or thread onto the shaft. The shaft sleeve is used to position the impeller correctly on the shaft, and it also protects the shaft.
The location where the shaft passes through the casing is called the stuffing box. A sealing arrangement must be used to seal the gap between the shaft and the wall of the stuffing box. Either packing or a mechanical seal may be used to seal this area.
How Do Mechanical Seals Work?
Mechanical seals vary in design, performance, and cost. The simplest seal consists of just a few parts: a stationary face, a rotating face, a gland, and a spring.
- The gland fits around the pump shaft and bolts directly to the face of the stuffing box directly onto the pump casing.
- The stationary seal ring, sometimes called the mating ring, is sealed to the gland and held in place around the pump shaft by the gland.
- The rotating seal ring, sometimes called the primary ring, is sealed to the shaft by an elastomeric element, and is pressed against the stationary mating ring by the spring.
- The spring applies pressure to the rotating primary ring by pressing against a retaining clip or collar fastened to pump shaft.
Since the stationary mating ring is sealed to the gland, and the rotating primary ring is sealed to the shaft, the only way for liquid to leak out of the stuffing box is to pass between the rings which are being pressed together by the spring.
As the pump shaft rotates, the rotating face rotates against the stationary face. A small amount of the pumped liquid does make its way between the faces but evaporates due to the heat generated by the rotating seal faces. This small bit of liquid is enough to keep the seal faces cool and lubricated. As long as the seal faces stay clean, smooth, and lubricated they will eliminate virtually all leakage between the shaft and the wall of the stuffing box.
The final part of the mechanical end is the bearing arrangement. Generally speaking, centrifugal pumps are equipped with standard ball-type anti-friction bearings. These are the same bearings used in everything from electric motors, to roller skates, to automobiles, and they are lubricated by grease or oil.
The pump shaft is supported and held in place by the bearings which have to be designed to handle all of the loads created by the rotation of the impeller, and sized to provide a reasonable service life. Bearing failures are one of the most common causes of pump downtime, so design Engineers and End-Users will often be particularly interested in the specific details surrounding the design of the bearing arrangement. Pump professionals are well served to spend time learning the ins-and-outs of the bearing systems of the pumps they deal with.
Pumps are simple machines. In their simplest form, they consist of two hydraulic components and a mechanical support system. Understanding the purpose of these parts is a critical step in the development of every pump professional.