Centrifugal Pumps

A centrifugal pump is a rotary machine in which flow and pressure are generated dynamically. Unlike positive displacement pumps, the inlet and outlet are not separated in this pump, and the fluid can flow freely through the pump. A centrifugal pump consists of two moving and stationary parts. The main moving part or rotor is the impeller, which is a wheel that transfers energy to the fluid. The stationary part or stator also consists of a diffuser, which comes in various types.

There are three types of centrifugal pumps: radial, mixed flow, and axial. These pumps do not differ significantly in operation, but each is suitable for a specific range of flow rate and head. To determine which of these pumps should be used, it is necessary to be familiar with a quantity called specific speed, which is a key criterion for pump selection. For each specific speed, only one type of pump will have maximum efficiency.

Components of a Centrifugal Pump

The components of a centrifugal pump that are in direct contact with the fluid determine the hydraulic performance of the pump. there are also mechanical components that hold the impeller in place and seal the casing, as well as components that enable the impeller to rotate. In the following, we will examine the different parts of a centrifugal pump.

Fluid-Contacting Parts: The primary function of this part is performed by the impeller, which, through rotation and the direct application of force to the fluid, causes it to be pumped.

Casing: This part is the casing and protective cover for all the pump components. The presence of a casing is also necessary to prevent fluid leakage and maintain fluid pressure.

Mechanical Components: These components include all the parts that enable movement and fluid sealing and include the following:

• Shaft: The impeller is attached to the shaft, which is usually made of steel or stainless steel. The size of the shaft is a critical design parameter. If the shaft size is selected too small, the pump vibration will increase, reducing the life of the bearings and other centrifugal pump components, and eventually causing the shaft to break. A large shaft size also increases the cost of the pump and the torque and energy required to rotate the impeller.
• Shaft Protector: Typically, the part of the shaft that is outside the sealing cover is covered by a protective piece. This piece is made of metals such as stainless steel or bronze and its geometry is such that it can slide over the shaft or be screwed onto it. In addition to protecting the shaft, the presence of a shaft protector ensures the correct placement of the impeller on the shaft.
• Seal: The part where the shaft passes through the casing is the stuffing box, and a seal must be used in this part to separate the wall of the stuffing box from the shaft. Mechanical seals have different performances, designs, and costs, but the simplest seal consists of a few parts: a gland, a stationary sealing ring, a rotating sealing ring, and a spring. The gland is located around the pump shaft and opposite the stuffing box bolts on the centrifugal pump casing. The stationary sealing ring is attached to the gland and is fixed around the shaft by the gland.

The rotating sealing ring is attached to the stationary ring by an elastomeric polymer element and is pressed against it by a spring. The spring applies the necessary pressure to the rotating ring. With the stationary ring attached to the gland and the moving ring attached to the shaft, the only way for fluid to leak from the stuffing box into the sealed chamber is to pass between the two rings, which are pressed together by the spring. As the pump shaft rotates, the rotating surface faces the stationary surface.

 A very small amount of fluid passes between these two surfaces, but the fluid that passes through evaporates due to the high heat generated by the friction between the surfaces. This small amount of fluid is sufficient to lubricate and cool the sealing components. When the sealing surfaces are cool, clean, soft, and lubricated, they can virtually eliminate all leakage between the shaft and the stuffing box.

• Bearings: Centrifugal pumps typically use standard anti-friction ball bearings lubricated with oil or grease. These bearings are similar to those used in other projects such as electric motors and automobiles. The shaft is held in place by the bearing and must be designed to withstand all the forces applied from the fluid to the impeller. The bearing must also be selected with the appropriate size to provide an acceptable service life. Failure of this part is one of the common causes of pump failure; therefore, engineers and users are interested in the details of the pump bearing when purchasing a centrifugal pump. There are several types of bearings, including:
  • Roller Bearings: This type consists of cylindrical rollers through which the shaft rotates. This type of bearing reduces friction and helps in withstanding axial and radial forces.
  • Ball Bearings: In this type, the shaft rotates with the help of balls inside the bearing. They are not difficult to design and are suitable for high speeds. They are also easy to maintain.
  • Sliding Bearings: This type, also known as a bushing in industry, is a hollow cylinder inside which the shaft rotates. There must always be enough oil between the shaft and the cylinder, and the rotational speed of the shaft must be above a certain value. Otherwise, the frictional force of this type of bearing will increase significantly.