Impeller
The impeller is a vaned disk mounted on the shaft. Its function is to increase the pressure of the liquid by means of its rotating action. The impeller is the main rotating part that provides the centrifugal acceleration to the fluid. The impeller may be of open, semi-open, or enclosed type, and may have anywhere between two to ten vanes. It may also be of single, or double, suction design. It may be radial flow, axial flow or mixed flow design.
Impeller nut
Function of Impeller nut is:
-To lock the impeller in its proper axial position
-To prevent axial movement due to hydraulic thrust
Seal
A pump may be provided with one of two types of sealing elements - packing rings or mechanical seal. The sealing elements prevent the leakage of the pumped liquid into the atmosphere.
Casing
Casings are generally of two types: volute and circular. The impellers are fitted inside the casings. The casing contains the liquid and acts as a pressure containment vessel that directs the flow of liquid in and out of the pump. In most cases the casing includes the suction and discharge nozzles of the pump that connect it to the external piping. In some vertical pumps the casing may be referred to as bowl, and in some small pumps they may it call it housing.
Function of Casing is to:
-Incorporates nozzles to connect suction & discharge piping
-Directs flow into & out of the impeller.
-Provides support to the bearing bracket
Volute casings build a higher head; circular casings are used for low head and high capacity. A volute is a curved funnel increasing in area to the discharge port.. As the area of the cross-section increases, the volute reduces the speed of the liquid and increases the pressure of the liquid. One of the main purposes of a volute casing is to help balance the hydraulic pressure on the shaft of the pump. Running volute-style pumps at a lower capacity than the manufacturer recommends can put lateral stress on the shaft of the pump, increasing wear-and-tear on the seals and bearings, and on the shaft itself. Double-volute casings are used when the radial thrusts become significant at reduced capacities.
Circular casing have stationary diffusion vanes surrounding the impeller periphery that convert velocity energy to pressure energy. Conventionally, the diffusers are applied to multi-stage pumps.
The casings can be designed either as split casings or solid casings. A split casing implies two or more parts are fastened together. When the casing parts are divided by horizontal plane, the casing is described as horizontally split or axially split casing. When the split is in a vertical plane perpendicular to the rotation axis, the casing is described as vertically split or radially split casing. Casing Wear rings act as the seal between the casing and the impeller. Solid casing implies a design in which the entire casing including the discharge nozzle is all contained in one casting or fabricated piece.
Wear rings
Wear ring provides an easily and economically renewable leakage joint between the impeller and the casing. Clearance becomes too large the pump efficiency will be lowered causing heat and vibration problems. Most manufacturers require that you disassemble the pump to check the wear ring clearance and replace the rings when this clearance doubles.
Function of wear ring is:
- To control the leakage losses across the annular path between impeller and wear ring
- To protect the rotating impeller from rubbing with the stationary casing.
- To provide a replaceable wear joint.
The type of wear rings used in pump is depending on the liquid being handled, pressure difference across the joint and impeller speed. The most common type of wear rings are the L-type, the labyrinth type and the flat type. In small single suction pumps the wear rings are generally pressed into the casing. Some may be locked by set screws.
Casing wear rings on larger pumps are sometimes held in place with a flange screwed to the casing. In axial pumps, the casing wear rings are just set on a pin.
Impeller wear rings are often press fit or shrink fit onto the impellers. Set screws are used with these fits at times to provide added strength to the fit.
Shaft
The shaft is usually the longest part of a pump and is made of one piece. Its function is to transmit the input power from the driver into the impeller. In a close-coupled pump, the motor has an extended length of shaft that acts as the pump shaft. In some vertical pumps, the so-called line shaft pump, the shaft may be supplied in more than one piece of ten Feet sections depending on the pump bowl setting below the ground surface.
The basic function of a centrifugal pump shaft is to transmit the torques encountered when starting and during operation while supporting the impeller and other rotating parts.
Shaft Sleeve
Function of shaft sleeve is:
-To protect the shaft from erosion & corrosion
-To enhance the stiffness of the rotating element
-To protect the shaft from abrasion wear at packed stuffing box or at leakage joints
Coupling
The function of a coupling is to connect the pump shaft and the driver shaft, and to transmit the input power from the driver into the pump.
Shaft couplings can be classified in to two groups:
1) Rigid couplings- These couplings are used in applications where there is absolutely no possibility or room for any misalignment.
2) Flexible couplings- These couplings are more prone to selection, installation and maintenance errors. Flexible shaft coupling can be divided in to two groups: elastomeric and non-elastomeric.
Tire and rubber sleeve designs are elastomer in shear couplings. Jaw and pin and bush coupling designs are elastomer in compression couplings.
Gear, chain and grid couplings are examples of non-elastomeric, lubricated couplings. Disc and diaphragm couplings are non-elastomeric and non-lubricated.
Bearings
The functions of the bearings are to support the weight of the shaft (rotor) assembly, to carry the hydraulic loads acting on the shaft, and to keep the pump shaft aligned to the shaft of the driver.
Stuffing Box
Stuffing box both refer to a chamber, either integral with or separate from the pump case housing that forms the region between the shaft and casing where sealing media are installed. When the sealing is achieved by means of a mechanical seal, the chamber is commonly referred to as a Seal Chamber. When the sealing is achieved by means of packing, the chamber is referred to as a Stuffing Box. Both the seal chamber and the stuffing box have the primary function of protecting the pump against leakage at the point where the shaft passes out through the pump pressure casing. When the pressure at the bottom of the chamber is below atmospheric, it prevents air leakage into the pump. When the pressure is above atmospheric, the chambers prevent liquid leakage out of the pump. The seal chambers and stuffing boxes are also provided with cooling or heating arrangement for proper temperature control.
Back Plate
The back plate is of pressed steel manufacture, which together with the pump casing forms the actual fluid chamber in which the fluid is transferred by means of the impeller.
Bearing Housing
The bearing housing is used to enclose and protect the shaft bearings, ensuring proper alignment. The housing will also include some type of method for lubricating the bearings and cooling the pump.
Gland
The gland is a very important part of the seal chamber or the stuffing box. It gives the packing’s or the mechanical seal the desired fit on the shaft sleeve. It can be easily adjusted in axial direction. The gland comprises of the seal flush, quench, cooling, drain, and vent connection ports as per the standard codes like API 682 and API 610.