2.1 Centrifugal pumps
As discussed previously, centrifugal pumps have prevalent application in liquid pipeline transmission systems as they are capable of handling variable heads and flow rates! These pumps can handle multi-products and other liquids over a wide range of viscose and other properties [7].
Centrifugal pumps are fluid-kinetic machines designed for power increase within rotating impeller. Therefore, it is also called the "hydrodynamic pumping principle. “According to this principle, dynamic pumps can be considered as continuous flow machine they use rotating impellers or propellers to sequentially accelerate the liquid (increasing its velocity) then decelerate it (trading kinetic energy for increased pressure). This may require a number of stages (maximum about 10), often within the same casing. Dynamic pumps always have an open flow route through the machine.
Typical dynamic pumps are:
Centrifugal Pumps
Axial Flow Pumps
Hybrid (Mixed Flow) designs also exist.
Centrifugal pumps are, by far, the most commonly used dynamic pumps.
Centrifugal pumps work by spinning an impeller submerged in a bath of fluid (Fig. 2.1) Impeller vanes impart rotary motion to the liquid; centrifugal effects push the liquid toward the outer edge of the impeller. This displaced liquid leaves the impeller and is collected by the shaped volute, which decelerates the fluid, converting kinetic energy to potential energy. This potential energy is expressed as increased pressure and decreased velocity. The liquid being displaced from the impeller creates a partial vacuum at the centre of the impeller, permitting more liquid to enter from the pump suction. The impeller is a close fit in the volute casing to reduce the amount of back-flow bypass in the impeller. Centrifugal pumps may include several stages in series, each comprising one impeller, stage casing and volute.
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Figure 2-1. Centrifugal pump actions |
The impeller provides velocity energy to the liquid. The high velocity at low pressure is converted into low velocity at high pressure in the volute or diffuser by gradually increasing the flow area. Generally, the impeller provides 40% of the energy and volute casing the remainder. There are different impeller design which are use depending on application.
These are:
straight vane, single suction closed
francis vane, double suction close
open mixed flow
axial flow
open impeller with partial shroud
radial vane, non-clogging impeller
paper pulp impeller
Centrifugal pump configuration includes the following:
horizontal single stage, end suction pump
vertical in-line single stage pump
high speed vertical in-line pump
horizontal two stage end suction pump
vertical in-line multi-stage pump
single-stage radial split casing pump
horizontal multi-stage split casing pump
multi-stage barrel casing pump
horizontal multi-stage ring section pump
high-pressure multi-stage barrel casing pump
Most process pumps are of end-suction design; these pumps have an axial flange and a radial delivery flange generally cast or fabricated into a single casing. Pipeline pumps are mostly side suction and discharge. The centrifugal action the pump takes place as follows:
The pump shaft is rotated by an external source (e.g., an electrical motor).
The impeller rotates inside the stationary casing.
Liquid flows into the impeller from the suction piping.
The impeller imparts velocity energy to the fluid.
The liquid velocity energy is converted into pressure energy as it passes through the volute or diffuser.
Pump components include the rear housing/cover, containing the mechanical shaft seal; that is clamped to the rear of the casing, with a static seal formed by a gasket or (The pump shaft passes through the rear cover and carries the impeller. The pump I s supported by a bearing housing fitted with two rolling element bearings. There is; air gap between the rear of the mechanical seal and the bearing housing to prevent p: liquid leakage past the seals and hence compromising the bearing system. The rear end of the shaft is normally fitted with a flexible coupling, linking the pump to the driver.
Modern end suction pumps are designed such that the working parts of the pump may be withdrawn in a single unit for maintenance. This "back pull out" unit is the con pump, including the seal, less the front casing. Back pull out units can be exchanged quickly but it is still necessary to inspect the front casing for damage and carry out an align check as part of the job.
Figure 2-2. Centrifugal pump impeller design/arrangements
Centrifugal impeller pump arrangements depend on fluid entry and discharge type, casing arrangement, and staging. Such impeller arrangements are indicated in Fig. 2-2.