Type of Pump
The selection of type and construction of a pump is very important to meet the process specification and proper application. Knowledge of the variety of pumps in the market should be reviewed and understood.A. Positive Displacement Pumps :-
Positive displacement (PD) pumps work by allowing a fluid to flow into some enclosed cavity from a low-pressure source, trapping the fluid, and then forcing it out into a high-pressure receiver by decreasing the volume of the cavity. This is done intermittently in the case of reciprocating pumps and continuously in the case of rotary gear and screw pumps. Some examples of PD pumps are: fuel and oil pumps in most automobiles, the pumps on most hydraulic systems, and the heart of most animals. Some general types of the positive displacement pumps are as below:
a) Reciprocating Pump
Reciprocating pumps create and displace a volume of liquid, their “displacement volumes”, by action of a reciprocating element. Liquid discharge pressure is limited only by strength of structural parts. A pressure relief valve and a discharge check valve are normally required for reciprocating pumps.
Reciprocating pumps can be further classified into three types of pump as below,
i) Piston Pumps
ii) Packed Plunger Pumps
iii) Diaphragm Pumps
b) Rotary Pump :-
Rotary pumps function with close clearances such that a fixed volume of liquid is displaced with each revolution of the internal element.
Rotary pumps include:
i) Gear Pump
ii) Lobe Pump
iii) Vane Pump
iv) Screw Pump
All those pumps above have the similar working principles: pumping the liquid with the help of rotating elements. The difference lies on the rotating elements; they could be gear, lobe, vane, or screw.
B. Dynamic Pumps :-
Dynamic pumps, in which energy is continuously added to increase the fluid velocities within the machines to values greater than those occurring at the discharge so subsequent velocity reduction within or beyond the pump produces a pressure increase. Major kinds which often used in many industries are centrifugal pumps .
a) Centrifugal Pumps
Centrifugal pumps are dynamic pumps. A centrifugal pump raises the pressure of the liquid by giving it a high kinetic energy and then converts it into pressure energy before the fluid exits the pump. It normally consists of an impeller (a wheel with blades), and some form of housing with a central inlet and a peripheral outlet. The impeller is mounted on a rotating shaft and enclosed in a stationary casing. Casings are generally of two types: volute and circular. The impeller design and the shape of the casing determine how liquid is accelerated though the pump.
Some general types of the centrifugal pumps are as below:
i) Overhung pump
A pump with the impeller(s) cantilevered from its bearing assemblies is classified as an overhung pump.
ii) Between bearings pump
A pump with the impeller(s) located between the bearings is classified as a between bearings pump. The pump may be single-stage (one impeller), twostage, or multistage. It can be axially (horizontally) split or radially split.
iii) Vertically suspended pump
A pump with the impeller(s) cantilevered vertically and the suction nozzle typically submerged is classified as a vertically suspended pump.
iv) Seal-less pump
Seal-less pumps are special pumps which do not require shaft seals. Construction for seal-less pumps is driven by canned motors or magnetic couplings. It is normally used in process involve extremely hazardous fluid, where leakage cannot be tolerated.
v) Submersible pump
Submersible pumps are designed to prevent pump cavitation. The driver components inside are completely surrounded by the pumped fluid.
vi) Horizontal self-priming pump
Horizontal self-priming pumps are designed to create a vacuum at the pump inlet. This enables the pump to “suck” fluid into its casing. The suction nozzle of the pump can therefore be located above the level of liquid being pumped.
Centrifugal pumps are used in more industrial applications than any other kind of pump. This is primarily because these pumps offer low initial and upkeep costs. Traditionally these pumps have been limited to low-pressure-head applications, but modern pump designs have overcome this problem unless very high pressures are required. The single-stage, horizontal, overhung, centrifugal pump is by far the most commonly type used in the chemical process industry . Basically, pump selection is made on the flow rate and head requirement and with other process considerations, such as material of the construction pumps for the corrosive chemical service or for the fluid with presence solids in the stream.
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