The pump is a mechanical device used to transfer liquids from one point to another by imparting energy supplied by a prime mover to the liquid. The prime mover can be an electric motor, an I.C. engine, a steam engine, or a turbine. The power required to pump the liquid depends on the quantity of the liquid, the height (head) to which it is to be lifted, and the viscosity of the liquid.
There are two main types of pumps:
- Positive Displacement Type
- Rotodynamic Type
1. Positive Displacement
A positive displacement pump makes a fluid move by trapping a fixed amount and forcing that
trapped volume into the discharge pipe. There are two types of positive displacement pumps:
- Rotary Type
- Reciprocating Type
1.1. Rotary Type PD Pump
1.1.1. Gear Pump
A gear pump uses the meshing of gears to pump fluid by displacement. As the gears rotate, they separate on the intake side of the pump, creating a void and
suction which is filled by fluid. The fluid is carried by the gears to the discharge side of the pump, where the meshing of the gears displaces the fluid.
- Cost-effectiveness, simple construction, and compactness.
- The rigid design of the gears and houses allows for very high pressures and the ability to pump highly viscous fluids.
- Usually limited to a maximum working pressure of 210 bars and a maximum speed of 3,000 rpm.
- Mostly used for hydraulic fluid power applications and chemical installations to pump high
- viscosity fluids
1.1.2. Vane Pump
A set of vanes is mounted in a rotor in which the vanes slide in and out of the rotor. The rotor is mounted eccentrically in the casing. As the vanes rotate past the suction port, they slide out of the rotor while maintaining constant contact with the casing. Springs or sealer rings help hold the vanes against the casing, thus the vanes make a close seal, or fit, against the casing wall. The trapped fluid is forced from the suction port to the discharge port.
- Rotary vane pumps have higher efficiencies than gear pumps.
- Generally used for mid pressures up to 180 bars.
- Compensates for wear through vane extension.
- Develops a good vacuum.
- Vane pumps are commonly used as high-pressure hydraulic pumps
- In automobiles for supercharging, power-steering, air conditioning, and automatic transmission pumps.
1.1.3. Screw Pump
A screw pump is a type of positive displacement pump that uses two or more screws that intermesh to pressurize fluids and move them in a system.
The screws take in the fluid then push it out from the other side while increasing its pressure
- Easy maintenance, long lifetime, constant high efficiency with variable capacity.
- Used for transport of viscous fluids with lubricating properties.
- Suited for a variety of applications such as fuel injection, oil burners, boosting, hydraulics, fuel, lubrication, circulating, feed, and so on.
1.1.4. Lobe Pump
Lobe pumps are like gear pumps in operation in that fluid flows around the interior of the casing. Unlike external gear pumps, however, the lobes do not make contact.
As the lobes come out of the mesh, they create an expanding volume on the inlet side of the pump and the liquid to be pumped flows into this cavity and is trapped by the lobes as they rotate. Liquid travels around the interior of the casing in these enclosed volumes between the rotor’s lobes and the casing. Finally, the meshing of the lobes forces liquid through the outlet port under pressure.
- High efficiency, reliability, corrosion resistance, handle low viscosity liquids with diminished performance.
- Loading characteristics are not as good as other designs, and suction ability is low.
- High-viscosity liquids require reduced speeds to achieve satisfactory performance.
- Lobe pumps are used in a variety of industries including pulp and paper, chemical, food, beverage, pharmaceutical, and biotechnology.
1.2. Reciprocating Type Positive Displacement Pump
In a positive displacement pump, fluid is moved by trapping a fixed amount of fluid and forcing the trapped volume into the pump’s discharge
Reciprocating pumps are best suited for high-pressure and low-volume applications. They frequently require pulsation dampeners because of the pulsating nature of the flow.
1.2.1. Piston & Plunger Pumps
A piston pump is a type of positive displacement pump where the high-pressure seal reciprocates with the piston and pushes the fluid from the cylinder. It is a type of positive displacement pump where the high-pressure seal is stationary and a smooth cylindrical plunger slides through the seal and pushed into and withdrawn from a liquid cavity.
- Better suited for handling viscous liquids.
- Capable of producing high pressures and large capacities with high efficiency and are self-priming.
- They require more maintenance because of the large number of moving parts.
- Piston and plunger pumps are mainly used for handling very low flow rates at very high pressures.
- Used in applications that could range from 70 to 2,070 bars
1.2.2. Diaphragm Pump
A diaphragm pump is a positive displacement pump that uses the reciprocating action of a membrane to pump the fluid. The membrane is mechanically pushed and pulled in and out of a pumping chamber. When the diaphragm is extended, the fluid being pumped is sucked into the diaphragm chamber through the intake line. When the diaphragm is collapsed, all the fluid is forced out of the diaphragm chamber. A pair of non-return check valves prevent the reverse flow of the fluid.
- Suitable for discharge pressure up to 1,200 bar.
- Have good dry running characteristics.
- Have good self-priming capabilities.
- Can handle highly viscous liquids.
- Compact size and weight
- Easy, low cost, and convenient servicing
- Diaphragm pumps are commonly called “mud suckers” because of their use in pumping slurries and wastewater in shallow depths.
- Capable of handling all sorts of aggressive media including gases and gas/liquid mixtures,
- and can achieve very high pressures.
- Larger models of this pump type are used to move heavy sludge and debris-filled wastes
- from trenches and catch basins.
- Smaller models are typically used in chemical metering or dosing applications where very
- constant and precise amounts of liquid delivery are required.
2. Rotodynamic Pump
A rotodynamic pump is a kinetic machine in which energy is continuously imparted to the
pumped fluid by means of a rotating impeller, propeller, or rotor.
2.1. Centrifugal Pump
A Centrifugal pump is used to transport fluids by the conversion of rotational kinetic energy
to the hydrodynamic energy of the fluid flow. The fluid enters the pump impeller along or near to the rotating axis and is accelerated by the impeller, flowing radially outward into a diffuser or volute chamber from where it exits. The fluid gains both velocity and pressure while passing through the impeller. The diffuser decelerates the flow and further increases the pressure.
- Common uses include water, sewage, petroleum, and petrochemical pumping.
Problems of Centrifugal Pumps
- These are some difficulties faced in centrifugal pumps:
- Cavitation – NPSH of the system is too low for the selected pump
- Wear of the impeller—can be worsened by suspended solids
- Corrosion inside the pump caused by the fluid properties
- Overheating due to low flow
- Leakage along the rotating shaft.
- Lack of prime—centrifugal pumps must be filled (with the fluid to be pumped) in order to
Affinity Laws for Centrifugal Pumps
- Flow is proportional to the pump speed
- The pressure is proportional to the square of the pump speed
- Brake horsepower (energy input) is proportional to the cube of the pump speed
2.2. Axial Flow Pump
An axial flow pump has a propeller-type of impeller running in a casing. The pressure in an axial flow pump is developed by the flow of liquid over the blades of the impeller. The fluid is pushed in a direction parallel to the shaft of the impeller, that is, fluid particles, in course of their flow through the pump, do not change their radial locations.
- The main advantage of an axial flow pump is that it has a relatively high discharge at a
- relatively low head.
- The effect of turning off the fluid is not too severe in an axial pump and the length of the
- impeller blades are also short leading to higher stage efficiencies.
- Axial flow pumps are used in applications that require very high flow rates and very low
- They are useful as circulating water pumps in power plants.
- They’re also commonly used in the chemical industry for circulating large amounts of fluids
- in evaporators.
- Very useful in flood dewatering applications.