Pumps and their Types
Pumps and their Types
Pumps of different types and sizes are used in large numbers in a steel plant. They have got a very wide range of application in the steel plant and in many areas, pumps form critical piece of equipment for the process.
A pump is a device that adds energy into the fluids (liquid or gases), or sometimes slurries. The energy can be expressed in two ways i.e. either an increase in pressure or an increase in flow. Pumps are used to move or raise fluids. They are not only very useful, but are excellent examples of hydrostatics.
Pumps operate by some mechanism, and consume energy to perform mechanical work by moving the fluid. Pumps operate via many energy sources, including manual operation, electricity, engines, or wind power. They come in many sizes, from microscopic to very large industrial pumps.
Pumps may be classified as positive displacement pumps and non-positive displacement pumps (Fig 1).
Fig 1 Types of pumps
Positive displacement pumps
These pumps deliver a constant volume of fluid in a cycle. The discharge quantity per revolution is fixed in these pumps and they produce fluid flow proportional to their displacement and rotor speed. These pumps are used in most of the industrial fluid power applications. The output fluid flow is constant and is independent of the system pressure (load). The important advantage associated with these pumps is that the high-pressure and low-pressure areas (means input and output region) are separated and hence the fluid cannot leak back due to higher pressure at the outlets. These features make the positive displacement pump most suited and universally accepted for hydraulic systems. The important advantages of positive displacement pumps include capability to generate high pressures, high volumetric efficiency, high power to weight ratio, change in efficiency throughout the pressure range is small, and wider operating range pressure and speed.
It is important to note that the positive displacement pumps do not produce pressure but they only produce fluid flow. The resistance to output fluid flow generates the pressure. It means that if the discharge port (output) of a positive displacement pump is opened to the atmosphere, then fluid flow will not generate any output pressure above atmospheric pressure. But, if the discharge port is partially blocked, then the pressure will rise due to the increase in fluid flow resistance. If the discharge port of the pump is completely blocked, then an infinite resistance will be generated. This will result in the breakage of the weakest component in the circuit. Therefore, the safety valves are provided in the hydraulic circuits along with positive displacement pumps.
Positive displacement pumps operate by alternating of filling a cavity and then displacing a given volume of liquid. These pumps deliver a constant volume of liquid against varying discharge pressure or head. They have relatively low discharge capacity.
The main types of positive displacement pumps are (i) rotary pump, (ii) reciprocating pump, (iii) steam pump, and (iv) power pump. Rotary pumps use gears, vanes, lobes, or screws to trap and convey fluid from inlet to outlet sides of the pump while reciprocating pumps use the back and forth motion of mechanical parts. Reciprocating pumps compress liquid in small chambers via pistons, plungers, or diaphragms. These pumps are typically used in low-flow and high-head applications. Piston pumps may have single or multiple stages and are generally not suitable for transferring toxic or explosive material. Diaphragm pumps are more commonly used for toxic or explosive materials.
The construction of the reciprocating pumps is similar to that of the four stroke engine. The crank is driven by an external rotating motor. The piston of pump reciprocates due to crank rotation. The piston moves down in one half of crank rotation, the inlet valve opens and fluid enters into the cylinder. In the second half of crank rotation the piston moves up, the outlet valve opens and the fluid moves out from the outlet. At a time, only one valve is opened and another is closed so there is no fluid leakage. Depending on the area of cylinder the pump delivers constant volume of fluid in each cycle independent to the pressure at the output port.
The positive displacement pump is often used where relatively small quantity of the fluid is to be handled and the delivery pressure is quite large. The fluid flow rate of these pumps ranges from 0.0005 to 70 cum/min, the pressure head ranges between 0.7 and 7,000 kg/sq cm and specific speed is less than 500.
Non-positive displacement pumps
These pumps are also known as hydro-dynamic pumps. In these pumps the fluid is pressurized by the rotation of the propeller and the fluid pressure is proportional to the rotor speed. These pumps cannot withstand high pressures and generally used for low-pressure and high-volume flow applications. The fluid pressure and flow generated is due to inertia effect of the fluid. The fluid motion is generated due to rotating propeller. These pumps provide a smooth and continuous flow but the flow output decreases with increase in system resistance (load). The flow output decreases because some of the fluid slip back at higher resistance. The fluid flow is completely stopped at very large system resistance and thus the volumetric efficiency becomes zero. Therefore, the flow rate not only depends on the rotational speed but also on the resistance provided by the system. The important advantages of non-positive displacement pumps are lower initial cost, less operational maintenance because of lesser moving parts, simplicity of operation, higher reliability, and suitability with wide range of fluid etc. These pumps are primarily used for transporting fluids and find little use in the hydraulic or fluid power industries. Centrifugal pump is the common example of non-positive displacement pumps.
The term ‘centrifugal pumps’ comprises radial, semi-axial and axial pumps, but also side channel, peripheral and liquid-ring pumps whose working principles are fundamentally different from that of the first group.
Centrifugal pumps operate by applying a centrifugal force to fluids. These pumps use the centrifugal force imparted to the fluid by one or more rotating elements (impellers) to increase the kinetic and pressure energy of the fluid. These pumps produce a head and a flow by increasing the velocity of the liquid through the machine with the help of a rotating vane impeller.
The centrifugal pump creates an increase in pressure by transferring mechanical energy from the motor to the fluid through the rotating impeller. The fluid flows from the inlet to the impeller centre and out along its blades. The centrifugal force hereby increases the fluid velocity and consequently also the kinetic energy is transformed to pressure.
Centrifugal pump uses rotational kinetic energy to deliver the fluid. The rotational energy typically comes from an engine or electric motor. The fluid enters the pump impeller along or near to the rotating axis, accelerates in the propeller and flung out to the periphery by centrifugal force. In centrifugal pump the delivery is not constant and varies according to the outlet pressure. These pumps are not suitable for high pressure applications and are generally used for low-pressure and high-volume flow applications.
The performance data of a centrifugal pump are described by (i) the flow rate which is normally defined as the useful volume flow through the discharge nozzle, (ii) the specific work or the head, (iii) the power consumption at the pump coupling (brake horsepower), (iv) the efficiency at the pump coupling, (v) the net positive suction head (NPSH) at the pump inlet, or the net positive suction energy (NPSE), and (vi) the speed of the pump rotor.
These pumps are typically used in moderate to high flow applications with low-pressure head, and are very common in process industries. Centrifugal pumps in the narrow sense are designed for flow rates from 0.001 to 60 cum/sec, heads of 1 m to 5,000 m and speeds from a few hundred to about 30,000 revolutions per minute (rpm). These values are intended to illustrate the broad range of applications; they do not define the absolute limits of actual or future pumps. Most of the centrifugal pumps are not self-priming and the pump casing needs to be filled with liquid before the pump is started.
There are three types of centrifugal pumps—radial, mixed, and axial flow pumps. In the radial pumps, pressure is developed completely through a centrifugal force, while in axial pumps pressure is developed by the lift generated by the impeller. Mixed flow pumps develop flow through a centrifugal force and the impeller. Various types of centrifugal pumps are (i) end suction pump, (ii) in line pump, (iii) double suction pump, (iv) vertical multistage pump, (v) horizontal multistage pump, (vi) submersible pump, (vii) self-priming pump, (viii) axial flow pump, and (ix) regenerative pump etc.
Normally the pump is placed over the fluid storage tank. The pump creates a negative pressure at the inlet which causes fluid to be pushed up in the inlet pipe by atmospheric pressure. It results in the fluid lift in the pump suction. The maximum pump lift can be determined by atmospheric pressure and is given by pressure head. Theoretically, a pump lift of 8 m is possible but it is always lesser due to undesirable effects such as cavitation. The cavitation is the formation of vapour cavities in a liquid. The cavities can be small liquid-free zones (bubbles or voids) formed due to partial vaporization of the liquid. These are usually generated when a liquid is subjected to rapid changes of pressure and the pressure is relatively low. At higher pressure, the voids implode and can generate an intense shockwave. Therefore, the cavitation is always to be avoided. The cavitation can be reduced by maintaining lower flow velocity at the inlet and therefore the inlet pipes have larger diameter than the outlet pipes in a pump. The pump lift is to be as small as possible to decrease the cavitation and to increase the efficiency of the pump.
The pressure regulation is the process of reduction of high source pressure to a lower working pressure suitable for the application. It is an attempt to maintain the outlet pressure within acceptable limits. The pressure regulation is performed by using pressure regulator. The primary function of a pressure regulator is to match the fluid flow with demand. At the same time, the regulator must maintain the outlet pressure within certain acceptable limits.
Selection of a suitable pump
There is no recipe for the selection of the most suitable pump. Various factors would influence the selection, but in the end, economics has to be the decisive factor and this includes capital, maintenance, replacement, and energy costs. The followings need to be considered for selecting the pump.
- Flow rate and pressure head – The two types of pumps behave very differently regarding flow rate and the pressure head. While the centrifugal pump has varying flow depending on the system pressure or head, the positive displacement pump has more or less a constant flow regardless of the system pressure or head. Positive displacement pump normally gives more pressure than a centrifugal pump.
- Capacity and viscosity – One of the major differences between the pump types is the effect of viscosity on the capacity. In centrifugal pumps the flow is reduced when the viscosity is increased while in the positive displacement pumps the flow is increased with the increase of viscosity. Liquids with high viscosity fill the clearances of the positive displacement pumps causing a higher volumetric efficiency. Hence a positive displacement pump is better suited for the high viscosity application. On the other hand, a centrifugal pump becomes very efficient at even modest viscosity.
- Mechanical efficiency – Both the types of pumps behaves differently with respect to the mechanical efficiency of the pumps. In case of a positive displacement pump, change of the system pressure or head has little or no effect on the flow rate, while in a centrifugal pump, changing of the system pressure or head has a dramatic effect on the flow rate of the pump.
- Net positive suction head – Another consideration in selecting between the two types of the pumps is the net NPSH. In positive displacement pumps, NPSH varies as a function of flow determined by the speed. NPSH gets reduced in the positive displacement pump with the reduction of the speed. In a centrifugal pump, NPSH varies as function of flow determined by the pressure.