Due to the wide range of applications across sectors for which they are used, pumps are available in a plethora of sizes and functions. Pumps range from very large to very small, from those handling gas to handling liquid, and from those suited to high volumes to others better for low volumes.
Pumps can be generally classified into either positive displacement pumps or dynamic pumps based on their operating principle. Displacement pumps can be further subclassified as rotary and reciprocating pumps, while dynamic pumps can be centrifugal or special effect pumps.
Despite the fact that all pumps can handle any type of liquid, the choice is based on two key parameters – efficiency and economy. Displacement pumps are generally more efficient than centrifugal pumps, but are costlier than the latter. Therefore, the benefit of higher efficiency tends to be offset by increased maintenance costs. Centrifugal pumps are generally the most economical followed by rotary and reciprocating pumps.
Dynamic pumps are characterised by their mode of operation. They have a rotating impeller that converts kinetic energy into pressure or velocity, which is needed to pump the fluid.
These pumps are of two types – rotary and special pumps. Of the rotary family of pumps, centrifugal pumps are the most commonly used for pumping water in industrial applications. Typically, more than 75 per cent of the pumps installed in an industry are centrifugal.
Centrifugal pumps, which are also known as roto-dynamic pumps, have two main components – one, a rotating component comprising an impeller and a shaft, and two, a stationary component consisting of a casing, casing cover and bearings. These pumps produce a head and a flow by increasing the velocity of the liquid through the machine. This is achieved with the help of a rotating vane impeller. Here, the liquid is forced into an impeller either by atmospheric pressure or by creating artificial pressure. The vanes of the impeller pass kinetic energy to the liquid, thereby causing the liquid to rotate. The liquid leaves the impeller at a high velocity, and this impeller, which in turn is surrounded by a volute casing or a stationary diffuser ring, converts kinetic energy into pressure energy. Centrifugal pumps include radial, axial as well as mixed-flow units. Special-effect pumps, on the other hand, are largely used for specialised conditions at industrial sites.
Positive displacement pumps
A positive displacement pump operates by alternately filling a cavity and then displacing a given volume of liquid. It delivers a constant volume of liquid in each cycle, independent of the discharge pressure or head. Pumping element designs include gears, lobes, rotary pistons, vanes, screws and hoses. These pumps are useful for industries like chemical processing, liquid delivery, marine, biotechnology, pharmaceuticals, food, dairy, and beverage processing.
The advantages of these pumps are that they are versatile, have compact designs and are capable of generating a continuous flow regardless of differential pressure. They also have high viscosity performance. Positive displacement pumps are capable of generating high pressures, which can be limited by the regulating or safety features installed in delivery/ discharge pipes. However, they are only capable of low discharge flows. This is primarily because of the limited volume swept by the displacement element as well as the element’s speed. Moreover, these pumps are mechanically complex and therefore tend to cost more. They are widely used for pumping viscous fluids.
Positive displacement pumps are classified into reciprocating and rotary depending upon the mode of displacement. In reciprocating pumps, the displacement is by reciprocation of a piston plunger. These are used only for pumping viscous liquids and in oil wells. In rotary pumps, the displacement is by the rotary action of a gear, cam or vanes in a chamber of the diaphragm in a fixed casing. Rotary pumps are further classified as internal gear, external gear, lobe and slide vane, etc. These pumps are used for special services in particular conditions existing at industrial sites.
Choice of pumps
One of the key factors that determines the choice of a pump is its size. The use of multiple pumps instead of a single pump helps contain energy consumption. For instance, during lower-demand periods, one small pump can be operated while two pumps running in parallel can cater to higher-demand periods. This averts the need to throttle down a large pump during periods of low demand.
It has been seen that oversized pumps account for the single largest source of energy wastage in industries. An oversized pump operates at a higher head or flow than required and operates at a lower efficiency when throttled down. Therefore, the feasibility of operating a large pump should be established at the onset of an operation, or else it should be replaced with a smaller, more efficient pump.
Notably, multiple pumps are commonly used in chilled water systems for boiler feeds and to meet cyclic production demands. This is one of the most effective methods of energy conservation in pumping systems and leads to large financial savings. Meanwhile, multiple pumps can be used in parallel to provide a wide-ranging flow at a relatively constant head or in series to provide a range of pressures at relatively constant flow.
In addition, the user has to effectively make a choice from among a multitude of positive displacement or centrifugal pumps. The latter is often considered the best choice for a number of applications. However, these pumps generate different amounts of flow at different pressures. Positive displacement pumps are generally more efficient than centrifugal pumps and should be considered whenever possible in low- flow, high-head applications and while handling viscous fluids. Further, other factors such as maintenance and installation costs considerably affect pump selection.
Emergence of new and efficient systems
Over the years, manufacturers have offered a variety of efficient high capacity pumps in the market. Pump dimensions and technology have improved manyfold. Recent innovations in this field include solar-based pumping systems, lowest life-cycle cost pumps, high capacity concrete volute pumps and vertical turbine pumps.
Besides, there have also been technological advancements for lowering pumping requirements. These include increasing the net positive suction head available to a pump, allowing a higher cooling water temperature rise across heat exchangers to reduce cooling water pumping requirements, and heating a viscous process fluid prior to pumping to reduce viscosity.
Further, variable-speed operation has been deployed as a measure to improve efficiency in pumping systems. In centrifugal pumps, variable speed operation can match the output of the pump to the system requirement. This results in saving of energy which is usually lost when pressure drops across a control valve or as excess flow in bypass systems. Thus, to obtain additional energy savings, variable-speed operations should be adopted in pumping systems where the head and/or flow requirements vary and are frequently below 75 per cent of maximum design conditions.
Applying energy management best practices and purchasing energy-efficient equipment can help achieve significant energy and cost savings in pump systems.
Despite several challenges, Indian pump manufacturers are continuously improving productivity, quality and service. The industry has demonstrated that it has the capability of producing a wide variety of efficient pumps matching international standards and requirements. Further, the market size for energy-efficient pumps is constantly increasing as a result of the government’s push towards the use of star-rated pumps.