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Agriculture being the most common activity in India, the pump on the farmer’s well is a commonly known pump. It is often a motor-driven centrifugal pump. But before electric power could reach the nook and corner of the country, farmers were using an engine-driven centrifugal pump. Even today there are many drought-prone areas and in severe summer, village folk have to trek miles everyday to get a pale of water. To alleviate the hardships of people in places, where electricity has still not reached and supply of diesel also cannot be managed, hand pumps are installed on tube wells. Pumping water from inaccessible depths was in fact the major motivation, which prompted the invention of pumps. Cities have piped water supply. But the municipal corporations would supply water only to underground storage tanks of housing societies. Supply to individual households is then managed by the housing societies by pumping water from the underground storage tanks to overhead tank. Thus everybody whether living in a village or in a city is familiar with pumps. Petrol or diesel is filled in cars, trucks, etc. again by pumps. These pumps are often gear pumps.
In hand pumps and gear pumps there is very little running clearance. So, there is nil or negligible slippage of the liquid displaced by the pumping element; by the piston of the hand pump or by the gear-train of gear pump. These pumps are hence classified as displacement pumps or positive displacement pumps. In centrifugal pumps there is distinct running clearance between the rotating impeller and the casing. Part of the liquid impelled by the impeller would slip back to pump’s suction. So displacement in centrifugal pumps is not positive. Pumps having impellers to impel the liquid are called as impeller pumps. The piston of the hand pump has a reciprocating motion, whereas the gears of the gear pump have a rotary motion. Pumps are hence either rotodynamic or reciprocating. Hand pumps are installed in tube wells are installed to work vertically. Engine-driven centrifugal pumps run with axis of the shaft horizontal. Pumps are hence vertical or horizontal. Coolant pumps in automobiles or pumps on ocean-going ships are on unsteady or moving foundation. Most other installations of pumps have rigid foundation. There would thus be many ways of classifying pumps. By a broad classification, pumps are of two types- impeller pumps and displacement pumps. Impeller pumps are primarily centrifugal. They are further of two broad types as mentioned in Ans.7, the commonplace centrifugal pumps and the regenerative type centrifugal pumps. Based on specific speed, the commonplace centrifugal pumps are further classified as radial flow, mixed flow and axial flow centrifugal pumps. In terms of number of stages (impellers) in a pump assembly, pumps are single stage or multi-stage. There are also axially split casing pumps, which most commonly are double suction, i.e. the impeller has two suction passages bifurcated within the casing. In terms of inclination of the axis, the pumps would be classified also as horizontal, vertical or inclined pumps. The displacement pumps are of two broad types; rotary and reciprocating. Rotary pumps are of various types, the gear pumps – external gear and internal gear pumps, screw pumps, single screw pumps, twin screw pumps multiple screw (or rotor) pumps, sliding vane pumps, flexible vane pumps, peristaltic pumps lobe p umps, shuttle block pumps, etc. The single screw pumps are either Archimaedian screw pumps or helical rotor, progressive cavity pumps, with the stator of elastomeric material. The reciprocating positive displacement pumps are of two types, the piston (or plunger) pumps and diaphragm pumps. The piston or plunger pumps would have one piston (simplex), two pistons (duplex), multiple number of pistons (triplex, quadruplex, etc.)
Because of better running clearances than in positive displacement pumps, centrifugal pumps can run at high speeds and yet they will have less wear and tear. Because of high running speeds, they also become compact in design. They can handle very high flows. Better running clearances also make machining, assembly and manufacturing readily amenable to cost-effective economics of scales of production.
Centrifugal pumps do have limitations i) Highly viscous and shear-sensitive liquids ii) Liquids with delicate solids iii) Liquids with very high percentage concentration of solids iv) Multi-phase flows, especially with entrainment of air or gas v) Metering and dosing duties, where precise, yet wide-range regulation of flow-rate is required vi) When very high pressures are required to be developed with small flow-rates vii) Flow-rates required are miniscule viii) Fail-safe self-priming capability is required ix) Certain times, even if a given application is within the capability of either a centrifugal pump or a positive displacement pump, a positive displacement pump may prove more energy-efficient than a centrifugal pump. A detailed Life Cycle Cost Analysis would be warranted in such cases.
A commonplace installation of a pump is to draw water from a well or a suction sump and lift it to a higher level, say into an overhead reservoir. When the pump is mounted above the level in the suction sump, it works against a suction lift. Static Suction Lift is the difference in elevations between the two levels – the level of liquid in suction sump and level of eye of impeller.
A relevant question is “What maximum static suction lift can pumps handle?” When pump starts pumping after priming it throws away all liquid at the eye of impeller. Thus vacuum is developed at the eye of the impeller. Since the liquid in the suction sump, when open to atmosphere, as is the most common case, is at atmospheric pressure, the atmospheric pressure forces the liquid to the low-pressure vacuum at the impeller eye. The maximum static suction lift then is equal to the column of liquid, which atmospheric pressure can make stand. At mean sea level, atmospheric pressure can make a column of mercury stand to 760mm. Since specific gravity of mercury is 13.6, the column of water, which atmospheric pressure can make stand would be 13.6*0,76 = 10.336m (33.91 ft). At altitudes above mean sea level (MSL), the atmospheric pressure itself reduces. In turn the liquid column, which the less atmospheric pressure can make stand, will be less, reducing approximately at 1m less for every 1000m increase in altitude. So, at a place at 500m MSL, atmospheric pressure will make 9.836m of water column (mWC = metres of water column; mLC = metres of liquid column) stand. ‘mLC’ is inversely proportional to the specific gravity of the liquid. For maximum suction lift one ought to leave margins for losses in hydraulic friction at the strainer, the foot valve, suction piping, bend(s), eccentric taper, if any, etc. What a gauge or manometer at the pump suction will show is called as the manometric suction lift. Maximum suction lift thus depends upon • the specific gravity of liquid to be handled, • the altitude at mean sea level and • the losses due to hydraulic friction in the elements of s uction piping.
Centrifugal Pumps with suction lift need to be primed. So they also need a foot valve and inherent losses through foot valve and strainer. For large pumps, priming the pump becomes quite some exercise. This can be made easier by use of vacuum pump for priming. Construction contractors face water percolating into the pit excavated at the site. To dewater the pit, they need to pump out the water. And the pump will have to be primed. To facilitate this need for priming at every shifting to a new pit, contractors’ pumps are made self-priming by designing a chamber integral with the pump casing. The chamber gets filled during pumping and the filled chamber serves the purpose of relieving the need for priming at every shifting. So, such types of centrifugal pumps (see IS-8412) with an integral priming chamber are called self-priming pumps or contractor pumps. The concept of priming chamber can as well be implemented by providing the chamber as an accessory to the pump. Priming chambers are useful, where liquids are hazardous for manual handling to do priming. Small domestic pumps are often regenerative turbine type centrifugal pumps (IS-8472). These pumps have some self-priming capability inherent by design. They are made mostly in small sizes and can hence handle only small flows. Also these pumps have poor efficiency and hence do not make energy-efficient pumping.
There are these two difficulties with pumps installed with a suction lift - the limitation of maximum suction lift and the need for priming. One logical approach is to submerge the pump. To keep the bearings and driver safely away from the liquid, the pump has to be connected to the bearing housing and driver with a long pump shaft and maybe, some intermediate shafts and intermediate support bushes. One prominent version of submerged pumps is with volute casing and a separate delivery pipe. In chemical industries such pumps are often installed, suspended into process tanks. Another prominent version of submerged pumps is Vertical Turbine Pumps (IS-1710). In these pumps, the pumps have diffuser casing(s) or bowl(s) instead of volute casing. So, the delivery flow rises along the pump shaft and is taken out from the discharge bend, which also is a part of the pump. If liquid to be handled is not a clear liquid and is likely to be offensive to the intermediate bush supports and for lubrication to the bushes, the shaft and the intermediate bushes are provided with a protection tube. Vertical turbine pumps are made in fairly large sizes, handling flows of the order of 20,000 m3/h, with delivery nozzle size of the order of 2200mm and with drive ratings of the order of 3 MW.
Dismantling the vertical turbine pumps or vertically submerged volute casing pumps for maintenance and/or overhaul becomes quite some exercise. Development of submersible motors eliminated the need for long shafting, facilitating also exploring deep ground water and facilitating also drilling only tube wells instead of bore wells. Submersible motors also made drainage and dewatering duties free of worries of failure of motors due to ingress of water into the motor. Pumps with submersible motors are also used in sewage handling. Monoset pumps with horizontal submersible motors present good option to the conventional surface monosets.
In a pump the rotating unit is the sub-assembly, which has the most wear-prone components and hence needs periodic attention.
In back pull out pumps, a spacer coupling between the motor and the pump helps to take out the rotating unit of the pump without disturbing either the motor or the suction and delivery piping. This demands the casing to be independently supported. Horizontal, end suction, single-stage centrifugal pumps (IS-13518 / ISO-2858) specifies dimensions and ratings for such back pull out pumps.
In these pumps removing away the top half of the casing facilitates complete inspection of the rotating unit. Because these pumps are most commonly installed with shaft axis horizontal, they are also called as Horizontal Split Casing (HSC) pumps. The shaft runs through from one end to another and has bearing supports at both ends and also two shaft sealing units. One common version of these pumps is double suction pumps. The impeller is virtually two impellers made integral back to back. Axial thrust is inherently balanced by this type of impeller design. These are also made in multi-stage version, incorporating crossovers leading outlet of one casing to inlet of another impeller.
These are most commonly made for vertical mounting and are popularly used as in-line boosters. Vertical mounting lends a space-saving advantage. Small units are also used, horizontally mounted on vertical pipelines, typically as heat pumps in cold climate areas. They are made both in single stage, volute casing pattern and in multistage construction. In vertical single stage pumps, they are made as moonsets, so that the rotating unit can be taken out along with the motor. This makes for ease of maintenance.
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