TYPES OF PUMP

 

TYPES OF PUMP

The mechanical device or arrangement by which fluid is caused to flow at increased pressure is known as a pump and the process of using a pump for this purpose is known as pumping. Irrigation pumps, in general, are driven either by engines or electric motors. Basically, the following four principles are involved in pumping fluid. Atmospheric pressure, centrifugal force, positive displacement and movement of columns of fluid caused by differences in specific gravity. 

1) DYNAMIC PUMP

 (a) Centrifugal Pump

 (b) Turbine Pump

     a. Deep well turbine 

     b. Submersible pump 

(c) Propeller Pump 

(d) Jet Pump 

(e) Air Lift Pump

2) Various positive-displacement pumps

The positive-displacement principle applies in these pumps:

• Rotary lobe pump
• Progressive cavity pump
• Rotary gear pump
• Piston pump
• Diaphragm pump
• Screw pump
• Gear pump
• Hydraulic pump
• Rotary vane pump
• Peristaltic pump
• Rope pump
• Flexible impeller pump

(a) Centrifugal Pump: 

Centrifugal pumps are used to transport fluids by the conversion of rotational kinetic energy to the hydrodynamic energy of the fluid flow. The rotational energy typically comes from an engine or electric motor. They are a sub-class of dynamic axisymmetric work-absorbing turbomachinery. 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 (casing), from which it exits.

Common uses include water, sewage, agriculture, petroleum and petrochemical pumping. Centrifugal pumps are often chosen for their high flow rate capabilities, abrasive solution compatibility, mixing potential, as well as their relatively simple engineering. A centrifugal fan is commonly used to implement an air handling unit or vacuum cleaner. The reverse function of the centrifugal pump is a water turbine converting potential energy of water pressure into mechanical rotational energy.

A centrifugal pump may be defined as one in which an impeller rotating inside a close – fitting case draws in the liquid at the centre and, by virtue of centrifugal force, throws out through an opening at the side of the casing. In operation, the pump is filled with water and the impeller rotated. The blades cause the liquid to rotate with the impeller and, in turn, import a high velocity to the water particles. The centrifugal force causes the water particles to be thrown from the impeller reduces pressure at the inlet, allowing more water to be drawn in through the suction pipe by atmospheric pressure. The liquid passes into the casing, where its high velocity is reduced and converted into pressure and the water is pumped out through the discharge pipe. The conversion of velocity energy into pressure energy is accomplished either in a Volute casing or in a Diffuser.

The centrifugal pumps are classified according to 

1. Type of energy conversion: 

     (a) Volute (b) Diffuser 

2. Number of stages 

     (a) Single stage (b) Multi stage 

3. Impeller types 

    (a) Single or double action (b) Open, semi-open or closed 

4. Axis of rotation 

    (a) Horizontal (b) Vertical 

5. Method of drive 

   (a) Direct connected (b) Geared (c) Belt or chain driven 

Common troubles and their remedies for a centrifugal pump are as follows:

1. Pump fails to deliver water: (i) Air leak in suction line, mainly in threaded connections are to be located with white lead (ii) Gaskets admitting air should be tightened (iii) Defective foot valve should be checked for its flap and replaced. 

2. Pump fails to develop sufficient pressure or capacity: (i) Pump speed should be checked and corrected (ii) Suction line and foot valve clogging to be checked (iii) Check the suction lift (iv) Check for worn out impeller.  

3. Pump takes for much power: (i) Speed may be high (ii) Head may be lower and pumping too much water (iii) Mechanical defects in installation. 4. Pump leaks excessively at the stuffing box: (i) Worn out packing or incorrectly inserted packing (ii) Worn out shaft to be renewed. 

5. Pump is noisy: (i) Too high suction lift (ii) Mechanical defects such as bent shaft, broken bearing etc. 

(b) Turbine Pumps: Turbine pumps consist of impellers placed below the water level and are driven by a vertical shaft rotated by an engine or motor placed at the ground level or under the water. 

1. Vertical Turbine Pump

Vertical Turbine Pump (or) Deep well Turbine Pump: is a vertical axis centrifugal or mixed flow type pump comprising of stages which accommodate rotating impellers and stationary bowls possessing guide vanes with the motor fixed on the ground level. The pump bowl is surrounded by a screen to keep coarse sand and gravel away from entering the pump. These pumps are adopted to high lifts and high efficiencies under optimum operating conditions. The pressure head developed depends on the diameter of the impeller and the speed at which it is rotated. Since the pressure head developed by a single impeller is not great, additional head is obtained by adding more bowl assemblies or stages. Turbine pumps could be water lubricated or oil lubricated. It is preferable to use oil lubricated pumps for wells giving fine sand along with water.

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2. Submersible Pump 

Submersible Pump is a turbine pump coupled to a submersible electric motor. A cable passing through the water supplies power to the motor. Both the pump and the motor are suspended and operate under the water, pumping water through the discharge column. The pump eliminates the long shaft and bearings that are necessary for a vertical turbine pump. Submersible pumps are cheaper than the vertical turbine pumps. Suitable for deep settings and also for crooked wells which are not perfectly vertical. The installation of the pup is easy and the initial cost of installation low. The repair of the submersible pumps, when they go out of order is not easy and require technical skill. Submersible pump requires little maintenance, after 6000 hours of operation or two years of service life, it may be necessary to with draw the pump from the bore hole and overhaul it. Selection of the submersible pump is mainly depending upon the bore well size, type, well discharge etc. 

3. Propeller Pumps: 

 The principal parts of the propeller pumps and method of operation are similar to the turbine pumps. The main difference is in design of the impellers, which give high discharges at low heads. Two types of impellers i.e. axial flow type and mixed flow type are used in this pump. In single stage pumps only one impeller is used and in multistage pumps more than one impeller is used. The selection of a propeller pump is done based on the characteristic curves compared with the well discharge and head.   

4. Jet Pumps: 

Jet Pumps Consist of a combination of a centrifugal pump and a jet mechanism or ejector. Jet pump is used when the suction lift of the centrifugal pump exceeds the permissible limits. A portion of the water from the centrifugal pump is passed through the drop pipe to the nozzle of the jet assembly. This water is forced through the throat opening of the diffuser, creating a vacuum which causes water to be drawn from the well. The water mixed with the boost water is carried up through the diffuser where the high velocity energy is converted into useful pressure energy, forcing the water up through the delivery pipe to the centrifugal pump.

5. Air-lift Pump

Air-lift Pump operates by the injection of compressed air directly into the water inside a discharge or eductor pipe at a point below the water level in the well. The injection of the air results in a mixture of air bubbles and water. This composite fluid is lighter in weight than water so that the heavier column of water around the pipe displaces the lighter mixture facing it upward and out of the discharge pipe. The piping assembly consists of a vertical discharge pipe called the educator pipe – and a smaller air pipe. Airlift pumping is extensively used in the development and preliminary testing and cleaning of tube wells. The advantages of air-lift pumps are simplicity, tube well need not be perfectly straight or vertical, and impure water will not damage the pump. The main disadvantage is its low efficiency about 30 per cent. 

Positive Displacement Pump

 In a positive displacement pump, the fluid is physically displaced by mechanical devices such as the plunger, piston, gears, cams, screws etc. In this type of pump, a vacuum is created in a chamber by some mechanical means and then water is drawn in this chamber. The volume of water thus drawn in the chamber is then shifted or displaced mechanically out of chamber,

Various positive-displacement pumpS

The positive-displacement principle applies in these pumps:

• Rotary lobe pump
• Progressive cavity pump
• Rotary gear pump
• Piston pump
• Diaphragm pump
• Screw pump
• Gear pump
• Hydraulic pump
• Rotary vane pump
• Peristaltic pump
• Rope pump
• Flexible impeller pump

A positive-displacement pump can be further classified according to the mechanism used to move the fluid:

• Rotary-type positive displacement: internal gear, screw, shuttle block, flexible vane or sliding vane, circumferential piston, flexible impeller, helical twisted roots or liquid-ring pumps

• Reciprocating-type positive displacement: piston pumps, plunger pumps or diaphragm pumps

• Linear-type positive displacement: rope pumps and chain pumps

(a) Reciprocating Pumps: 

In this type of pump, a piston or a plunger moves inside a closed cylinder. On the intake stroke, the suction valve remains open and allows water to come into the cylinder. The delivery valve remains closed during intake stroke. On the discharge stroke, the suction valve is closed and water is forced in delivery pipe through delivery pipe through delivery valve which opens during discharge stroke.

Reciprocating pumps move the fluid using one or more oscillating pistons, plungers, or membranes (diaphragms), while valves restrict fluid motion to the desired direction. In order for suction to take place, the pump must first pull the plunger in an outward motion to decrease pressure in the chamber. Once the plunger pushes back, it will increase the pressure chamber and the inward pressure of the plunger will then open the discharge valve and release the fluid into the delivery pipe at a high velocity.

Pumps in this category range from simplex, with one cylinder, to in some cases quad (four) cylinders, or more. Many reciprocating-type pumps are duplex (two) or triplex (three) cylinder. They can be either single-acting with suction during one direction of piston motion and discharge on the other, or double-acting with suction and discharge in both directions. The pumps can be powered manually, by air or steam, or by a belt driven by an engine. This type of pump was used extensively in the 19th century—in the early days of steam propulsion—as boiler feed water pumps. Now reciprocating pumps typically pump highly viscous fluids like concrete and heavy oils, and serve in special applications that demand low flow rates against high resistance. Reciprocating hand pumps were widely used to pump water from wells. Common bicycle pumps and foot pumps for inflation use reciprocating action.

These positive-displacement pumps have an expanding cavity on the suction side and a decreasing cavity on the discharge side. Liquid flows into the pumps as the cavity on the suction side expands and the liquid flows out of the discharge as the cavity collapses. The volume is constant given each cycle of operation and the pump's volumetric efficiency can be achieved through routine maintenance and inspection of its valves.

Typical reciprocating pumps are:

• Plunger pumps – a reciprocating plunger pushes the fluid through one or two open valves, closed by suction on the way back.
• Diaphragm pumps – similar to plunger pumps, where the plunger pressurizes hydraulic oil which is used to flex a diaphragm in the pumping cylinder. Diaphragm valves are used to pump hazardous and toxic fluids.
• Piston pumps displacement pumps – usually simple devices for pumping small amounts of liquid or gel manually. The common hand soap dispenser is such a pump.
• Radial piston pumps - a form of hydraulic pump where pistons extend in a radial direction.

(b) Rotary Pumps: In this type of pump, the reciprocating motion is substituted by the rotary motion. The rotary motion is achieved by cams or by gears. There are two cams or gears which fit with each other. They rotate in opposite directions. The water enters through the suction pipe and it is trapped between cams or teeth of gears and casing. It is then thrown with force into the discharge pipe. This type of pump is useful for moderate heads and small discharges not greater than 40 litres per second.  

Rotary positive-displacement pump

Rotary vane pump

These pumps move fluid using a rotating mechanism that creates a vacuum that captures and draws in the liquid.

Advantages: Rotary pumps are very efficient , because they can handle highly viscous fluids with higher flow rates as viscosity increases.

Drawbacks: The nature of the pump requires very close clearances between the rotating pump and the outer edge, making it rotate at a slow, steady speed. If rotary pumps are operated at high speeds, the fluids cause erosion, which eventually causes enlarged clearances that liquid can pass through, which reduces efficiency.

Rotary positive-displacement pumps fall into 5 main types:

• Gear pumps – a simple type of rotary pump where the liquid is pushed between two gears
• Screw pumps – the shape of the internals of this pump is usually two screws turning against each other to pump the liquid
• Rotary vane pumps
• Hollow disk pumps (also known as eccentric disc pumps or Hollow rotary disc pumps), similar to scroll compressors, these have a cylindrical rotor encased in a circular housing. As the rotor orbits and rotates to some degree, it traps fluid between the rotor and the casing, drawing the fluid through the pump. It is used for highly viscous fluids like petroleum-derived products, and it can also support high pressures of up to 290 psi.
• Vibratory pumps or vibration pumps are similar to linear compressors, having the same operating principle. They work by using a spring-loaded piston with an electromagnet connected to AC current through a diode. The spring-loaded piston is the only moving part, and it is placed in the center of the electromagnet. During the positive cycle of the AC current, the diode allows energy to pass through the electromagnet, generating a magnetic field that moves the piston backwards, compressing the spring, and generating suction. During the negative cycle of the AC current, the diode blocks current flow to the electromagnet, letting the spring uncompress, moving the piston forward, and pumping the fluid and generating pressure, like a reciprocating pump. Due to its low cost, it is widely used in inexpensive espresso machines.

1) Gear pumps

Gear pumps are also widely used in chemical installations to pump high viscosity fluids. There are two main variations: external gear pumps which use two external spur gears, and internal gear pumps which use an external and an internal spur gears (internal spur gear teeth face inwards, see below). Gear pumps are positive displacement (or fixed displacement), meaning they pump a constant amount of fluid for each revolution. Some gear pumps are designed to function as either a motor or a pump.

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. The mechanical clearances are small— in the order of 10 μm. The tight clearances, along with the speed of rotation, effectively prevent the fluid from leaking backwards.

The rigid design of the gears and houses allow for very high pressures and the ability to pump highly viscous fluids.

2) screw pump

A screw pump, also known as a water screw, is a positive-displacement (PD) pump that use one or several screws to move fluids or solids along the screw(s) axis. In its simplest form (the Archimedes' screw pump), a single screw rotates in a cylindrical cavity, thereby moving the material along the screw's spindle. This ancient construction is still used in many low-tech applications, such as irrigation systems and in agricultural machinery for transporting grain and other solids.

Development of the screw pump has led to a variety of multiple-axis technologies where carefully crafted screws rotate in opposite directions or remains stationary within a cavity. The cavity can be profiled, thereby creating cavities where the pumped material is "trapped".

In offshore and marine installations, a three-spindle screw pump is often used to pump high-pressure viscous fluids. Three screws drive the pumped liquid forth in a closed chamber. As the screws rotate in opposite directions, the pumped liquid moves along the screws' spindles.

Three-spindle screw pumps are used for transport of viscous fluids with lubricating properties. They are suited for a variety of applications such as fuel-injection, oil burners, boosting, hydraulics, fuel, lubrication, circulating, feed and so on.

3) Roots-type blower

The Roots-type blower is a positive displacement lobe pump which operates by pumping a fluid with a pair of meshing lobes resembling a set of stretched gears. Fluid is trapped in pockets surrounding the lobes and carried from the intake side to the exhaust. The most common application of the Roots-type blower has been as the induction device on two-stroke diesel engines, such as those produced by Detroit Diesel and Electro-Motive Diesel. Roots-type blowers are also used to supercharge Otto cycle engines, with the blower being driven from the engine's crankshaft via a toothed or V-belt, a roller chain or a gear train.

The Roots-type blower is simple and widely used. It can be more effective than alternative superchargers at developing positive intake manifold pressure (i.e., above atmospheric pressure) at low engine speeds, making it a popular choice for passenger automobile applications. Peak torque can be achieved by about 2000 rpm. Unlike the basic illustration, most modern Roots-type superchargers incorporate three-lobe or four-lobe rotors; this allows the lobes to have a slight twist along the rotor axes, which reduces pulsing in the input and output 

4) plunger pump

A plunger pump is a type of positive displacement pump where the high-pressure seal is stationary and a smooth cylindrical plunger slides through the seal. This makes them different from piston pumps and allows them to be used at higher pressures. This type of pump is often used to transfer municipal and industrial sewage

Rotary piston and plunger pumps use a crank mechanism to create a reciprocating motion along an axis, which then builds pressure in a cylinder or working barrel to force gas or fluid through the pump. The pressure in the chamber actuates the valves at both the suction and discharge points. Plunger pumps are used in applications that could range from 70 to 2,070 bar (1,000 to 30,000 psi). Piston pumps are used in lower pressure applications. The volume of the fluid discharged is equal to the area of the plunger or piston, multiplied by its stroke length. The overall capacity of the piston pumps and plunger pumps can be calculated with the area of the piston or plunger, the stroke length, the number of pistons or plungers and the speed of the drive. The power needed from the drive is proportional to the pressure and capacity of the pump

General maintenance of pumps for maximum working efficiency 

1. The suction lift should be periodically checked and it should be within the permissible limits. 

2. The gland packing in the pump should be checked and replaced if necessary. The water should drip through the packing at a rate of 15 to 30 drops minute. 

3. Periodical inspection of impeller of the pump is necessary for wear. 

4. The rpm of the prime mover should be at the rated valve. 

5. The alignment of the pimp and motor shaft should be checked. 

Pumping power

The power imparted into a fluid increases the energy of the fluid per unit volume. Thus the power relationship is between the conversion of the mechanical energy of the pump mechanism and the fluid elements within the pump. In general, this is governed by a series of simultaneous differential equations, known as the Navier–Stokes equations. However a more simple equation relating only the different energies in the fluid, known as Bernoulli's equation can be used. Hence the power, P, required by the pump:

${\displaystyle P={\frac {\Delta pQ}{\eta }}}$

where Δp is the change in total pressure between the inlet and outlet (in Pa), and Q, the volume flow-rate of the fluid is given in m³/s. The total pressure may have gravitational, static pressure and kinetic energy components; i.e. energy is distributed between change in the fluid's gravitational potential energy (going up or down hill), change in velocity, or change in static pressure. η is the pump efficiency, and may be given by the manufacturer's information, such as in the form of a pump curve, and is typically derived from either fluid dynamics simulation (i.e. solutions to the Navier–Stokes for the particular pump geometry), or by testing. The efficiency of the pump depends upon the pump's configuration and operating conditions (such as rotational speed, fluid density and viscosity etc.)

${\displaystyle \Delta P={(v_{2}^{2}-v_{1}^{2}) \over 2}+\Delta zg+{\Delta p_{\mathrm {static} } \over \rho }}$

For a typical "pumping" configuration, the work is imparted on the fluid, and is thus positive. For the fluid imparting the work on the pump (i.e. a turbine), the work is negative. Power required to drive the pump is determined by dividing the output power by the pump efficiency. Furthermore, this definition encompasses pumps with no moving parts, such as a siphon.

Efficiency

p efficiency is defined as the ratio of the power imparted on the fluid by the pump in relation to the power supplied to drive the pump. Its value is not fixed for a given pump, efficiency is a function of the discharge and therefore also operating head. For centrifugal pumps, the efficiency tends to increase with flow rate up to a point midway through the operating range (peak efficiency or Best Efficiency Point (BEP) ) and then declines as flow rates rise further. Pump performance data such as this is usually supplied by the manufacturer before pump selection. Pump efficiencies tend to decline over time due to wear (e.g. increasing clearances as impellers reduce in size).

When a system includes a centrifugal pump, an important design issue is matching the head loss-flow characteristic with the pump so that it operates at or close to the point of its maximum efficiency.

Pump efficiency is an important aspect and pumps should be regularly tested. Thermodynamic pump testing is one method.

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