Pumps Updated

PUMPS INTRODUCTION Pumps are fluid motive devices that increase the mechanical energy of liquid, increasing its velocity or elevation or all three. CLASSIFICATION

Pumps are classified as followed

DYNAMIC In these the energy is continuously added to increase the fluid velocities within the machine to values in excess of those occurring at the discharge such that subsequent velocity reduction within or beyond the pump produces a pressure increase.

DISPLACEMENT In the energy is periodically added by application of force to one or more movable boundaries of any desired number of enclosed, fluid containing volumes, resulting in a direct increase in pressure.

CENTRIFUGAL PUMPS Centrifugal pumps convert kinetic energy (created by centrifugal force) into pressure energy. However, the propeller and turbine types also combine this principle with that of mechanical impulse (generated by mechanical force of the impeller blade acting directly on the liquid). A pump which combines the uses of centrifugal force with mechanical impulse to produce an increase in pressure is the axial-flow pump. In this pump the fluid travels roughly parallel to the shaft through a series of alternately rotating and stationary radial blades having airfoil cross sections. The fluid is accelerated in the axial direction by mechanical impulses from the rotating blades; concurrently, a positive-pressure gradient in the radial direction is established in each stage by centrifugal force. The net pressure rise per stage results from both effects.

PRINCIPLE OF OPERATION The centrifugal pump works by centrifugal forces. Centrifugal force is the force in a rotating body which makes particles move away from the center of rotation. This is illustrated in Figure (14) which shows a stone being whirled in a circle at the end of a piece of string. Because of the circular motion, there is a centrifugal force acting on the stone, tending to push it outwards, and at any moment the stones direction is at a tangent to the circle (i.e. the direction of movement is such that if the stone were free then it would not pass through the circle but just brush against its edge). If the string was suddenly broken then the stone would, fly off in this tangential direction to the circle.

Figure.14 In the centrifugal pump an impeller containing curved vanes (or a similar construction) receives liquid feed through a central hole or eye. The particles of fluid move outward from the eye of the impeller towards the outer rim of impeller (See figure.15).

The rotating vanes of the impeller cause liquid to move in a circular path, the rotation generating centrifugal force. The centrifugal force propels particles of liquid outward through the rotating vanes as shown.

Figure (15) impeller In any circle which is rotating, a point at the circumference travels a greater distance in a given time than a point nearer the center does in the same period of time. The distance traveled around a circular path in a given length of time is called the tangential velocity. This means that tangential velocity is greater at the rim of the impeller, than it is at the eye of the impeller. It also means that the greater the diameter of the impeller, the higher the tangential velocity for the same r.p.m of the impeller. The suction of liquid outwards from the eye of the impeller creates a low pressure area at that position and this in turn causes fresh liquid to flow from the pump inlet into the impeller eye. The resultant direction in which the liquid leaves the impeller vane is shown on the right-hand side of figure (16) the liquid experiences two velocities, one in the tangential direction of vane tip, and another in a direction following the vane curvature. These two directions complete and the liquid take the resultant direction shown.

Figure –16 When the liquid leaves the rim of the impeller it is traveling at a high velocity. The shape of the casing into which the liquid flows is designed in such a manner that the high velocity liquid slows down and eventually leaves the pump outlet at a lower velocity than that at which it left the rim of the impeller. This means that the liquid has given up some of its kinetic energy, and what in fact happens is that the kinetic energy given up is converted into static pressure energy (energy is neither created nor destroyed, but is converted. The suction at the eye of the impeller and the conversion of kinetic energy to static pressure energy explain why a vacuum may exist at the inlet to a centrifugal pump, and, why the static pressure at the outlet of a centrifugal pump is much greater than the static pressure at any point along the inlet line.

TYPES OF IMPELLERS Some of the more common kinds of impellers are shown in figure (17). The impeller vanes are curved to ensure a smooth flow of liquid. Both turbulence and internal circulation are reduced as the number of vanes is increased. This circulation decreases the developed head. To a large extent, the characteristics of a centrifugal pump depend on the angle of the tip of the blades.

Figure – 17 Closed impellers generate head between the two walls of the rotating impeller. However, semi open impellers generate head between the one wall of the rotating impeller and one stationary wall of the casing. Open impellers generate head between two stationary walls of the casing. Closed impellers have the following advantages: their maintenance is low, their wearing surfaces are relatively uncritical, and their original efficiencies are maintained over most of their lives. Open and semi open impellers require close clearances between the rotating vanes and the corresponding wall of the casing. Wear results in increased clearances, greater leakage losses and lower efficiencies. Open impellers are used for pumping liquids containing suspended solids. Many pumps, particularly those of large capacity, have double-suction impellers. This means that they have an inlet on either side. These impellers impart no appreciable thrust to the shaft and are commonly used in pumps which have a horizontally split case. Other impellers are single suction, having only one inlet. These are used in vertically split pumps. A single suction impeller is always subjected to axial thrust.

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In double suction pump, liquid is fed from identical suction chambers located at each side of the impeller. This design substantially eliminates hydraulic imbalance which is always present with a single suction impeller. Thus double suction pumps can be used for higher operating pressures than single suction pumps. Double suction pumps also allow lower liquid velocities at the eye of the impeller. This results in relatively low NPSH requirements.

WEAR RINGS In a centrifugal pump there are two distinct pressure zones, low pressure at the center of the impeller and high pressure at it circumference. To prevent high pressure liquid leaking into the low-pressure zone the minimum clearance possible has to exist between the rotating impeller and the stationary casing. With close clearances a certain amount of wear will take place and as the gap between the impeller and the casing increases so will the amount of liquid being re-circulated to the suction. The efficiency of the pumps will be reduced and in order to regain it, the gap has to be reduced again. To avoid the replacement of impeller, casing, or both it is normal practice to install separate wear rings on the impeller and in the casing at the points of contact (see fig. 19). When wear takes place, which seriously affects the pump’s efficiency, the rings can be replaced that are relatively cheap.

Figure – 19 Wear Ring Mounting

BEARINGS: The shaft which is fixed to the impeller, rides on the bearings. It is connected to the motor with a coupling. As you know that centrifugal pumps operate at high speeds of rotation and can involve equipment of some considerable weight. The effect of the above is the development of radial loading due to the weight of the equipment. With thrust loading being developed due to operation and type of pump. To reduce the frictional, wearing & tearing tendencies of rotating equipment, bearings, have been developed to support radial loads and work against thrust (axial) loads as they develop.

TYPES OF CENTRIFUGAL PUMPS

1.

VOLUTE PUMP

The volute pump is by far the most common type of centrifugal pumps. The liquid leaves the impeller into a progressively widening spiral casing called the volute as shown in fig. (20). This design facilitates the conversion of the liquid velocity to pressure as it flows from the impeller to the discharge line.

Figure – 20 Volute type Pumps

2.

DIFFUSER PUMP

In diffuser pumps, after the liquid has left the impeller, it is passed through a ring of fixed diffuser vanes. This provides a more controlled flow and allows a more efficient conversion of velocity head into pressure head. The change from high velocity to pressure takes place gradually. This eliminates shock losses. Thus diffuser pumps have high efficiencies. Some of the larger pumps have efficiencies over 90 percent. See figure (21).

Figure –.21: Section of Diffuser Pump

Diffuser type centrifugal pumps are commonly used for high head application. As with volute pumps, they are available in more than one stage.

3.

MULTI STAGE PUMP

To obtain a high discharge pressure, a multiple impeller pump is used with two or more impellers mounted on one shaft. The impellers are linked in series so that the liquid which leaves the first impeller is guided into the inlet of the next impeller and so on to the last impeller from which the liquid leaves through the discharge line. See figure (22) below;

CAPACITY It is expressed in terms of volumetric flow rate.

HEAD Height of fluid column equivalent to the total pressure differential (under adiabatic conditions) measured immediately before and after the device.

STATIC SUCTION LIFT Vertical distance from the liquid supply level to the pump centre line. This is valid only when the pump is above the liquid level

STATIC SUCTION HEAD Vertical distance from the pump center line to the liquid supply level

FRICTION HEAD Pressure head required to overcome the resistance to flow in pipes

NET POSITIVE SUCTION HEAD (NPSH): A pump can only effectively transport liquid if there is adequate pressure on the suction side to force the fluid into the inlet nozzle of the pump casing. Any pump, which is, for instance, lifting water from an open pit below it has the liquid pushed into the impeller or inlet valves by the atmosphere pressure. We know that a liquid may exert a vapor pressure above its free surface in a container. For example, the vapor pressure of water at 71 oC is 4.7 psia. If the pressure at the suction to a pump pumping water at 71 oC falls below 4.7 psia, then the water will vaporize (i.e. vapor will be formed) and the pump will fill with vapor and not with liquid. The pump is “vapor locked”. To prevent this happening, the absolute pressure at the pump suction must be greater than the vapor pressure of the water at the temperature at which it is being pumped. This means that the absolute pressure at the suction end of the pump in the case mentioned above should not be allowed to fall as low as 4.7 psia. The net positive suction head is the pressure above liquid vapor pressure at pump suction, converted to a head of liquid. For example, let’s suppose that the

absolute pressure at the suction of pump transferring water at 71 oC is 8.7 psia. Now the vapor pressure of water at that temperature is found to be 4.7 psia. Therefore, the pressure at the pump suction is 8.7 – 4.7 which equals 4.0 psia above the vapor pressure of the water. This pressure difference can be converted to a head of water of approximately 9 feet. The net positive suction head is therefore 9 feet. The net positive suction head is usually expressed as N.P.S.H.

REQUIRED N.P.S.H. It is a function of the pump design and is the absolute pressure needed to overcome frictional and other losses occurring in the pump suction nozzle and inlet pumping elements. This will give the minimum liquid head required to get liquid into the pump without vapor being formed. The required value of N.P.S.H. increases with pump capacity, impeller speed and discharge pressure.

AVAILABLE N.P.S.H. It is a term expressing the absolute pressure value available at the pump suction valve after accounting for, the pressure on the liquid surface, the vapor pressure exerted by the liquid at that temperature, system suction frictional losses, the static lift involved and the pressure loss due to the change in the liquids velocity head. If the N.P.S.H. available is less than the N.P.S.H. required then the pump will not operate correctly. If this does occur then the phenomena is known as cavitation. To avoid this pump manufacturers always build a safety factor into the design of the system.

CAVITATION When the NPSH is below the required minimum as specified by the manufacturer, then vapor bubbles start forming in the pump suction (whether centrifugal, rotating or reciprocating). These are then forced into the higherpressure regions towards the discharge. Here the bubbles collapse violently creating noise, vibration and wear. This phenomenon is called cavitation. It can also be caused by sucking air in through worn pump shaft seals. Pitting of pump parts is a common result of cavitations. Every one who works in a refinery needs to have a clear idea of its causes and consequences.

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The greater the liquid slip in a pump the smaller is the effect of cavitation. Cavitation is more serious with viscous liquids than thin ones. High-speed pumps are more prone to cavitation. Cavitation reduces the head and capacity performance of a pump, and may effectively prevent it from pumping at all.

POSITIVE DISPLACEMENT PUMPS In this class of pumps definite volume of liquid is trapped in a chamber, which is alternately filled from the inlet and emptied at a higher pressure through the discharge. There are two sub classes of these A) Reciprocating Pumps: 1. The plunger pump 2. The piston pump 3. The diaphragm pump B) Rotary Pumps: 1. The gear pump 2. The screw pump

A)RECIPROCATING PUMPS In it the chamber is stationary cylinder that contains a piston or plunger. Piston pumps, plunger pumps, and diaphragm pumps are examples of reciprocating pumps.

1. PISTON PUMPS In a piston pump liquid is drawn through an inlet check valve into the cylinder by the withdrawal of a piston and then forced out through a discharge check valve on the return stroke. Most piston pumps are double acting with liquid admitted alternately on each side of the piston so that one part of the cylinder is being filled while the other is being emptied. Often two or more cylinders are used in parallel with common suction and discharge headers, and the configuration of the pistons is adjusted to minimize fluctuations in the discharge rate. The piston may be motor driven through reducing gears or a steam cylinder may be used to drive the piston rod directly. The maximum discharge pressure for commercial piston pumps is 50atm Figure (25) is a diagram to show the pump section of a single-acting piston pump. The working of this pump is, in principle, similar to that of a single-acting plunger pump. The piston is sealed off by using one or more piston springs or piston rings that fit in circular grooves in the piston. The springs move against the cylinder wall when a slight pressure is applied and thus seal off the discharge

area. The pistons can be driven in a similar way to the plunger pump. The piston pump is made to handle larger volumes than the plunger pump.

Figure (25), Single-acting piston pump Figure (26) is a diagram showing double-acting piston pump. Every stroke of the pump supplies fluid to the system as it does in the double-acting plunger pump. The pump now has two piston valves and two discharge valves. Higher demands are made on the sealing around the piston because of the higher pressure difference to the right and left of the piston. The piston rod in the cylinder lid must also be sealed to prevent liquid leaking outside on the piston rod side during discharge stroke of the piston. Since the liquid is divided over two strokes, the liquid flow is fairly constant. The output of a double-acting piston pump is almost twice as large as output of a single-acting piston pump.

Figure (26), Double-Acting Piston Pump

2.PLUNGER PUMPS For higher pressures pluger pumps are used. They contain a heavy walled cylinder of small diameter with a closed fitting reciprocating plunger, which is merely an extension of the piston rod. At the limit of its stroke, the plunger fills nearly all the space in the cylinder. Plunger pumps are single acting and are usually motor driven. They can discharge against a pressure of 1500atm or more Figure (23) shows a diagram of a single-acting plunger pump. The pump casing has a suction and discharge valves. When the plunger moves to the right, the suction stroke takes place. The volume of the liquid in the casing will increase; the pressure in the cylinder will decrease causing the suction valve to open. The liquid will be drawn from the suction line to the pump cylinder.

Figure –23 The plunger will then move to the left making a discharge stroke. The volume of the liquid in the cylinder will decrease, causing the pressure to increase and the discharge valve to open, delivering the liquid into the discharge line. Then another suction stroke will take place and the process continues. Figure (24) shows a double-acting plunger pump, in which the suction stroke on one side occurs at the same time as the discharge stroke on the other side. A double-acting plunger pump delivers almost twice as much as a single- acting pump of the same plunger diameter and stroke length.

Figure – 24, Double Acting Plunger Pump

3.DIAPHRAGM PUMPS In a diaphragm pump, the reciprocating member is a flexible diaphragm of metal, plastic, or rubber. This eliminates the need for packing or seals exposed to the liquid being pumped, a great advantage when handling toxic or corrosive liquids. These handle small to moderate amounts of liquid upto 100gal/min, and can develop pressures in excess of 100 atm. Figure (27) shows the principle of a single-acting diaphragm pump. The displacement device here is not a plunger or a piston but a diaphragm. A diaphragm is a disc made of elastic material, which is clamped around the circumference and which is connected in the middle to a rod which can move the diaphragm backwards and forwards. The volume increases and decreases similar to those of the plunger and piston pumps. The purpose of a diaphragm pump is to transport chemical liquids which could corrode metal.

Figure (27), Diaphragm pump It is also used as a feeder pump when measured quantities are needed. For this reason the diaphragm is made of a material that is not only elastic, but resistant to the action of chemical liquids. The discharge and suction valves are frequently ball valves, possible made of synthetic material, which give a reasonable good sealing at a relatively low pressure

METERING PUMPS Because of the constancy of volume flow, plunger and diaphragm pumps are widely used as “metering pumps” injecting liquid into a process system at controlled but adjustable volumetric rates

B.ROTARY PUMPS: The displacement bodies in this type of pump are meshed gears or screws, rotating discs with internal teeth and occasionally other forms of rotating bodies. In the pump casing more displacement bodies are found than in the plunger pump. Because of this, there is a number of small “suction and discharge strokes” in one rotation. This makes the output of these pumps more constant and the use of an air chamber superfluous.

These pumps have been used with success where a constant output under a relatively low pressure is needed, for example the supply lubricating oil to bearings. They discharge pressures upto 200atm or more. The construction of these pumps is relatively simple and other than the rotors themselves, they contain very few moving parts. A disadvantage is that its parts must be machined very accurately to keep leakage losses small.

1.THE GEAR PUMP Essentially this pump consists of a casing A and in it two accurately meshed gears I & II. The casing is closed off by the covers B and C in such a way that the gears are allowed very little play. Gear I is driven by the gear shaft D. Gear II can turn freely in bearing and is driven by gear I. To prevent leakage along the drive shaft D, cover C is provided with a seal E. At the front and back of the casing, openings are provided to supply and remove the liquid, i.e. the suction and discharge ports. Both gears have the same diameter and the same number of teeth.

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2.THE SCREW PUMP

The screw pump consists of one or more screws which fit closely in the casing and can rotate (see figure.29). There are some types of pumps, in which one (driven) screw turns the other, i.e. like the gear pump. In most screw pumps, however both screws are turned by external gears because the pitch angle of the screws is not large enough to drive each other.

Figure 29 In figure 29 the grooves of the screw are easily recognizable. During the rotation of the screw, the liquid in the grooves will be transported from left to right at the direction of rotation as shown above. In this case the suction line must be attached on the left and the discharge line on the right.

CHOICE OF A PUMP

Choice of a pump depends upon many factors

CONSTANT AND VARIABLE CAPACITY Constant speed reciprocating pumps are suitable for applications where the required capacity is expected to be constant over a wide range of system head variations. However the output may be pulsating and it must be considered Centrifugal pumps are often used in variable, variable capacity applications

SELF PRIMING •

Rotary and reciprocating pumps are self priming but centrifugal unless specifically designed as such are not

SYSTEM LAYOUT •

Sometimes the system layout can influence the choice of a pump. In general, centrifugal will require less space than reciprocating and vertical less space than horizontal. However more headroom may be required for handling the vertical pump’s maintenance and installation.

FLUID CHACTERISTICS • •

Fluid characteristics such as viscosity, density, volatility chemical stability and solid content are also important factors consideration. Rotary pumps are suitable for use with viscous fluids such as oil or grease whereas centrifugal pumps can be used for clean, clear fluids and fluids with high solid content