Unit 1 Pump Terminology
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Module No. 5 : Pumps Unit No. 1 - Pumps technology Page 1/21
description
pump
Transcript of Unit 1 Pump Terminology
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UNITS IN THIS COURSE
UNIT 1 PUMP TERMINOLOGY.
UNIT 2 PUMP CLASSIFICATION.
UNIT 3 RECIPROCATING PUMPS.
UNIT 4 ROTARY PUMPS.
UNIT 5 CENTRIFUGAL PUMPS.
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TABLE OF CONTENTS
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Para Page
1.0 COURSE OBJECTIVES 3
1.1 PUMP TERMINOLOGY 4
1.1.1 Positive Displacement 4
1.1.2 Capacity 5
1.1.3 Efficiency 6
1.1.4 Reciprocating Motion 6
1.1.5 Stroke 7
1.1.6 Centrifugal 8
1.1.7 Suction 9
1.1.8 Discharge 9
1.1.9 Suction Head 10
1.1.10 Suction Lift 11
1.1.11 Axial Thrust 11
1.1.12 Radial Movement 13
1.1.13 Vibration 13
1.1.14 Cavitation 14
1.1.15 Vapour Lock 15
1.1.16 Multi-Stage 16
1.1.17 Series and Parallel Operation 18
1.1.18 Bypass 19
1.1.19 Standby 19
1.1.20 Prime Movers 19
1.1.21 Couplings 20
1.1.22 Upstream and Downstream 20
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1.0 COURSE OBJECTIVES
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This course describes the different types of pumps commonly found in process plants. The course has five Units. These Units describe the basic terminology of pumps and pumping, the types of reciprocating, rotary and centrifugal pumps, start up of centrifugal pumps, pump problems, and pump control systems. When he has finished the course, the student will be able to:
Explain the basic terminology of pumps.
Identify different kinds of pumps and explain their functions.
Identify the differences between the main types of reciprocating pumps and explain the end uses of these pumps in process plants.
Explain the function of pulsation dampeners.
Identify the differences between the main types of rotary pumps and explain the end uses of rotary pumps in process plants.
Explain how centrifugal pumps increase pressure.
Explain how pump shafts are sealed and how axial and radial thrust are balanced in single stage centrifugal pumps.
Explain the reasons for, and the advantages of, multi-stage centrifugal pumps.
Explain how axial thrust is balanced in a multi-stage centrifugal pump.
Explain how cavitation and vapour lock occur in a centrifugal pump and explain how these problems are overcome.
Explain why centrifugal pumps are connected in parallel or in series.
Explain the pre-start checks and start-up procedure for centrifugal pumps with electric motor and steam turbine prime movers.
Explain the procedure for centrifugal pump changeover.
Use pump readings to identify pump problems.
Identify and explain pump control systems.
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1.1 PUMP TERMINOLOGY
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Pumps and pumping systems use special wording. In this Unit the following wording will be explained:
Positive Displacement Radial Movement
Capacity Vibration
Efficiency Cavitation
Reciprocating Motion Vapour Lock
Stroke Multi-stage
Centrifugal Series and Parallel Operation
Rotary Bypass
Suction Standby
Discharge Prime Movers
Suction Head Couplings
Suction Lift Upstream and Downstream
Axial Thrust
1.1.1 Positive Displacement
Positive displacement happens when a liquid is forced to move by the movement of a solid object into it. When this happens, the liquid is forced out of whatever is containing it.
As an example, fill a plastic cup to the top with water. Now take a second plastic cup and slowly push it into the water. All the water in the first cup will be forced out (displaced) by the second cup.
A similar thing happens in positive displacement pumps. The liquid is:
• Trapped in a confined space (the cylinder of the pump).
• Forced out of the space by the movement of a solid object (the piston or plunger of the pump).
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Reciprocating pumps and rotary pumps (both of which are covered in more detail in
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other Units of this course) are examples of positive displacement pumps.
Figure 1-1 Positive Displacement
1.1.2 Capacity
The capacity of a pump is the volume, or amount, of liquid that the pump can move from one point to another in a set amount of time. The units used for capacity are a unit of volume and a unit of time.
The unit of volume changes depending on the units used to measure the area of the piston and the length of the piston stroke. For example, cubic centimetres or cubic feet are two of the units that can be used. The unit of time used could be seconds, minutes, hours or days.
Standard units of volume used by the Operator are:
gallons per minute (gal/min), gallons per hour (gal/h),US gallons per minute (American gallons per minute - USgal/min), litres per minute or per hour (I/min, ]/h), barrels per day (bpd).
The equation to find maximum pump capacity (liquid output) of a pump is:
Capacity = Piston area x Length of stroke x Full strokes per minute.
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This formula is for a single-acting pump. If the pump is 'double acting' it pumps on
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each stroke of the piston so the capacity is doubled.
The piston area and length of stroke is usually given in the pump manufacturer's manual. They are also printed on the pump name plate.
Capacity is a measurement of volume which uses units such as cubic inches (in 3),
cubic centimetres (CM3), cubic metres (M3).
In the normal operation of a pump some liquids will leak out of the pump cylinder. To get a true figure for the capacity of the pump it is necessary to multiply the calculated capacity by the 'efficiency' of the pump. Efficiency means how well something works.
1.1.3 Efficiency
Pump Efficiency = Real Pump Output x 100 Calculated Pump Output
We multiply by 100 because efficiency is expressed as a percentage.
The Operator tries to keep the real pump output as close to the calculated pump output as possible. This gives the best pump efficiency.
Factors that affect the efficiency of a pump are:
Wear of pump parts.
Viscosity of the pumped liquid.
Dirty liquids.
The temperature of the pumped liquid.
1.1.4 Reciprocating Motion
Reciprocating motion is straight line motion in two directions, but in only one plane. The plane may be vertical with the motion upwards and downwards or horizontal with the motion from side to side.
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Figure 1-2 Reciprocating Motion
1. 1. 5 Stroke
The stroke of a reciprocating pump is the distance that the piston or plunger moves inside the cylinder.
A 'single-acting' pump only forces the liquids out of the cylinder discharge once for each complete turn of the crank shaft. A 'double acting' pump empties the cylinder two times for each revolution of the crank shaft, one on the forward stroke of the piston and one on the back (or return) stroke of the piston.
Figure 1-3 Single and Double Acting Pumps
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1.1.6 Centrifugal
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Centrifugal means moving outwards from the centre. Centrifugal force is a force caused by rotating an object. This force is used in centrifugal pumps.
Figure 1-4 shows the principle of centrifugal force as applied to an object and to a pump.
Figure 1-4 Centrifugal Force
Figure 1 -4A shows a shaft. At the top of the shaft is an arm with a small ball on the end of it. The arm is attached to the shaft by a hinge. In Figure 1 -4A the shaft is not turning and the arm hangs down vertically.
In Figure 1-4B the shaft is turning. Centrifugal force causes the arm to rise up until it is horizontal (at 90 degrees to the shaft).
Figure 1-4C shows how centrifugal force can be applied to a liquid. This time the shaft has a small dish on the top of it. Liquid flows into the dish from the supply pipe.
If the shaft is not turning the liquid will fill the dish and flow over the side of the dish. When the shaft is turning centrifugal force acts on the liquid and forces it to leave the dish at 90 degrees to the shaft.
It is this principle which is used in centrifugal pumps.
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1.1.7 Suction
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The suction 'side' of a pump is the place where the liquid comes into the pump.
Liquids flow naturally from a high pressure area to a lower pressure area. The movement of the pumping device (piston, plunger, impeller) makes a low pressure area at the pump suction inlet.
A simple example of suction is the use of a drinking straw. One end of the straw is put in the liquid you want to drink, the other end is put in your mouth. You change the shape of your mouth to reduce the pressure in your mouth. The liquid flows from the can or glass, through the straw and in to your mouth. Your mouth has made suction.
Suction pressure must be low enough so the liquid flows into the pump with a steady volume.
1.1.8 Discharge
The discharge side of a pump is the place where the pumped liquid leaves the pump.
The discharge pressure is always higher than the suction pressure. It must be high enough to overcome any pressure which is in the equipment and pipelines after the pump.
The difference between the suction and discharge pressure is caused by the pump adding 'energy' to the liquid. Energy takes many different forms. Pressure is one form of energy.
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1.1.9 Suction Head
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If the liquid to be pumped is at a higher level than the pump, then this is called a 'suction head' system. Figure 1-5 shows a simple suction head system.
Figure 1-5 Suction Head System
The vertical distance from the liquid level to* the centre of the pump suction is called the suction head. The centre of the pump suction is sometimes called the 'datum' or 'datum point'. The suction head is often called the Net Positive Suction Head and abbreviated to NPSH. Suction head is normally measured in metres or feet.
In a suction head system the pressure of the liquid (because the liquid is higher than the pump) is available to move the liquid to the pump.
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1.1.10 Suction Lift
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If the liquid to be pumped is at a lower level than the pump, then this is termed a 'suction lift' system. Figure 1-6 shows a simple suction lift system.
In a suction lift system the pump must create enough suction (low pressure at the pump inlet) to draw the liquid into the pump.
Suction lift is normally measured in metres or feet.
Figure 1-6 Suction Lift System
1.1.11 Axial Thrust
Axial thrust is a force on the shaft of a centrifugal pump. It is caused. by the difference in pressure from pump suction to pump discharge. The force acts along the shaft of the pump from discharge to suction.
Centrifugal pumps are protected from axial thrust by two methods.
EITHER
A thrust bearing which holds the pump impeller in the correct position and stops -any axial movement. See Figure 1-7.
OR
A double suction impeller. This is an impeller with 2 suction inlets which balances out the forces. See Figure 1-8. Double suction impellers are used in high volume/high pressure pumps.
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Figure 1-7 Axial Thrust and Thrust Bearing
Figure 1-8 Single- and Double-Suction Impellers
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1.1.12 Radial MovementM
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Radial movement is movement at right angles (90) to the shaft. It happens only in centrifugal pumps.
Radial movement is movement outwards from the centre point of rotation, as shown in Figure 1-9.
Radial movement is caused by:
1. The impeller being driven by a long, unsupported shaft. .1. The impeller being out of balance.
Movement of the impeller away from its correct position reduces the efficiency of the pump and may cause damage to the casing or internal parts of the pump. Radial movement is prevented by using bearings to support the shaft.
Figure 1-9 Radial Movement
1.1.13 Vibration
Vibration is the rapid movement of one or more parts of a piece of equipment, including pumps. It is usually caused by one or more of the rotating parts not being correctly balanced, so that a centrifugal force is generated at one point on the rotating part.
Vibration is bad for rotating equipment because it causes serious damage to internal parts of the equipment, particularly to bearings.
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Vibration can be caused by:
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1. Suction or discharge problems.1. Cavitation.
Vibration can be prevented by:
1. Correct pump construction.1. Proper motor and pump balancing.1. Proper use of bearings.
If vibration occurs:
1. The pump must immediately be stopped.1. The cause of the vibration must be found and repaired
1.1.14 Cavitation
Cavitation is the formation and collapse of very many small vapour bubbles in the liquid being pumped.
When cavitation happens, some of the liquid being pumped changes to vapour. This is known as 'flashing'. If this happens in the suction section or at the suction inlet of the impeller (the eye of the impeller), the vapour bubbles will be carried into the impeller.
As the pressure increases, the vapour bubbles collapse in the vanes of the impeller. The liquid rushes in to the space left with so much force that it knocks off tiny particles of metal from the vanes of the impeller. This causes pitting and erosion of the vanes and body of the impeller and it may be necessary to replace the impeller. Cavitation can also be a cause of vibration.
The collapse of the vapour bubbles causes a crackling noise inside the pump. This noise tells the Operator that cavitation is happening.
Cavitation is caused by:
1. The pump operating near to its minimum Net Positive Suction Head (NPSH). The NPSH of any pump is given in the manufacturer's manual usually on what is called a 'pump curve'.
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1. A restricted pump inlet. This could be caused by;
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1. a valve in the suction line being partially closed,1. the pump suction line being blocked.
1. The liquids being pumped are too hot.1. The pump casing is too hot.1. The pump discharge valves are blocked in.1. The discharge pressure is too high.
To correct cavitation:
1. The NPSH available must be increased.1. The pumping rate must be decreased.
NPSH available may be increased by raising the level of liquid on the suction side of the pump.
The pumping rate can be decreased by partly closing the discharge valve of the pump. This is known as 'throttling'. Decreasing the pumping rate of the pump will also increase the NPSH available.
Cavitation in a pump is an operational problem. The cause must be found and corrected immediately.
1.1.15 Vapour Lock
Vapour lock usually only happens in centrifugal pumps, although it can sometimes happen in some types of rotary pumps.
Vapour lock happens when gas enters the pump with the liquid. The gas separates from the liquid inside the pump and fills all or part of the area in which the impeller is housed. This area is known as the I volute' in centrifugal pumps.
A pump can only move liquids. If gas enters the pump the flow of liquid stops.
When a pump is vapour locked, the discharge pressure gauge has almost the same reading as the suction pressure gauge while the pump is running.
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Vapour lock can be caused by one or more of the following:
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1. Low liquid level in the vessel being pumped out.1. * Low flow through the pump.1. e Gases trapped in the suction liquids.
To clear a vapour lock the gas must be removed from the pump. If the pump is fitted with a vent valve on the casing, clearing the gas can be done with the pump running by opening the vent valve. If a casing vent valve is not fitted, the discharge vent can be used.
Vapour lock can be prevented by preventing gas from entering the pump. Preventing gas from entering the pump can be done by:
1. Raising the suction pressure to the pump.1. Raising the level of liquid in the vessel that is being pumped.1. Degassing (removing the gas from) the suction liquids.
1.1.16 Multi-Stage
When describing centrifugal pumps, multi-stage means that there are two or more impellers arranged in series on a single drive shaft. Figure 1-10 shows a single-stage pump and a multi-stage pump with two impellers.
Figure 1-10 Single- and Multi-Stage Pumps
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In a multi-stage pump the discharge of liquid from the first stage impeller is directed
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to the suction inlet of the second stage impeller. If there are more than two stages then the discharge from the second stage impeller is directed to the suction of the third stage impeller and so on, until final discharge from the pump. As the liquid moves from one stage to the next the pressure of the liquid is increased in steps.
Multi-staging is used to give higher discharge pressures. The more stages or impellers, the higher will be the final discharge pressure of the pump.
Most Multistage pumps have the impellers set on the shaft so that their suction and discharge pressures will balance the axial thrust forces.
Figure 1-11 Opposed Impellers
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1.1.17 Series and Parallel Operation
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When two or more pumps are used together in the same system, they can be connected so that they operate either in series or in parallel. Figure 1-12 shows how the connections are made. All types of pumps can be connected in series or parallel although it is more usual to see centrifugal pumps connected in this way.
Figure 1-12 Series and Parallel Operation
When pumps are connected in series, the discharge of one pump is fed to the suction of another pump. When pumps are connected in this way the discharge pressure of the system is increased.
When pumps are connected in parallel, they discharge into the same pipeline. When pumps are connected in parallel the quantity of liquid which can be pumped (the flow rate) is increased.
SERIES PARALLEL
Increases discharge pressure Discharge pressure the same
Discharge volume the same Increases discharge volume
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1.1.18 Bypass
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A bypass is a piping system which allows the flow of liquid to travel around the pump instead of through the pump. Bypassing a pump is an option. A bypass pipeline together with the suction and discharge lines for the pump have valves so that the pump can be in service or bypassed.
Figure 1-13 shows a typical bypass arrangement for a pump. Note that the bypass pipeline starts before the pump suction valve, and finishes after the pump discharge valve. This is so that the pump can be isolated from the system.
Figure 1-13 Typical Bypass Arrangement
1.1.19 Standby
Standby is a word used to show that a pump is not in service, but is ready to be brought into service very quickly.
In very important processes where the liquids must continue flowing, two pumps are placed in parallel. One of the pumps operates or is 'on line'. The other pump is on 'standby', ready for immediate service if the 'on line' pump fails.
1.1.20 Prime Movers
Prime movers are the engines used to drive the pumps. They can be electric motors, pneumatic motors driven by compressed air or another gas, steam turbines, gas turbines, or even a diesel or petrol engine.
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The type of prime mover used is determined by:
1. Location (where the pump is).1. Power available.1. Fuel costs.
1.1.21 Couplings
Some pumps are attached directly to the drive shaft of the prime mover. However, it is more usual for the pump and prime mover to be separate units. The two units are joined together by a coupling. Although this sounds like a simple device, couplings are often quite complicated.
The function of a coupling is to transmit the energy of the prime mover to the pump.
The coupling must be able to handle:
1. Stop and start forces.1. Shaft misalignment.1. Thermal expansion.
1.1.22 Upstream and Downstream
Upstream and downstream are words commonly used when talking about pumps. They provide a clear way to describe the location of other equipment, valves or controllers connected to the pump.
Upstream means the pipework, equipment and liquid in the system before the pump.
Downstream means the pipework, equipment and liquid in the system after the pump.
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Figure 1-14 Upstream and Downstream
Figure 1-14 shows a very simple pumping system. The flow of liquid is from the storage tank, through the pump and its suction, check and discharge valves, through the pressure control valve, to the process plant.
The locations of these units can be related to each other using 'upstream' and 'downstream. Each of the following statements is correct.
1. The storage tank is upstream of the pump suction valve.1. The storage tank is upstream of the pump.1. The suction valve is upstream of the pump.1. The process plant is downstream of the pump.1. The check valve is downstream of the pump.1. The check valve is upstream of the pump discharge valve.1. The pressure control valve is downstream of the discharge valve.
