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CHARACTERISTICS OF PUMPS AND PIPE SYSTEMS
133
CHARACTERISTICS OF PUMPS AND PIPE SYSTEMS Pump Characteristics The characteristics of head, power, efficiency and cavitation plotted against flow for a typical centrifugal pump at two speeds so as to illustrate the effect of speed on the performance of the pump are shown in Fig 22.1. The maximum efficiency line for variation of speed is clearly shown. As will be seen, there are two lines for cavitation, namely, the operation line, which contains a margin of safety, and the cavitation line which is determined and plotted so as to ensure that the margin of safety is adequate. 200 r - - - - - - - - - - - - - - ,
E 150 -0
~ 100 ::I:
50
o
100
,soO _- --
Figure 22.1 Effect ofspeed on pump characteristics
"
~
--"1000
60 20
~ ~
E
80
....,
~
o>-
c
c
o 40 :;::
....,
CD
0
10 6 0
~
w
o
30
60
Flow (I/s)
90
o
~
(/)
CENTRIFUGAL PUMPS
134
For such curves showing moderate changes of speed, the cavitation performance is approximately constant for a given quantity, although, as will be seen from the inlet shape number curves, a slight change of cavitation performance does occur with change of speed. (See Chapter 9).
Consideration of Pump and Pipeline As opposed to a reciprocating pump, a centrifugal pump generates a head which forces a certain quantity of liquid through a system. For any centrifugal pump, flow of water will onl y commence if the generated head at no discharge exceeds the pressure difference between the suction and delivery branches of the pump. When flow takes place the head of the system will be increased by the pipe, valve and other friction and by the velocity head. The head generated by a pump may increase as the discharge valve is opened and then fall (Fig 22.2) or may fall progressively with increase in quantity (Fig 22.1). The latter characteristic, a constantly falling one, is essential for certain pressure systems where the flow is controlled at a considerable distance from the pump, eg in boiler feeding.
I
Stable
Figure 22.2 Pump and system characteristics tV
E o
z Quantity
Falling Characteristic A falling characteristic is not usually necessary where valves are full open or where throttling, if any, takes place at the pump discharge. Regulation at the end of a long pipe, as in boiler feeding, demands a falling characteristic, since the change in pipe volume due to elastic deformation under pressure variation permits a momentary storage of water with consequent water hammer at the rising part of the curve (Fig 22.2). It is, however, essential on all systems to ensure that the closed valve head of the pump exceeds the static lift.
Static Head and Flow Conditions When a pump is operating on a given system, the flow will stabilize at a point where the generated head is equal to the static head plus friction and velocity head, the operating point being determined by the intersection of the pump characteristic and the total friction
CHARACTERISTICS OF PUMPS AND PIPE SYSTEMS
135
characteristic of the system (Fig 22.2). It is assumed that the suction lift and suction piping to the pump are such as to permit the pump to operate well within its cavitation limit.
Physical Explanation of Pressure Oscillations When a pump having an unstable characteristic (Fig 22.2) is operated on a long pipeline with a control valve remote from the pump, water hammer may result. This is due to the fact that the elasticity of the pipe permits a small amount of water to be stored in the pipe. The storage volume corresponds to the expansion of the pipe between the closed valve head and the peak of the characteristic. When operating at the unstable part of the curve, the quantity increase to peak head point momentarily, the excess quantity being stored in the pipe. When peak is reached, the pipe can accept no more water and the pump flow reduces. In consequence of the characteristic, the head also reduces. The pipe then has a higher pressure than the pump and a back flow is induced, closing the non-return valve on the pump. The pump flow and head again increase to peak and the cycle repeats indefinitely. The only way to avoid this water hammer is to provide a characteristic which falls progressively with increase of quantity so that the pump is always higher in pressure than the pipeline. Such a characteristic also permits parallel operation with equally shared loads. A falling characteristic is essential for boiler feed duties where control is by boiler valves remote from the pumps and where parallel operation over the whole characteristic is required. Where pumps operate on an accumulator, a falling characteristic is only essential if the pumps run at the top to the stroke on closed valve (the accumulators are generally close to the pump) or if parallel operation is required. On tappet control a falling curve is less important. Closed valve head, however, must exceed maximum accumulator pressure. Where pumps discharge to nozzles, ie all friction head, and where the control valves are not likely to operate below approximately half quantity, (eg on descaling or debarking duties), a falling characteristic is not quite so important, but it is essential that the closed valve head should materially exceed the duty head. Waterworks pumps on a static plus friction duty do not require a falling characteristic, but where more than one pump is installed, the closed valve head must exceed every possible operating head, otherwise it would be impossible to bring into load a second or third pump. This can only be determined by drawing the pump and system characteristic (as Fig 22.2) for every condition of load.
CHARACTERISTICS OF PUMPS AND PIPE SYSTEMS Pump Characteristics The characteristics of head, power, efficiency and cavitation plotted against flow for a typical centrifugal pump at two speeds so as to illustrate the effect of speed on the performance of the pump are shown in Fig 22.1. The maximum efficiency line for variation of speed is clearly shown. As will be seen, there are two lines for cavitation, namely, the operation line, which contains a margin of safety, and the cavitation line which is determined and plotted so as to ensure that the margin of safety is adequate. 200 r - - - - - - - - - - - - - - ,
E 150 -0
~ 100 ::I:
50
o
100
,soO _- --
Figure 22.1 Effect ofspeed on pump characteristics
"
~
--"1000
60 20
~ ~
E
80
....,
~
o>-
c
c
o 40 :;::
....,
CD
0
10 6 0
~
w
o
30
60
Flow (I/s)
90
o
~
(/)
CENTRIFUGAL PUMPS
134
For such curves showing moderate changes of speed, the cavitation performance is approximately constant for a given quantity, although, as will be seen from the inlet shape number curves, a slight change of cavitation performance does occur with change of speed. (See Chapter 9).
Consideration of Pump and Pipeline As opposed to a reciprocating pump, a centrifugal pump generates a head which forces a certain quantity of liquid through a system. For any centrifugal pump, flow of water will onl y commence if the generated head at no discharge exceeds the pressure difference between the suction and delivery branches of the pump. When flow takes place the head of the system will be increased by the pipe, valve and other friction and by the velocity head. The head generated by a pump may increase as the discharge valve is opened and then fall (Fig 22.2) or may fall progressively with increase in quantity (Fig 22.1). The latter characteristic, a constantly falling one, is essential for certain pressure systems where the flow is controlled at a considerable distance from the pump, eg in boiler feeding.
I
Stable
Figure 22.2 Pump and system characteristics tV
E o
z Quantity
Falling Characteristic A falling characteristic is not usually necessary where valves are full open or where throttling, if any, takes place at the pump discharge. Regulation at the end of a long pipe, as in boiler feeding, demands a falling characteristic, since the change in pipe volume due to elastic deformation under pressure variation permits a momentary storage of water with consequent water hammer at the rising part of the curve (Fig 22.2). It is, however, essential on all systems to ensure that the closed valve head of the pump exceeds the static lift.
Static Head and Flow Conditions When a pump is operating on a given system, the flow will stabilize at a point where the generated head is equal to the static head plus friction and velocity head, the operating point being determined by the intersection of the pump characteristic and the total friction
CHARACTERISTICS OF PUMPS AND PIPE SYSTEMS
135
characteristic of the system (Fig 22.2). It is assumed that the suction lift and suction piping to the pump are such as to permit the pump to operate well within its cavitation limit.
Physical Explanation of Pressure Oscillations When a pump having an unstable characteristic (Fig 22.2) is operated on a long pipeline with a control valve remote from the pump, water hammer may result. This is due to the fact that the elasticity of the pipe permits a small amount of water to be stored in the pipe. The storage volume corresponds to the expansion of the pipe between the closed valve head and the peak of the characteristic. When operating at the unstable part of the curve, the quantity increase to peak head point momentarily, the excess quantity being stored in the pipe. When peak is reached, the pipe can accept no more water and the pump flow reduces. In consequence of the characteristic, the head also reduces. The pipe then has a higher pressure than the pump and a back flow is induced, closing the non-return valve on the pump. The pump flow and head again increase to peak and the cycle repeats indefinitely. The only way to avoid this water hammer is to provide a characteristic which falls progressively with increase of quantity so that the pump is always higher in pressure than the pipeline. Such a characteristic also permits parallel operation with equally shared loads. A falling characteristic is essential for boiler feed duties where control is by boiler valves remote from the pumps and where parallel operation over the whole characteristic is required. Where pumps operate on an accumulator, a falling characteristic is only essential if the pumps run at the top to the stroke on closed valve (the accumulators are generally close to the pump) or if parallel operation is required. On tappet control a falling curve is less important. Closed valve head, however, must exceed maximum accumulator pressure. Where pumps discharge to nozzles, ie all friction head, and where the control valves are not likely to operate below approximately half quantity, (eg on descaling or debarking duties), a falling characteristic is not quite so important, but it is essential that the closed valve head should materially exceed the duty head. Waterworks pumps on a static plus friction duty do not require a falling characteristic, but where more than one pump is installed, the closed valve head must exceed every possible operating head, otherwise it would be impossible to bring into load a second or third pump. This can only be determined by drawing the pump and system characteristic (as Fig 22.2) for every condition of load.