8. Operation limits of pump-pipeline system
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Transcript of 8. Operation limits of pump-pipeline system
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October 13, 2004
Vermelding onderdeel organisatie
1
oe4625 Dredge Pumps and Slurry Transport
Vaclav Matousek
Dredge Pumps and Slurry Transport
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October 13, 2004 2
8. OPERATION LIMITS OF PUMP-PIPELINE SYSTEM
REQUIRED MANOMETRIC PRESSURE
MAXIMUM VELOCITY – INITIAL CAVITATION
MINIMUM VELOCITY – STATIONARY BED
Dredge Pumps and Slurry Transport
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October 13, 2004 3
REQUIRED MANOMETRIC PRESSURE
DETERMINATION OF SUCTION PRESSURE
DETERMINATION OF DISCHARGE PRESSURE
Dredge Pumps and Slurry Transport
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October 13, 2004 4
Hman-Q CURVE OF A CENTRIFUGAL PUMP
( ) ( )2 2
2m p s
man p s m p s
V VP P P h h
ρρ
−= − + + +
•A rotating impeller of a centrifugal pump adds mechanical energy to the medium flowing through a pump. •As a result of an energy addition a pressure differential occurs in the pumped medium between the inlet and the outlet of a pump. •The manometric pressure, Pman, that is delivered by a pump to the medium, is given as
The manometric head, Hman, that is delivered by a pump to the medium, is man
manf
PHgρ
=
Dredge Pumps and Slurry Transport
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October 13, 2004 5
REQUIRED MANOMETRIC PRESSURE
( )2 2
2p s
man p s m
V VP P P ρ
−= − +
hs,pump
Ps
Pump-pipeline system: mean velocity V
mixture density mr
Patm hd,pipe
hs,pipe
hd,pump
Pp = ?
Ps = ?
Dredge Pumps and Slurry Transport
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October 13, 2004 6
REQUIRED MANOMETRIC PRESSURE
( )2
, , , , , 2s
s atm f s pipe m s pipe s pump f totloss s m fVP P gh g h h gHρ ρ ρ ρ= + − − − −
hs,pump
Ps
Pump-pipeline system: mean velocity V
mixture density mr
Patm hd,pipe
hs,pipe
hd,pump
Ps = absolute suction pressure
Dredge Pumps and Slurry Transport
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October 13, 2004 7
REQUIRED MANOMETRIC PRESSURE
( )2
, , , , 2p
p atm m d pipe d pump f totloss d m f
VP P g h h gHρ ρ ρ= + + + −
hs,pump
Ps
Pump-pipeline system: mean velocity V
mixture density mr
Patm hd,pipe
hs,pipe
hd,pump
Pp = absolute discharge pressure
Dredge Pumps and Slurry Transport
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October 13, 2004 8
MAXIMUM VELOCITY IN THE SYSTEM
THE UPPER LIMIT FOR A SYSTEM OPERATION:VELOCITY AT THE INITIAL CAVITATION OF A PUMP
Dredge Pumps and Slurry Transport
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October 13, 2004 9
Static Pressure Variation Along SystemThe static-pressure (P) variation along suction and discharge pipes connected with a pump (schematic).
The static pressure varies due to changes in - the geodetic height (suction pipe)- the velocity [head] (change inpipe diameter in front of the pump)
and…due to the losses (both in suction and discharge pipes).
Dredge Pumps and Slurry Transport
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October 13, 2004 10
MAXIMUM VELOCITY IN THE SYSTEM
Criterion for non--cavitational operation of a systemThe no cavitation condition for a certain pump-suction pipe
combination is:
(NPSH)REQUIRED < (NPSH)AVAILABLE
The available Net Positive Suction Head is a total available energy head over the vapour pressure at the suction inlet to the pump during an operation at velocity Vm in a suction pipe of a certain geometry and configuration.
( )2
2s vapour m
AVAILABLEf
P P VNPSHg gρ
−= +
Dredge Pumps and Slurry Transport
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October 13, 2004 11
MAXIMUM VELOCITY IN THE SYSTEM
Criterion for non--cavitational operation of a systemThe no cavitation condition for a certain pump-suction pipe
combination is:
(NPSH)REQUIRED < (NPSH)AVAILABLE
The required Net Positive Suction Head is a minimum energy head a certain pump requires to prevent cavitation at its inlet. This is a head value at the incipient cavitation.
( )2
,min
2s vapour m
REQUIREDf
P P VNPSHg gρ
−= +
Dredge Pumps and Slurry Transport
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October 13, 2004 12
MAXIMUM VELOCITY IN THE SYSTEM
Criterion for non--cavitational operation of a systemThe no cavitation condition for a certain pump-suction pipe
combination is:
(NPSH)REQUIRED < (NPSH)AVAILABLE
The (NPSH)REQUIRED-Q curve is a characteristic specific for each pump and it must be determined by a pump cavitation test. A design (dimensions, shape) and an operation (specific speed) of a pump decide the absolute suction pressure at the initial cavitation. ( )
2,min
2s vapour m
REQUIREDf
P P VNPSHg gρ
−= +
Dredge Pumps and Slurry Transport
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October 13, 2004 13
MAXIMUM VELOCITY IN THE SYSTEM
Criterion for non--cavitational operation of a systemThe no cavitation condition for a certain pump-suction pipe
combination is:
(NPSH)REQUIRED < (NPSH)AVAILABLE
At the incipient cavitation, the absolute suction pressure Ps,minat the pump inlet is equal to the difference between the atmospheric pressure Patm and the so-called “decisive vacuum” (Dutch: maatgevend vacuum) (Vac)d, i.e.
(Vac)d = Patm – Ps,min.
Dredge Pumps and Slurry Transport
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October 13, 2004 14
MAXIMUM VELOCITY IN THE SYSTEM
Criterion for non--cavitational operation of a system
The upper limit for the working range of a pump-pipeline system is given by points of intersection of a pump decisive vacuum curve and a set of vacuum curves of a suction pipefor various mixture densities.
The vacuum curve of a suction pipe :
atm S
f f
P PVacg gρ ρ
−=
Dredge Pumps and Slurry Transport
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October 13, 2004 15
MAXIMUM VELOCITY IN THE SYSTEM
Criterion for non--cavitational operation of a system
The upper limit for the working range of a pump-pipeline system is given by points of intersection of a pump decisive vacuum curve and a set of vacuum curves of a suction pipefor various mixture densities.
The decisive vacuum curve of a pump :
( ) 2
2atm vapourd m
REQUIREDf f
Vac P P V NPSHg g gρ ρ
−= + −
Dredge Pumps and Slurry Transport
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October 13, 2004 16
MAXIMUM VELOCITY IN THE SYSTEM
( ) 2
2atm vapourd m
Rf f
Vac P P V NPSHg g gρ ρ
−= + −
atm S
f f
P PVacg gρ ρ
−=
Dredge Pumps and Slurry Transport
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October 13, 2004 17
MAXIMUM VELOCITY IN THE SYSTEM
HOW TO AVOID CAVITATION
In the design of a pump-pipeline system:• to reduce the static head that the pump must overcome, i.e.
to put the pump as low as possible
• to reduce the head lost due to flow friction, i.e. to minimize local losses and a suction pipe length
• to increase pressure by using a larger pipe at the suction inlet of a pump.
Dredge Pumps and Slurry Transport
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October 13, 2004 18
MAXIMUM VELOCITY IN THE SYSTEM
HOW TO AVOID CAVITATION
During the operation of a system (the position of a pump and a geometry of a suction pipeline can not be changed):
• to reduce the head lost due to flow friction either by• diminishing the mean mixture velocity or by • reducing the mixture density in a suction pipeline.
Dredge Pumps and Slurry Transport
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October 13, 2004 19
MINIMUM VELOCITY IN THE SYSTEM
THE LOWER LIMIT FOR A SYSTEM OPERATION:VELOCITY AT THE INITIAL STATIONARY BED IN A PIPE
Dredge Pumps and Slurry Transport
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October 13, 2004 20
MINIMUM VELOCITY IN THE SYSTEM
Criterion for the deposit-free operation of a systemMean mixture velocity must be higher than deposition-limit
velocity:
Vm > Vdl.
Dredge Pumps and Slurry Transport
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October 13, 2004 21
Empirical Model for Critical Velocity (MTI)
C. Diagram for Vcr:
Dredge Pumps and Slurry Transport
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October 13, 2004 22
Empirical Model for Critical Velocity (MTI)
B. Correlation for Vcr:
16 111.7 5
0.1 1.65vd s
critvdmf
C SV DCd
− = − +
In the equation dmf [mm], D [m] and Vcr [m/s].
Dredge Pumps and Slurry Transport
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October 13, 2004 23
MINIMUM VELOCITY IN THE SYSTEM
HOW TO AVOID STATIONARY BED1. If the pipeline is composed of sections of different pipe sizes:
the mixture flow rate must be maintained at the level assuring a super-critical regime (Vm > Vdl) in the pipe section of the largest pipe diameter.
2. If the solids concentration fluctuates along a pipeline:the mixture flow rate must be maintained at the level assuring a super-critical regime in the section of an extreme concentration. For a prediction, use the highest value of the deposition-limit velocity from the entire range of expected solids concentrations.
Dredge Pumps and Slurry Transport
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October 13, 2004 24
MINIMUM VELOCITY IN THE SYSTEM
HOW TO AVOID STATIONARY BED3. If during a job a pipeline is prolonged or coarser solids are
pumped:the flow rate supplied by a dredge pump might become insufficient to assure a super-critical regime in a pipeline. Then two solutions must be considered:
• to pump mixture at much lower concentration; this will lead to lower frictional losses and thus higher flow rate that might be high enough to avoid a thick stationary bed in a pipeline
• to install a booster station; this increases the manometrichead provided by pumps and increase a flow rate.
Dredge Pumps and Slurry Transport
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October 13, 2004 25
EFFECT OF PUMP POSITION ON
OPERATIONAL LIMITS
If a pump (e.g. a submerged pump) is placed to a lower position within a pump-pipeline system:
• the suction pipe becomes shorter
• the geodetic height over which a mixture has to be lifted becomes smaller.
Dredge Pumps and Slurry Transport
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October 13, 2004 26
EFFECT OF PUMP POSITION ON
OPERATIONAL LIMITS
Dredge Pumps and Slurry Transport
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October 13, 2004 27
EFFECT OF PUMP POSITION ON
OPERATIONAL LIMITS
Dredge Pumps and Slurry Transport
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October 13, 2004 28
OPERATION LIMITS AND PIPE LENGTH
0 1 2 3
25
50
75
100
450
550
650
750
850
vacu
üm (k
Pa)
man
omet
risch
e dr
uk (k
Pa)
debiet (m /s)3
constant toerental
maatgevend vacuüm
zuigleiding-karakteristiek
Qkritisch Qmaatg. vacuüm
kortste persleiding
langste persleiding
werkgebied
onderkritisch
bovenkritisch
The maximum length of a discharge pipeline is limited by the deposition-limit velocity.
The minimum length of a discharge pipeline is limited by the decisive vacuum of a pump.
Dredge Pumps and Slurry Transport