chapter 10-equipment for fluid mechanic
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7/27/2019 Chapter 10-Equipment for Fluid Mechanic
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CHE 493: fluid mechanics
Chapter 10
Equipment for fluid mechanics
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Course learning outcome
1. Able to explain the types and selection of pumps (Eg. NPSH, etc.)
2.Able to calculate the work done by the pump.
3. Describe the types, selection of compressorand design calculation.
4.Able to calculate the work done by thecompressor.
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Pump
Very important mechanical devices in industry,agriculture and in daily live.
Pump has 2 important functions :1. Ability to supply liquid at required rate of flow.
2. Impart energy to the liquid, thereby increasing
the pressure- by rotating shaft
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Types of pump•
DISPLACEMENT PUMP• Piston pump
• Diaphragm pump
• Rotary pumps
• Gear pump• Peristaltic pump
•
DYNAMIC PUMP• Centrifugal pump
• Axial Flow pump
• Mixed Flowpump
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Pump•
In the turbomachineryindustry, capacity of
the pump/volume flow
rate
• Performance of the pump
is characterize by nethead, H .
mQ
inout
z g
V
g
P z
g
V
g
P H
22
22
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PumpEfficiency
Brake horse power, bhp=
WHP < BHP
shaft
horsepower water
shaft
horsepower water
pumpT
H V g
bhp
W
W
W
(rad/s)= rotational speed of the shaft
Torque supply to the shaft
shaft shaft T W
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It must be higher because there is irreversible losses
due to friction
Power delivered to
the fluidMechanical energy
supply to pump
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Pump performance curve
Free delivery Shut off Net head, H=0 ; maximum
flowrate through the pump
Net head ,H=max, Q=0
Is achieve when there is no
flow restriction at thepump inlet/outlet-no load
to the pump
Is achieve when the outlet
port of pump is blockedoff.
Q is very large, but H=0;
the pump efficiency is zero
because the pump did not doany useful work.
H is very large but Q
=0, the pump efficiency is
zero because the pump didnot do any useful work.
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• For given piping system, major &minor
losses, elevation changes caused the
required net head increase with the volumeflow rate.
• The pump efficiency reach the maximum value
between shut off condition and the free
delivery condition. (Note as H*, bhp* and
V@Q *) : BEP=best efficiency point
• For steady condition, the pump can operate
only along the performance curve
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Pump performance curve
Pump performance curves for a centrifugal pump 9
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Required net head of
pumps increases with
flowrate
Available net head of
pumps decreases with
flowrate
Operating point of a
piping system isestablished as the
volume flow rate where
the system curve and the
pump performance curveintersect.
BEP should be close to
operating point for best
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In unfortunate situations the system curve and the pump
performance curve intersect at more than one operating
point.
It can happen when the system curve is almost flat & meet apump that has a dip on its net head performance curve.
This situation should be avoided because the system may
“hunt “ for an operating point, leading to unsteady flow
situation
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Required net head, H required
This equation is evaluated from
inlet(upstream) to outlet (downstream).
total Lturbineu pumprequired hh z z g
V V
g
P P
h H ,12
2
11
2
2212
, )(2
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Pump head delivered to the fluids does4 things:-
1. It increase it static pressure ofthe fluid from P1 to P2
2. It increases the dynamic pressureof the fluid from P1 to P2
3. It raises the elevation of thefluid from P1 to P2
4. It overcomes irreversible head
losses in the piping system
Thus at operating point:-
availablerequired H H
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PUMP CAVITATIONS & NPSH
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Pump cavitations & NPSH
When pumping liquids, it is possible forthe local pressure inside the pump tofall below the vapor pressure of theliquid.
When , vapor filled bubblescalled cavitations bubbles appear. Inother words, the liquid boils locally.
After cavitations of bubbles are formedthey are transported through the pumpregion where the pressure is higher,causing collapse of the bubble.
v P P
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This will cause noise, vibration, reduce
efficiency and damage to impeller blades.
Repeat collapse leads to erosion of the blade
and causing blade failure.
To avoid cavitations , pressure of the pump
should above vapor pressure.
NPSH- net positive suction head, define as the
difference between the pump inlets stagnation
pressure head and the vapor pressure head .
g
P
g
V
g
P NPSH v
inlet pump
2
2
v P P
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Pump manufacturers test their pump for
cavitations in the pump test facilities
by varying the volume flow rate and
inlet pressure.
They publishes performance parameter as
required net positive suction head
(NPSHrequired).(minimum NPSH necessary toavoid cavitations in the pump)
NPSHrequired increases with volume flow
rate.
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Cavitations damage in an impeller Source : http://www.controlengeurope.com/article/18406/Balancing-pump-efficiency-with-NPSH.aspx
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• At the point whereby the NPSH and NPSH required
intersect, the maximum volumetric flowrate can
be estimated
•
To make sure there is no cavitations , actualNPSH should be higher then NPSC required.
• Value of NPSH varies not only with flow rate,
but also with liquid temperature.
• It also depend on type of the liquid being pump.18
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How to increase available NPSH?
• Lower the pump/ raised the inlet
reservoir level.
• Use larger diameter of pipe.
• Reroute the piping system such thatfewer minor losses
• Shorten the length of the pipe upstream
of the pipe• Use smoother pipe
• Use elbow with minor loss coefficient.
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• To avoid
*catastrophic
situation
NPSH(A) > NPSH(R)
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NPSH(A)= the Net Positive
Suction Head available at
the pump
NPSH(R)= The NPSH that
required by the pump to
operate without
cavitation occurring
*Not taking the suction static headinto account fully can cause
catastrophic cavitation to occur in the
pump
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Pump in series & parallel
We need to consider pump in series or using largerpump to increase the volume flow rate / pressure.
Arranging dissimilar pump in series/ parallel may lead
a problem especially when the other pump is larger
than another.
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In series
In series-the combine net head is the sum of the nethead of each pump (at given flow rate)
n
i
icombined H H 1
Pump 3 is the strongestand pump 1 is the weakest.
The shut off head of the 3pumps combined in seriesis equal to the sum of theshut off head of individualpump.
Net head of the three
pumps in series =H1+H2+H3.
The individual pumpshould be shut off and bypasses at flow rates larger
than that pump delivery.22
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Parallel Combine capacity for n pumps in parallel
n
i
combined V V
1
1
The free delivery ofthe three combinedpumps are equal to thesum of the freedelivery of each
individual pump.
To avoid pump damageand loss of combinedcapacity, anyindividual pump should
be shut off at net headlarger than that pump’sshut off head.
That pump branch shouldbe block with valve.
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Example
The 11.25 inch impeller option of the centrifugal pump is
used to pump water at 25o
C from a reservoir whosesurface is 1.2 m above the centerline of the pump
inlet. The piping system from the reservoir to the pump
consist of 3 m cast iron pipe with an ID of 0.1m and an
average inner roughness height of 0.5mm. There are
several minor losses: a sharp edge inlet (KL =0.5),
three flange smooth 90o regular elbow (KL=0.3 each),
and fully open flanged globe valve (KL=6).Estimate the
NPSH when the flowrate is 400gpm.(α=1.05)
1
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