chapter 10-equipment for fluid mechanic

7/27/2019 Chapter 10-Equipment for Fluid Mechanic

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CHE 493: fluid mechanics

Chapter 10

Equipment for fluid mechanics

1



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.

2



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

3



Types of pump•

DISPLACEMENT PUMP• Piston pump

• Diaphragm pump

• Rotary pumps

• Gear pump• Peristaltic pump

•

DYNAMIC PUMP• Centrifugal pump

• Axial Flow pump

• Mixed Flowpump

4



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

5



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

6

It must be higher because there is irreversible losses

due to friction

Power delivered to

the fluidMechanical energy

supply to pump



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.

7



• 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

8



Pump performance curve

Pump performance curves for a centrifugal pump 9



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

efficiency 10



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

11



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

12



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

13



PUMP CAVITATIONS & NPSH

14



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

15



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

16



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.

17

Cavitations damage in an impeller Source : http://www.controlengeurope.com/article/18406/Balancing-pump-efficiency-with-NPSH.aspx



• 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



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.

19



• To avoid

*catastrophic

situation

NPSH(A) > NPSH(R)

20

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



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.

21



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

ll l



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.

23

l



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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chapter 10-equipment for fluid mechanic

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