Chapter 10 Pump

8/3/2019 Chapter 10 Pump

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Equipment for

Fluid Mechanics


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AssignmentWith a group of three students, prepare areport on types of pump and compressor.

All references should be cited.

# of pages : more than 13 pages not including

references.Font: Arial 11, spacing 1.5

Due date: 15/10/09 (Thursday) before 5pm.


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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 compressor

and design calculation.

4.Able to calculate the work done by the

compressor.


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Pump

Very important mechanical devices in industry,

agriculture and in daily live.

Pump has 2 important functions :

a) ability to supply liquid at required rate of flow.

b) impart energy to the liquid, thereby

increasing the pressure.

Types of pump?


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Pump

Capacity of the pump/Volume flow rate

Performance of the pump is characterize by nethead, H.

Efficiency

Brake horse power,bhp=

mQ

inout

z

g

V

g

Pz

g

V

g

PH

22

22

shaft

horsepowerwater

shaft

horsepowerwater

pumpT

HVg

bhp

W

W

W

(rad/s)= rotational speed of the shaft

Torque supply to the shaft

shaftshaft TW


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Pump performance curve

Free delivery Shut off

H=0 Q=0

Is achieve when there is no flowrestriction at the pump inlet/outlet-noload to the pump

Is achieve when the outlet port of pumpis blocked off.

Q is very large, but H=0; the pumpefficiency is zero because the pump didnot do any useful work.

H is very large but Q =0, the pumpefficiency is zero


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For given piping system, major +minor losses, elevation changes

caused the required net head increase with the volume flow rate.

The pump efficiency reach the maximum value between shut off

condition and the free delivery condition. (Note as H*, bhp* and

V@Q *)

For steady condition, the pump can operate only along the

performance curve

Available net head of pumps decreases with flowrate

.


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V

Operating point of a piping system is established as thevolume flow rate where the system curve and the pumpperformance curve intersect.

BEP should be close to operating point for best efficiency


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In unfortunate situations the system curve and the pumpperformance curve intersect at more than one operating

point. It can happen when the system curve is almost flat & meet a

pump 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 flowsituation


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Required net head, H required

This equation is evaluated from

inlet(upstream) to outlet (downstream).

totalLturbineupumprequired hhzz

g

VV

g

PPhH

,12

2

11

2

2212

,)(

2


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Pump head delivered to the fluids does 4 things:-

1. It increase it static pressure of the fluid from P1

to P2

2. It increases the dynamic pressure of the fluid

from P1 to P2

3. It raises the elevation of the fluid from P1 to P24. It overcomes irreversible head losses in the

piping system

Thus at operating point:-availablerequired HH


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Ex 1:

A local ventilation system is used to remove air andcontaminated product by dry cleaning operation. Theduct is round and is constructed of galvanized steelwith longitudinal seams and with joints every 0.76m.The inner diameter (ID) of the duct is D=0.23m, and itstotal length, L =13.4m. There are 5 elbows along theduct. The equivalent roughness height of this duct is0.15mm, and each elbow has minor loss coefficient ofKL = 0.21.To ensure adequate ventilation, theminimum required volume flowrate through the duct is0.283 m3/s at 25C.From manufacturer list, hood entry

loss coefficient is 1.3 based on duct velocity. When thedamper is fully open, its lost coefficient is 1.8.


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A centrifugal fan with 0.23 inlet and outletdiameter is available. Its performance dataare shown below. Predict the operating

point of this local ventilation system anddraw a plot of required and available fanpressure rise as functions of volume flowrate. Is the chosen fan adequate.

Q cmm (P fan,mm H2O)

0 22.9

7 24.114 22.9

21 19

28 10.2

34 0


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Pump cavitation & NPSH

When pumping liquids, it is possible for thelocal pressure inside the pump to fall belowthe vapor pressure of the liq,

When , vapor filled bubbles calledcavitations bubbles appear. In other words, theliquid boils locally.

After cativation of bubbles are formed they aretransported through the pump region wherethe pressure is higher, causing collapse of thebubble.

vPP

vPP


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This will cause noise, vibration, reduce

efficiency and damage to impeller blades.

Repeat collapse leads to erosion of the bladeand 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

PNPSH v

inletpump

2

2

vPP


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Pump manufacturers test their pump for

cativation 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 cativation in the pump)

NPSHrequired increases with volume flow rate.


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At the point whereby the NPSH and NPSH requiredintersect, the maximum volumetric flowrate can beestimated

To make sure there is no cativation, actual NPSH should behigher then NPSC required.

Value of NPSH varies not only with flow rate, but also withliquid temperature.

It also depend on type of the liquid being pump.


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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 that fewer minorlosses

Shorten the length of the pipe upstream of thepipe

Use smoother pipe

Use elbow with minor loss coefficient.


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Pump in series & parallel

We need to consider pump in series or using

larger pump to increase the volume flow rate /

pressure.

Arranging dissimilar pump in series/ parallel

may lead a problem esp 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 net head of each pump (at given flow rate)

n

i

icombined HH1


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In figure above pump 3 is the strongest and

pump 1 is the weakest. The shut off head of

the 3 pumps combined in series is equal to thesum of the shut off head of individual pump.

Net head of the three pumps in series =

H1+H2+H3.The individual pump should be shut off and by

passes at flow rates larger than that pump

delivery.


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ParallelCombine capacity for n pumps in parallel

n

i

combined VV1

1


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The free delivery of the three combined pumps

are equal to the sum of the free delivery ofeach individual pump.

To avoid pump damage and loss of combined

capacity, any individual pump should be shut offat net head larger than that pumps shut off

head.

That pump branch should be block with valve.


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Example

The 11.25 inch impeller option of the

centrifugal pump is used to pump water at25oC from a reservoir whose surface is 1.2 m

above the centerline of the pump inlet. The

piping system from the reservoir to the pumpconsist of 3 m cast iron pipe with an ID of 0.1m

and an average inner roughness height of

0.5mm.


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There are several minor losses: a sharp edge

inlet (KL =0.5), three flange smooth 90o

regular elbow (KL=0.3 each), and fully openflanged globe valve (KL=6) estimate the

maximum volume flow rate(in gpm) that can

be pumped without cativation. If the waterwere warmer, would this maximum flow rate

increase/ decrease?

Chapter 10 Pump

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