5 - Pump-pipeline System Analyses Design

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Pump Pipeline

Analyses & Design


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1 - Introduction

This chapter : analysis & design of pipe systemincorporating rotodynamic pumps

Pump selection CE concerned with: Design of river abstraction

Borehole supplies from g/w & surface water

Foulwater drainage from low-lying land

3 categories of rotodynamic pumps (according to shapeof impellers): Centrifugal (radial flow)

Mixed flow Propeller (axial flow


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Specific Speed (Ns):

(1)

where Q: discharge

H: total head

N: rotational speed (rev/min) Expression interpreted as speed in rev/min

Total head generated by a pump is also called the manometrichead (Hm) since it is the difference in pressure head recorded

by pressure gauges connected to the delivery and inlet pipeson either side of the pump, provided that the pipes are of samediameter.

4/3H

QNNs

Pump type Ns range

Centrifugal

Mixed flow

Axial flow

Up to 2600

2600 to 5000

5000 to 10 000


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2 Head terms in pumping

Static suction lift Vertical distance fr water level in source of tank to centerline ofpump

If pump lower than source tank, static suction lift is -ve

Static discharge head Vertical distance fr centerline of pump to w/l in discharge tank

Total static head Static suction lift + static discharge head

Total dynamic head (TDH) Static head + friction loss + minor losses a.k.a total head


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Figure 1 Head terms in pumping


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3 System head curve For any piping system, the friction loss, hf and the

minor loss, hm can be expressed in terms of the flowthrough the system.

(2)

or,

(3)

Plot of eq (3) between Hp versus Q, is known as thesystem head curve.

The curve represents behavior of the piping system,important in selection of a pump.

g

kV

gD

LVZHp

22

22

4

2

5

2

81.0DKQ

DLQ

gZHp


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Figure 2 Typical system head curve


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4 Pump Characteristics Curve

For a given pump at a given speed, there are definite relationshipsamong the pump discharge capacity, head, power, and efficiency.

Relations are derived from actual tests on a given pump or similarunit and are usually depicted graphically by the pump characteristicsor performance curves, comprising the following:

Pumping head versus discharge Power input (P) versus discharge

Efficiency () versus discharge

Energy imparted to fluid is gQHm, & pump efficiency may be

derived as

(4)P

gQHm


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General shape of curves varies with size, speed &

design of particular pump Important feature : increase in head reduces capacity

At given speed, pump is rated at the head and Q, whichgives max efficiency, referred as best efficiency point (A)

Figure 3 Pump Characteristic Curve


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5 Single Pump & Pipeline System Primary requirement : to determine a suitable pump and

pipe combination for the required design discharge.

Fig 4 : pump must generate total head equal to Hst plus

the pipeline head losses at Q. Manometric head is defined as rise in total head across

pump.

(5)

with:

Thus, (6)

g

V

g

P

g

V

g

PH ssddm

22

22

g

VhZ

g

Ph

g

VZ

g

P dld

d

ls

ss

2;

2

2

2

2

1

lsldstm hhHH


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Fig 4 Simple pumping main


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Discharge is a function of both pump andpipeline.

For a given system, head-discharge

characteristic curves for the pump may besuperimposed on that for the pipeline (Fig 5).

Point of intersection of the two characteristics

curves locates one possible combination of headand discharge for the system under steady flowconditions.

The intersection point is referred to as theoperating point.


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Figure 5 Pump and pipeline characteristics curves


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Example 1Calculate the steady discharge of water between the tanks

in the system shown in Figure below and the power

consumption. Pipe diameter (Ds = Dd) = 200 mm; Length =

2000 m; k = 0.03 mm (uPVC). Losses in valves, bends plus

the velocity head amount to 6.2 V2/2g. Static lift = 10.0 m.


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Example 1

Pump characteristics

The efficiencies given are the overall efficiencies of the

pump and motor combined.

Discharge

(l/s)

0 10 20 30 40 50

Total Head(m)

25 23.2 20.8 16.5 12.4 7.3

Efficiency

(%)

- 45 65 71 65 45


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6 Multiple Pump SystemsA. Pumps in parallel

Pumping stations frequently contain several pumps in parallelarrangement. (Fig 6a). Any number of the pumps can be

operated simultaneously

Objective: deliver a range of discharges. Common feature of

sewage pumping stations where inflow rate varies. By automatic switching according to the level in the suction well,

any number of the pumps can be brought into operation.

Fig 6 (a) Pumps operating in parallel


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In predicting H vs Q curve for parallel operation, it isassumed that head across each pump is the same.

Thus, at any arbitrary head, individual pump dischargesare added (Fig 6b).

(7)pnppnp nQQHH

6 (b) Characteristic curves foridentical pumps operating in

parallel


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B. Pumps in Series Basis of multistage and borehole pumps; the discharge from the

first pump (or stage) is delivered to the inlet of the second pump,

and so on.

Same discharge passes through each pump receiving a pressureboost in doing so.

All the pump in series system must be operating simultaneously.

(8)pnppnp QQnHH

Figure 7

Pumps operating

in series


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7 Variable speed pump operation By using variable speed motors, Q of single

pump can be varied to suit operating

requirements of the system. Using dimensionless analysis and dynamic

similarity criteria, it can be shown that if the

pump delivers a discharge Q1 at manometrichead H1 when running at speed N1; thecorresponding values when the pump is runningat speed N2 are given by:

(9)2

1

212

1

212 )()(

N

NHH

N

NQQ


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In constructing the characteristic curve for speed N2,several pairs of values of Q1, H1 from the curve for N1

can be obtained and transformed into homologous pointsQ2, H2 on the N2 curve. (Fig 8).

Figure 8 Effect of speed

change on pump

characteristics


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Example 2

A centrifugal pump delivers 30 liters ofwater per second against a head of 12

metres and running at 1200 r.p.m. requires

6 kW power. Determine the discharge,head of the pump and power required, if

the pump runs at 1500 r.p.m.


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8 Suction lift limitations Cavitation : phenomenon consists of local vaporization of

a liquid

Occurs when absolute pressure falls to the vapourpressure of the liquid at the operating temperature

Can occur at the inlet to a pump and on the impellerblades, particularly if the pump is mounted above the

level in the suction well. Cavitation causes physical damage, reduction in

discharge and noise.

To avoid: pressure head at inlet should not fall below a

certain minimum which is influenced by the furtherreduction in pressure within the pump impeller. (Fig 9)


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If Ps represents the pressure at inlet, then is

the absolute head at the pump inlet above the vapour

pressure (Pv) and is known as the net positive suctionhead (NPSH).

Figure 9 Head conditions in suction pipe

g

pp vs


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NPSH = (10)

Where Pa : ambient atmospheric pressure; hs : suction lift;

hls : total head loss in suction pipe; Vs : velocity head in suction pipe; :density of liquid.

Value of NPSH can be obtained from the pump manufacturer;values must not be exceeded to avoid cavitation.

Thoma introduced a cavitation number:

(11)

In recent years electro-submersible pumps in the smallto medium size range have been widely used. Theyeliminates the need for suction pipes; problems ofcavitation and cooling are avoided.

g

Vhh

g

pp

g

pp sfss

vavs

2

2

mH

NPSH


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9 Installation of Centrifugal Pumps

Pump is installed between two valves for easy removal in case ofmaintenance.

On suction side, a combined bellmouth entry and strainer are necessary,

together with a non-return valve to ensure self-priming. On delivery side, a second non-return valve is necessary to prevent

damage from possible surge pressures.

In addition, an air valve and flow meter (venturi type) are desirable.

Figure 10 Typical centrifugal

pump installation

5 - Pump-pipeline System Analyses Design

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