CEA 4001 - Suction conditions

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60

9.6.1 General

Wherever

possible,

horizontal

centrifugal

pumps

should

be

used,

installed

with

a

positive

suction

head.

i.e.

in

accordance

with

the

following:

-

At

least

two

thirds

of

the

effective

capacity

of

the

suction

tank

shall

be

above

the

level

of

the

pump

centre

line;

-

The

pump

centre

line

shall

be

no

more

than

2

m

above

the

low

water

level

of

the

suction

tank

(level

X

in

8.3.5).

NOTE:

If

this

is

not

feasible,

the

pump

may

be

installed

under

suction

lift

conditions

or

vertical

turbine

pumps

may

be

used.

NOTE:

Suction

lift

and

submersible

pump

arrangements

should

be

avoided

and

only

used

when

it

is

not

practicable

to

arrange

positive

suction

head.

It

is

critical

that

suction

lift

arrangements

should

not

vary

from that

stated

in

9.6.2

and

as

shown

in

figures

5

and

6.

9.6.2

Suction

pipe

9.6.2.1

General

The

pump

suction

shall

be

connected

to

a

straight

or

taper

pipe

at

least

two

diameters

long.

The

taper

pipe shall

have

horizontal

topside

and

a

maximum

included

angle

not

exceeding

20º.

Valves

shall

not

be

fitted directly

to

the

pump

inlet.

The

suction

piping,

including

all

valves

and

fittings,

shall

be

designed

in

such

a

way

as

to

ensure

that

the available

NPSH

at

the

pump

inlet

exceeds

the

required

NPSH

by

at

least

1

m

with

the

maximum

demand flow

and

maximum

water

temperature

(see

table

15).

T

Table 15: Pump pressure and flow rating

Pipework

Hazard

Class

Rated

pump

flow

Pump

inlet

condition

Pre-calculated

LH/OH

Maximum

demand

flow

from

table

7

For

tanks,

with

water

supp

ly

at

low

water level

(see

X

in

Figure

4). For

b

ooster

pumps,

with

mi

nimum

town

main

pressure.

HH

1,4

x

Flow

Required

from

table

8

Fully

calculated

All

Maximum

demand

flow

Suction

piping

shall

be

laid

either

horizontal

or

with

a

continuous

slight

rise

towards

the

pump

to

avoid

the possibility

of

air

locks

forming

in

the

pipe.

A

foot

valve

shall

be

fitted

where

the

centre

line

of

the

pump

is

above

the

low

water

level

(see

8.3.5).

9.6.2.2 Positive head

The

diameter

of

the

suction

pipe

shall

be

no

less

than

65

mm.

Furthermore,

the

diameter

shall

be

such

that

a velocity

of

1,8

m/s

is

not

exceeded

when

the

pump

is

operating

at

maximum

demand

flow.

This

velocity may

be

increased

to

2,5m/s,

if

the

pressure

in

the

suction

pipe

is

not

more

than

0,4

bar

below

ambient pressure.

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