81 General Pump Selection

1.3NPSH value

The NPSH value (Net Positive

Suction Head) is significant in

assessing the suction

characteristics of a centrifugal

pump: It describes the minimum

inlet pressure required by each

centrifugal pump for reliable

and cavitation-free operation

[1.13]. We need to distinguish

between the permissible

cavitation, which causes the

developed head of the pump to

drop by 3 percent (also referred

to as NPSHr = required) and the

NPSH value of the system (also

referred to as NPSHa =

available).

Cavitation-free pump operation

generally requires the following

condition to be met:

The safety allowance is defined in

accordance with ATV (German

Association for Water, Waste

Water and Waste) and HI

(Hydraulic Institute) as 30 % of

NPSH3% of the pump. NPSHa can

be calculated using equation (10a).

NPSHa ^ NPSHr , NPSHr = NPSH3% + safety allowance

(9)

NPSHa = ze + + - HI

pe + pb - pD

g 2 gve

2

(10a)

This equation can be simplified

for an open system installed at

altitudes up to 3300 ft AOD and

a fluid temperature of 86 F:

NPSHa = ze + 10 m

(10b)

To be able to document the

NPSH3% value of a pump in the

sales literature, pump manufac-

turers conduct a test run in a test

field using a special dry pump

installation. In a wet installation

this NPSH value is

practically impossible to measure.

As the NPSH3% value changes

in relation to the flow rate it is

entered as a function of the

flow rate NPSHr = f(Q). This

value describes the pressure head

in meters or feet required above

the vapor pressure of the fluid

at the impeller inlet (reference

point for NPSH = intersection

of pump shaft axis and the

perpendicular plane through

the extremities of the vane

leading edge, sometimes

referred to as the impeller eye,

see Fig. 1.3).

PS'

PS'

PS' PS'PS' PS'

Fig. 1.3: Position of the reference point Ps for various impeller types (Source: KSB publication Selecting Centrifugal Pumps)