Xylem Lowara Scuba SC205C, Manual

22

TECHNICAL APPENDIX

The minimum operating values that can be reached 

at the pump suction end are limited by the onset of 

cavitation.

Cavitation is the formation of vapour-filled cavities 

within liquids where the pressure is locally reduced to 

a critical value, or where the local pressure is equal to, 

or just below the vapour pressure of the liquid.

The vapour-filled cavities flow with the current and

when they reach a higher pressure area the vapour

contained in the cavities condenses. The cavities 

collide, generating pressure waves that are transmitted 

to the walls. These, being subjected to stress cycles, 

gradually become deformed and yield due to fatigue. 

This phenomenon, characterized by a metallic noise

produced by the hammering on the pipe walls, is called

incipient cavitation.

The damage caused by cavitation may be magnified by

electrochemical corrosion and a local rise in 

temperature due to the plastic deformation of the 

walls. The materials that offer the highest resistance to

heat and corrosion are alloy steels, especially austenitic

steel. The conditions that trigger cavitation may be 

assessed by calculating the total net suction head, 

referred to in technical literature with the acronym 

NPSH (Net Positive Suction Head).

The NPSH represents the total energy (expressed in m.)

of the liquid measured at suction under conditions of

incipient cavitation, excluding the vapour pressure 

(expressed in m.) that the liquid has at the pump inlet.

To find the static height hz at which to install the 

machine under safe conditions, the following formula

must be verified:

hp

+

hz

≥

 (NPSHr + 0.5) + hf + hpv

where:

hp

   is the absolute pressure applied to the free liquid

        surface in the suction tank, expressed in m. of 

        liquid; hp is the quotient between the barometric

        pressure and the specific weight of the liquid.

hz

   is the suction lift between the pump axis and the

        free liquid surface in the suction tank, expressed

        in m.; hz is negative when the liquid level is

        lower than the pump axis.

hf 

   is the flow resistance in the suction line and its

        accessories, such as: fittings, foot valve, gate

        valve, elbows, etc.

hpv

 is the vapour pressure of the liquid at the

        operating temperature, expressed in m. of

        liquid. hpv is the quotient between the Pv vapour

        pressure and the liquid’s specific weight.

0,5

  is the safety factor.

The maximum possible suction head for installation

depends on the value of the atmospheric pressure

(i.e. the elevation above sea level at which the pump

is installed) and the temperature of the liquid.

To help the user, with reference to water temperature

(4° C) and to the elevation above sea level, the 

following tables show the drop in hydraulic pressure

head in relation to the elevation above sea level, and

the suction loss in relation to temperature.

Friction loss is shown in the tables at pages 117-118 of

this catalogue. To reduce it to a minimum, especially

in cases of high suction head (over 4-5 m.) or within

the operating limits with high flow rates, we recommend 

using a suction line having a larger diameter than that 

of the pump’s suction port. It is always a good idea to 

position the pump as close as possible to the liquid 

to be pumped.

Make the following calculation:

Liquid: water at ~15°C 

γ

 = 1 kg/dm

3

Flow rate required: 30 m

3

/h

Head for required delivery: 43 m.

Suction lift: 3,5 m.

The selection is an FHE 40-200/75 pump whose NPSH

required value is, at 30 m

3

/h, di 2,5 m.

For water at 15 °C

hp =  Pa / 

γ

 = 10,33m, hpv =  Pv / 

γ 

= 0,174m (0,01701 

bar)

The Hf flow resistance in the suction line with foot 

valves is ~ 1,2 m.

By substituting the parameters in formula      with the

numeric values above, we have:

10,33 + (-3,5) ≥ (2,5 + 0,5) + 1,2 + 0,17

from which we have: 6,8 > 4,4

The relation is therefore verified.

 Water 
 temperature (°C)

   20     40     60   80    90    110    120

 Suction 
 loss (m)

                 0,2    0,7   2,0   5,0   7,4   15,4   21,5

 Elevation above

 sea level (m)

   500   1000  1500   2000    2500   3000

 Suction

 loss (m)

               0,55    1,1    1,65     2,2      2,75     3,3

NPSH

1

1