Net Positive Suction Head.

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, characterised 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 code 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 difference in height 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 friction loss in the suction line and its accessories, such as: fittings, foot valve, gate valve, elbows, etc.
hpv 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.

Water Temperature (°C) 20 30 40 50 60 70 80 90 100
Suction Loss (m) 0.1 1.0 2.3 3.6 6.3 10.5 15.5 23 33.5
Elevation Above Sea Level (m) 500 1000 1500 2000 2500 3000
Suction Loss (m) 0.55 1.1 1.65 1.65 2.75 3.3

Friction loss must be calculated using a recognised formula. To reduce it to a minimum, especially in cases of high suction head (over 4-5m.) or within the operating limits with high delivery values, we recommend using a suction line with a larger diameter than that of the pump’s suction inlet. 

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 g = 1 Kg/dm3
Delivery Required:
30 m3/h
Head for Required Delivery:
43 m
Suction Difference in Height:
3.5 m

The selection is a FHE 40-200/75 pump whose NPSH required value is, at 30 m3/h, 2.5 m

For water at 15°C the hpv term is PV = 0.174 m (0.01701 bar)

eh = Pa/g ÷ 8 = 10.33m

The hf friction loss in the suction line with foot vales is 1.2m

By substituting the parameters in the 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 relationship is therefore verified.


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