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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 metres) of the liquid measured at suction under conditions of incipient cavitation, excluding the vapour pressure (expressed in metres) 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 metres of liquid; hp is the quotient between the barometric pressure and the specific weight of the liquid.
hf
is the friction loss in the suction line and its accessories, such as fittings, foot valve, gate valve, elbows, etc.
hz
is the difference in height between the pump axis and the free liquid surface in the suction tank, expressed in metres; hz is negative when the liquid level is lower than the pump axis.
hpv
is the vapour pressure of the liquid at the operating temperature expressed in metres 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 understand the influence of water temperature and elevation, 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 must be calculated using a recognised formula. To reduce it to a minimum, especially in cases of high suction head (over 4-5 m) 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 good 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).
e h = Pa / g = 10.33 m
The hf friction loss in the suction line with foot vales is 1.2 m.
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.
