ITT BPHV manual Npsh

NPSH

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 vapor-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 vapor pressure of the liquid.

The vapor-filled cavities flow with the current and when they reach a higher pressure ares the vapor 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 de- formed and yield due to fatigue. This phenomenon, char- acterized 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 feet) of the liquid measured at suction under conditions of incipi- ent cavitation, excluding the vapor pressure (expressed in feet) that the liquid has at the pump inlet.

To find the static height (hz) at which to install the ma- chine under safe conditions, the following formula must be verified:

hp + hz ≥ (NPSHr + 2 feet) + hf + hpv

where:

hp	is the absolute pressure applied to the free liquid sur-
	face in the suction tank, expressed in feet 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 feet;
	hz is negative when the liquid level is lower than the
	pump axis.
hf	is the flow resistance in the suction line and its acces-
	sories, such as: fittings, foot valve, gate valve, elbows,
	etc.

hpv is the vapor pressure of the liquid at the operating temperature, expressed in feet of the liquid. hpv is the quotient between the Pv vapor pressure and the liquid‘s specific weight.

0.5is the safety factor.

The maximum possible suction head for installation de- pends 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 (40ºF) 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	68	104 140 176 194 230			248
Temperature (°C)
Suction	-.7	2.3	6.6	16.4 24.3 50.5	70.5
Loss (ft)

Elevation Above 1600 3300 4900 6500 8200 9800

Sea Level (ft)

Suction	1.8	3.6	5.4	7.2	9.0	10.8
Loss (ft)

To reduce it to a minimum, especially in cases of high suc- tion head (over 13 – 16 feet) 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.

47