System NPSHavailable

NPSHavailable = NPSH of system pe = Pressure available system inlet liquid level pb = Barometric pressure pD = Vapour pressure of pumped fluid at the pump suction inlet ρ = Density of the pumped fluid at the pump suction inlet g = Local gravitational acceleration (9.81 m/s2) ze = Static level difference between system inlet liquid level and a reference level, the negative sign becoming applicable if the reference level is above the system inlet liquid. The reference point is the centre of the impeller. Hv = Friction loss in suction-side system.The reference point for the NPSH value is the centre of the impeller, i.e. the intersection of the pump shaft axis with the plane perpendicular to it through the outer points of the blade leading edge.The duty point of a centrifugal pump can only be a permanent duty point if complying with:NPSHavailable > NPSHrequired + safety margin.

Self-Priming Centrifugal Pumps

The pump suction pipe is then air-evacuated without any special external suction devices.Centrifugal pumps without external or internal suction devices can be operated at a suction lift if the pump body is filled with water before starting the actual fluid displacement. Due to the function of a non-return valve the fluid will be retained within the pump body after switch-off.The construction required for for self-priming capability has a negative effect on the pump efficiency.

Rated Pressure

It is stated without a unit quantity. Its numerical value states the permissible working gauge pressure in bar at a temperature of 20°C (68°F) to which a component or specified material may be subjected. In the field of pump technology, it is common to use pressure ratings according to DIN 2401 for the range from PN 1.6 to PN 200.Preferential designation for pressure ratings are 1.6; 2.5; 4; 6; 10; 16; 25; 40….

Cavitation

This occurs if the static pressure in the fluid falls below the temperature-related vapour pressure. A forceful implosion of the bubbles occurs when the fluid stream reaches a region of higher pressure.Cavitation can lead to premature material wear and noise emission. Cavitation should therefore be avoided as far as possible.

Minimum Inlet Pressure

Pumps require, in order to ensure trouble-free operation, a minimum static inlet pressure at the pump suction port, which is generally termed the minimum available net positive suction head (NPSH).This available inlet pressure decreases with the flow through the pump on account of the velocity change of the liquid in the suction section and the impeller. The critical location is the entry into the impeller suction neck. If the pressure then falls below the operating vapour pressure of the fluid, the fluid flashes into a vapour and forms bubbles.The ensuing flow through the impeller leads to an hydraulic pressure build-up. Within this region the bubbles collapse or implode with a tremendous shock on the adjacent surfaces causing vibration and even destruction in the form of pitting or erosion. This process, termed Cavitation, is accompanied by low rumbling or sharp rattling noise with rising intensity as cavitation increases.The pumping performance is also impaired as the pump is now handling a water/gas mixture. A minimum static inlet pressure must thus be maintained at the pump suction port in order to avoid cavitation.The level of this required pressure depends on:

– the temperature of pumped liquid – required net positive suction head HH (termed: NPSHrequired) corresponding to the pump duty point.

The required net positive suction head (HH or NPSHrequired) relates specifically to the pump and is normally depicted by pump manufacturers as an NPSH curve in the pump duty chart.Generally there is a strong relationship with the pump speed. For constant pump types the following is true:

– High speed -> High NPSHrequired – Low speed -> Low NPSHrequired

It is recommended practice to raise these levels by a safety margin of 0,5 m (1,6 ft) to compensate for possible variations on duty point calculations when selecting the pump. General standards allow a minimum level of cavitation at the determined NPSHrequired which:

– reduces the Pumping Head at the duty point by 3% or less – does not cause any material deterioration, performance reduction or reduce service life.

Such acceptable levels of cavitation may however cause unacceptable noise.To ensure best performance, it is common practice to load the NPSHavailable calculation with a safety margin of approx. +1 to +5 m (+3 to +15 ft), depending on speed and Duty Point of the pump.