Tag: Avoiding Cavitation

Suction operation with normal priming centrifugal pumps

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This means that the local air pressure p<sub>b</sub> is higher than the product of the holding pressure head HH and the vapour pressure and makes a supply pressure at these temperatures unnecessary. This correlation is causally related to the drastic decrease in vapour pressure at cold water. In practice this means:Pumps with negative minimum head H<sub>req</sub> are able to operate in suction mode (not self-priming).The size of the suction capacity corresponds approximately to the value of the negative minimum suction head minus 1m safety range.Since the pumps normally used in building services engineering do not normally self-priming, the following conditions must be met to ensure suction operation:
  • Filling and venting of the suction-side pipeline including the pump before commissioning.
  • Prevention of air intake during pump operation (in case of air pockets, collapse of the suction function).
  • Prevention of the suction line running empty when the pump is at a standstill by using a foot valve (danger of leakage in case of contamination).
Non-return valves in the discharge line are not sufficient, as air can be sucked in via the shaft seal (mechanical seal or stuffing box) when the pump is at a standstill.In general, the suction capacity of normally priming pumps is limited to a range of max. 2 to 4 m due to their design. For higher suction heights max. 8 to 9 m and for self-priming special pumps must be used.

System NPSHavailable

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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.

Suction Head With Non-Selfpriming Centrifugal Pumps

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That means that the local atmospheric pressure pb is higher than the sum of net positive suction head HH and vapour pressure pv; the inlet pressure is thus no longer required. This interrelation is based on the drastic reduction of the vapour pressure of cold water. In practice, that means:Pumps operating at a negative minimum inlet pressure Hreq are capable of creating a suction lift (not self-priming).The suction capacity is approximately equal to the level of the negative minimum inlet pressure minus 1m safety factor.As pumps normally used in conjunction with building services are generally not of selfpriming characteristics, the following conditions for suction lift operation must be met:
  • Filling and venting of the suction-side pipework including the pump before commissioning.
  • Avoiding air entrainment during pump operation (aeration will result in break-down of suction capability).
  • Avoiding drainage of suction-side piping on standdown of pump by providing and installing a footvalve (danger of leakage due to dirt particles).
Reliance on non-return valves in the discharge pipe is not sufficient, as air can be entrained by way of the shaft seal (mechanical or packed gland seals) on pump standdown.The suction capability of non-selfpriming pumps is, on account of their construction features, generally limited to the range of max. 2 to 4 m. Higher suction lifts (max. 8 to 9 m) and selfpriming operation require the use of Special Pumps.

Self-Priming Centrifugal Pumps

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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.

Cavitation

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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.

Net positive suction head (NPSHr)

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Generally recognizable is the strong dependence on the pump speed. If the construction is unchanged:High speed -> High holding pressure head Low speed -> Low holding pressure headIn order to take account of any uncertainties in the design of the duty point, these values must be increased by a safety margin of 0.5 m when selecting the pump.By definition a minimum cavitation is permissible at NPSH, whereby the following conditions are allowed:
  • The head of the pump at the nominal point is reduced by 3%.
  • No material damage impairing the function and service life occurs.
Due to the permissible cavitation, cavitation noises can still occur, some of which are perceived as disturbing.In order to eliminate the residual cavitation, it is necessary to provide the calculated minimum inlet head with a surcharge of approx. + 1 to + 5 m. The minimum inlet head must be calculated with a surcharge of approx. This surcharge depends on the speed and the operating point of the pump.

Minimum Inlet Pressure

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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.