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.
It is highly recommended to always select the smaller pump if the specified system duty point is between two possible pump curves. The resulting capacity reduction has, in heating systems, no appreciable effect on the effective heating performance. The positive effects are lower noise levels, lower investment costs and improved economy. For heating installations it is customary to undersize pump capacities up to 10% below the specified duty.
To avoid Cavitation (vapour formation within the pump) it is necessary to maintain at the pump suction port an adequately high positive pressure (static head) in relation to the vapour pressure of the fluid being handled. The minimum required inlet heads for Glandless pumps are generally listed in pressure charts. Glanded pumps require calculations in accordance with the NPSH information.
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.
It is calculated from the absolute energy level minus the evaporation pressure level. The evaporating head shall be calculated with the evaporating pressure corresponding to the temperature prevailing in the inlet cross-section of the pump.
The NPSHavailable value is the system-specific NPSH relating to the given flow rate and the fluid characteristics.
The NPSHrequired is the lowest NPSH value at which definite cavitation criteria (i.e. wear due to cavitation, vapour formation, vibration, noise, head loss) can be contained.
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.
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.
Increasing the available static pressure at the pump location (positioning the pump at a lower level, e.g. from a roof plant room into the basement to benefit from the increased static head from the higher water column. The pump performance remains unchanged).
Lowering the fluid temperature (reducing the vapour pressure pD).
Changing the pump characteristics (reducing the driven speed and/or different pump design with a lesser Pump NPSHrequired).
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.
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 head
In 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.
The vapour pressure of the pumped fluid greatly influences Cavitation Behaviour and thus the System NPSHavailable.
The vapour pressure is one of the important liquid properties for pump selection and depends on the fluid temperature.
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