Tag: Self-Priming Centrifugal Pumps

Suction operation with normal priming centrifugal pumps

Posted on by bs
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.

Suction Head With Non-Selfpriming Centrifugal Pumps

Posted on by bs
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.