What is a self-priming pump?
We can define this as a pump that will lift fluid from a level below the pump and is capable of evacuating air from the pump suction line without any external auxiliary devices.
Self-priming Pump
We will cover the centrifugal self-primer in this article, with a brief mention of other types.
Standard pump (not self-priming)
The commonly found centrifugal self-primers have a built-in reservoir in the pump casing, giving it a ‘boxy’ appearance.
Self-priming pumps are generally used for emptying sumps and, as such, solid particles would frequently be encountered, so the pump impeller and seal must be suitable for handling these.
A semi-open impeller or one with generous passageways for passing the solids will be required. Likewise, the seal chamber should have a means of modifying the fluid flow path in this area and keeping the solids away from the intricate seal parts and faces.
The sequence of operation during the priming cycle is shown above.
When designing an installation intended for self-priming pumps, there are some basic rules which must be followed to avoid a costly disappointment!
- Generally, centrifugal self-primers can lift water about six metres (higher for lighter fluids, less for more dense fluids).
Ensure that the priming line is kept as short as possible between the free surface of the liquid and the suction nozzle of the pump in order to keep priming time reasonable.
Side Channel Pump
Remember that the priming time is a function of the suction piping volume. Ensure correct submergence to avoid vortexing/ingress of air which will reduce pump performance drastically.
- Check that there is sufficient NPSH (net positive suction head) available.
You may have to decrease the static lift, lower the fluid temperature or change the suction pipe size to satisfy the NPSH required by the pump and allow for a safety margin.
Air Operated Diaphragm Pump (AODD Pump)
You may have to play around with the size of the suction pipe in order that NPSH available is acceptable as well as keeping priming time low. An oversize suction pipe may contain too much volume!
- The suction line should slope towards the pump suction so that there is no possibility of air pockets which will impeded priming. No long horizontal runs!
- In addition, if the discharge line is not open ended i.e. no possibility of any back pressure, an air bleed line immediately after the pump discharge nozzle must be installed.
This enables the air from the suction line to be easily evacuated (the pump is not a compressor!). It must be open to atmosphere in order to work, and usually spills back to the sump.
Peristaltic Pump (Hose Pump)
The integral priming chamber ensures that adequate liquid is retained so that priming can be repeated.
Other types of pumps that can self-prime are side-channel pumps (lifts up to 10m, but limited solids handling ability), air operated diaphragm pumps (wet lift c5.5m, dry 2.5m) & peristaltic (hose) pumps, which can lift up to 9.5m.
There is a related article, on what is a sump pump? is available to view here: https://www.flexachem.com/what-is-a-sump-pump/
If in doubt ask Phil Soltan, Flexachem’s Pump Specialist, better known as 'Dr Pump'. Phil has been working in the pump industry for more than 40 years. He has an accumulated wealth of knowledge in all aspects of pump system analysis and addressing 'bad actors'. He has also worked for two major pump manufacturers along his career journey. Mobile: 086 1853782. Email: phil.soltan@flexachem.com Website: www.flexachem.com