Self-priming pumps - how do they work?

[almacphe]

Hello,

I am curious about the principle of self-priming pumps. From what I can find they can't actually "prime" a pump for start-up but rather keep it primed in the event air gets into the line. Is this true or is it possible to get a self-priming pump that will prime a pump for start-up so that a second priming pump for manual pump isn't required?

 

[MikeHalloran]

The garden variety self- priming centrifugal pump has an impeller and a volute just like any other centrifugal, but the pump's actual suction port draws from the bottom of a large cavity. The suction port on the pump's exterior fills that cavity, near the top. When the pump stops, the cavity retains enough liquid to get the pump running the next time it starts.

Note that the pump won't fill itself from a dry condition, i.e., you have to prime it once when it's new.

You can get nearly the same effect by fitting a large trap to the suction of an ordinary pump.





Mike Halloran
Pembroke Pines, FL, USA

 

[Zapster]

Is this true or is it possible to get a self-priming pump that will prime a pump for start-up so that a second priming pump for manual pump isn't required?

Yes they are readily available in many styles. What are you trying to pump and what is the suction lift required. Positive displacement pump will create a vacuum lift however; ensure that the pump you select is suited for the dry running that takes place during the vacuum lift. Some turbine pumps are also suitable for suction lifts.

Gear pumps used to be the standard for lifting fuel from below grade tanks. These self priming pumps have all but disappeared from the automotive filling stations, but are still used for fuel oil burners on boiler for homes and industry. Now submersibles are common-place at the filling stations.

 

[Artisi]

There are many manufacturers throughout the world who make "true" self-priming pumps - many have standard designs while some will custom built to suit your requirements.

There are 2 main types of priming systems - the first usually consists of a vacuum pump positioned at the pump inlet and protected from the pumped product by a air/ water valve system to evacuate all the air from the pump, the second uses an air-compressor and an ejector to achieve the same thing.



Naresuan University
Phitsanulok
Thailand

 

[ekschwab]

Many positive displacement pumps will truly self-prime.

Check out page 2.324 of the Pump Handbook (2nd Ed.) for an automatic priming system for centrifugal pumps.

 

[DiaphragmPumps]

Self priming pumps generally all use the same principle of reducing the pressure on the suction side, so atmospheric pressure pushes the product into the pump.
Centrif's generally have a trap in the volute as described above, which holds a small amount of liquid, so upon starting this liquid is ejected making the suction. PD pumps generally use an increase in volume of a chamber to ensure this pressure drop. see the faq section for a diaphragm pump here

http://www.diaphragmpumps.co.uk

here is another good site demonstrating principles

http://www.animatedsoftware.com/ascodesc/pumpdesc.htm