Suction pressure of a pump lower than atmospheric pressure

No the main reason that the suction pressure is lower than atmospheric pressure is because the action of the impeller creates an area of lower pressure ie: vacuum in the center of the impeller. Atmospheric pressure acting on the surface of the liquid forces the liquid into the suction line attempting to fill the area of lower pressure. The area of lower pressure is maintained by the continued action of the impeller in evacuating the liquid from the center of the impeller.

No centrifugal pump is truly self priming because they all require liquid to be evacuated from the center of the impeller by the rotation of the impeller to cause the area of lower pressure. Therefore they must initially start full of liquid. Self priming centrifugal pumps are often of a style where the impeller sits in the housing below both the suction and the discharge thereby remaining full of liquid all the time. Provided the suction lift is moderate they can often "self prime" using the volume in the pump case.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

I don't what you mean by "main" reason, there can be different factors of varying preponderance when combined.
The friction losses in the suction line and the geometric head at the suction may effect the pressure at the suction. The liquid being pumped may also be in pressurized or vacuum condition in a tank or reservoir which would contribute in having a suction pressure at the pump inlet varying from Patm.

By the way, having suction pressure lower than atm. pressure does not necessarily mean cavitation or inception of cavitation occur. That phenomenon relates to the vapor pressure of the liquid, thus the concept of net positive suction head available vs. required (given by the manufacturer based on operation and pumps characteristics and the acceptable drop of head - to make it short...) - keeping in mind a certain safety margin to be observed.

Self-priming is, I would say, a design feature or option and is not the only way to prime the pump. It can be a manual procedure for instance that needs to be applied.

I agree with my posters above, but clearly at no flow conditions if the inlet line is full of water then having the water level below pump inlet level will be registering a negative pressure compared to atmospheric pressure for an "open" pond. If your liquid level is inside a tank which can be pressurized, then this might not be the case depending on the pressure above the liquid level.

Centrifugal pumps require a flooded suction to start and work effectively. There are a number of ways this can happen.

Self priming pumps, either centrifugal or PD type. Note the self priming centrifugal pumps always need some water stored inside them to work.
Just turn it on and hope the pump will self charge
Use a vacuum line or air pump to produce a negative pressure in the inlet to raise water from the water level
Use a "foot valve" aka non return valve on the inlet to prevent draining back ( tend not to work too well after a while)
Lower the pump or use a submersible pump or a shaft driven pump

Lots of options and your choice will depend on your flow rates, size, amount of "lift" required.

LI



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Not all so called "self-priming pumps" need water storage within the casing to effect prime, those that do are not true self-priming pumps, Vacuum assisted self-priming pumps are the only TRUE self-priming pumps and can be primed from a dry situation.
Vacuum assisted self-pumps can be equipped with an integral vacuum pump or a air driven ejector or operated by an externally vacuum or high air pressure source.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

Artisi,

Had not come across these before now - thanks for info. Always thought these bits cam separate from each other, but the powers of Mr Google show me differently.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

There are two broad categories of self primers: dry prime, and wet prime. Wet prime require an initial priming, and then maintain liquid in the casing to allow the pump to reprime. Dry prime units use a vacuum or a venture device to create a vacuum in the casing to prime the pump.

Obviously, the 'wet prime' version may be a poor choice for outdoor unheated locations in a freezing climate.