Self priming centrifugal pump

[Jenya]

Hello,

Could you please help me with the following question.

We have a self priming centrifugal pump. The type that retains part of liquid inside the casing such as goulds 3796. We have tested the pump with two liquids, water and hexane. The suction side was blinded off with a blind flange with pressure gauge. Discharge side was also blinded of with a blind flange with pressure gauge and air bleed line. Bleed line is 8mm tube going into hexane tank.

Test 1: water 10C

Casing is filled with initial prime (water). pump started and discharged air through air bleed line. Firther pressure at the suction side dropped to -0.8 barg.

Test 2: hexane 15C

Casing is filled with initial prime (hexane). pump started and discharged air through air bleed line. Firther pressure at the suction side dropped to -0.2 barg. Hexane boiling temperature at -0.2 barg is 50C.

The question is why created suction pressure with water in the casing is lower than with hexane? Density? Viscosity?

 

[EdStainless]

What is the vapor pressure of water vs hexane?
Once you suck hard enough to boil the liquid you can't go any lower.
And remember, pumps deliver head, not pressure. so even if the head was the same the hexane would only be half or the pressure because it is half of the density.

I hope that you only ran for a few seconds in this condition. This is a good way to destroy seals and bearings.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[LittleInch]

This is not the way to test a pump like this. It will be about vapour pressure. Be aware that as the fluid recirculated it will heat up so although you start at 15C, it won't be that after for very long.

Why are you doing this. You should be able to work this out from the pump spec.

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

 

[Artisi]

EdStainless: he is talking about a self priming pump unit, and without checking I would say with double mech. seal arrangement with between seal lubrication - either grease or oil - no seal faces requiring product lubrication. As for bearings, none requiring any lubrication from pumped product, so would suggest this unit could run all day in a dry condition.

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.)