Jacking Oil Pump Air Lock Bleed Valve

[jgleeson]

Hi All,

I have recently installed a small bleed valve on our turbine jacking oil pump suction pipework to allow for the bleeding of built up air before operation, we have sloped the suction pipework to a high point and that is where the bleed valve is installed. This was due to previous air lock issues affecting our discharge pressure, currently our operators have to manually open this valve to bleed the air out but i was wondering if we install pipework as per the attached photo, whether we can leave it permanently open?

My thinking is that with the bleed valve open and the vent line raised to a high point above the oil level of the tank, the air can constantly vent out to atmosphere without the need for manual intervention. However i am having a hard time rationalising whether the oil that will be in the vent line will be sucked through the pump during operation, and then air ingress through the valve? Or will it always remain at the oil level of the tank even in operation? It's relatively simple but i cant seem to come up with a solid answer for what will happen?

Any guidance would be appreciated.

 

[Snickster]

The vent line oil level will equal the tank oil level minus the pressure drop between the tank and vent line and minus the velocity head in the suction line. Per Bernoulli equation:

Ht = Hv + V^2/2g + Hf

rearranging:

Hv = Ht - V^2/2g - Hf

Ht = static liquid head in tank (ft)
Hv = static liquid head in vent (ft)
Hf = friction head loss(ft)

V^2/2g is Kinetic energy due to velocity of fluid (ft)

V = velocity (ft/sec)
g = acceleration constant (32.2 ft/sec^2)

If your suction has relatively low velocity and length, then Hv should not be much less than Ht, and therefore liquid level in vent will be maintained.

 

[LittleInch]

It depends on the level in the tank and the frictional or head losses in the pipe.

You would be better off without the high point, but install a short piece of much larger pipe to allow the air to separate with a vent line off the to of this larger pipe.

But where is this air coming from? If you flow at a reasonable velocity it will sweep the air out of the pipe.

 

[pierreick]

Hi,
A few comments:
-Nothing wrong to start a pump from field, except for lazy operators,
-To me the location of the bleed is wrong, assuming centrifugal pump, should be on the discharge line of the pump, between the manual valve (isolation valve) and the check valve this will allow to bleed air/gas trapped in the suction line and the body of the pump to prevent air/gas lock.
Before start of the motor line up the pump, open the bleed valve until you visualize oil coming, close the bleed valve and then start the motor.
My view
Pierre

 

[Doug Hunter]

I agree with Snickster that you need to work out your math. If there is low enough relative flow resistance in the vent line, you might suck air.

I agree with LittleInch that you would be best served by understanding the source of the air as part of your problem solving process. The system will be more robust with a de-aerated sump tank.

 

[LittleInch]

An inlet line like this should either be flat or slope upwards back to the tank. Having a high spot is normally not considered good practice.

The issue of entrained air or vortexing at low level in the tank needs to be investigated first