re: Pump Gas-locking

[TurbineBlade]

Hi there,

I am gathering info to understand the issues of gas-locking in a pump. What cause gas-locking, typical scenario of a gas-locking in a plant application, methods to prevent and mitigation etc.


Thanks

 

[dcasto]

We could narrow it down if we knew if the pump is a centrifical or positive displacement type and what fluids you are pumping (don't need all the details).

Some common things are not having enough NPSH, as pumps wear, they can require more than the original design.

Piping on the suction to small (NPSH again) due to more volume than originally design.

On PD pumps, bad valves can contribute.

Piping on the inlet has pockets.

The minimum flow bypass valve causing flashing.

There are loads more.

 

[Artisi]

Entrained air in the pumped product, either naturally occuring or introduced by the process.

Air entering thru' the shaft seal.



Phitsanulok
Thailand

 

[TurbineBlade]

Hi,

The scenario is: Two cooling vertical centrifugal pumps running during a maintenance condition with the suction coming from water tanks or headers with hugh surface area. The pump bypass valves are also partially open. System fluid is heavy water (D20). When during maintenance on some plant equipment, these headers are drained to low level. One pipe is connected on the side of the tank at about 1/3 of the tank. The pipe is open atmospheric during maintenance. Normally, the submergence level is higher than 1/3 so that no air entrapment. However, system disturbances may cause the water level to fluctuate into the low level below the open pipe connection.

To what extent of air entrapment will cause the gas-locking of the pump? I am thinking that if one cooling pump tripped, it may cause the water level to drop temporarily to the extent that gas-lock of another pump.

 

[dcasto]

the D2O will become saturated with air at some point in time. After saturation air can become gas bubbles because of either a drop in pressure or increase in temperature. It would be hard to imagine that the line from the tanks to the pump were so small that friction in the line would cause to much of a decrease in pressure, but there may be enough of a drop as the water enters the impellar. The flow pypass is adding heat. The net effect is that you could have air breaking out and it will eventually vapor lock the pump.

If everything is just right, the water level is 1/8" over the tank nozzel, I've seen where: when the pump starts, the level drops in a vortex and you suck in enough air to lock the pump. That might be something to look at. A vortex breaker may help.

 

[pleckner]

Centrifugal pumps don't like much more than about 3% dissolved gas in the liquid.