Air bubble when main drain valve closed

[shiyang100]

Update:

1. There are always VERY VERY small air bubble in the pump basket. A few here there. I try not to worry about it too much. Although I guess this indicates some air leak, but very small.

2. The bigger problem: There are always some bigger air bubbles appear upon the start of pump. Even the pump was just off a min ago with literally no air. Sometimes, these bigger air bubbles are gone after a few minutes. While other times, you can see the bubble is just hanging on the top. Did water testing on all vavles, connectors, pump lid, no change.

3. I started to close the valves. Close MD, air bubble is still there. Close one skimmer, the air bubble is gone. Reopen that valve, the air didn't return.

my questions:
1. why did air appear upon starting the pump? I assume there are air got sucked in after pump off. Should that be on pressure side?

2. I don't understand why turning off skimmer line valve and reopening it got rid of bubbles. If there is skimmer line suction leak, air should return.

 

[mas985]

1. There are always VERY VERY small air bubble in the pump basket. A few here there. I try not to worry about it too much. Although I guess this indicates some air leak, but very small.

That can also be due to outgassing. Low pressure forces dissolved gasses out of solution. But usually, a pump on full speed will flush these bubbles out the return.

1. why did air appear upon starting the pump? I assume there are air got sucked in after pump off. Should that be on pressure side?

Leaks after the pump is shut off can be either side. But if the leak were on the pressure side of the pump, most of the water gets siphoned out of the pump. Not just a bubble.

2. I don't understand why turning off skimmer line valve and reopening it got rid of bubbles. If there is skimmer line suction leak, air should return.

Right but if the air was only caused by the pump being off (i.e. already there), a change in flow rate and velocity in the pump basket, caused by opening and closing a valve, could be enough to flush out the air. If the air never returns again while the pump is on, then the leak might be dependent on the pressure in the pump basket.

 

[shiyang100]

That can also be due to outgassing. Low pressure forces dissolved gasses out of solution. But usually, a pump on full speed will flush these bubbles out the return.


Leaks after the pump is shut off can be either side. But if the leak were on the pressure side of the pump, most of the water gets siphoned out of the pump. Not just a bubble.


Right but if the air was only caused by the pump being off (i.e. already there), a change in flow rate and velocity in the pump basket, caused by opening and closing a valve, could be enough to flush out the air. If the air never returns again while the pump is on, then the leak might be dependent on the pressure in the pump basket.

Thanks Mark. I feel the same way. The leak is dependent on the pump pressure. Still struggling to find out where the leak is though

 

[mas985]

I think most pump basket designs are such that the lid seals better with a higher vacuum in the pump basket so that is where I would focus. Are you sure that the lid is completely tightened against the stop?

 

[shiyang100]

I assumed so. Did water testing, the bubble is still there.