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Spool valve, temporary exhaust leak

Spool valve, temporary exhaust leak

(OP)

To all,

I have a spool valve that will show a small leak under certain parameters and I can’t explain the phenomenon. I was hoping someone had an idea or thoughts on what I am seeing.

It’s a 4-way spool valve; 1 inlet, 2 outlets. Operated at 80-100PSI. Viton O-rings.

What I am seeing is a temporary leak out the exhaust port, after the spool has fully shifted, depending on the volume of air the spool valve is pressurizing (outlet).
For example.

Case one:
If the two outlets on the spool valve are pressurizing a small volume of air or the outputs are just completely sealed; The spool valve will shift and once the shift is complete, the valve is fully sealed with no air escaping the exhaust port. (This is good, what I would like to see)

Case two:
If the two outlets on the spool valve are pressurizing a larger volume of air, after the spool valve shifts, I can observe a temporary leak out the exhaust port.

To observe this I had attached a tube to the exhaust port where I submerged the other end underwater so that I can watch for bubbles. When pressurizing a small volume of air I see no bubbles after the shift. When are pressurizing a larger volume of air I observed 7-12 total bubbles over roughly a 4 minute time span. These bubbles decrease expediently from the shift till the four minute mark, so I see the majority of the bubbles within the first 15-30 seconds.

The only correlation that I am aware of for when I observed this temporary leak is the volume of air I talked about. I believe something is causing the O-rings to be compromised for a short time after the shift with the larger volume of air.

My theory right now is that this is a temperature effect caused by the large volume of air flowing over the O-ring while exhausting. This is why I would see a leak immediately, then when the O-ring slowly gets back up to temperature, the leak will slow until the full seal is made.

Does anyone have thoughts or theories on this?
Thank you all for your time, I appreciate it.

RE: Spool valve, temporary exhaust leak

There is no leak. The exhaust air gets cold from expansion. The more air you exhaust the colder the valve and exhaust line get. When the exhaust flow stops the air in the exhaust line warms and expands.

RE: Spool valve, temporary exhaust leak

(OP)
Ahh I see. I knew the exhaust air would get cold from expansion, however I was focusing on how that would affect the O-rings. I didn't stop to think that it could just be the air in the line coming back to ambient temp.

Thank you for the response, I do appreciate it.

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