Pressure testing fuel tank

[CaptnKingfisher]

I've got a polyethylene tank from 1989. Water in my fuel had me concerned I decided to replace the vent and filler hose, and the filler o-ring. I pulled the tank out the boat to inspect it. After a power washing and visual inspection it looked a lot better than I expected.

I set up to pressure test first by plugging the fill hose, setting up a gauge at the vent port, and hooking up my air compressor to the 3/8 fuel line port. I read online to stop the test at three psi, so I turned on my compresser and watched my gauge. The tank began to swell and get quite hard, yet my gauge still read zero psi. To me that's illogical that my tank has enough pressure to swell and feel firm yet have 0 psi. I stopped the test and used the same gauge to pressure test my radiator on my truck. Gauge worked normally as its supposed to so I know my gauge is not bad.

I decided to hold off til I get more information. I am afraid of damaging my tank. While it was swollen with air I sprayed around with soapy water and did not find any leaks and it held pressure for an hour. When I finally took my gauge off the tank did not immediately depressurize like I expected. I was hooked up at the anti-siphon valve and air was coming out incredibly slowly almost imperceptible. Next I removed my plug at the fuel vent port and now air came rushing out as the tank depressurized. Does this mean my anti-siphon valve is bad? Also why was I reading zero psi on my gauge inline at the vent port, yet when I pulled the plug on that line I could hear/feel the tank depressurize. Do I really need more air?

Any help is appreciated. I'm starting to think this tank is still usable and I'm beyond happy about that, the last thing I wanna do is ruin it during a pressure test

 

[CaptnKingfisher]

Solved. Sort of. I took the anti-siphon fitting off, connected directly to the port for the fuel line and used my hand pump to pressure test. I fearlessly kept pumping despite the enormous swelling of the tank. I eventually got to three psi, it took a ton of air. Found a leaking sending unit gasket. I'm going to replace my anti-siphon fitting cuz I don't understand it well enough to know if it's functioning correctly or not, it's a cheap part and I'm tired.

 

[alldodge]

Glad your ok, you could have been hurt or worst. Take the surface area in square inches then multiply that by the pressure in the tank.

 

[dingbat]

As a kid right out of trade school I was as assigned to weld up a 36” x 18”x 12”, doubled walled cooling jacket for system using 1/4” steel plate.

Got it all welded up, The cooling water supply was 10 gpm @ 60 psi.

Hooked up a regulator and put 60 psi of air in it and went to find my boss to show him my handy work.

By the time we returned to inspect the cover, the sides had bulged and the entire thing was starting to distort.

My boss about had a heart attack when he saw it. That’s the day I learned the difference between pressure testing and hydrostatic pressure testing

 

[CaptnKingfisher]

As a kid right out of trade school I was as assigned to weld up a 36” x 18”x 12”, doubled walled cooling jacket for system using 1/4” steel plate.

Got it all welded up, The cooling water supply was 10 gpm @ 60 psi.

Hooked up a regulator and put 60 psi of air in it and went to find my boss to show him my handy work.

By the time we returned to inspect the cover, the sides had bulged and the entire thing was starting to distort.

My boss about had a heart attack when he saw it. That’s the day I learned the difference between pressure testing and hydrostatic pressure testing

Never heard of hydrostatic.. on my way to YouTube now.. at least yanno your welds were good lol