overfill prevention problems. NEED HELP!

[mudd11]

Specifics:

I have a 6k gallon tank, 120" diameter. It is manifold with a 10k gal 120" tank, On-rd diesel. 2" fiberglass manifold line. Single STP in 10k tank. Probe in each tank. Separate Vents.

Issue:

When the truck begins to drop fuel, the overfill prevention valve closes immediately, regardless how much fuel is in the tank. There is also back-pressure on the 4" truck hose as soon as the hose is buttoned to the fill.

Troubleshooting:

- Tried new opw 71so and and Emco a1100 non-coaxial overfill tubes, both did the same thing.
- Cut emco overfill bottom on 45* angle, 8" from bottom of tank
- Blew back on vent line
- removed atg cap to try and release pressure while dropping.
- Removed overfill, installed striaght tube (truck can then drop, but slower)
- Removed overfill, no tube, splash drop (truck can then drop)

Being that the bottom of the tube is normally submerged in fuel at all times, how can there be pressure on the truck hose?

*** It took approx. 3 drops after both tubes were installed for this to start to occur each time. Could this weird back-pressure be damaging the tubes, thus causing them to close as soon as fuel hits them? But, even when the tubes weren't shutting, the drop flow was slower than it should be.

Tube in the tank prior to these didn't work, and was cut too short on the top side.

[1onemarty]

Sure you don't have bad P/V Vent caps, or a clog in the line from the tank to vent cap?

[mudd11]

The tanks have standard vent caps and we successfully blew the vent line back to the tank.

But even if the vent was bad, how could the overfill tube shut without having fuel raising the float on the tube?

[petrodood1]

What height is the actual valve & float on the 71so set at (in relation to the tank riser and tank) ? Should be at 95% of full tank capacity (not the tank diameter) somewhere between 107 and 108 inches from the bottom of the tank..

[bigbadjohn]

How do you know the overfill valve is closing?

Sounds to me like the vent is partially obstructed. Put a pressure gauge on the vent when you are blowing it back, use 5-10 psi when you blow it back to match the kinds of pressure you get from a truck drop and see if the pressure decays to zero right away when you turn off the nitrogen. If the vent holds any pressure at all for any amount of time it's an indicator of a problem.

[mudd11]

What height is the actual valve & float on the 71so set at (in relation to the tank riser and tank) ? Should be at 95% of full tank capacity (not the tank diameter) somewhere between 107 and 108 inches from the bottom of the tank..

The overfill tubes are both cut at 11.5" below the top of the tank. Which are in-line with your measurements above.

[petrodood1]

Is there a possibly there is a ball float at the riser/tee where the vent is attached? Possibly the ball is stuck partially and when you blow back thru the vent it allows the flow but closes if dropping a load...just a thought.

[ZMiller]

Tank when being delivered to is not venting. Driver hooking up the phase 1 when dropping? Probably not. Ball float partly closed. Station tank configurations changed lately? Has receintly, site been tested and the tester short one or two bladder plugs.

[mudd11]

Got on the canopy Friday during a drop. Both tanks are venting fine. The 10k tank has a 3” vent, 6k has a 2”. Could there possibly be a manifold problem that is creating a vacuum and damaging the overfills? Since the 6k tank would naturally fill faster, the siphon would be moving product into the 10k tank while dropping. Could this damage the overfill?

I know that the overfill (out of the 6k tank) is now damaged, as I put it in another single compartment 20k gallon tank that has never had a problem, and it shut immediately in it as well.

[DelmarvaPetroSrv]

Did you ever find out what is causing this issue? Is there stage II vapor recovery at this site? We have been having this problem at a few sites with the OPW valves, but it seems to have started after we decommissioned the stage II and brought the site up to EVR compliant, so there is now a vacuum on the tanks. Of course OPW has never had this problem before.