I can't find this problem anywhere via google. It's not solved by resetting the regulator (and I'll argue that it shouldn't be below), and doesn't seem to be related to this cooking.se question.
Of my last four propane tanks, I've had this problem with two: I turn off all (five) burners on my grill and (obviously) leave the tanks valve closed, but as soon as I screw the regulator onto the tank valve, there's a hissing sound of gas and the accompanying smell. This lasts a few seconds, then stops. Note that this is before any valves are open.
Then, as soon as I turn on the tank valve (still leaving the burners off), there's more gushing gas sound. This doesn't seem to stop, even if I let it go for a good few seconds (maybe thirty in total). I've used soapy water to track the source of this leak, and found it to come from the back of the tank valve — the side opposite the threads, which I gather is the pressure relief valve.
In each case, I've attached a different tank with no trouble, then tried the bad tanks again and had the problem. So I assume that this is not a problem with my equipment. Maybe it is, and only turns up with some tanks, but I doubt it. My main point of evidence is that the gas is actually exiting the propane tank valve itself, not my regulator, hoses, etc.
Last time, I just exchanged the tank and ate the loss of money because I was in a big hurry. But now I'm wondering, is this a known problem? Should the store I bought it from agree to just replace my tank for free? Is there some reason this might actually be a problem with my equipment?
EDIT:
I took the tank back to where I got it, told them the problem, and they just replaced it for free, no hassle.
Best Answer
I'm no expert on flammables, but I've grilled a ton in my day on various equipment, and feel I've learned a couple things that may help here. However, your mileage may vary, and take this all with a grain of salt.
Is this a known problem?
Depends on what this is. I agree with your read that this likely isn't your equipment, and is likely something on the tank. If so, it is not known to me, but I have a different approach on tanks than you. More below.
Should the store I bought it from agree to just replace my tank for free?
Yes, I think they should do so happily; it leaks gas after all. If they don't, you should insist (say something about how you were hoping to grill some steaks, not start a house fire).
Is there some reason this might actually be a problem with my equipment?
The only thing I could fathom is if your equipment had some significant downstream pressure, but even if it did, I believe that would cause propane to flow more slowly to your burners, not leak out the back of the tank valve.
Is this a pressure release value?
This doesn't sound like a pressure release valve issue to me. If it were, the valve would open when the pressure increased inside the tank (usually a function of tank temperature increase), completely unrelated to when you opened the tank valve.
So then what is it?
If I had to guess, I'd say that the bleed screw is not tightened down well. This screw is opened when they fill the tank (allowing the air to escape while LPG is pumped into the tank). When they are done filling the tank, the bleed screw should be tightened, and perhaps that wasn't done when these tanks were filled. With the tank valve closed, check the bleed valve on the back of the tank valve to ensure it is closed.
So what should I do?
For what it is worth, I've never been a fan of those tank exchanges for a handful of reasons:
I have had excellent luck in buying several new, empty tanks (from Home Depot or the like) and having them filled myself (usually at either a U-Haul, AirGas, or gas station). It's slightly less convenient, but I get a longer effective tank life (usually 33% more gas in my fill vs. exchanged tanks), better value (the gas is cheaper!), and my tanks don't look nor operate poorly.