What makes you believe that turning a valve handle back half a turn from the fully seated postion is good industry practice?
It also probably depends on the type of valve. For example, with a ball valve that only has 1/4 turn from fully open to fully closed, I can't believe such practice is recommended.
I've never heard this "rule" before but I have practiced it with non-rising stem gate valves. There is no visual indication whether the valve is open or closed so if the handle does not move right away, you do not know if it is jammed, or which way to turn it to break it free. You may be just jamming the valve tighter and it is easy to break the valve stem this way. I've always hated these valves and would never specify them myself.
My barbecue wouldn't light when my BIL tried to start it.
He assumed that the tank valve was stuck closed.
So he got out a pipe wrench to open it.
Of course, the tank was empty, and the valve was full open and backseated.
The valve is still backseated and the tank is empty, and nobody can close the valve.
I don't know about an industry rule, but I've adopted the habit of fully seating (globe) valves to close them, as is required, and of slacking them away from the backseat a bit after opening them fully. So I can grab the valve handle and know instantly that it's open if it moves easily, and it's most likely closed if it doesn't move easily.
WRT a gate valve, I wouldn't back off the wheel after seating the valve in the closed position, for fear that the gate might wiggle loose later, but I would slack the wheel after backseating the valve in the open position, again just so I can easily distinguish open|closed|stuck.
I don't bother to fully open propane tanks, my BBQ doesn't need that much flow, but I always fully close the tank between uses. I can't imagine the valve sealing without being cranked down tight.
----------------------------------------
The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.
I do this with domestic valves which are rarely used, but only when fully opening them. You can easily jam these open if they are infruquently used and I agree with composite pro - At least you'll know if the valve is open or closed as you'll either turn it half a turn then stop or keep turning it closed.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
