All 1/2" steel piping with #2 fuel oil flowing

There is an actuated shutoff valve then about a 20' run of pipe then a 10' drop and another 8' run to an oil gun with tip out in the furnace.

There is a negative pressure coming from the furnace, and there is a small additional negative pressure created during a 30 second purge with the oil off and air still on... probably equals about 3 psi total...

So why would there need to be a check valve installed immediately before the oil gun?

Whouldnt the pressure after the valve be 0 and the pressure from the atmosphere be like 15-3 = 12psi + friction and such... so the oil would all stay in the line except for maybe a little that would come out that was in around that last 8' run?

I think a check valve would do little to almost nothing here... a couple people are insisting that there HAS to be a check valve there... am I missing something? 

It is most likely to prevent the atomization air (compressed air) from backing up through the burner nozzle.   Fuel oil and compressed air connections will be near the windbox, and these go to the oil gun which uses the pressurized air to atomize the oil.

I am not familiar with the specifics of how oil burner guns are configured, but it seems possible that the air could backup through the oil line if the oil pump was suddenly shut off or a valve closed.

The primary purpose of the check valve may be to introduce a forward pressure drop (cracking pressure to open the spring) and therefore prevent the oil lines from draining into the furnace when it is shutdown.

For one thing it will keep air that replaces the oil that has run out the end of the pipe from making its way back to wherever it can make its way back to.

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