There are 3 LPG Gas Tanks 2000 Liters (530 Gallons) each supplying a restaurant in a big Housing complex. There is an external underground valve box right outside the restaurant, before going inside the building, having the following valves.
1)Solenoid Valve with 2 ball valves together with a by-pass ball valve . The solenoid valve will close under these 4 cases
2)2nd Stage regulator to reduce from 20 psi down to 0.5 psi before going into the building.
3)Low pressure switch whose perpose is to give a signal to the solenoid valve(through the control panel) to shut down in case of low pressure in the tanks.
The shut down on low pressure in the tanks is to prevent oxygen or air from entering the tanks,
If there is air in the tanks it needs to be purged prior to refilling, a process the gas supplier normally does.  purging can be done by pulling vacuum or by repeated low level filling and venting, either case it cost time, money and gas.
The purging is for safety reasons and to provide a consistant flow of gas to the fixtures
NFPA 54 states
"5.11 Low Pressure Protection.  A protective device shall be installed between the meter and the appliance if the operation of the appliance is such (i.e., a gas compressor)that it could produce a vacuum or a dangerous reduction in gas pressure at the meter; such devices include, but are not limited to, mechanical, diaphragm operated, or electrically operated low pressure shutoff valves."
You should always install these devices on underground propane delivery lines as you can never tell if the restaurant isn't going to move and another business moves in with a vacuum-developing appliance. Safety first.

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