Propane Fill Valve 

Figure 1

When propane is delivered to a tank, the delivery driver hooks up the hose from the truck to the propane fill valve. The hose end coupling is screwed onto the filler valve and seated firmly against a gasket to prevent gas from escaping during the delivery process. If this gasket is not present in the fill valve, the connection between the hose and tank will not be secure and the delivery driver will not be able to fuel the propane tank. This black gasket is visible around the edge of the filler valve wall.

For more details see Figure 2 below

Figure 2

Propane Fill Valve Operation

After the hose coupling is securely attached to the tank fill valve, the valve is the hose-end valve is opened and liquid propane begins to flow into the tank. The pressure from the gas in the hose alone is enough to open the fill valve. Practically, Inside there is a nozzle ( check valve) that is pressed through by the pressure of the source and released when the pressure is removed to keep close the tank Figure 3 shows all details This is located inside the tank and is not visible when looking at a propane tank. The part that is outside the tank also has a one-way valve that opens and closes with the pressure of the hose. This valve is visible by looking into the valve outside the tank. These two "backflow prevention" valves ensure that propane is securely stored in the tank opened only when a delivery is made.

Figure 3

below all details

1. Seal cap made of tough, resilient moulded plastic. Protects the threads and internal working parts (nozzle). Caps are designed to contain normal tank pressures and must be kept on valves at all times.
2. Long-wearing gasket permits hand-tight connection of cap and hose coupling
3. Safety groove is designed to shear below the ACME thread, leaving the valve seats closed and unaffected if the delivery truck pulls away with the hose connected
   
4. Seat disc of special synthetic composition is extra thick for longer life
   
5. The valve guide is precision machined to ensure a positive seal
6. Check valve for extra fast filling

Tank Fill Valve Protection

The protection of the filler valve is very important as it operates as the first line of supply to your gas system. The purpose of the protective cap is to keep water and debris from entering the valve. If debris enters the valve, it could keep the discs from seating properly causing the valve to remain in an open position allowing gas to escape. Water in the valve can cause the hose end coupling to be frozen onto the filler valve making it difficult for the delivery driver to disconnect the propane delivery hose from the fill valve.

Recommendations

Do not try to make any kind of modification to the fitting, it could be fatal for those who work around it