It is very important that you reduce system
pressure to zero before performing service or maintenance on a CNG vehicle. If high pressure gas is allowed to escape during maintenance, it could cause severe injury or death. Every technician working with CNG vehicles must know the proper method of relieving pressure. Always follow the current manufacturer’s instructions
for the vehicle that you are working on. In any vehicle, you must be aware
of the areas where high pressure gas is present. Unless the entire system, including the fuel
cylinders, is completely empty, high pressure gas is trapped somewhere. You must be familiar with the system you that are working on to know if high pressure exists
at the point where you need to work. Here are some examples. In this system, you can safely service the coalescing filter if you close the manual shut off and run the vehicle until the engine dies. Any point downstream of the shut off will be safe to work on once the engine stops running, but pressure upstream of the shut off is still high. If there is a problem upstream of the shutoff, you must close the cylinder valve, and open
the shut off before running the engine. Once the engine dies, pressure is trapped in the cylinder only, and you can safely work on the rest of the system. If the system uses multiple cylinders, be sure to close all of the cylinder valves before running the engine.