It seems we have all sorts of filters on our trucks that all have separate functions while the truck is working away. Fuel filters, diesel particulate filters (DPF), oil filters, and even air filters all have their separate functions that equal a clean and smooth running truck. Sure, some of these are recent additions in truck technology and all have undergone some technology changes over the years. Some oil filters even claim that under careful observation they can drastically increase drain intervals. What I would like to focus on however is the role a fuel filter plays in all of this and how a CNG fuel filter system is both similar and different in some respects.
A diesel fuel filter, of course, keeps harmful contaminants from entering your engine from the fuel delivery system. Whether it is particles of dirt or water, it will keep it out of the engine and prevent your engine from stalling out, fouling an injector, or possibly even worse. Periodic maintenance and replacement of this element is recommended per OEM manuals at certain intervals, differing of course between different makes and models.
When it comes to CNG, the principle of fuel filter protection remains the same, but it is what you are trying to keep out that may differ slightly. Of course, moisture in any fuel system can only be a bad thing, but what about oil? If a little oil got into a diesel fueled engine, it would probably just burn it up and not even let out so much as a hiccup. One general rule when running a CNG powered truck is "Oil Is Bad"! As I have mentioned briefly in some previous articles, some smaller/older stations are lacking the 
compressor power to fuel large trucks. It is imperative that a CNG driver be very selective as to where he/she fuels on a regular basis. That being said, an old or overworked compressor can leak oil, which usually finds a direct route into your fuel system via your fuel tanks! If it weren't for a few trusty fuel filters like the ones pictured here, it could have a devastating effect and hefty bill along with the repair for what it would do to a CNG powered engine.
These filters need to be changed during routine oil change PM services and drained with the valve shown here during daily pre-trip inspections. Any oozing sludge-like fluid leaking from these valves when drained could be a very good indication that you have taken on some oil and need to have your fuel system looked at.
Another filter that needs changing regularly to maintain effectiveness, which is often overlooked by drivers I’ve talked to, is put in by the tank manufacturer on the high-pressure fueling side of your tank. This is another safeguard built in to help eliminate fuel system contamination when fueling up your CNG truck. A bulletin for change intervals and information on changing out this particular filter on an Agility brand system can be found at their web site under the heading "product support". This type of filter may differ in its exact location depending on who your tank manufacturer was and what model of truck you drive, but be sure to look for it as well. Most of these troubling moments can be avoided by using a good fueling station designed for big trucks and dispensing large volumes of natural gas, but it always gives me peace of mind to know that the fuel filters I keep in top condition are there doing there job just in case!