What is the cause for water iin the gas line?

There is no reason that moisture enters a gas line but it happens.
That's why we install a drip loop. Consisting of a inverted tee with as 6" capped nipple, just after the gas meter to intercept and collect any moisture before it enters your house. Regards, Tom

Hi Alexyates...

Some natural gas has a higher water content than other natural gasses. In my area, for example, natural gas is obtained via coal processes and the final product has a high moisture content. In other areas, natural gas is found from other sources and purified in a manner that leaves the natural gas with a lower moisture content. My guess is that your area gets its gas from a coal process, too.

As Tom said, this moisture, no matter the percentage of moisture present in the gas, is handled in gas systems by pitching the gas pipe to DRIP TEES. Drip tees can be at the meters or as in my area, at each individual appliance being served by gas.

Good luck!

Mark