Some basic information here Mohammad

https://www.thefreelibrary.com/New+technology+in+drip+odorization.-a0143774628

https://naturalgasodorization.com/odorant-injection-systems/ (There is a drop odouriser section)

Thanks John Machell

But I couldn't find mechanism of Drip odorizers!

How does odorant drop in a line with pressure of 60 or 250 psi without any pump?

Here is one example Mohammad

https://patents.google.com/patent/US20190070627A1/en

The 'drip', as you say, cannot drip against the pressure of the gas main. In the attached example, the 'drip' goes into a special chamber temporarily isolated from the pressure of the main. The chamber is then opened to the pressure of the main and the odouriser is evaporated into the gas flow. The drawings in the linked document explain the mechanism

Thanks again John Machell

One type of drip odorizer may be similar to what you said. But I think in odorizer mentioned in my question there is not a chamber. My friend manufactured and used this type of odorizer that is consist of a tank and a needle valve without middle chamber! But he doesn't know how his odorizer works!