Is it illegal to pretend to be a natural gas company personnel to install a self-closing valve?

Illegal. It is illegal to pretend to be a natural gas company personnel to install a self-closing valve and sell gas safety products to users on the grounds of gas safety. In order to avoid the recurrence of this kind of behavior, we must call the police and let the police investigate, and strictly put an end to endangering society and affecting the social environment again.