The problem is found ...
The outlet pipes of pressure regulating and metering are welded with seamless steel pipes, with DN51 and below double argon arc. Each branch leading to the stove is: main pipe welding reducing tee+welding pipe hoop (DN25), and then galvanized pipe is connected by thread. In some cases, such as the renovation project where the user does not stop business, metal hose is added. See another answer for the pipe diameter of the interface.