Before the use of the joint coating heat shrinkable sleeve, it is necessary to preheat the steel pipe. Generally, the preheat temperature is 70-90 ℃.
The preheating of the joint can be done by flame or electric induction heater. Flame requirements for joint coating with different pipe diameters: the pipe diameter is less than or equal to 450mm, medium strength flame or electric induction heater is used, and the minimum flare specification is 150000btu / HR (1btu = 1.055kj). If the pipe diameter is greater than or equal to 450mm, medium to strong flame heater or electric induction heater shall be used, and the minimum flare specification is 300000btu / hr. When the pipe diameter is greater than or equal to 760mm, electric induction heater or infrared heating equipment shall be used. Of course, many on-site construction is not so good conditions, generally we recommend liquefied gas gun for preheating.
Preheating has two main purposes. One is to remove the moisture on the surface of the steel pipe. If there is moisture, if it is not removed, the moisture will form a layer of diaphragm after the epoxy primer is applied, which may cause the primer to fall off. Second, there will be a certain temperature on the surface of the steel pipe after preheating. This temperature will make the epoxy primer react chemically and cure slowly before installing the heat shrinkable tape and heat shrinkable sleeve.
Before sandblasting and derusting the surface, we should preheat the steel pipe at the joint to at least 30 ℃ or 40-50 ℃ above the dew point. It can be properly adjusted according to the ambient temperature. If there is a strong wind or the ambient temperature is lower than 15 ℃, the preheat temperature at the joint should be increased to more than 70 ℃.