What type of fitting fits inside a pipe or tubing, commonly used with polyethylene?

The type of fitting that fits inside a pipe or tubing, especially in systems using polyethylene, is known as an insert fitting. These fittings are designed to be inserted directly into the end of the pipe or tubing, allowing for a secure connection that maintains the integrity of the flexible material. This method is particularly advantageous in irrigation systems, where flexibility and adaptability are essential for efficient water distribution.

Insert fittings are utilized for their ease of installation and ability to create a strong, leak-free joint when properly attached to the pipe. They can be used in various applications, including connecting different lengths of pipe, making transitions from one size of pipe to another, and attaching other components such as valves or emitters.

In contrast, the other choices do not fit the description provided in the question. A swing joint is a configuration of piping designed to allow flexibility at the connection point, but it does not specifically refer to a fitting that fits inside a pipe. Globe valves and gate valves are types of valves used to control fluid flow, and they are not designed to fit directly inside pipes in the same manner as insert fittings.