Final answer:
A nipple is a pipe fitting threaded at both ends, used for making closed connections in a piping system. Other options like elbows, return bends, and tees serve different purposes such as changing direction, making U-turns, or merging and splitting flows.
Step-by-step explanation:
The type of pipe fitting described in the student's question, which is threaded at both ends for making closed connections, is known as a nipple. This fitting is a short piece of pipe usually with male threading on both ends to connect two other fittings. Pipe nipples are commonly used to allow plumbing to be connected and extended or for other purposes like as a connector or a flow control component within a piping system. The three other options given in the question serve different functions:
- Elbows are fittings that change the direction of the piping.
- Return bends are U-shaped fittings that allow the pipe to turn 180 degrees.
- Tees are T-shaped fittings used to split or combine fluid flow or to connect pipes of different diameters.
It's important to choose the correct pipe fitting for the task at hand to ensure a proper and secure connection in any plumbing or piping system.