Which type of plug is typically used for plugging boiler tubes?

The standard taper plug is utilized for plugging boiler tubes because its design allows for a secure and effective seal within the tube. The tapered shape enables a tight fit when inserted, accommodating the variations in tube diameters and providing a reliable closure to prevent leaks. This is particularly important in boiler maintenance, as even minor leaks can lead to significant operational issues and safety hazards. The robust construction and effective sealing capabilities of a standard taper plug make it ideal for handling the high-pressure environment of boiler systems.

The other choices do not provide the same level of effectiveness for this specific application. A blind nipple, while it can close off a connection, is not designed to create the same level of seal and may not fit well in the constraints found in boiler tubes. Tapered phenolic plugs are often used for different purposes, particularly where chemical resistance is necessary, but they may not be suitable for the high temperatures and pressures typically found in boilers. A threaded plug can secure a closure but lacks the adaptability and assurance of a perfect seal in the dynamic environment of a boiler tube.