Which of the following is NOT an appurtenance?

An appurtenance in the context of utilities or plumbing refers to an accessory or a supplementary component that is attached to a primary system, such as pipelines, to help it function properly. In this context, a mounting bracket serves primarily as a support structure designed to hold or secure other components in place rather than directly aiding in the control or management of the system's operation.

Conversely, devices like pressure relief valves, regulators, and connector fittings all serve critical functions directly related to the operation and safety of systems. For example, pressure relief valves help prevent overpressure occurrences in systems, regulators control the pressure and flow of gases or liquids, and connector fittings facilitate the joining of different pipes or hoses. These items are integral to ensuring that the primary system operates safely and efficiently, thus qualifying them as appurtenances.

In summary, the mounting bracket supports, while the others are functional components that directly improve or control the system’s performance, making it clear why the mounting bracket does not fit the definition of an appurtenance in this context.