Which of the following valves is not required in a steam system?

In steam systems, the inclusion of various valves serves specific functions critical to safety and operational efficiency. The main stop valve is essential for controlling the flow of steam and isolating the system during maintenance or emergencies. The pressure relief valve is a safety device that prevents excessive pressure from building up, which can lead to dangerous situations such as explosions or equipment failure. The feed water valve is necessary for regulating the water entering the boiler, ensuring that it operates at the correct levels for steam generation.

In contrast, while a temperature control valve can be beneficial for regulating the temperature of steam or the process it is serving, it is not universally required for all steam systems. Some applications may not necessitate precise temperature regulation, making this valve optional depending on the system's specific requirements. Therefore, the temperature control valve is not an obligatory component in the same way that the other valves are, which are crucial for maintaining safe and reliable operation within a steam system.