Which type of valve opens automatically when the pressure in the line becomes too high?

A relief valve is designed specifically to automatically open when pressure within a system reaches a predetermined level. The primary function of this valve is to protect equipment and piping from damage due to excessive pressure. When the pressure exceeds the set point, the relief valve actuates, allowing fluid to escape from the system, thus reducing the pressure back to safe levels.

This function is critical in many applications, including boilers, gas cylinders, and various hydraulic systems where maintaining a specific pressure is necessary for safe operation. The relief valve not only safeguards components but also helps in maintaining system integrity and operational safety.

In contrast, a shut-off valve is used to manually stop the flow of fluid when needed, while a check valve allows fluid to flow in one direction and prevents backflow, without any automatic pressure regulation. A ball valve also serves to control fluid flow but requires manual operation to open or close. The focus of these other valve types is on flow control or prevention rather than on automatic pressure relief.