Which type of valve may be used as a vacuum breaker if installed backwards on a steam system?

In a steam system, a check valve is designed to prevent backflow, ensuring that the fluid only travels in one direction. When installed in reverse, however, it can function as a vacuum breaker. This is due to its ability to allow air to enter the system when there is a negative pressure or vacuum, thereby equalizing pressure and preventing potential collapse of the piping or equipment.

Check valves operate with a disc or ball that seals the flow path when the fluid travels in the designated direction. If the pressure on the inlet side drops below a certain level (which occurs during a vacuum condition), the valve can open to atmospheric pressure, effectively breaking the vacuum. This function is particularly crucial in steam systems where maintaining pressure equilibrium is essential for operational stability.

In contrast, other valve types such as globe, ball, and triple duty valves are not designed to perform this vacuum-breaking function when reversed. Their mechanisms do not permit the flow of air into the system effectively under negative pressure conditions, which is critical in maintaining the integrity and proper functioning of steam systems. Thus, the check valve's specific design and operation allow it to serve as a vacuum breaker when installed backwards.