What type of relief device must all pressure vessels be equipped with?

All pressure vessels must be equipped with safety mechanisms to prevent hazardous situations; thus, the inclusion of both a pressure relief valve and a pressure-limiting device is essential. A pressure relief valve is designed to open and release excess pressure from the vessel when it exceeds a pre-set level, helping to prevent explosions or catastrophic failures. This valve is crucial because it ensures that the internal pressure remains within safe limits by venting excess pressure.

In conjunction with the pressure relief valve, a pressure-limiting device acts as an additional safeguard, regulating the pressure of the system by controlling the flow of fluid into the pressure vessel or by stopping the operation of systems if pressure thresholds are reached. Together, these components provide a two-fold approach to maintaining safe operating conditions within the pressure vessel.

Other options, while they may provide important safety functions, do not offer the comprehensive protection that combining both a pressure relief valve and a pressure-limiting device can provide. Hence, having both is a requirement to ensure that pressure vessels operate safely and effectively under various conditions.