In a vented relief valve, the pilot stage is drained externally at a pressure:

In a vented relief valve, the pilot stage is designed to control the operation of the main relief valve by responding to pressure changes. For a relief valve to function properly and relieve excess pressure from a system, it is essential that the pilot stage is drained at a pressure lower than the relief valve setting.

When the external drain pressure is lower than the setting of the relief valve, it allows the valve to open at the appropriate moment, preventing a buildup of pressure that could exceed safe operating limits. This differential pressure ensures that the valve can respond accurately to increases in system pressure, thus ensuring effective relief and maintaining safety within the system.

If the drain pressure was higher than, or even equal to, the relief valve setting, it could prevent the pilot from operating correctly, resulting in the relief valve failing to open when needed. Therefore, maintaining a lower pressure in the pilot stage is crucial for the reliable performance of a vented relief valve.