Which valve must be present between the air supply and the shutoff valve of the dry system?

In a dry sprinkler system, the pressure relief valve plays a crucial role in maintaining the integrity and operational safety of the system. This valve is essential as it allows for the controlled release of excess pressure that may build up within the system due to thermal expansion, changes in air temperature, or other operational factors. By doing so, it helps prevent potential damage to the pipeline and associated components, as excessive pressure could compromise the functionality of the dry system.

The pressure relief valve is strategically placed between the air supply and the shutoff valve to ensure that any excess pressure is adequately managed before it reaches the shutoff valve. This placement is key in protecting the overall system from failures that could occur if pressure were to exceed safe operational limits.

In contrast, the other components listed support different functions within the sprinkler system and do not serve the specific role of managing pressure between the air supply and shutoff valve. Therefore, understanding the pressure relief valve's purpose is essential for anyone involved with the operation and maintenance of dry sprinkler systems.