What is one essential part of a double check valve assembly?

The essential part of a double check valve assembly is a shutoff valve. This valve is crucial because it allows for isolation of the assembly from the rest of the water system for maintenance, testing, and replacement of components without disrupting the entire system. The shutoff valve provides a means to ensure that the water supply can be halted while work is being performed, thus enhancing safety and operational efficiency.

In contrast, a single check valve is not sufficient for backflow prevention on its own, as it does not offer the same level of protection as a double check valve. An air gap, while important in certain plumbing applications, is not a part of the double check valve assembly itself; rather, it serves as a physical separation to prevent backflow. PVC pipe is simply a type of material used for plumbing and does not constitute a functioning component of the assembly.