Which of the following is not permitted to be installed between a fire hydrant and its control valve?

In the context of NFPA 24, the primary consideration for the installation between a fire hydrant and its control valve is ensuring that the system remains reliable and efficient in delivering water during emergencies. While check valves are essential components in many plumbing systems to prevent reverse flow, their installation between a fire hydrant and its control valve is not permitted because they could prevent the hydrant from flowing properly. During an emergency, if water needs to be withdrawn from the hydrant, any impediment to that flow could hinder firefighting efforts.

By contrast, flushing valves, hydrostatic valves, and flow control valves serve specific purposes that align with maintaining the proper operation of a fire protection system. Flushing valves allow for the removal of sediment and debris that may accumulate in the system, ensuring clean and usable water is available. Hydrostatic valves, when used appropriately, can help control pressure and maintain the integrity of water flow. Flow control valves manage the rate of water flow, which is essential for system performance but does not interfere with the connection to the hydrant.

Thus, the prohibition of check valves in this location is a design choice that prioritizes optimal function and reliability in emergency situations.