How should normally closed valves be secured as per applicable NFPA standards?

The appropriate method for securing normally closed valves, as indicated in NFPA standards, is by means of a seal. Seals are utilized to provide a tamper-evident, non-intrusive means of ensuring that the valve remains in its intended position until it is necessary to operate it. This method serves both to restrict unauthorized access and to maintain the integrity of the fire protection system.

Using seals helps to prevent accidental or deliberate operation of the valve, which could compromise the system's effectiveness. This method aligns with NFPA guidelines to ensure that the system operates reliably when needed while offering a straightforward visual cue that the valve has not been manipulated.

Other options, although they may be relevant in different contexts, do not specifically align with the NFPA’s established practices for securing normally closed valves. For instance, locking a valve may not provide the same level of assurance against unauthorized adjustments as a seal, while a manual override is more about the operation of the valve rather than its security in a closed state. Regular maintenance checks are important for overall system integrity but do not specifically address how the valves should be secured. Thus, the use of seals for normally closed valves is the correct approach defined by NFPA standards.