Under what condition should a valve not be used in a piping system?

A valve should not be used as a bulkhead in a piping system because it is critical to maintain a clear separation between gas and test mediums. The presence of a valve at this junction could impede the ability to achieve and maintain a proper seal during testing procedures, leading to potential leaks or incorrect readings. The role of a bulkhead is to ensure safety and effectiveness by providing a solid, leak-proof barrier, which valves typically do not guarantee. Valves are designed to control flow rather than to act as a permanent separation between different mediums, which is essential for the integrity of the system during testing. Therefore, utilizing a valve in this context compromises both the safety and reliability expected in a gas piping installation.