What type of valve must be used when installing systems involving multiple ASME pressure vessels not in a manifold?

When installing systems involving multiple ASME pressure vessels that are not in a manifold, it is essential to utilize a shutoff valve. This type of valve is vital because it provides a means to completely stop the flow of natural gas in the event of a maintenance need or emergency. By allowing operators to isolate specific vessels from the system, shutoff valves enhance safety and compliance with regulatory requirements.

Shutoff valves are particularly important for ensuring that each pressure vessel can be individually isolated without needing to depressurize the entire system, thereby allowing for safer operation and maintenance practices. In cases where pressure vessels are not interconnected (i.e., not part of a manifold), having direct control over the flow to each vessel helps to manage risks associated with potential leaks or mechanical failures.

The other types of valves listed serve different purposes: relief valves are used to prevent overpressure conditions by discharging excess gas, overflow valves manage liquid levels in tanks, and excess-flow valves automatically shut off flow in the event of a hose rupture or significant leak, but they do not provide the same level of operational control as shutoff valves for isolating vessels.