What type of valve should be installed to isolate a dual pressure relief valve while keeping the system in service?

A three-way valve is the appropriate choice for isolating a dual pressure relief valve while keeping the system in service because it allows for the redirection of flow from one path to another. This design enables maintenance or inspection of the relief valve while the system remains operational by diverting the flow away from the isolated part.

The configuration of a three-way valve facilitates the connection of two different lines, giving the operator the ability to maintain functionality in one section of the system even while performing necessary work on another. This is crucial in maintaining safety and efficiency within the system, especially in environments where pressure control is critical.

The other types of valves do not provide the same level of flexibility needed in this scenario. A two-way valve merely opens or closes a single flow path, making it impractical for isolating a relief valve without shutting down the entire system. Automatic valves may control flow based on specific conditions but do not inherently isolate components. Shut-off valves are typically all-or-nothing, meaning they would cut off flow entirely rather than allowing for continued service while isolating a specific component. Thus, the three-way valve stands out as the most suitable option for the given situation.