What type of valve must be installed on fuel-oil piping where more than one fuel-oil-burning appliance is present?

The requirement for a shutoff valve in fuel-oil piping when multiple fuel-oil-burning appliances are present is rooted in safety and operational efficiency. A shutoff valve allows for quick and easy termination of fuel flow to the appliances in case of an emergency or during maintenance. This is crucial for preventing uncontrolled fuel release, which can lead to hazardous situations such as fires or spills.

In systems with multiple appliances, having a shutoff valve ensures that you can isolate any one appliance without affecting the others. This maintainability is important for routine inspections and repairs, allowing technicians to work safely on a single unit while keeping the entire system operational.

Other types of valves, like relief valves, check valves, and pressure valves, serve different purposes. Relief valves are primarily used to prevent overpressure conditions, check valves exist to prevent backflow, and pressure valves help regulate pressure within a system. While these valves are important in their respective roles, they do not provide the necessary functionality for controlling flow to different appliances in a scenario with multiple fuel-oil burners. Hence, the use of a shutoff valve is specifically mandated for the application described.