Which valves on a boiler allow steam to escape when pressure exceeds a certain limit?

The correct choice is safety valves. Safety valves are specifically designed to prevent pressure from exceeding a predetermined limit within a system, such as a boiler. When the internal pressure rises beyond the set threshold, the safety valve opens to allow steam to escape, thereby protecting the boiler and associated equipment from potential damage due to overpressure.

Safety valves are essential in maintaining safe operating conditions and preventing catastrophic failures, which is why they are crucial in the design and operation of pressure vessels like boilers. This functionality is distinct from the roles of other types of valves.

Relief valves also serve pressure safety functions but often operate in a different context, such as managing changes in pressure gradually rather than abruptly like safety valves do.

Check valves are meant to prevent backflow in piping systems and are not designed to relieve pressure.

Control valves regulate flow and pressure within a system but do not provide the emergency pressure relief that safety valves do. Therefore, the specific function of safety valves to release steam when pressure exceeds safe limits underlines their importance in boiler safety management.