Final answer:
The purpose of an outside stem and yoke valve is to control fluid or gas flow with a visual indicator via its stem movement and to provide a reliable seal with easy maintenance.
Step-by-step explanation:
The purpose of an outside stem and yoke (OS&Y) valve is to control the flow of fluid or gas in a system. This type of valve is often used in fire sprinkler systems and in many industrial settings. The distinguishing feature of an OS&Y valve is the stem, which moves in and out of the valve when it is opened or closed, providing a visual indication of the valve's position. The yoke of the valve is the support structure that holds the stem packing and the actuating mechanism, which adjusts the stem. This type of valve is known for providing a reliable seal and being easy to maintain since the stem packing can be replaced without fully removing the valve from the pipeline.