What is O S & Y Gate valve and how is it different from a normal Gate valve?
OS&Y means "outside screw and yoke" The designation has to do with the location of the screw nut and the shaft screw threads.
In this style, the screw nut is supported by an A-frame type "yoke" outside of the enclosed body of the valve. Thrust faces of the nut and the yoke hold the nut in a fixed position as the shaft (or "stem") rises of lowers without turning. The valve wheel is attached to this nut either directly or through gearing and as they both turn, the nut, acting on the external screw threads causes the shaft to rise or lower into the valve body pulling or pushing the valve disk to open or close the valve. There is a packing gland around the where the shaft enters the valve body. You can look at an OS&Y gate valve
and tell if it is open or closed by the position of the shaft. It is more difficult with an OS&Y globe valve because the shaft movement is short.
In a non-rising stem (NRS) gate or globe valve, the shaft screw nut is built into the INTERNAL valve disk or the valve plug. The handle and shaft turns and the inner part of the valve opens and closes as the inside nut rides up and down internal shaft threads turning within the nut. These types of valve are more compact than the OS&Y. In both types the valve packing also acts as a guide.
Both types are used extensively in all kinds of municipal and industrial water service.
A fire hydrant is a specialy OS&Y , globe valve where the shaft is totally encased and the nut is turned with a special wrench. As it is a globe valve the movement is a very short distance compared to a gate valve.