OS&Y means outside stem and yoke. The threaded part of the valve stem is exposed and the yoke (or bearing bracket) allows the stem to rise through. That is the stem is attached to the gate, but free to rotate, with the valve being opened or closed by the gate/stem assembly rising through the yoke.
A NRS valve on the other hand has a stem fixed at the yoke. but free to rotate with the movement of the gate being effected by the stem screwing in or out of the gate.
Relatives features of each -
NRS cheaper.
NRS valves require clean fluid which will not clog the internal stem/ gate threaded connection.
OSY valves are higher temperature tolerant due to the mechanical threaded connection being out of the process fluid and able to obtain some cooling to the environment.
NRS valves require the fluid to have some lubricating properties while OS&Y valves require external lubrication.
OS&Y threads may be clogged from external factors such as dust, but may be cleaned in service.
The most important difference is that the position of the gate (i.e open or closed) is readily determinable with the OS&Y valve. When the valve is fully opened, the stem will be protruding through the handwheel and flush (or nearly) when closed. The NRS valve does not have this advantage.
Hope this helps,
Bill Birch
