SD and SC valves select and isolate one of 4 to 16 streams, with the remainder dead-ended in the SD and flowing to a common outlet in the SC. The SF selector is similar, but carries the evolution a step further with the non-selected streams flowing through individual outlets.
Low pressure SF selectors are made of Nitronic 60, with a rotor made of Valco E, a polyaryletherketone/PTFE composite. Valves are rated from 100 to 200 psi gas and 75° to 200°C, depending on the fitting size and number of positions.
Valves can be actuated manually or automated with pneumatic or electric actuators. GC valves usually require a standoff - an extension between the valve and actuator which allows the valve to be mounted in the heated zone while the actuator or handle remains outside at ambient temperature. Standoffs are available in lengths of 2", 3" 4", and 6".