A Control Valve is a valve used to control fluid flow by varying the size of the flow passage as directed by a signal from a controller. This enables the direct control of flow rate and the consequential control of process quantities such as pressure, temperature, and liquid level.
All valves are designed to meet a designated internal permissible leak rate standard (FCI/ANSI). There are six (6) permissible leak rates or Classes numbered 1 through VI. The higher the leak rate number, the lower permissible internal leak rate.
Identifying the permissible internal leak rate for a valve has to be one of the top priorities when choosing the correct valve. A valve with a high leak rate, will cause excessive wear. The valves with a high leak rate will internally pass more medium and result in premature valve failure.
There are six different seat leakage classifications as defined by ANSI FCI 70-2.