SECTION - 4: CONTROL VALVES IN PROCESS DRAWINGS Engineers document their work using many complementary methods. A Piping and Instrumentation Diagram (P&ID) is the most common method of illustrating the functional relationship of piping, valves (automated and manual, pipe sizes, sample points and instrumentation. A moderate level of mechanical detail is provided for process equipment, so that the piping and instrumentation can be precisely documented.International symbol standards are used for piping, equipment and instruments (ISA, 1992). A typical arrangement used to represent control valves on P&ID is depicted below. The figure below shows a list of typical valve symbols used in industry. Control valves may fail in various positions open, closed, locked, or indeterminate, and it needs to be shown on the P&ID. The fail positions maybe identified using letters below the valve symbol FO for Fail Open FC for Fail Closed FL for Fail Last or Locked and FI for Fail Indeterminate. The fail positions
may also be identified by an arrow. An arrow up signifies the valve fail open and an arrow down is fail close. A crossing line is fail indeterminate. Two crossing lines indicate fail locked or last position.