11If you examine this diagram closely, you will notice an error in it: it shows the top and bottom of the piston actuator connected together by air tubing, which if implemented in real life would prevent air pressure from imparting any force to the valve stem at all! Connecting the top and bottom of the actuator together would ensure the piston always sees zero differential pressure, and thus would never develop a resultant force. The output tube of PY-590 should only connect to the bottom of the piston actuator, not to the bottom and the top. A more minor error in this diagram snippet is the labeling of SOV-590A: it actually reads “SOV-59DA” if you look closely enough! My first inclination when sampling this real P&ID for inclusion in the book was to correct the errors, but I think an important lesson may be taught by leaving them in: documentation errors are a realistic challenge you will contend with on the job as an instrumentation professional!