Final Control Elements and Control Valves
Final Control Elements and Control Valves tests how a controller output becomes physical action. Expect valve types, trim, actuators, positioners, Cv, flow characteristics, cavitation and flashing, fail-safe action, stroking, and calibration.
Core rules
- Valve flow capacity is expressed as Cv: the flow of water in gpm at 1 psi drop. For water, Q = Cv√(ΔP/SG); size for the normal and maximum case, not only the maximum.
- A positioner compares the input signal with stem position and adjusts air to reduce error. A common calibration maps 4–20 mA to 3–15 psi and 0–100% stroke.
- Fail-safe action is set by actuator spring and air failure mode. "Fail closed" means the valve closes on loss of air or signal; verify by nameplate, P&ID, and physical test.
- Flashing occurs when the downstream pressure stays below vapor pressure; cavitation occurs when pressure drops below vapor pressure then recovers above it. Both damage trim; cavitation causes noise and erosion.
- Stroke the valve 0, 25, 50, 75, and 100% up and down; record deadband, hysteresis, and linearity.
Worked example
A linear positioner is calibrated 4–20 mA to 0–100% stroke. At 12 mA, the signal is 50% of the 16 mA span: (12−4)/16 = 0.50. The expected output is 3 + 12 × 0.50 = 9 psi, and the stem should be at 50% travel. If the valve is at 42% while the positioner reads 9 psi, the positioner feedback linkage or calibration is wrong, or the actuator is sticking. Do not "fix" it by changing the controller output; correct the final element. Also confirm the valve is fail closed: on air failure, the stem should move to the closed position.
Traps
Reversing fail-safe action, calibrating a positioner with the bypass open, ignoring actuator spring range, and diagnosing cavitation as a tuning problem are common. Also do not assume a valve is linear because the signal is linear.
Key points
- Cv relates flow, pressure drop, and specific gravity.
- Positioner maps signal to stroke and corrects friction.
- Verify fail-safe action physically.
- Distinguish flashing from cavitation.
- Record up/down stroke and deadband.