Which of the following in not a basic part of a flow control valve?

• A. Body
• B. Adjustment knob
• C. Check valve poppet
• D. Piston

Answer: (D)

Flow control valves regulate how fast hydraulic fluid can move by throttling the through-path, so the essential parts are the body (the housing) and an adjustable mechanism that sets how open or closed the throttling port is. In many designs, a check valve poppet is included to control direction or to complement the throttling action. A piston, on the other hand, is not part of the basic flow-restriction element; pistons are typical inside actuators or other valve types where moving parts create displacement rather than simply throttling flow. So the element that isn’t a basic part of a flow control valve is the piston.