The part of a pilot-operated relief valve that controls the flow of oil is the ___________.

• A. Poppet
• B. Orifice
• C. Spring
• D. Spool

Answer: (D)

Spool. In a pilot-operated relief valve, the flow is regulated by the main valve’s spool, which slides inside the valve body to vary the opening between the pressure port and the relief path. The pilot valve senses system pressure and, when the set point is reached, directs hydraulic pressure to shift the main spool. As the spool moves, it exposes more (or less) of the relief passage, controlling how much oil can flow back to the tank and thus keeping the system pressure near the set value. The other parts play supporting roles: a spring sets the pressure at which the valve begins to open; a poppet is another type of sealing element used in different designs; an orifice is just a fixed restriction and does not actively regulate flow in the same way as the spool does.