Working of Self-operated valve

instrumentation

#1

A “control valve” works to restrict the flow of fluid through a pipe at the command of a remotely sourced signal, such as the signal from a loop controller or logic device. A self-operated valve regulates the flow by its self without any external actuation.

self-operated valve

A self-operated valve regulates the flow using the flow pressure of the fluid in the pipeline. The outlet pressure of the valve is feedbacked to the diaphragm, the feedbacked excess pressure at the downstream actuates the diaphragm and moves the valve stem.

Feedback is taken from downstream of the valve and give to the lower chamber of the diaphragm.The diaphragm-stem system is attached to a spring. This spring establishes the pressure-regulating “setpoint” value for the regulator. If a different setpoint is desired, the spring must be replaced with one having a different stiffness.

When regulated pressure increases the at the downstream, the pressure acts on the lower chamber accordingly. The diaphragm-stem system moves up. At the time flow area between valve plug valve seat reduces and closes the valve opening thus restrict the fluid flow.

When the pressure at the downstream reduces the diaphragm goes back to normal position as the spring adjusted setpoint.

There are self-operated valves which can regulate the setpoint manually, which doesn’t use spring for the upper chamber.Instead of a stiff internal spring establishing the regulator’s pressure setpoint, the externally supplied loading pressure does that. Since this loading pressure is easily adjusted by turning the knob on the manual-set pressure regulator, the main regulator now becomes adjustable as well. The pilot mechanism controls the main gas throttling mechanism, hence the name pilot.