When controlling the downstream pressure of the self-acting control valve (figure 7 b), the signal is taken over the control line to the downstream of the valve pipeline and maintains the set pressure downstream constantly. This magnitude of set pressure depends on the weight of the heavy hammer, on the other hand, if the weight of the heavy hammer is changed, the set pressure value will be changed correspond.
The control line passes the downstream pressure on the diaphragm 1 of the main valve is increased, the valve plug moves downward and the passage of flow area becomes smaller. When the torque exerted by downstream pressure on lever 2 is equal to the torque exerted by the heavy hammer on the lever, it is in a balanced state then the valve remains to open.
When controlling the upstream pressure of the self-acting control valve (figure 7 a), the signal is taken over the control line to the upstream of the valve pipeline and maintains the set pressure upstream constantly. The control line passes the upstream pressure on the diaphragm 1 of the main valve is increased, the valve plug moves downward and the passage of flow area becomes larger, then the inlet pressure corresponds decreases, it will make the lever and weight 3 force plug moves upward, the passage of flow area becomes smaller until the upstream pressure meets the set pressure.
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