|  The Model 730 Pressure Relief / Sustaining Valve is an automatic control valve designed to relieve excess pressure or sustain a minimum upstream backpressure.
It is a pilot controlled, hydraulically operated, diaphragm actuated globe valve in either the oblique (Y) or angle pattern design. Valve differential pressure powers the diaphragm actuator open or closed.
The lower control chamber is connected through a fixed orifice to the downstream pressure, which serves to cushion the closing of the valve. The upper control chamber, which operates on a two-way control principle, has varying pressure produced by the regulating pilot and the pilots internal upstream restriction needle valve.
 The pressure regulating pilot senses upstream pressure and modulates open or closed. This varies the pressure in the upper control chamber causing the main valve to throttle thus maintaining constant upstream pressure.
When the upstream pressure rises above the pilot setting the pilot opens, pressure in the upper control chamber decreases and the main valve modulates open to relieve upstream pressure and maintain pilot setting.
Should the upstream pressure fall below the pilot setting the pilot closes, pressure in the upper chamber increases and the main valve throttles closed to maintain the preset pilot setting.
The pressure relief/sustaining pilot has an adjusting screw to preset the desired upstream pressure and an internal needle valve to control the closing speed. For easy identification the pressure relief/sustaining pilot is stamped with the number "3" on the side of the pilot body.
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