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1.3 DESCRIPTION OF THE OPERATION
Make reference to figure 1.
Under ordinary conditions the controlled pressure, that is the valve inlet pressure reaches the
pilot chamber (A) and (C) (1) through the connection pipe (1) and the filter (2), and, through
the nozzle (3) it reaches the valve main chamber (B).
On the pilot diaphragm (4) the controlled pressure acts on one side and the load of the
setting spring acts on the other side (5); on the pilot safety diaphragm (14) the controlled
pressure acts on the chamber (C), the spring (16) and the leakage pressure from the
diaphragm, if any (4) ) acts on the chamber (D); on the valve main plug (6) the controlled
pressure and the positioning spring (7) act on the chamber (B) on one side, on the other side
the same pressure, but applied on a smaller section, corresponding to the main diameter of
the valve bore acts. Therefore, contrary to what happens in the traditional spring safety
valves, when the controlled pressure is near the setting pressure, the load applied on the
plug seat increases instead of decreasing thus ensuring a perfect seal also with values very
near to the intervention values.
Until the controlled pressure is lower than the setting value, the spring action (5) prevails on
the value of the fluid thrusts on the diaphragm (4) and the plug-pilot (8) keeps the discharge
connection shut (9).
When instead the pressure reaches the setting value, the plug-pilot (8) opens allowing to
discharge the fluid in the chamber(E) and the consequent full opening of the plug (6)
When the plug-pilot (8) begins to discharge, a flow is formed thus determining a pressure
drop on the orifice (3) and this determines the value of the valve blowdown The equipment is
designed in such a way that in case one of the main components of the valve or of the pilot
device breaks the valve opens; in fact:
a) the breakage of the small connection pipes (1), (10) or of the relevant fittings causes the
emptying of the chamber (B) and therefore the opening of the plug (6).
b) the breakage of the spring (7) does not determine considerable operating variations,
because it only keeps the plug (6) in the closing position when the valve is commissioned.
c) the breakage of the spring (5) determines the opening of the plug-pilot (8) and
therefore the emptying of the chamber (E) and the consequent opening of the plug
(6).
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