Samson 33-7, Mounting And Operating Instructions

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EB 2551-2 EN 3-1
Design and principle of operation
3 Design and principle of oper -
ation
Î
See Fig. 3-1
The Type 33-7 Excess Pressure Valve (SEV)
consists of the main valve with actuator and
the pilot valve attached to it.
The safety excess pressure valve is used to
maintain the pressure upstream of the main
valve to a set point adjusted at the pilot
valve.
The safety excess pressure valve opens as the
upstream pressure p
1
rises.
When the safety excess pressure valve starts
to open, the process medium first flows
through the pilot valve (8) before the main
valve (1) opens. During which, the forces
acting on the plug in the main valve are
eliminated by the balancing bellows (5).
The upstream pressure p
1
is fed through the
control line (10) to the pilot valve (8). The
pilot valve (8) creates a control pressure p
S
. It
functions as an excess pressure valve and
controls the main valve (1).
The control pressure p
S
is transferred through
the control line (12) and acts on the operat-
ing diaphragm (6).
The control pressure p
S
drops when the
upstream pressure p
1
rises. The force
resulting from the control pressure p
S
acting
on the operating diaphragm (6) and the
forces created at the positioning springs (7)
fall below the force of the prevailing
upstream pressure p
1
. The main valve (1)
opens.
A decrease in upstream pressure p
1
causes
the control pressure p
S
to rise. As a result,
the control pressure p
S
gradually reaches the
same level of the upstream pressure p
1
causing the main valve (1) to be closed by
the positioning springs (7).
Upon failure of the operating diaphragm (6),
the safety excess pressure valve opens and
the pressure falls below the adjusted pres-
sure p
1
.
To ensure proper functioning of the regulator,
the minimum differential pressure Δp
min
must
correspond to the pressure specified in Table
3-1.
Note