Samson 33-1, Mounting And Operating Instructions

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EB 2551-1 EN 3-1
Design and principle of operation
3 Design and principle of oper-
ation
Î
See Fig. 3-1
The Type 33-1 Safety Shut-off Valve (SSV)
consists of the main valve with actuator and
the pilot valve attached to it.
The safety shut-off valve is used to maintain
the pressure downstream of the main valve
to a set point adjusted at the pilot valve.
The shut-off valve closes when the down-
stream pressure p
2
rises.
When the shut-off 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 bal-
ancing bellows (5).
The upstream pressure p
1
is fed through the
control line (10) to the pilot valve (8) and
serves as the pilot supply pressure. The pilot
valve (8) creates a control pressure p
S
. It
functions as a pressure reducing 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).
A drop in downstream pressure p
2
causes
the control pressure p
S
to rise. The force re-
sulting from the control pressure p
S
acting on
the operating diaphragm (6) overcomes the
forces created at the set point springs (7)
and the prevailing downstream pressure p
2
.
The main valve (1) opens.
A rise in downstream pressure p
2
causes the
control pressure p
S
to decrease. As a result,
the control pressure p
S
gradually reaches the
same level of the downstream pressure p
2
causing the main valve (1) to be closed by
the set point springs (7).
Upon failure of the operating diaphragm (6),
the safety shut-off valve closes and the pres-
sure falls below the adjusted pressure set
point.
To ensure proper functioning of the regulator,
the minimum differential pressure Δp
min
must
correspond to the pressure specified in Table
3-1.
Note