Design And Principle Of Operation; Fail-Safe Positions - Samson 3595 Mounting And Operating Instructions

3 Design and principle of oper-
ation
See Fig. 3-1
Valve style and actuator
The Type 3595 Valve is a cage valve de-
signed as globe or angle valve. The valve
can be combined with various electric, elec-
trohydraulic and pneumatic actuators. This
documentation includes examples of the
combination with the Type 3276 or
Type 3271 Pneumatic Actuator.
Cage valve
Different valve trims are used in the
Type 3595 Valve to meet application re-
quirements. All valve trims have a plug
which is guided through a cage with very lit-
tle clearance over the entire stroke. This cage
guide suppresses vibration of the closure
member, resulting in a better control perfor-
mance at high energy forces created by
changes in pressure and flow rate. The plug
is available with or without pressure balanc-
ing.
Design
The plug is connected to the plug stem using
a form-fit connection. The plug stem is con-
nected to the actuator stem by a stem con-
nector which transfers the linear motion of
the actuator to the valve plug. The actuator is
connected using a special assembly, which
varies depending on the mounted actuator.
These assemblies are fitted with an external
anti-rotation fixture for the plug stem.
EB 8079 EN

Design and principle of operation

The plug stem in the valve bonnet is sealed
by either a PTFE or graphite packing, which
is either self adjusting or can be adjusted
manually.
The valve has a clamped-in seat. The seat is
inserted into the seat bridge. The seat and
cage are clamped into place by tightening
the body nuts.
Function
The medium flows through the valve as indi-
cated by the arrow on the valve body. A
change in the pneumatic signal acting on the
actuator changes the plug travel and how
far the valve is opened as a result. The plug
position and cage design determine the re-
leased cross-section and the resulting flow
rate.

3.1 Fail-safe positions

The fail-safe position of the control valve up-
on air supply or control signal failure de-
pends on the actuator used (see associated
actuator documentation).
Depending on how the compression springs
are arranged in the SAMSON Type 3271
Pneumatic Actuator, the valve has two differ-
ent fail-safe positions:
− Stem extends (FA)
When the signal pressure is reduced or
the air supply fails, the springs move the
actuator stem downward and close the
valve. The valve opens when the signal
pressure is increased enough to over-
come the force exerted by the springs.
3-1