Fail-Safe Positions; Versions - Samson 3248 Mounting And Operating Instructions

Design and principle of operation

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
and Type 3277 Pneumatic Actuator, the
valve has one of two different fail-safe posi-
tions:
− Actuator 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.
− Actuator stem retracts (FE)
When the signal pressure is reduced or
the air supply fails, the springs move the
actuator stem upwards and open the
valve. The valve closes when the signal
pressure is increased enough to over-
come the force exerted by the springs.
Tip
The actuator's direction of action can be re-
versed, if required. Refer to the mounting
and operating instructions of the pneumatic
actuator:
u EB 8310-X for Type 3271 and Type 3277
3-4

3.2 Versions

Tip
We recommend the use of positioners with
integrated diagnostic firmware (see sec-
tion 3.4) for valves used for on/off service.
The partial stroke test included in this soft-
ware helps prevent a shut-off valve normally
in its end position from seizing up or getting
jammed.
Larger valve sizes
The Type 3248 Valve is available up to valve
size NPS 8.
Actuators
In these instructions, the preferable combina-
tion with a SAMSON Type 3271 or
Type 3277 Pneumatic Actuator is described.
The pneumatic actuator (with or without
handwheel) can be replaced by another
pneumatic actuator in a different size, but
with the same travel.
Î Observe the maximum permissible actu-
ator force.
Note
If the travel range of the actuator is larger
than the travel range of the valve, the spring
assembly in the actuator must be preloaded
so that the travel ranges match. See associ-
ated actuator documentation.
The basic pneumatic actuator can be re-
placed by a pneumatic actuator with addi-
tional handwheel.
EB 8093-1 EN