Design And Principle Of Operation - Samson 3351 Mounting And Operating Instructions

3 Design and principle of oper-
ation
Fail-safe action
In the Type 3351 Valve, the connected pneu-
matic control pressure (5.8) opposes the
force of the spring (5.5) to open or close the
valve.
Depending on the design of the valve seat
(2) and where the plug (3) is located inside
the valve, the valve has two different fail-safe
actions which become effective when the
pressure is relieved from the diaphragm
(5.4) or the supply of control pressure fails.
− Fail-close valve: upon failure of the con-
trol pressure, the valve is closed by the
spring.
− Fail-open valve: upon failure of the con-
trol pressure, the valve is opened by the
spring.
For versions with the optional handwheel (6),
a fail-close valve can be opened and a fail-
open valve can be closed in the event of con-
trol pressure failure.
Direction of flow (see Table 3-1)
The direction of the medium flow in the valve
depends on the process medium and the se-
lected fail-safe action.
For fail-close valves which are used to con-
trol gases and vapors, the medium must flow
in the flow-to-close direction (A  B).
Except for the DN 100 version: the medium
must flow in the flow-to-open direction
(B  A).
EB 8039 EN

Design and principle of operation

For control applications with liquids, the me-
dium must flow in the flow-to-open direction
(B  A).
In fail-open valves, all media must flow in
the flow-to-open direction (A  B).
Table 3-1: Direction of flow
Fail-safe ac- Process
tion medium
Fail-close Vapor/gas
Fail-close Vapor/gas
Fail-close Liquid
Vapor/
Fail-open
gas/liquid
FTO (flow-to-open)
1)
FTC (flow-to-close)
Control pressure and max. differential
pressure Δp
Table 3-5 shows the correlation between
control pressure and max. differential pres-
sure Δp based on the process medium used.
Flow direction
Valve size A  B B  A
DN 15 to 80
FTC
1)
NPS ½ to 3
DN 100
– FTO
NPS 4
DN 15 to
100 – FTO
NPS ½ to 4
DN 15 to
100 FTO
1)
NPS ½ to 4
–
1)
1)
–
3-1