Samson 3351 - Design and Principle of Operation

EB 8039 EN 3-1
Design and principle of operation
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ow(seeTable3-1)
Thedirectionofthemediumow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ow
intheow-to-closedirection(AB).
ExceptfortheDN100version: the medium
mustowintheow-to-opendirection
(BA).
For control applications with liquids, the me-
diummustowintheow-to-opendirection
(BA).
Infail-openvalves,allmediamustowin
theow-to-opendirection(A B).
Table3-1: Direction of ow
Fail-safe ac-
tion
Process
medium Valve size
Flow direction
AB BA
Fail-close Vapor/gas
DN15to80
NPS½to3
FTC
1)
–
Fail-close Vapor/gas
DN100
NPS4
– FTO
1)
Fail-close Liquid
DN15to
100
NPS½to4
– FTO
1)
Fail-open
Vapor/
gas/liquid
DN15to
100
NPS½to4
FTO
1)
–
1)
FTO(ow-to-open)
FTC(ow-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.