7.5.3 BACK FLOW PREVENTER
• Prevents accidental reverse flow through safety valve.
This option, sometimes called a ‘vacuum block,’ prevents a
pilot operated safety valve from reverse flow, when
sufficient vacuum is present at the inlet flange. The
backflow preventer also prevents reverse flow when the
pressure at the outlet flange (superimposed back
pressure) is greater than the current system pressure.
Reverse flow will occur with any standard type or design of
pilot operated safety valve, when sufficient reverse
differential pressure exists. Reverse flow, should it be
induced by a reverse differential pressure, will be
prevented by this option. All backflow preventers operate
by permitting the introduction of outlet pressure into the
dome of the main valve, thereby holding the piston firmly
onto the nozzle, overcoming the effect of a reverse
differential pressure across the safety valve. The option
also includes a built-in provision to prevent reverse flow
through the pilot that would otherwise pass through the pilot supply line, back into the system.
A Backflow Preventer should be specified whenever:
• A vacuum may be present at the inlet connection due to unusual operating conditions or a temporary
vacuum condition that may occur under startup conditions.
• The discharge of the safety valve is connected to a downstream pressure vessel, where pressure may
vary from time to time, in excess of the pressure in the upstream system.
!Pay attention to connect the
back flow preventer correctly!
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78 SERIE SAFETY RELIEF VALVE
SARASIN-RSBD
™
