3.15 PRESSURE/VACUUM VALVES AND IGS VENTING
To prevent over pressure of cargo tanks, the pressure/vacuum (P/V),
valves must be properly set, or inert gas system (IGS), vents correctly
adjusted when loading begins. The venting procedure used must comply
with the method indicated to the shore terminal. The objective of the
venting system procedure is to disperse the gasses and prevent an explosive
mixture from forming on or near the ship.
3.15.1 P/V valve setting
Tankers without IGS, which will closed-load cargo, must manually lift
the P/V relief valve weights (using the handwheel or lever provided),
to permit free exhaust of the tank atmosphere while the tanks are filling.
Immediately after a tank has completed loading, its P/V valve should
be reset to the normal position.
Both operations are important. If the valves are not lifted for loading,
a cargo tank may be over pressured, causing structural damage to the
deck. If the valves are not reset after loading, significant vapour
losses will occur. Both operations should be completed using a checklist
and verified by a second independent check. Enter both inspections in
the log book.
3.15.2 Inert gas system
The cargo tanks on IGS ships should be fully inerted before loading
begins and throughout the loading, with the system pressurised to between
500 and 800 mm water gauge. Several tanks should be checked using an
oxygen analyser to verify that the cargo tanks contain 8% or less oxygen.
The IGS plant is then shut down and the deck isolation valve closed.
It should not be necessary to operate the IGS plant during loading.
The IGS vent valve(s) should be opened just before or immediately after
loading begins and used to maintain an appropriate pressure on the IGS
system throughout the loading operation. The IGS pressure should not
exceed 1400 mm (2 psig), at any time.
The liquid levels in the deck water seal and the P/V breaker, and the
proper function of the IGS deck non-return valve, should be verified
before loading begins. The effects of over-pressuring the cargo tank
vent system may be loss of liquid from the P/V breaker and the deck
water seal. The cargo officer must understand the actions required if
3.15.3 IGS tank block valves
Most tankers with IGS are provided with valves or blinds which can be
used to shut off IGS flow to the individual tanks. Each of these valves
must be verified to be open before loading commences. If the vessel
is CCR operated, there may be no indication of the position of these
valves in the CCR. In that case, each valve must be verified to be open
during the pre-loading inspection and confirmed open by the deck watch
before loading any tank can begin. An IGS valve position status board
should be located in the CCR and carefully maintained.
3.15.4 Vent valve defects
Any defect in vent valve operation requires immediate attention. The
valves must be maintained in perfect condition to prevent tank damage
due to over pressure or vacuum. Leaking vent valves can permit oxygen
to enter the tanks (a safety/explosion hazard), or cargo vapours to
escape (with resulting cargo loss claim). Leaking valves make it difficult
to maintain required minimum IGS pressure and cause extra running of
the IGS system.
3.15.5 Vapour return lines
Some ports require the recovery of vapour expelled from cargo tanks
while loading. Ships loading at such ports must have a vent collector
system and a manifold connection for the shore vapour recovery hose.
The vent system valves are adjusted and all P/V valves set to the automatic
(or voyage), position so that gasses expelled as loading proceeds are
displaced ashore. On shore, the hydrocarbon vapours are condensed and
recovered as liquids. The shore terminal pre-transfer inspection and
checklist must include verification that the vapour receiving system
is lined up correctly. The chief officer should verify that this item
appears on the shore declaration of inspection.