1.1 The tank outlet to the pressure relief valves
(PRVs) should remain in the vapour phase at the 98% liquid level and
Code specified list and trim.
1.2 PRVs, which have been sized using the GC Codes,
have adequate capacity.
1.3 To assure adequate relieving capacity condition,
1.3.1 is required and to assure adequate blowdown condition, 1.3.2
is required.
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1.3.1 The pressure drop in the vent pipe from
the cargo tank to the PRV inlet (Δ pinlet
)
should not exceed 3% of MARVS, as the Code PRV capacity from equation (1) at 1.2 × MARVS on
all vapour flow.
-
1.3.2 The blowdown (Δpclose
)
should not be less than (Δpinlet
) plus
0.02 × MARVS at the installed rated vapour capacity
where required to assure stable operation of the PRV. This calculation
should be performed at MARVS on all vapour flow.
Pilot-operated valves can tolerate higher inlet-pipe pressure
losses when the pilot senses at a point that is not affected by the
inlet-pipe pressure drop.
1.4 The built-up back pressure in the vent piping
from the PRV outlet to the location of discharge to the atmosphere,
and including any vent pipe inter-connections which join other tanks,
should not exceed the following values:
-
.1 for unbalanced PRVs: 10% MARVS
Special consideration may be given in cases where the back pressure
exceeds 10% of the MARVS at a tank pressure of 1.2 × MARVS; and
-
.2 for balanced PRVs and pilot operated PRVs as
advised by the manufacturer; normally 30% of MARVS for
balanced PRVs and 50% of MARVS for pilot operated PRVs.
when assuming isenthalpic expansion of saturated liquid, at
1.2 × MARVS, through the PRV with the vent piping under fire
exposure. A heat flux of 108 kW/m
2 is assumed
for uninsulated vent piping.
1.5 The built-up back pressure in the vent piping
may be estimated by the procedures outlined in section 2.
1.6 A more accurate procedure of evaluating tank
vent systems on flashing two-phase flow should be consulted if these
simplified procedures do not demonstrate compliance with the requirements
stated in 1.3 and 1.4 above.
1.7 MARVS means the maximum allowable relief valve
setting of a cargo tank (gauge pressure).