2.0 The following procedures will demonstrate the adequacy
of a tank vent system to limit the pressure rise in a cargo tank to not greater than 1.2
× MARVS during all conditions, including fire conditions implicit in 8.5.2 of the IGC Code.
2.1 Prepare a simplified flow sheet of the cargo
tank vent system, identifying the fittings and the actual diameters
and lengths of pipe. (See annex 2 for
an example).
Divide the system into sections between nodes at changes in
pipe diameter and at inter-connections with flows from other relief
valves.
List the fittings and their dynamic loss coefficients.
Calculate the external surface area of the piping sections between
the nodes.
2.2 Calculate the COD PRV capacity of (QGCC)
of each tank PRV, in m3/s of air at standard conditions in
accordance with 8.5.2 of the IGC Code and not the installed rated capacity
(Q
IR) of each PRV in m3/s air at standard conditions at 1.2
× MARVS. The calculation should be done for the highest gas factor of the
products included in the cargo list. N-butane has often the has often the highest value
for gas factor “G” in the Code and usually determines the Code minimum capacity.
Determine the mass flows for cargo conditions 1.2 × MARVS through each PRV for the Code PRV capacity and for the installed
rated capacity for both all vapour flow and for two-phase cargo flow.
Also calculate the mass flow at MARVS for the installed
rated capacity on all vapour flow.
Equation (1) may be used for all vapour
mass flow and equations(2), (3) and (4) may be used for two-phase mass flow. Equation (2) may be applied to multicomponent
mixtures whose boiling point range does not exceed 100 K.
2.3 Estimate all the vapour flow pressure drop
in the pipe from the cargo tank connection to the PRV inlet flange,
working from the known tank pressure towards the PRV. This pressure
drop is calculated by using the difference in stagnation pressures.
Therefore, the second term of equation
(5) may be used may be used for pipe sections of constant diameter.
For contractions equation (5.1) may
be used.
2.4 Check that the pressure drop at each PRV inlet
complies with 1.3.1 at the Code
PRV capacity for all vapour flow to assure adequate relief capacity.
For the calculation, the vapour mass flow of product (Wg)
from equation (1) should
be used.
For control purposes, 1.3.1 should
be repeated using the Code PRV two-phase flow (W', equation (4)) at 1.2 × MARVS and 1.3.2 by using the installed rated two-phase
flow at MARVS. Both calculations should give a smaller inlet pressure
loss than the corresponding all vapour pressure loss.
Check
that the blowdown Δpclose
complies with 1.3.2 to assure stable operation.
2.5 Estimate the two-phase flow pressure in the
discharge pipe at the location of discharge to the atmosphere. Equation (6) may be used, with the Code
PRV two-phase mass flow (W', equation
(4)) to assure adequate relief capacity, to check if the exit
pressure is greater than 1 bar a.
2.6 Estimate the vapour fraction and two-phase
density in the vent pipe at the exit to the atmosphere, assuming transfer
of the fire heat flux of 108 kW/m
2) through
the uninsulated vent piping. Equations(7) and (8) may be used.
2.7 Estimate the built-up back pressure at the
PRV outlet flange, commencing from the known vent pipe exit pressure,
calculating the pressure drop between pipe nodes and working, section
by section, back up the pipe to the PRV.
Equations (7), (8), (9) and (5) may be used with iteration until
the upstream node absolute pressure, vapour fraction and specific
volume are justified and assuming that vapour is saturated.
At
pipe diameter expansion fittings where fluid velocity is reduced,
a pressure recovery generally occurs. This recovery is overestimated
in case of two-phase flow when dynamic loss coefficients for single-phase
flow are used. For the purpose of these guidelines, the static exit
pressure of a conical expansion fitting is assumed to be equal to
the static inlet pressure.
2.8 Estimate the chocking pressure (p
ec) at the exit of every section with the mass-flux (Gp)
in that section for the pipeline between the PRV and the vent exit. Equation (6) may be used.
Compare the pressure distribution along the vent line as derived
from items 2.5 to 2.7, with the
different choking pressure for each section as derived from equation (6).
If choking
pressure at any location exceeds the corresponding calculated pressure
derived from 2.5 to 2.7, the calculation as described in 2.5 to 2.7
should be repeated commencing from choking point location and corresponding
choking pressure, working back up the pipe to the PRV.
If
choking pressure at more than one location exceeds the corresponding
calculated pressure derived from 2.5 to 2.7, the commencing point
of the recalculation should be taken as the choking location point
giving the highest built-up back pressure.
2.9 Check that the built-up back pressure at each
PRV outlet complies with 1.4,
at the Code PRV capacity for two-phase mass flow (W', equation (4)), to assure stable operation
of the valves, thus assuring adequate relief capacity.
2.10 For conventional unbalanced valves only:
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.1 If back pressure is derived from items 2.5 to 2.8 is within the range of 10%
go 20% MARVS, an additional evaluation should be performed
in order to decide whether the system is acceptable.
-
.2 The system should perform with the following
requirement: With one valve closed and all others discharging at the
installed rated PRV capacity, the back pressure should be less than
10% of MARVS.