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 x MARVS during all conditions, including fire
conditions implicit in 8.5.2 of the IGC Code.
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 interconnections
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.
Calculate the Code PRV capacity ( Q GCC
) of each tank PRV, in m3
of air at standard conditions in accordance with 8.5.2 of the IGC Code and note the installed rated capacity ( Q IR
) of each PRV in m3
air at standard conditions at 1.2 x MARVS. The calculation should be done for the highest gas factor of the products included in the cargo list. n-Butane 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 at 1.2 x 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 100K.
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 for pipe sections of constant diameter. For contractions, equation (5.1) may be used.
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 x 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
Check that the blowdown Δ P close
complies with 1.3.2 to assure stable operation.
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.
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/m2
through the uninsulated vent
piping. Equations (7) and (8) may be used.
Estimate the built-up backpressure 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.
Estimate the choking pressure ( pec
) 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 item 2.5 to 2.7, with the
different choking pressures 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 backpressure.
Check that the built-up backpressure 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.
For conventional unbalanced valves only:
- If backpressure as derived from 2.5 to 2.8 is within the range of 10% to 20% of MARVS, an additional evaluation should be performed in order to decide whether the system is acceptable.
- The system should perform with the following requirement: with one valve closed and all others discharging at the installed rated PRV capacity, the backpressure should be less than 10% of MARVS.