**THE ASSEMBLY,**

RECALLING Article 15(j) of the Convention on the International Maritime Organization concerning the functions of the Assembly in relation to regulations and guidelines concerning maritime safety,

RECALLING ALSO resolution MSC.32(63) On the adoption of amendments to the International Code for the Construction and Equipment of Ships Carrying Liquefied Cases in Bulk (IGC Code),

RECALLING FURTHER paragraph 8.2.18 Of the revised IGC Code which provides that the adequacy of the vent system fitted on tanks is to be demonstrated by using the guidelines developed by the Organization,

HAVING CONSIDERED the recommendation made by the Maritime Safety Committee at its sixty-fifth session,

**1**. ADOPTS the Guidelines for the Evaluation of the Adequacy of Type C Tank Vent Systems set out in the annex to the present resolution;

**2.**INVITES Governments to apply the Guidelines when establishing the adequacy of the vent systems fitted on tanks in accordance with relevant provisions of the IGC Code;

**3.**REQUESTS the Maritime Safety Committee to keep the Guidelines under review and to amend them as necessary.

## Annex 1 Amendments to the International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code) (Resolution MSC.32(63))

## Annex 2 Worked example of the procedures

**Procedures**

**Reference**

**No.**

By 2.1

Figure 1 - Simplified flow sheet of a cargo tank vent system with one vent stack connected to two tanks

Table 1List of vent pipe lengths and surface areas and dynamic loss coefficients

_________

* Ignored under procedure 2.7.

Table 2 - Typical values for dynamic loss coefficient (N) for vent system fittings. N may vary with pipe diameter

**Summary of predictions**

The predicted two-phase propane properties are shown at five node points in the PRV discharge vent piping, in fixture 2 at the Code PRV flow-rate, and in figure 3 at the installed rated flow-rate. The flowing pressure drop in the piping to the PRV inlet is less than Guideline 1.3 requires. The built-up backpressure at the PRV outlet is also less than Guideline 1.4 requires for the pilot-operated PRVs installed.

The flowing pressure drop in the PRV inlet piping is well within Guideline 1.3 for tile Code PRV all vapour flow-rate but exceeds the requirement for the installed rated all vapour flow-rate. However, the pressure drop is acceptable for reasons explained in the footnote to paragraph 1.3.2 above. The blowdown and closing pressure should be set to assure stable operation when both PRVs are open.

These procedures are now applied to example case 3B in Dow Chemical Company's Report to CTAC using their RELief DESign program, February 25, 1992 (BCH 22/INF.6). Per RELDES RESULTS on page 9, the last two-phase flow of 106 lbs/sec (48.1 kg/s) occurs at a tank pressure of 169 prig (12.66 bar a), Quality (percent vapour by mass) is stated to be 0.10% and Vessel inventory is 76.2%, liquid propane. The PRV discharge vent pipe is assumed to be 10 ft long by 8 inches diameter (3.04 m length x 0.203 m diameter) and PRV Orifice Area is 12 .3 sq. in. (7.935 x 10

^{- 3}m

^{2}), K

_{d}= 0.953.

Figure 2 - Two-phase propane properties at Code PRV relief flow-rate-Simplified flow sheet of a cargo tank vent system with one vent stack connected to two tanks

Figure 3 - Two-phase propane properties at Code PRV relief flow-rate-Simplified flow sheet of a cargo tank vent system with one vent stack connected to two tanks

## Annex Guidelines for the evaluation of the adequacy of type C tank vent systems

### 1 General

**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.

**1.3.1**The pressure drop in the vent pipe from the cargo tank to the PRV inlet (Δ P

_{inlet}) should not exceed 3% of MARVS, at the Code PRV capacity from equation (1) at 1.2 x MARVS on all vapour flow.

**1.3.2**The blowdown (Δ P

_{close}) should not be less than (Δ P

_{inlet}) plus 0.02 x 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 backpressure in the vent piping from the PRV outlet to the location of discharge to the atmosphere, and including any vent pipe interconnections which join other tanks, should not exceed the following values:

- for unbalanced PRVs: 10% MARVS. Special consideration may be given in cases where the back pressure exceeds 10% of MARVS at a tank pressure of 1.2 x MARVS; and
- for balanced PRVs and pilot-operated PRVs as advised by manufacturer; normally 30% of MARVS for balanced PRVs and 50% of MARVS for pilot-operated PRVs,

^{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 for 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).

### 2 Procedures

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.

**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 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.

**2.2**Calculate the Code PRV capacity ( Q

_{GCC}) of each tank PRV, in m

^{3}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 m

^{3}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.

**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 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 ( W

_{g}) 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 loss.

Check that the blowdown Δ P

_{close}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 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.

**2.8**Estimate the choking pressure ( p

_{ec}) at the exit of every section with the mass-flux ( G

_{p}) 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.

**2.9**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.

**2.10**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.

### 3 Equations

The following equations may be used to demonstrate the adequacy of the vent system.

### 4 References

**1 General**

**1.1**IGC, GC Codes 8.2.17 draft text of amendments BCH 22/14, annex 8 Code for Existing Ships 8.2.15 draft text of amendments BCH 22/14, annex 8

**1.2**BCH 20/7, annex 4, validated by BCH 21/INF.3, annex 2

**1.3**IGC Code 8.2.16 draft amendment BCH 22/14, annex 9; API RP 520 Part II, Third Edition, November 1988, 2.2.2 on page 2

**1.4**IGC Code 8.2.16 draft amendment BCH 22/14, annex 9; API RP 521, Third Edition, November 1990, 5.4.1.3.1 on page 45 and API RP 520, Part I, Fifth Edition, July 1990, 2.2.4.1 on page 7 and 4.3.2.1 Fig. 27 on page 30

**1.5**BCH 20/7, annex 5 as referenced in 3. Equations 2.4 Frank J. Heller: " Safety relief valve sizing: API versus CGA requirements plus a new concept for tank cars": API-Proceedings 1983, Refining Department, Vol. 62, API, W.D.C, pp. 123-135

**3 Equations**

(2) "Flashing flows or: Some practical guidelines for emergency releases", Fauske and Associates, Plant/Operations Progress, July 1985, private communication SIGTTO/Fauske and Associates, June 1st 1994

(5) "The discharge of two-phase flashing flow in a horizontal duct", Fauske and Associates, AIChE Journal, March 1987 on pages 524 (equation) and 526 (Fanning friction factor in two-phase flows), private communication SIGTTO/Fauske and Associates, January 21, 1993

(6) "Size safety relief valves for flashing liquids"; J.C. Leung (Fauske and Associates), Chemical Engineering Progress, Feb. (1992), pp. 70-75

(7) BCH 20/7, annex 5

(8) BCH 20/7, annex 5

(9) BCH 20/7, annex 5