Relief system capacity

The relief system capacity should be calculated at the same pressure as was the required relief rate in A6.2 above, so that they can be compared. 

Gases are compressible, so their density decreases and velocity increases with pressure drop through a relief system. The increasing velocity leads to choking when the velocity reaches the speed of sound in the gas. This is discussed in more detail in 9.2. 

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The relief system capacity per unit area should be evaluated as discussed in... 

Two possible effects of solids on the relief system capacity are ... 

The presence of solids will act to reduce the relief system capacity below that for... 

SuperChems Expert 161 is a code developed by Arthur D Little Inc. for risk assessment consequence analysis, which also has a relief system sizing option. The code has a physical properties package that can handle highly non-ideal properties. It can also consider the effect of chemical reaction in the relief system piping. The code uses the DIERS drift flux methods for level swell and has the option of a rigorous two-phase slip model for the. relief system capacity. 

In the original Boyle method1181, it was recommended that the relief system capacity be calculated on the basis of non-flashing liquid flow, and a safety factor of 3 applied to the result. The modified Boyle method1161 uses a relief system capacity calculated on the basis of two-phase flow. The modified Boyle method is therefore ... 

Figure A6.1 is a decision tree to aid selection of a calculation method for relief system capacity.

A number of proprietary computer codes for gas flow exist (see Annex 4). Many of these are intended for low velocity flow and do not handle choking. Before using such a code to evaluate relief system capacity, it should be checked that it is valid for high velocity choked flow (unless the use of equation (A6.5) indicates that flow will not be choked). 

The above calculation indicates that the vapour flow rate is 2.63 kg/s and the gas flow rate is 0.4536 kg/s. This can be used to obtain weighted average values of physical properties required for calculating the relief system capacity. 

In applying this rule, the capacity of the pressure relief system must also be sized to handle the quantity of fluid released at this pressure (together with other expected loads during this contingency), so that the built-up back pressure will not result in exceeding 1.5 times the design pressure. This additional load need not, however, be considered in calculations of flare and PR valve radiant heat levels. 

Liquid Drainage from Closed Relief System - Accumulation of liquid in closed rehef systems can impose appreciable back pressure and reduce relieving capacity. The following design features must be included to avoid these problems ... 

The resistance to flow method (K[ ), which has been employed by industry for years, has now been adopted by the ASME Code for rupture disc. Sizing is performed on a relief system basis and not by capacity of individual components. The key elements of tills metliod are ... 

The reactor system in a pilot plant contains stock tanks that are 24 in in diameter and 36 in high. A relief system must be designed to protect the vessel in the event of fire exposure. The vessel contains a flammable polymer material. What rupture disc diameter is required to relieve the vessel properly Assume a discharge pressure of 10 psig. The molecular weight of the liquid is 162.2, its boiling point is 673°R, the heat of vaporization is 92.4 Btu/lb, and the heat capacity ratio of the vapor is 1.30. 

In sizing depressuring valves, it should be assumed that heater burners are shut-off, reboilers are shutdown, and normal flow in the vessel has ceased. Vapor depressuring valves should be designed such that the initial, instantaneous depressuring flow rates do not exceed the capacity of the closed pressure relief system and the flare. 

The logic given in Figure 5.1 can be used to check that this section is the correct one for relief system sizing for any particular case. As explained in Chapter 5, the required relief rate, W, should first be calculated using the methods described in this Chapter. A two-phase mass flow capacity per unit area, G, should then be calculated using the methods described in Chapter 9 (or Chapter 10 in special cases). The required relief flow area can then be calculated using equation (5.1). 

The relief sizing methods detailed in Chapters 6-8 (and most methods in Annexes 4 and 5) yield an average two-phase required relief rate, W. In order to calculate the required relief flow area, A, using equation (5.1), the two-phase mass flow capacity per unit cross-sectional area of the relief system, G, is needed. This Chapter is concerned with methods for the, calculation of G. 

The capacity of a pressure relief system depends on its design and layout. It is recommended that relief systems are designed to be as short and as straight as practicable, with the minimum number of constrictions. This will minimise the required relief system diameter ias well as simplifying the calculation of G. 

The HEM method will tend, if anything, to underestimate the relief flow capacity and so to oversize relief systems. This is provided the upstream conditions have been correctly specified (see 9.3.3). Another possible exception to the HEM tending to underestimate flow is when there is a large upwards static head change (equivalent to greater than about 10% of the pressure in the reactor), in which case a slip flow model could be more conservative. 

The capacity of the relief system can be obtained from a two-phase flow calculation for nozzle flow. If the flow is not choked, then the Omega method (see Annex 8) or suitable computer code must be used to calculate flow capacity. For choked flow a larger range of methods may be applicable, e.g. ERM for vapour pressure systems (see 9.4.2) or Tangren et al. s method for gassy systems (see 9.4.3), together with the application of a discharge coefficient. The capacity can then be obtained from ... 

The effect of laminar flow in the relief system can be to reduce the flow rate compared with that for turbulent flow by an order of magnitude. 4.4 and Annex 2 describe how to determine whether or not flow will be laminar. In the case of laminar flow, this section gives methods for calculating the mass vent capacity per unit area,... 

Effect of solids on relief system flow capacity... 

A growing number of computer codes are available for relief system sizing or for the evaluation of the flow capacity (and hence the mass relief capacity per unit area, G) for a given relief system. The types of code available are discussed below under the following headings ... 

For simple relief systems and reacting systems for which it is applicable, the Omega method (see Annex 8) can be used to find the mass flow capacity of the relief system. However, if the Omega method is inapplicable, or if the relief system has sections of different diameter, then the use of a suitable computer code to obtain the mass relief capacity may be required. 

Relief sizing for single-phase relief can be done assuming steady-state, i.e. the gas/ vapour needs to be removed by the pressure relief system at the rate at which it is generated by the reaction. A required relief rate, Wg, can be specified, and the relief system can be sized to give a capacity which exceeds it. If the capacity is expressed as mass capacity per unit flow area, Gg, then the required relief area can be obtained from ... 

If relief is via a bursting disc, the flow capacity of the relief system will normally depend on friction and choke points in the relief system. The only exception is where friction is not important (LE/D less than about 40), where equation (A6.4) can be used.) Where friction is significant, an isometric sketch of the route of the relief system will be required to determine the capacity. If the system is to be of constant diameter, then using the sketch, the total equivalent length, LE, of the route, including the frictional resistance of bends and fittings can be determined111. This can also be expressed in terms of total frictional velocity head loss, K ... 

Relief/flare system. Investment may be reduced in relief drums by using a high-pressure and a low-pressure relief system instead of a single low-pressure relief system. Investment may also be lowered by designing relief drums by analysis of liquid dump capacity, and not by the droplet size criteria of API S21. 

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