Vent sizing methods

Vents are usually sized on the assumption that the vent flow is  

The first two eases represent the smallest and largest vent sizes required for a given rate at inereased pressure. Between these eases, there is a two-phase mixture of vapor and liquid. It is assumed that the mixture is homogeneous, that is, that no slip oeeurs between the vapor and liquid. Furthermore, the ratio of vapor to liquid determines whether the venting is eloser to the all vapor or all liquid ease. As most relief situations involve a liquid fraetion of over 80%, the idea of homogeneous venting is eloser to all liquid than all vapor. Table 12-3 shows the vent area for different flow regimes. 

These systems are ealled tempering (i.e., to prevent temperature rise after venting) systems beeause there is suffieient latent heat available to remove the heat of reaetion and to temper the reaetion at the set pressure. The vent requirements for sueh systems are estimated from the Leung s Method [31,32]  

Required vent area as a multiple of all vapor vent area 

964 Modeling of Chemioal Kinetios and Reaotor Design Alternatively, the vent area ean be expressed as  

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Dow Chemical Co. (1973) The Dow Safety Guide, a reprint from Chemical Engineering Progress (AIChE). Duxbury, H. A. (1976) Loss Prevention No. 10 (AIChE) 147. Gas vent sizing methods. 

Both vent sizing methods for gases and dusts require values for the deflagration indexes, KG or KSt. We discussed the experimental procedure to determine these values and also provided tables of typical values for gases and dusts in chapter 6. 

J C Leung, "Simplified Vent Sizing Methods Incorporating Two-phase Flow", International Symposium on Runaway Reactions and Pressure Relief Design, 200-236, AlChE, 1995, ISBN 0-8169-0676-9... 

Duxbury, H. a. (1976) Loss Prevention No. 10 (AIChE) 147. Gas vent sizing methods. 

Vent sizing method based on the dust type and plant to be protected. In the United States, NFPA 68 provides the required information for vent sizing calculations. 

Vent sizing methods require data on both the pressure/temperature relationship and rate of heat release/temperature relationship that occur during the course of the runaway reaction. This data can be obtained using techniques such as adiabatic pressure Dewar calorimetry or other special equipment described in Chapters 3 and 4. 

Vapour-pressure systems, where the pressure in the reactor is entirely due to the vapour pressure of the reaction mixture, are common in the chemical industry. A large number of vent sizing methods, both pre- and post-DIERS, are applicable to vapour pressure systems. The methods of Leung - and Fauske M both derived from DIERS work, are now widely used. 

Vent Sizing Package (VSP) The VSP is an extension of ARC technology. The VSP is a bench-scale apparatus for characterizing runaway chemical reactions. It makes possible the sizing of pressure relief systems with less engineering expertise than is required with the ARC or other methods. 

Equation 12-18 gives a eonservative estimate of the vent area, and the simple design method represents overpressure (AP) between 10%-30%. Eor a 20% absolute overpressure, a liquid heat eapaeity of 2,510 J/kg K for most organies, and eonsidering that a saturated water relationship exists, the vent size area per 1,000 kg of reaetants is ... 

The major method of vent sizing for gassy system is two-phase venting to keep the pressure eonstant. This method was employed before DIERS with an appropriate safety faetor [34], The vent area is expressed by ... 

If Fauske s method yields a signifieantly different vent size, then the ealeulation should be reviewed. 

Choose the smaller of the vent size from the two methods. 

A. Available Design Methods for Vent Sizing. Several methods are available to size the vent with a wide range of sophistication. The FIA chart, Fig. 1 prepared by the Factory Insurance Association in the mid 1960 s is a simple chart summarizing a wealth of experience. Reactions are classed by the degree of exothermic reaction. With vessel size and a judgment of reaction type a vent size range can be selected. 

In 1972 a paper on venting by Huff 2J documented concerns that many designers suspected that to truly be safe the vent sizing of many systems should be based on assuming two-phase flashing flow in the vent system. A two-phase flow vent method developed by Huff was compared with Boyle s all-liquid method, and values from the FIA chart in Figure 2. It can be seen that under many conditions, previous methods were not... 

Use of thermal stability tests (DTA s) to determine the heat sensitivity of a given process mixture is desirable. Recent advances in analytical methods permit good calorimetric determination of heat of reaction. Heat of reaction data are critical for exothermic reactor vent sizing. Heat impact from fire is usually small in comparison, but should not be neglected. 

Experimental data can be obtained from the DSC and from reaction calorimeters for the conditions of the desired reactions, and from the DSC, the ARC, the Reactive System Screening Test (RSST—Fauske and Associates) and from the Vent Size Package (VSP) for conditions allowing undesired reactions. The pressure effect can be studied using the ARC or DIERS methods. From the results of these tests, the rate of temperature rise and the maximum acceptable conditions for specific equipment can be calculated. The same holds for the pressure rise rate. 

With the assumption of two-phase turbulent flow, a simplified method has been developed [195] for estimating emergency vent sizes which is discussed in Section 3.3.4.7. 

It should be emphasized that the results from RSST experiments use simplified calculation methods and give estimated values for stability and vent sizing. For large scale purposes, more dedicated and accurate measuring techniques may be needed for further hazard evaluation determinations. Due to the relatively recent development of this apparatus, comparison with other stability test methods is not yet comprehensive. 

The use of the sizing method above for gassy systems assumes that case (iii) is not a tempered hybrid. If, during detailed relief sizing, case (iii) does turn out to be a tempered hybrid system, and the vent size is significantly smaller, then the worst case would need to be reassessed, by carrying out detailed relief sizing for both cases (ii) and (iii). 

For tempered systems, the pressure relief system will almost always need to be bigger if two-phase flow occurs, and DIERS[1] recommended that two-phase relief should normally be assumed for vent sizing purposes using the type of hand calculation methods given in this Workbook. This is explained in 4.3.2, Subsection... 

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