Sizing of vapor relief

The procedure of vapor relief valve sizing depends on the type of vapor flow. Sizing for critical flow is different from fliat for subcritical flow. It is therefore required to establish the flow behavior first. 

If the critical pressure is equal to or more than the downstream (back) pressure, the flow will be critical otherwise, the flow will be subcritical. 

The general equation to estimate the orifice area for critical flow is 

W = design flow, kg/h T = relieving temperature, K Z = compressibility factor, dimensionless MW = vapor molecular weight 

Process engineering and design using Visual Basic 

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Thus, the size of the relief deviee is signifieantly smaller than for two-phase flow. Sizing for all vapor relief will undoubtedly give an ineorreet result, and the reaetor would be severely tested during this runaway oeeurrenee. Table 12-7 gives the results of the VENT software program of Example 12-4. 

For fire reliefs with single-phase vapor flow the equations provided in sections 9-2 and 9-4 are used to determine the size of the relief. 

It should be noted that vapor depressuring may not be practical when the vessel design pressure is less than 100 psig (690 kPa) because valves and piping can become unreasonably large and costly or when the vapor depressuring load governs the size of pressure relief and flare headers. Refer to API RP 521,... 

THIS PROGRAM PRINTS THE RESULTS OF VAPOR RELIEF SIZING ONTO A PRINTER... 

Credit can be taken for a circumstance that is certain to occur during a relief situation and that will act to lower the column relief load. Credits are applied for reducing the calculated relief vapor rate, and thus the size of the relief device. Extreme caution is required in deciding whether credit should be taken for a given circumstance. If a credit is taken for a circumstance which cannot be relied on during a relief situation, the column may be overpressured. 

It is the rate of temperature inerease (i.e., power output) between the set pressure and the maximum allowable pressure, whieh determines the vent size and not the peak rate. Boiling is attained before potential gaseous deeomposition (i.e., the heat of reaetion is removed by the latent heat of vaporization). The reaetion is tempered, and the total pressure in the reaetor is equal to the vapor pressure. The prineipal parameter determining the vent size is the rate of the temperature rise at the relief set pressure. 

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. 

The following eonsiders a situation that involves all vapor relief. The size of a vapor phase rupture disk required is determined hy assuming that all of the heat energy is absorbed by the vaporization of the liquid. At the set temperature, the heat release rate q is... 

When a flammable liquid is sprayed as fine droplets into the air, a flammable mixture can result, which may burn or explode. The mist or spray may be formed by condensation of saturated vapors or by mechanical means [40]. As the particle sizes of the liquid become greater than 0.01 mm diameter, the lower flammability limit of the material becomes lower while above 0.01 mm, the LEL is about the same as the vapor. Mechanical engine crankcase explosions of oil mist in air are hazardous, and current practice is to apply explosion relief valves to the crankcase. 

Latent heat of vaporization 169 BTU/lb Tank size (nominal) 15 diameter and 20 high Operating pressure 1.5 oz/in.2 Maximum allowable tank pressure 3.5 oz/in.2 Maximum allowable vacuum 1.0 oz/in.2 Relief valve settings ... 

The method used for the safe installation of pressure relief devices is illustrated in Figure 8-1. The first step in the procedure is to specify where relief devices must be installed. Definitive guidelines are available. Second, the appropriate relief device type must be selected. The type depends mostly on the nature of the material relieved and the relief characteristics required. Third, scenarios are developed that describe the various ways in which a relief can occur. The motivation is to determine the material mass flow rate through the relief and the physical state of the material (liquid, vapor, or two phases). Next, data are collected on the relief process, including physical properties of the ejected material, and the relief is sized. Finally, the worst-case scenario is selected and the final relief design is achieved. 

After the relief type has been chosen and the relief size computed, the engineer takes the responsibility for completing the design of the relief system, including deciding how to install the relief in the system and how to dispose of the exiting liquids and vapors. 

This requires a relief device with a diameter of 0.176 m, a significantly smaller diameter than for two-phase flow. Thus, if the relief were sized assuming all vapor relief, the result would be physically incorrect and the reactor would be severely tested during this runaway event. 

As mentioned previously, two-phase flow discharges for fire scenarios are possible but not likely. To size the relief for fire and a single-vapor phase, use the heat input determined from Equations 9-36 to 9-38, and determine the vapor mass flow rate through the relief by dividing the heat input by the heat of vaporization of the liquid. This assumes that all the heat input from the fire is used to vaporize the liquid. The relief area is then determined using Equations 9-3 to 9-12. 

A horizontal vessel, 10 ft long and 3 ft in diameter, contains water. What relief size is required to protect the vessel from fire exposure Assume the following vapor relief only, M AWP of 200 psig, conventional spring-operated relief. 

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