Sizing of liquid relief

Pressure relief valves in liquid service that require ASME certification can be sized using the following equation  

65 when PRV is installed witii or witiiout rupture disk 

Process engineering and design using Visual Basic 

Note The curve above represents values recommended by various manufacturers. This curve may be used when Ihe manufacturer is not known. Otherwise, the manu cturer should be consulted for the applicable correction fector. 

The general calculation requires a trial-and-error method. As a first step, the PRV size is calculated by using Equation 4.40, with = 1. Once the area is known, the next orifice size is established, and then the value of A in Equation 4.42 is ascertained. From Equation 4.42, the Reynolds number is established, and the viscosity correction factor is calculated using Equation 4.41. Once the value of is known, the area is recalculated using Equation 4.40 and compared with the previous value. 

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The purpose of this Workbook is to give information on methods available for the sizing of emergency relief systems for exothermic runaway reactions in liquid-phase che mical reactors f % n. ... 

A proper sizing criteria Is to determine the liquid volume that could be dumped before the ESD system shuts in the relief valve source of overpressure. Thus, a dynamic liquid holdup determines the size of the relief drum. It will generally be smaller with the liquid holdup sizing criteria than a comparable horizontal API sized vessel. 

THIS PROGRAM PRINTS THE RESULTS OF LIQUID RELIEF VALVE SIZING WITH VISCOSITY GREATER THAN lOcP. 

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. 

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. 

The recommended method is from Guidelines for Pressure Relief and Effluent Handling Systems (AIChE-CCPS, 1998). It is an improvement over the method presented in the 7th edition of this Handbook. The procedure involves calculating a terminal velocity for a selected droplet size, then providing enough residence time in the vapor space to allow the droplets to fall from the top of the vessel to the level of liquid collected. Also, the vapor velocity in the separator must not exceed the value above which liquid may Be entrained from the liquid surface in the separator. The tank is treated as a simple horizontal cylinder, neglecting the volume of liquid in the heads. 

As the gas or vapour production rate increases, the flow regime may change from churn-turbulent to droplet flow, in which a fluidised bed of liquid droplets is present in the reactor (see Figure A3.1). This is of less practical interest for relief system sizing because if the gas or vapour rate is so high as to give droplet flow, the relief system size is likely to be impractically large. 

This example uses the same relief sizing problem as has been presented in 6.5.1. A relief system is to be sized for a relief pressure of 2.0 bara and a maximum accumulated pressure of 2.6 bara with a 793 kg charge in a 2 m3 reactor. However, in this case, the runaway is expected to be caused by external fire. Also, vapour/ liquid disengagement is not expected. 

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. 

The API 521 sizing criteria is appli-W cable for continuous relief when recovering small amounts of liquid hydrocarbon in the gas. 

Density of liquid - useful in design of storage vessels and shipping containers, useful in many engineering applications involving liquids, helpful in relief valve sizing for safety... 

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