Applicable Scope of Hazardous Area Classification　

We perform risk evaluation for items and complex conditions for which specific calculation methods are not described in IEC Ed3.0 by conducting engineering studies such as chemical engineering, thermodynamics, fluid dynamics, etc.

　Application to actual analysis　

1. Since the case released in the gas state can be easily calculated according to the standard, the risk evaluation could be carried out without much difficulty.
   
   
2. The case that liquid leaks, liquid pool is made on the ground and liquid evaporates is difficult.
   
   
   1. For the cases that liquid pool is non-boiling and no thermodynamic input is required from the surface on which the liquid is spilt
      • Since the evaporation rate calculation is based on mass transfer calculation by wind, we calculate the evaporation rate based on the formula specified by IEC Ed 3.0.
      • We set the leakage duration in this case according to the actual conditions such as process conditions and on-site patrol with reference to API.
        
        
   2. For the cases that liquid pool is boiling, or thermodynamic input is required from the surface on which the liquid is spilt
      • Since IEC Ed3.0 does not stipulate the evaporation rate regarding boiling liquids and thermodynamic input, we calculate it with heat balance calculation methods with solar heat, atmosphere, ground, etc. adopted by the ALOHA program developed by U.S. NOAA (National Oceanic & Atmospheric Administration) and EPA (Environmental Protection Agency).
        
        
   3. For the cases that leaked liquid is flashed
      • Since IEC Ed3.0 does not stipulate the evaporation rate in this case as above, we calculate flash rate with flash calculation and evaporation rate of the remaining liquid pool with heat balance calculation methods. Note that the hazardous distance cannot be calculated unless the amount of flash gas and evaporative gas are separately calculated.
        
        
3. Estimating the physical properties of leaked liquids are also important for the risk assessment. The properties must be calculated at each temperature and physical property estimation method must also be utilized.
   
   
   1. Pure component liquid
      • Specific heat at constant pressure of gas and liquid, specific heat ratio, density, latent heat of vaporization, vapor pressure, etc.
      • Vapor pressure is calculated from the Antoine coefficient.
        
        
   2. Multiple component liquid
      It is more complicated, since compositional variations must be considered.
      
      • Specific heat at constant pressure of gas and liquid, specific heat ratio, compression factor, density, latent heat of vaporization, vapor pressure, molecular weight, lower explosion limit(LEL), boiling point, liquid composition, etc.
      • For the vapor pressure, the partial pressure of each component is calculated from the Antoine coefficient, and Raoult's law is applied to obtain the partial pressure of the multicomponent system.
      • The LEL is calculated by Le Chatelier's law.
      • If the multiple component liquid contains water, the flash and evaporation calculations must include water in the calculations, but the properties such as molecular weight, gas density, LEL and the calculation of the volumetric release characteristic of the source must exclude water.
        
        
4. Regarding leakage from flanges, IEC Ed 3.0 indicates the range of the leakage port area, but we divide the range into 4 parts based on the (operating pressure / rated pressure) ratio and determine the appropriate value.
   
   
5. The suggested value of the leak port area differs depending on whether or not the leak port area may expand in the event of a leak, but we decided it in consideration of the maintenance conditions, the age of plant construction, operating pressure, in sound velocity or not, etc. 
   
   
6. If the outdoor ventilation velocities are applied form actual measured values, it should be assessed as the velocity that is exceeded 95% of the time by IEC Ed3.0. Note that applying mean wind velocity as the ventilation velocity may underestimate the risk evaluation.
   
   
7. Hazardous area
   The spatial shape of the hazardous area for each release source is determined by selecting from the spatial shapes of hazardous areas defined by IEC Ed3.0 according to the leakage and evaporation conditions of the leaked liquid.
   
   

    

8. Since primary grades of releases are various depending on how to handle hazardous materials, the rate of gas release or evaporation must be calculated on a case-by-case basis.
   
   [Evaluation examples for primary grades of releases]
   
   
   1. Tank vent
      　   Estimation of gas release rate by gas phase space expansion
      
      • Assumed that the little remaining liquid is in vapor-liquid equilibrium at that temperature when a tank is almost empty and the gas volume is maximum. The heat balance with solar heat to a tank roof and a sidewall, radiant heat from the tank, convective heat transfer by wind, etc.is calculated. The rate of temperature rise in the gas phase in the tank is calculated, and then the release rate of the vent gas is obtained from the volume expansion rate of the gas phase.
        The solar heat is determined by the latitude and longitude of the location of the tank, the date, and the altitude of the sun. By calculating the change over time in the gas release rates, we can learn the change in the hazardous area over time.
        
        
        Estimation of boiling evaporation rate by yank side wall temperature

      • Assumed that the liquid in tank is heated by the through-flow heat of the tank side wall in the case of a full tank.If the temperature of the tank side wall is below the boiling point of the liquid, the heat input is used to increase the whole liquid temperature. If the temperature reaches the boiling point, all the heat input is used to evaporate the liquid and the gas release rate is calculated.
        In the case of floating roof tanks, since there is almost no space in the gas phase , only boiling evaporation is considered without considering gas phase space expansion.
        As above, by calculating the change over time of the altitude of the sun, we can learn the change in the hazardous area over time.

        
        
        
        
   2. Evaporation from open surfaces
      When evaporating from a liquid surface that is open to the atmosphere, check the operating conditions of the liquid and the presence or absence of wind on the liquid surface (e.g. if the liquid surface is deep in the containers, less wind blows.), and then determine the gas release rate by mass transfer calculation or heat balance calculation.
      
      
      
   3. Evaporation from painted surfaces
      Assumed that a paint is applied evenly at one time on the painted surface, and then volatile components evaporate from the surface. As a risk assessment, this is the maximum rate of gas release and will be considered as a stricter assessment.
      
      
      
   4. Mixing liquids in open vessels
      Assumed that the most volatile liquid (liquid 1) is in the open vessel and then the other liquid (liquid 2) is poured in it. So, the evaporated gas from the liquid 1 is released with the same volume of the liquid 2 poured from the vessel.
      The gas release rate in this case is determined by the concentration of the gas evaporated from the liquid 1, the poured volume and poured duration of the liquid 2.
      
      
      
9. Secondary grade of releases is those that infrequently occur during normal operation and is basically the following types of fittings as indicated in IEC Ed 3.0.

   1. Flanges with compressed fibre gasket or similar

   2. Flanges with spiral wound gasket or similar

   3. Ring type joint connections

   4. Small bore connections up to 50 mm

   5. Valve stem packings

   6. Pressure relief valves

   7. Pump and compressor shaft seals

10. In addition to Section 9, Secondary grade of releases is considered leaks associated with human error, depending on the work situation at the hazardous material facilities.
    As an example, there are cases where a hazardous material container (200-litre drum, 18-litre drum, etc.) being transported to a manufacturing site is accidentally tipped over, resulting in leaks.
    In this case, we assume that the entire volume in the container leaks at one time and evaporates from the surface of the spread liquid pool.
    If the leakage amount is large, the area is often classified as a hazardous area. If the area is to be classified as a non-hazardous area, It is necessary to consider measures to prevent containers from tipping over or to change to containers with smaller capacities.

11. In any case, it is important to show how to determine the values in the risk assessment as required by IEC Ed 3.0.
    For each assessment case, we summarize the risk evaluation results in the table below, and the details of the risk evaluation are clearly recorded. (Below samples are about equipment handling LPG. Click on each picture to enlarge.)

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