ERPG-2

ERPG-2 The maximum airborne concentration below which it is believed that nearly all individuals coiild be exposed for up to one hour without experiencing irreversible or other serious health effec ts or symptoms that could impair their ability to take protective action. 

Probit models have been found generally useful to describe the effects of incident outcome cases on people or property for more complex risk analyses. At the other end of the sc e, the estimation of a distance within which the population would be exposed to a concentration of ERPG-2 or higher may be sufficient to describe the impact of a simple risk analysis. 

Dispersion modeling of credible worse case scenarios indicates the one-hour exposure to nearest human receptor exceeds ERPG-2 level or equivalent. 

The results indicate that 2000 people will be exposed to a concentration of ammonia greater than the emergency response planning guideline concentration (e.g., ERPG-2). 

Number Exposed Above ERPG-2 FIGURE 9. Example of consequence analysis results. 

AIIIA Emergency Response Planning Guidelines (ERPGs) ERPG-1 ERPG-2 ERPG-3 0.1 ppm 30 ppm 100 ppm AIHA 1991... 

Material CAS no. ERPG-1 ERPG-2 ERPG-3 IDLH WEEL-8 WEEL-C TLV-TWA Oral, mg/kg... 

For illustrative purposes, the hazardous properties of the materials used in the examples have been modeled to the Emergency Response Planning Guideline, ERPG-2 (see Section 3.1.1) concentration for toxic materials, or to one half of the lower flammable limit (LFL/2) for flammable material. Lastly, only the first 3600 seconds of the event are considered in these examples. For an actual situation, different concentration levels for toxic or flammable concentrations may be considered and different durations as well. 

To demonstrate the effectiveness of diking as a postrelease mitigation measure, an accidental release of carbon disulfide from a vertical storage tank will be evaluated. In the example, isopleths to the ERPG-2 concentration of 50 ppm (AIHA, 1992) will be considered. 

Figure 7.7. Hazard zone for ERPG-2 (50 ppm) for undiked carbon disulfide release F/2.

The dispersion modeling of the resultant ammonia vapor hazard zones to the ERPG-2 value for ammonia of 200 ppm was done using the SLAB model. The isopleths for the D/5 and F/2 meteorological conditions found in these examples are shown in Figure 7.17 and 7.18. 

Figure 7.17. Hazard zone from refrigerated anhydrous ammonia release for ERPG-2 concentration at D/5 conditions.

In this section we will demonstrate the use of water sprays as a postrelease mitigation measure. The theory and design of water spray systems has been detailed in Chapter 4. The scenario will involve the accidental release of hydrofluoric acid (HF) from a storage tank. Again we will be considering the distance to reach the ERPG-2 concentration for HF of 20 ppm (AIHA, 1992). 

Figure 7.22 shows the effect the reduced input for the mitigated case, caused by the capture of HF by the water spray, has on the centerline concentration of HF to the ERPG-2 value of 20 ppm as compare to the unmitigated case to the same concentration. 

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