Page 44 - CHEF Guide

Introduction

Table 1-1 Tier 1 Process Safety Event Material Threshold Quantities

Threshold Recommended
Threshold Quantity (TQ) Threshold Quantity
Release Material Hazard Classification (TQ) for indoor
Category for outdoor
releases releases

≥ 5 kg ≥ 0.5 kg
1 Toxic Inhalation Hazard (TIH) Zone A Materials
(11 lb) (1.1 lb)
≥ 25 kg ≥ 2.5 kg
2 Toxic Inhalation Hazard (TIH) Zone B Materials
(55 lb) (5.5 lb)
≥ 100 kg ≥ 10 kg
3 Toxic Inhalation Hazard (TIH) Zone C Materials
(220 lb) (22 lb)
≥ 200 kg ≥ 20 kg
4 Toxic Inhalation Hazard (TIH) Zone D Materials
(440 lb) (44 lb)
Flammable Gases, or
Liquids with Initial Boiling Point  35 °C (95 °F) and ≥ 500 kg ≥ 50 kg
5
Flash Point < 23 °C (73 °F), or (1100 lb) (110 lb)
Other Packing Group I Materials excluding strong acids / bases

Liquids with Initial Boiling Point > 35 °C (95 °F) and ≥ 1000 kg ≥ 100 kg
6 Flash Point < 23 °C (73 °F), or (2200 lb) (220 lb)
Other Packing Group II Materials excluding moderate acids / bases or ≥ 7 bbl or ≥ 0.7 bbl

Liquids with Flash Point 23 °C (73 °F) and 60 °C (140 °F), or
Liquids with Flash Point > 60 °C (140 °F) released at temperature at or ≥ 2000 kg ≥ 200 kg
7 above Flash Point, or (4400 lb) (440 lb)
Strong acids/ bases or Other Packing Group III Materials, or or ≥ 14 bbl or ≥ 1.4 bbl
Division 2.2 Nonflammable, Nontoxic Gases (excluding Steam, hot
condensate, and compressed or liquefied air)
These are for reference to this CHEF Guide, only. Actual Table details are provided in [11]

The barrier-based approach utilizes the Swiss cheese model which shows how a series of safety barriers (or protection
layers) can break down and allow hazards to propagate to loss events. These protection layers are represented by slices
of Swiss cheese, with the holes representing failures in the barrier (Figure 1-5, [11]). If an initiating event occurs and the
holes in the protection layers align, a loss event will occur.

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