An Introduction to Chemical Process Safety

By Daniel A. Crowl and Joseph F. Louvar
Jun 4, 2019

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This chapter is from the book 

Chemical Process Safety: Fundamentals with Applications, 4th Edition

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1-9 Risk Tolerance/Acceptance and Risk Matrix

Risk tolerance or acceptance is defined as “the maximum level of risk of a particular technical process or activity that an individual or organization accepts to acquire the benefits of the process or activity.”⁶ We cannot eliminate risk entirely—all activities inevitably involve risk. Indeed, people accept risks many times during their daily activities. For instance, simply crossing the street involves a risk assessment as to where and when to cross. People accept risks based on their perceived risk—which may or may not be the actual risk. The risk accepted is voluntary based on the perceived risk, while any additional actual risk not perceived will be involuntary.

Engineers must make every effort to minimize risks within reasonable constraints. No engineer should ever design a process that he or she knows will result in certain human loss or injury. For a chemical plant, at some point in the design stage or at every point in the operation of the plant, the corporation (this decision involves both the workers and management) must determine whether the risks are acceptable. The risk acceptance must be based on more than just perceived risks.

Risk tolerance may also change with time as society, regulatory agencies, and individuals come to expect more from the chemical industry. As a consequence, a risk that was considered tolerable years ago may now be deemed unacceptable.

A risk matrix is a semi-quantitative method to represent risk and to help companies make risk acceptance decisions. A typical risk matrix is shown in Table 1-14. The consequence or severity of the incident is found in columns 1, 2, and 3, and the likelihood of that incident occurring appears in columns 4 through 7. The incident severity is used to estimate the severity category and the safety severity level. The likelihood level is selected based on the frequency of the incident, as shown in columns 4 through 7. The combination of the severity category row and the likelihood column is used to determine the risk level, A through D.

Table 1-14 Risk Matrix for Semi-Quantitative Classification of Incidents

Risk Matrix Select the severity from the highest box in either of columns 1, 2, or 3. Read the Category and Safety Severity Level from the same row. Select the likelihood from columns 4 through 7. Read the Risk Level from the intersection of the severity row and the likelihood column. TMEF: Target mitigated event frequency (yr–1). TQ: Threshold quantity—see Table 1-15.						Likelihood
						4 LIKELY Expected to happen several times over the life of the plant	5 UNLIKELY Expected to happen possibly once over the life of the plant	6 IMPROBABLE Expected to happen possibly once in the division over the life of the plant	7 IMPROBABLE, BUT NOT IMPOSSIBLE Not expected to happen anywhere in the division over the life of the plant
Severity	1 Human health impact	2 Fire, explosion direct cost ($)	3 Chemical impact	Severity category	Safety severity level	0–9 years	10–99 years	≥ 100 years	> 1000 years
	Public fatality possible, employee fatalities likely	Greater than $10 million	≥ 20 × TQ	Catastrophic	4 TMEF = 1 × 10^–6	Risk level A	Risk level A	Risk level B	Risk level C
	Employee fatality possible, major injury likely	$1 million to< $10 million	9 × to< 20 × TQ	Very serious	3 TMEF = 1 × 10^–5	Risk level A	Risk level B	Risk level C	Risk level D
	Lost time injury (LTI) likely^a	$100,000 to< $1 million	3 × to< 9 × TQ	Serious	2 TMEF =1 × 10^–4	Risk level B	Risk level C	Risk level D	Negligible risk
	Recordable injury^b	$25,000 to< $100,000	1 × to< 3 × TQ	Minor	1 TMEF =1 × 10^–3	Risk level C	Risk level D	Negligible risk	Negligible risk
Risk level A: Unacceptable risk; additional safeguards must be implemented immediately. Risk level B: Undesirable risk; additional safeguards must be implemented within 3 months. Risk level C: Acceptable risk, but only if existing safeguards reduces the risk to as low as reasonably practicable (ALARP) levels. Risk level D: Acceptable risk, no additional safeguards required.

The severity levels are listed under columns 1, 2, and 3 in Table 1-14. They include human health impacts; direct costs of fire and explosion in dollars; and chemical impacts. The chemical impact is based on a chemical release quantity called a threshold quantity (TQ). Table 1-15 lists TQs for a number of common chemicals.

Table 1-15 Threshold Quantities (TQ) for a Variety of Chemicals

2000 kg = 4400 lb_m	1000 kg = 2200 lb_m	500 kg = 1100 lb_m
Acrylamide	Acetic anhydride	Acetaldehyde
Ammonium nitrate fertilizer	Acetone	Acrylonitrile
Amyl acetate	Acetonitrile	Calcium cyanide
Amyl nitrate	Aldol	Carbon disulfide
Bromobenzene	Ammonium perchlorate	Cyclobutane
Calcium oxide	Aniline	Diethyl ether or ethyl ether
Carbon dioxide	Arsenic	Ethane
Carbon, activated	Barium	Ethylamine
Chloroform	Benzene	Ethylene
Copper chloride	Benzidine	Furan
Kerosene	Butyraldehyde	Hydrazine, anhydrous
Maleic anhydride	Carbon tetrachloride	Hydrogen, compressed
n-Decane	Copper chlorate	Lithium
Nitroethane	Copper cyanide	Methylamine, anhydrous
Nitrogen, compressed	Cycloheptane	Potassium
Nitrous oxide	Cycloheptene	Potassium cyanide
Nonanes	Cyclohexene	Propylene oxide
Oxygen, compressed	Dioxane	Silane
Paraldehyde	Epichlorohydrin	Sodium
Phosphoric acid	Ethyl acetate	Sodium cyanide
Potassium fluoride	Ethyl benzene	Sodium peroxide
Potassium nitrate	Ethylenediamine	Trichlorosilane
Sulfur	Formic acid
Tetrachloroethylene	Heptane	100 kg = 220 lb_m
Undecane	Hexane	Hydrogen bromide, anhydrous
	Methacrylic acid	Hydrogen chloride, anhydrous
200 kg = 440 lb_m	Methyl acetate	Hydrogen fluoride, anhydrous
Ammonia, anhydrous	n-Heptene	Methyl bromide
Carbon monoxide	Nitrobenzene	Methyl mercaptan
	Nitromethane	Sulfur dioxide
5 kg = 11 lb_m	Octanes
Acrolein	Phenol, molten or solid	25 kg = 55 lb_m
Arsine	Propylamine	Chlorine
Diborane	Pyridine	Cyanogen
Dinitrogen tetroxide	Silver nitrate	Germane
Methyl isocyanate	Sodium permanganate Tetrahydrofuran	Hydrogen sulfide
Nitric oxide, compressed	Tetrahydrofuran	Nitric acid, red fuming
Nitrogen trioxide	Toluene	Sulfuric acid, fuming
Phosgene	Triethylamine
Phosphine	Vinyl acetate
Stibine	Zinc peroxide

Source: AICHE/CCPS. Details on how to compute the TQ are available from AICHE/CCPS Process Safety Metrics: Guide for Selecting Leading and Lagging Indicator (New York, NY: American Institute of Chemical Engineers, 2018).

The target mitigated event frequency (TMEF) listed with the safety severity level is the minimum frequency level desired for this level of severity. It defines the frequency for acceptable risk.

Some risk matrixes include a severity column based on environmental impacts. However, the environmental impact is implicitly related to the quantity of chemical released: The greater the chemical release, the greater the environmental impact. Thus, environmental impact is implicit in this risk matrix.

The procedure for using the risk matrix of Table 1-14 is as follows:

Select the severity levels from columns 1, 2, and 3 and select the highest level from any of these columns.
Read the Risk Category and Safety Severity Level from the highest row.
Select the likelihood from columns 4 through 7.
Read the risk level from the intersection of the Safety Severity Level row and the Likelihood column.

The risk levels are identified just below the table and define the risk and the required response. The Safety Severity Level contains the TMEF. The TMEF will be useful for the layer of protection analysis (LOPA) method presented in Chapter 11.

EXAMPLE 1-2

A risk analysis is performed on an incident involving a hole in a storage vessel containing a specific chemical. The chemical has a TQ of 5 lb_m. Calculations for this hole release estimate a total release of 50 lb_m of chemical. An employee fatality is possible with such a release, and the fire and explosion direct cost is estimated at $150,000. This incident is expected to occur once over the life of the plant. Use the risk matrix in Table 1-14 to determine the risk category, safety severity level, TMEF, and risk level.

Solution

Using Table 1-14, the following severity levels are selected under columns 1, 2, and 3:

Human Health Impact: Employee fatality possible
Fire, Explosion Direct Cost: $100,000 to < $1 million.
Chemical Impact: 9× to < 20× TQ

Selecting these three levels under columns 1, 2, and 3, respectively, results in the highest severity category of “Very serious,” with a safety severity level of 3 and a TMEF of 1 × 10⁻⁵. The likelihood is selected from column 5, since this is expected once over the lifetime of the plant.

Combining the Severity Category row of “Very serious” with Likelihood column 5 gives a risk level of B. Risk level B is defined as an “Undesirable risk; additional safeguards must be implemented within 3 months.”

Note that if we could drop the severity level or decrease the likelihood, we can reduce the risk level. The LOPA method presented in Chapter 11 is a formalized method to add more safeguards to reduce the risk level to the TMEF.

The risk matrix provided in Table 1-14 is one specific example; that is, most companies customize the risk matrix to work for their particular situation. Additional methods for determining risk are presented in Chapter 12 on risk assessment.

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