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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

  1. 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.

  2. Select the likelihood from columns 4 through 7.

  3. 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) likelya

$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 injuryb

$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 lbm

1000 kg = 2200 lbm

500 kg = 1100 lbm

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 lbm

Undecane

Hexane

Hydrogen bromide, anhydrous

 

Methacrylic acid

Hydrogen chloride, anhydrous

200 kg = 440 lbm

Methyl acetate

Hydrogen fluoride, anhydrous

Ammonia, anhydrous

n-Heptene

Methyl bromide

Carbon monoxide

Nitrobenzene

Methyl mercaptan

 

Nitromethane

Sulfur dioxide

5 kg = 11 lbm

Octanes

 

Acrolein

Phenol, molten or solid

25 kg = 55 lbm

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:

  1. Select the severity levels from columns 1, 2, and 3 and select the highest level from any of these columns.

  2. Read the Risk Category and Safety Severity Level from the highest row.

  3. Select the likelihood from columns 4 through 7.

  4. 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.

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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