Home > Articles

  • Print
  • + Share This
This chapter is from the book

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.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020