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Technical risk assessment

Chemical and Process Engineering Risk Assessment: Reaction, Containment and Procedural Controls

A worked example of how a teaching chemical processing unit identifies, scores and controls hazards, from reactor runaway and loss of containment to maintenance errors, utility loss, supplier quality and permit compliance.

Updated November 2026 8 hazards assessed Review every 6 months Highest risk: Critical to High
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Executive summary

This assessment shows how a teaching chemical processing unit can find, rate and control the hazards that matter most to people, equipment and the environment. Eight risks were chosen because they are plausible in everyday process engineering work, including reactor control, loss of containment, relief and instrument integrity, contractor maintenance, operator error, utility failure, supplier quality and environmental records. Loss of containment and runaway reactions carry the highest impact, so their controls receive the most attention. Each risk is scored before controls and again after existing and added controls. Most risks fall to Medium or Low once controls are in place, but runaway reaction and loss of containment stay at High because their consequences are severe even when rare. The first actions are a verified reactor interlock, a corrosion review, a physical isolation check for permits and a supplier qualification step. The unit is a teaching example, so every score illustrates method, not a real plant.

Focus area
Chemical and process engineering
Setting
Batch and continuous chemical processing unit with tank storage, utilities and contractor maintenance
Method
Five by five likelihood and severity risk register with barrier review

Scope

This assessment covers a teaching chemical processing unit, from raw material receipt and storage through reaction, separation, product handling, utilities and maintenance. It looks at risks to people, equipment, the environment, supply and legal duties. Jurisdiction, process chemistry, plant size and the design basis are assumptions made for teaching only, and no real plant is described.

Context

The unit has a jacketed batch reactor, a distillation column, solvent and product storage tanks, a control room with a distributed control system, and a shared utilities area. Operators follow written procedures and permits to work, while maintenance is partly outsourced to contractors. Raw materials come from a small number of suppliers. Each hazard is scored before controls to show its natural level, then again after existing controls and the additional controls proposed here.

Objectives, assets and stakeholders

The objective is to operate the unit safely, keep product within specification, protect the environment, meet legal duties and maintain production without harm. Assets in scope include the reactor, column, storage tanks, pressure relief devices, piping, instrumentation, control and safety systems, utilities, product quality records and the site environment. Stakeholders include operators, maintenance staff, contractors, process engineers, the plant manager, neighbors, customers, suppliers and the regulator. Every risk has one owner role, and the plant manager reviews the register with the site safety committee each quarter.

Risk matrix: before and after controls

Each cell shows hazard numbers placed by likelihood and impact. Compare the two grids to see how controls move each hazard.

Before controls

Inherent risk

Risk before controls: rows are likelihood from almost certain to rare, columns are impact from insignificant to catastrophic. Cells list hazard numbers.
1
Insignificant		 2
Minor		 3
Moderate		 4
Major		 5
Catastrophic
5 Almost certain
4 Likely Likelihood 4, impact 3, score 12, high: H05 Likelihood 4, impact 4, score 16, high: H04 Likelihood 4, impact 5, score 20, critical: H02
3 Possible Likelihood 3, impact 3, score 9, medium: H06 Likelihood 3, impact 4, score 12, high: H03 H07 Likelihood 3, impact 5, score 15, high: H01
2 Unlikely Likelihood 2, impact 4, score 8, medium: H08
1 Rare

After controls

Residual risk

Risk after controls: rows are likelihood from almost certain to rare, columns are impact from insignificant to catastrophic. Cells list hazard numbers.
1
Insignificant		 2
Minor		 3
Moderate		 4
Major		 5
Catastrophic
5 Almost certain
4 Likely
3 Possible
2 Unlikely Likelihood 2, impact 2, score 4, low: H06 Likelihood 2, impact 3, score 6, medium: H03 H05 H07 H08 Likelihood 2, impact 4, score 8, medium: H04 Likelihood 2, impact 5, score 10, high: H01 H02
1 Rare
  • Low 1 to 4
  • Medium 5 to 9
  • High 10 to 16
  • Critical 17 to 25

Hazard register

A summary first, then the detail for each hazard.

Summary of hazards with scores before and after controls
No.HazardBeforeAfterOwner
H01 Runaway exothermic reaction in the batch reactor 15 High 10 High Process safety engineer
H02 Loss of containment from vessel, pipework or seal failure 20 Critical 10 High Inspection and integrity lead
H03 Failure of pressure relief and instrumentation 12 High 6 Medium Maintenance and reliability engineer
H04 Maintenance isolation and permit errors by contractors 16 High 8 Medium Maintenance supervisor
H05 Procedural deviation and operator error 12 High 6 Medium Operations supervisor
H06 Utility failure of power, cooling water or inert gas 9 Medium 4 Low Utilities engineer
H07 Off-specification raw material from a critical supplier 12 High 6 Medium Procurement and quality lead
H08 Environmental release and waste records non compliance 8 Medium 6 Medium Environmental compliance officer
H01

Runaway exothermic reaction in the batch reactor

Safety Before 15 After 10

Heat from an exothermic reaction builds faster than cooling can remove it, for example after a cooling failure, a charge added too quickly or an incorrect reactant ratio, so temperature and pressure rise beyond design limits.

Who or what is affected: Operators, maintenance staff, neighbors and nearby process areas

Causes

Cooling loss, dosing error, poor mixing and reaction chemistry not fully understood at production scale.

Consequences

Vessel rupture, release of hot toxic material, fire, injury and plant shutdown.

Existing controls

  • Reactor temperature alarms alert the operator at a set high limit.
  • Reagent addition is limited by a written charging sequence and a dosing pump rate.

Control gaps

The interlock is not proof tested against the reaction timescale, and the quench route is not recently exercised.

Additional controls

  • Prevent: Calorimetry data sets the maximum charge rate and cooling duty for each batch.
  • Detect: Interlock trips reagent feed when temperature rises faster than the set rate of change.
  • Respond: Emergency quench and relief procedure is rehearsed every quarter by the shift team.
Likelihood 3 x impact 5 before; 2 x 5 after Owner: Process safety engineer Target: Within 30 days Early warning: Temperature rising faster than the normal rate during a batch charge.
H02

Loss of containment from vessel, pipework or seal failure

Safety Before 20 After 10

Solvent or product escapes from a corroded vessel, flange, pump seal or pipe, forming a flammable or toxic cloud or a spill that reaches the ground and drains.

Who or what is affected: Operators on the unit, contractors, nearby residents and the local environment

Causes

Corrosion, fatigue from thermal cycling, gasket failure and vibration at pumps.

Consequences

Fire, toxic exposure, environmental clean up, injuries and regulatory action.

Existing controls

  • Vessels and pipework are inspected on a planned schedule by the inspection team.
  • Gas detectors and bunds are fitted around the storage tanks and the reactor area.

Control gaps

Inspection findings are recorded but not always linked to the corrosion model, and small leaks may go unreported.

Additional controls

  • Prevent: Use a corrosion review that matches materials to each process stream and temperature.
  • Detect: Fit leak detection on flanged joints and pump seals with alarms to the control room.
  • Recover: Keep spill kits, drain isolation valves and a containment plan at each tank farm.
Likelihood 4 x impact 5 before; 2 x 5 after Owner: Inspection and integrity lead Target: Within 45 days Early warning: Gas detector alarm or a rise in make up solvent use without a matching batch.
H03

Failure of pressure relief and instrumentation

Operational Before 12 After 6

A relief valve sticks, an instrument drifts or a transmitter fails silently, so the protective function does not act when a process limit is exceeded.

Who or what is affected: Operators, process engineers and the plant equipment

Causes

Scaling or fouling of valves, drift in sensors, poor calibration records and vibration.

Consequences

Overpressure, vessel damage, release and loss of production.

Existing controls

  • Relief valves are tested at a planned interval by the maintenance team.
  • Critical transmitters have a second independent reading for comparison.

Control gaps

Calibration records are not linked to the protective function list, so overdue items can go unnoticed.

Additional controls

  • Prevent: Record instrument drift limits and replace instruments that exceed them before the next test.
  • Detect: Compare redundant readings each shift and alarm on any disagreement.
  • Respond: Use a manual isolation procedure when a relief function is doubtful.
Likelihood 3 x impact 4 before; 2 x 3 after Owner: Maintenance and reliability engineer Target: Within 60 days Early warning: A relief valve test that fails or an instrument reading that differs from its paired sensor.
H04

Maintenance isolation and permit errors by contractors

Safety Before 16 After 8

Equipment is opened or energized while still under pressure, hot or connected to a live stream, because the isolation is incomplete, the permit is unclear or the work differs from the plan.

Who or what is affected: Maintenance contractors, operators and supervisors

Causes

Time pressure, unclear drawings, contractor turnover and poor communication at shift change.

Consequences

Burns, chemical exposure, serious injury, release and regulatory investigation.

Existing controls

  • A permit to work is issued for each job and signed by the operations supervisor.
  • Lock and tag isolation is required before any line break.

Control gaps

Isolation checks depend on the drawing being current, and permit audits are not routine.

Additional controls

  • Prevent: Require a physical isolation check against the drawing before the permit is signed.
  • Detect: Audit a sample of permits each month against the work actually performed.
  • Respond: Stop the job at once when conditions change and reissue the permit.
Likelihood 4 x impact 4 before; 2 x 4 after Owner: Maintenance supervisor Target: Within 30 days Early warning: Permits issued without a completed isolation check.
H05

Procedural deviation and operator error

People Before 12 After 6

Operators skip a step, use an outdated procedure or misread an alarm, so the process moves outside its safe operating window during start up, shutdown or an upset.

Who or what is affected: Operators, shift teams and plant neighbors

Causes

Fatigue, complex procedures, alarm floods and changes not communicated clearly to staff.

Consequences

Off specification product, process upset, release or injury.

Existing controls

  • Written operating procedures are kept in the control room and reviewed after changes.
  • Operators complete a shift handover checklist.

Control gaps

Procedure changes are not always checked against training records, and alarm reviews are informal.

Additional controls

  • Prevent: Make the latest procedure version the only one available at the control panel.
  • Detect: Review alarm logs each shift for repeated acknowledgements without action.
  • Respond: Train shift teams on abnormal situation drills every quarter.
Likelihood 4 x impact 3 before; 2 x 3 after Owner: Operations supervisor Target: Within 60 days Early warning: Repeated alarm acknowledgements without a logged response.
H06

Utility failure of power, cooling water or inert gas

Operational Before 9 After 4

Loss of electrical power, cooling water or nitrogen supply stops pumps, agitators, cooling and purge systems, so the unit cannot be held in a safe state.

Who or what is affected: Operators, maintenance staff and the process equipment

Causes

Grid outage, pump trips, valve faults and weather or storm damage.

Consequences

Loss of temperature control, product loss, equipment damage and restart delays.

Existing controls

  • Critical loads are on an uninterruptible supply tested at planned intervals.
  • A written plan sets the safe shutdown sequence for utility loss.

Control gaps

Backup supply is tested but not under full plant load, so its capacity is unproven.

Additional controls

  • Prevent: Check that critical cooling loads have an independent supply path.
  • Recover: Rehearse restart after utility loss and record the time taken.
Likelihood 3 x impact 3 before; 2 x 2 after Owner: Utilities engineer Target: Within 90 days Early warning: Voltage dips or low cooling water flow alarms on the utility header.
H07

Off-specification raw material from a critical supplier

Supplier Before 12 After 6

A supplier sends material with a different impurity profile, concentration or moisture level from its certificate, so reactions proceed differently or off-specification product is made.

Who or what is affected: Operators, quality staff, customers and the procurement team

Causes

Single source supply, changes in the supplier process and weak checks on certificates.

Consequences

Off specification product, customer complaints, rework and reaction hazards.

Existing controls

  • Each delivery is sampled and checked against the certificate of analysis.
  • The supplier list is reviewed by procurement at a set interval.

Control gaps

Testing covers the main assay only, and impurity limits are not defined for each grade.

Additional controls

  • Prevent: Qualify critical suppliers with audits and trial batches before any change of source.
  • Detect: Test incoming material for the impurities that most affect the reaction.
  • Respond: Quarantine suspect lots and notify affected batches to the quality team.
Likelihood 3 x impact 4 before; 2 x 3 after Owner: Procurement and quality lead Target: Within 60 days Early warning: A certificate of analysis that differs from the in house test result.
H08

Environmental release and waste records non compliance

Legal and compliance Before 8 After 6

Wastewater, air emissions or waste solvent exceed the limits set in the site consent, or records are incomplete, so the site cannot show that it is compliant.

Who or what is affected: Neighbors, the local environment, regulators and the site operator

Causes

Process upsets, incomplete logs, changing waste streams and unclear responsibilities.

Consequences

Penalties, permit review, reputational harm and local complaints.

Existing controls

  • Discharge and emission monitoring is recorded at set frequencies.
  • Waste is sent to licensed carriers with signed consignment notes.

Control gaps

Consent conditions are not mapped to each waste stream, and monitoring data is not reviewed promptly.

Additional controls

  • Prevent: Compare each waste stream against the site consent before it is sent off.
  • Detect: Review monitoring data monthly against consent limits and trends.
  • Respond: Stop discharge and report any exceedance to the regulator as the consent requires.
Likelihood 2 x impact 4 before; 2 x 3 after Owner: Environmental compliance officer Target: Within 90 days Early warning: Monitoring results trending toward a consent limit.

Rating method and evaluation

Each risk is rated on two five-point scales. Likelihood runs from Rare to Almost certain, and severity runs from Negligible to Catastrophic, covering harm to people, release to the environment, damage to equipment, production loss and legal exposure. Multiplying the two gives a score from 1 to 25, which falls into one of four bands: Low from 1 to 4, Medium from 5 to 9, High from 10 to 16 and Critical from 17 to 25. Inherent risk is the score before any controls, assuming they are absent. Residual risk is the score after existing and additional controls, and it must never be higher than the inherent score. Severity is judged on the worst credible outcome, and a hazard is never rated down only because it has not yet happened. Critical and High risks need senior attention, a dated plan and verification of the safety functions relied on. Medium risks need routine controls with a named owner. Low risks are accepted and monitored. Operators, maintenance staff and engineers should challenge every rating, and a rating is reviewed whenever an indicator reaches its threshold or the process changes.

Likelihood, impact and rating bands

Likelihood

  • 5 Almost certain: Expected to happen in most circumstances.
  • 4 Likely: Will probably happen in many circumstances.
  • 3 Possible: Could happen at some time.
  • 2 Unlikely: Not expected, but it could happen.
  • 1 Rare: Only in exceptional circumstances.

Impact

  • 1 Insignificant: No lasting effect. Handled within routine work.
  • 2 Minor: Small effect that is quickly corrected.
  • 3 Moderate: Noticeable effect that needs managed effort to correct.
  • 4 Major: Serious effect on people, delivery or reputation.
  • 5 Catastrophic: Severe or lasting effect that threatens the whole activity.

Bands (likelihood x impact)

  • Low (1 to 4): Accept and monitor.
  • Medium (5 to 9): Manage with routine controls.
  • High (10 to 16): Senior attention and a treatment plan.
  • Critical (17 to 25): Act first. Do not proceed until reduced.

Monitoring, escalation and review

Review this assessment every 6 months, and sooner when an indicator below reaches its threshold.

Early-warning indicators and escalation thresholds
Indicator to watchEscalate when
Safety system proof test overdue or bypassed.Escalate to the process safety engineer at once when a test is overdue, and to the plant manager if a bypass lasts more than one shift.
Reactor temperature or pressure excursions above the alarm setpoint.Escalate to the shift supervisor immediately, and to the process safety engineer for any trip or excursion not explained within the shift.
Permits to work issued without a completed isolation check.Escalate to the maintenance supervisor at once and stop the job until the isolation is verified.
Certificate of analysis mismatches or late supplier deliveries.Escalate to the procurement lead for any mismatch, and to the quality manager if a product is held or released in doubt.
Open management of change actions past their due date.Escalate to the plant manager when any action is more than 14 days overdue.

Contingency arrangements

If a release, fire or runaway reaction occurs, the shift supervisor activates the emergency response plan, isolates the unit and moves non essential people to the muster point. Emergency services are called, and the plant manager informs the regulator and neighbors as local rules require. Maintenance and process engineers secure the equipment, preserve data and records, and do not restart until a competent review is complete. Medical care, spill containment and contractor communication follow the written plan, and the register is updated after any event.

Prioritized action plan

  1. Verify the reactor high integrity trip and relief system, owner process safety engineer, within 30 days.
  2. Enforce management of change for every modification, owner plant manager, within 45 days.
  3. Audit permits and isolations against the drawings, owner maintenance supervisor, within 60 days.
  4. Qualify critical suppliers and check certificates on receipt, owner procurement lead, within 60 days.
  5. Refresh operator training on abnormal situations, owner operations supervisor, within 90 days.

Conclusion

Safe chemical processing rests on a few reliable habits: verified safety systems, disciplined management of change, sound isolation and permits, controlled maintenance, checked supply and clear procedures. The highest scores sit with runaway reactions and loss of containment, and each has barriers that bring residual risk down. Treat the register as a living record, review it on the stated cycle, and seek qualified process safety and engineering advice for any real plant decision.

Assumptions and limitations

Assumptions

  • The unit operates under a documented design basis with hazard studies completed for the main vessels.
  • Operators are trained and competent, and the control system includes basic alarms and interlocks.
  • Jurisdiction is not specified, so legal duties are described in general terms only.

Limitations

  • The scores are examples for learning and do not describe any real plant or process.
  • Chemical hazards depend on specific substances, quantities and conditions, so specialist process safety review is needed.
  • This is educational content and does not replace a formal site-specific assessment or licensed engineering advice.

Related frameworks to study

  • Hazard and operability study method
  • Layer of protection analysis concept
  • Hierarchy of controls
  • Management of change process
  • Bow tie risk diagram

Technical Risk Assessment

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For Learning Reference Only: This assessment demonstrates how risk assessment principles are applied in a practical setting, providing a useful reference for learning and understanding. Actual assessments may be adapted to suit specific site conditions and circumstances.

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