Plant modification / Change procedures

This Technical Measures Document refers to the Plant Modification and Change Procedures.

Related Technical Measures Documents include:

The relevant Level 2 Criteria are:

Safety Management systems

The Chemical Industries Association Safety Advisory Group outlines the following good practice related to plant modification / change procedures:

  • Members of staff must be aware of the hazards associated with the work they carry out and be able to determine that the risks involved are acceptable;
  • Risk assessment must be carried out to determine the possibility and consequence of the hazards being realised; if necessary, appropriate precautions must be taken to minimise the risk;
  • All modifications – whether involving procedures, plant and equipment, people or substances – should be subject to formal management procedures;

The procedure should draw reference to:

  • Evidence from previous incidents – their cause and means of preventing them;
  • The intrinsic link between process definition and the validity of the hazard evaluation;
  • The options that are available in the design of safety measures;
  • Preventative measures (process control, instrumentation etc.);
  • Protective measures (containment, reactor venting, quenching, reaction inhibition).

Good industry practice requires that process and plant modifications should not be undertaken without having undertaken a safety, engineering and technical review. This review should be traceable and identify changes proposed to the following factors:

  • Process conditions;
  • Operating methods;
  • Engineering methods;
  • Safety;
  • Environmental conditions;
  • Engineering hardware and design.

A form of risk assessment should then identify what hazards have been created by the change that may affect plant or personnel safety, and what action can be implemented to reduce or eliminate the risk. Additional hazards that may be introduced which need to be considered are fire, explosion and loss of containment.

Changes may affect other parts of the plant which may be quite remote from the source of the change. Therefore all parts of the plant should be considered in undertaking hazard identification and risk assessments.

Factors that are crucial to the success and safe implementation of a plant modification procedure include:

  • Corporate history;
  • Communication between different departments;
  • Recognition of authorised personnel;
  • Accurate recording and monitoring of changes to plant and process.


Various stages of Hazard and Operability Study are generally undertaken. These are summarised in the following table:

HAZOP study Description
1 Identify major hazards and check for availability of key hazard data
2 Coarse HAZOP using flowsheet and block diagram
3 Full HAZOP on frozen P&I diagram
4 Check that all intended actions have been implemented, including hardware and software
5 Pre-commissioning check including statutory requirement
6 Safety audit after a few months operation

These are equally applicable to plant modifications as new plant. However, written procedures should be in place to determine what level of HAZOP (if any) should be applied. These procedures should take account of:

  • The degree of hazard and extent of the change;
  • The worst case accident likely to arise as a result of the modification,
  • Previous HAZOPs relating to the item being changed;
  • The appointment of a competent person (excluding the HAZOP chairman) to decide the requirements for HAZOP.

A conventional hazard and operability study will identify potential hazards, but gives no likelihood of an incident occurring, or the loss suffered. The methodology has been developed to address likelihood and risk to assist in resource and priority allocation. Where hazards are not eliminated by actions placed during the HAZOP, Hazard analysis should be employed to determine if the risk is acceptable.

Preliminary Hazard Analysis (PHA)

Preliminary hazard analysis is performed as the first step in a hazard assessment. The essential stages of a systematic procedure to maintain a safe manufacturing situation include:

  • Definition of the process / operating conditions / plant design;
  • Characterisation of the process with respect to chemical reaction hazards;
  • Selection and specification of safety measures;
  • Implementation and maintenance of safety measures.

These stages should be applied:

  • During initial (research) process development work;
  • Prior to transfer to pilot plant scale;
  • Before full scale manufacturing is established;
  • When modifications to the process / plant are undertaken.

Operating procedures during change

Poor management and control of changes to plant and process often results in increased risk to plant, people and environment. Consequently, control of operating procedures during change is a critical task. Only authorised personnel should amend existing operating procedures or issue temporary operating instructions during plant or process changes. It is good practice for the operating procedures to be authorised by representatives from several different departments. These may include:

  • Operations;
  • Technical;
  • Health, Safety and Environmental;
  • Quality.

An assessment of change to risk should be an integral part of generation of procedures. The degree of control of change will depend upon whether the proposed change may be classed minor and major procedural changes

Commissioning procedures

Commissioning procedures are covered in more detail in the Technical Measures Document on Operating Procedures.

The continued integrity of the plant needs to be upheld by adequate maintenance, inspection and avoidance of unauthorised design or operational changes. To avoid hazards caused by modifications, it is necessary that any proposal for change be identified, and that the proposal is formally authorised after, after technical investigation, by competent personnel of senior status. Modifications should be designed, constructed, inspected, tested and proved to have achieved the design intent and should be maintained at least to the standard of the design criteria required by the process.

Decommissioning procedures

The requirements for decommissioning will vary depending upon the nature of the plant items to be decommissioned and the duty the plant items fulfilled. Operating procedures should be provided for decommissioning of hazardous plant in the same way as for commissioning. These procedures should be subject to hazard review and risk assessment. General measures that should be adopted for a common approach to decommissioning include:

  • Establish communication with plant personnel to ensure surrounding plant areas are prepared for decommissioning activity;
  • Undertake removal of hazardous substance via a cleaning procedure to ensure plant item is clean and empty with particular consideration where there may be dead-legs where material may be trapped;
  • Consideration of the disposal of items which may be contaminated by absorption of hazardous substances and chemical change;
  • Mechanically isolate plant item from other surrounding plant items by physical disconnection or fitting of blanks;
  • Electrically isolate plant item from power sources by physical disconnection.

Status of guidance

Plant modification and maintenance procedures are covered in general guidance, however, no guidance is available that specifically covers plant modification. Most companies usually adopt internally generated plant modification procedures that have been developed through:

  • Corporate history and experience;
  • Good industry practice;
  • Input from Safety, Health and Environmental department;
  • Input from Operations department;
  • Input from Technical department;
  • Input from Engineering department.

Codes of Practice relating to Plant modification

  • CISHEC/8906/1000, CIA, Guide to Hazard and Operability Studies.
  • Kletz T.A., 'HAZOP and HAZAN: Identifying and Assessing Process Industry Hazards', 3rd Edition, IChemE, 1992.
  • HS(G)176 The storage of flammable liquids in tanks, HSE, 1998.
    Paragraph 23 to 27 give guidance on the requirements for HAZOP and Risk Assessment. Paragraph 26 requires that risk assessments are carried out for modifications and demolishing.
    Paragraphs 158 to 161 give guidance on testing and commissioning.
    Paragraphs 201 to 205 give guidance on decommissioning.
  • HS(G)28 Safety advice for bulk chlorine installations, HSE, 1999.
    Paragraph 210 requires that modifications are only carried out after conducting a risk assessment (possibly HAZOP) and discussions with the chlorine supplier.
  • HS(G)30 Storage of anhydrous ammonia under pressure in the UK : spherical and cylindrical vessels, HSE, 1986.
    Paragraphs 98 to 117 give guidance on commissioning and decommissioning.
    Paragraph 121 requires that a competent person authorise returning the plant to service after modification.
  • LPGA Code of Practice 1, Bulk LPG Storage at Fixed Installations (Part 1, Part 2, Part 3 and Part 4), LP Gas Association.
    Supersedes HS(G)34, The Storage of LPG at Fixed Installations. 1987, HSE.
    Part 3, Section 2.6 requires that modification or repairs for whatever purpose, including change of duty shall comply with the design codes in Parts 1 and 2.
  • HS(G)34 Storage of LPG at fixed installations, HSE, 1987.
    Superseded by the above.
    Paragraphs 172 to 191 give guidance on commissioning, decommissioning and maintenance, including that modifications should be certified by a competent person.

Further reading material

  • ILO, Major hazard control: A practical manual, 1988.
  • Section 3.3.5 provides guidance on procedures for carrying out repair work (e.g. welding of components containing flammable substances).
  • European Federation of Chemical Engineering, EFCE Publication Series No. 59, Hazards from Pressure: Exothermic Reactions, Unstable Substances, Pressure Relief and Atmospheric Discharge, The Institution of Chemical Engineers, 1987.
  • Knowlton, R.E., An Introduction to Hazard and Operability Studies, Chemetics International, Vancouver, 1981.
  • Coulson, J.M., & Richardson J.F., Chemical Engineering Volume 6, Pergamon Press, Third Edition, 1983.
  • Lees, F.P., Loss Prevention in the Process Industries: Hazard Identification, Assessment and Control, 1996, Second Edition.

Case Studies illustrating the importance of Plant modification / Change procedures

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