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Plant Layout

This Technical Measures Document refers to Plant Layout.

Other relevant Technical Measures Documents are:

The relevant Level 2 Criteria are (29) b, (29) g, h, i and (38) d.

General principles

Plant layout is often a compromise between a number of factors such as:

The most important factors of plant layout as far as safety aspects are concerned are those to:

In determining plant layout designers should consider the factors in outlined in the following sections.

Inherent safety

The major principle in Inherent Safety is to remove the hazard altogether. The best method to achieve this is to reduce the inventory of hazardous substances such that a major hazard is no longer presented. However, this is not often readily achievable and by definition no COMAH facility will have done so. Other possible methods to achieve an Inherently Safer design are:

Plant layout considerations to achieve Inherent Safety are mainly those concerned with domino effects (see below).

The Dow / Mond Indices

These hazard indices are useful for evaluating processes or projects, ranking them against existing facilities, and assigning incident classifications. They provides a comparative measure of the overall risk of fire and explosion of a process, and are useful tools in the plant layout development stage since they enable objective spacing distances to be taken into account at all stages.

The methodology for undertaking a rapid ranking method that is based on the Dow / Mond index is detailed in ILO, PIACT, Major Hazard Control: A practical manual, 1988.

Although these are useful rule-of thumb methodologies for first consideration of plant layout, they do not replace risk assessment. The distances derived between plant units using these systems are based upon engineering judgement and some degree of experience rather than any detailed analysis.

Domino effects

Hazard assessment of site layout is critical to ensure consequences of loss of containment and chances of escalation are minimised. Domino may be by fire, explosion (pressure wave and missiles) or toxic gas cloud causing loss of control of operations in another location.


A fire can spread in four ways:

The spread of fire from its origin to other parts of the premises can be prevented by vertical and horizontal compartmentation using fire-resisting walls and floors. Further information may be found in BS 5908 : 1990. Consideration should also be given to the spread of flammable material via drains, ducts and ventilation systems. Delayed ignition following a release may result in spread of flames through such systems via dispersed flammable gases and vapours.

Protection against domino effects by convection, conduction and radiation can be achieved by inherent safety principles i.e. ensuring that the distances between plant items are sufficient to prevent overheating of adjacent plants compromising safety of those plants also. Where this is not possible due to other restrictions, other methods such as fire walls, active or passive fire protection may be considered.


Explosion propagation may be directly by pressure waves or indirectly by missiles. As for fires, inherently safe methods that should be considered are:

However, the latter may not provide practical solutions, particularly against missiles, and risk analysis may be required to prove adequate safety.

Toxic gas releases

Toxic gas releases may cause domino effects by rendering adjacent plants inoperable and injuring operators. Prevention/mitigation of such effects may be affected by provision of automatic control systems using inherently safer principles and a suitable control room (see section below on Occupied Buildings).

Reduction of consequences of event on and off Site

In addition to the measures described in the sections above, Plant Layout design techniques applicable to the reduction of the risks from release of flammable or toxic materials include:

Risk management techniques should be used to identify control measures that can be adopted to reduce the consequences of on or off site events. See references cited in further reading material.

Positioning of occupied buildings

The distance between occupied buildings and plant buildings will be governed by the need to reduce the dangers of explosion, fire and toxicity. In particular, evacuation routes should not be blocked by poor plant layout, and personnel with more general site responsibilities should usually be housed in buildings sited in a non-hazard area near the main entrance. Consideration should be given to siting of occupied buildings outside the main fence. In all cases occupied buildings should not be sited downwind of hazardous plant areas. Further guidance is available in standard references.

Aggregation / trapping of flammable vapours

To avoid aggregation and trapping of flammable / toxic vapours which could lead to a hazardous event, buildings should be designed so that all parts of the building are well ventilated by natural or forced ventilation. Flammable storages should be sited in the open air so that minor leaks or thermal outbreathing can be dissipated by natural ventilation. Maintenance procedures should include the displacement of vapours from hazardous areas before work begins (see Technical Measures Document on Permit to Work Systems).

Segregation of incompatible substances (particularly in warehouses / storage areas)

This is detailed in the Technical Measures Document on Segregation of Hazardous Materials.

Status of guidance

Additional material providing much insight into analysis of offsite consequences through a risk management program is now available from the United States Environmental Protection Agency. This provides guidance on offsite consequence analysis for toxic gases, toxic liquids, and flammable substances.

Codes of Practice relating to Plant Layout

Further reading material

Case studies illustrating the importance of Plant Layout