Plant Layout

This Technical Measures Document refers to Plant Layout.

Other relevant Technical Measures Documents are:

The relevant Level 2 Criteria are 5.2.1.3 (29) b, 5.2.1.3 (29) g, h, i and 5.2.1.6 (38) d.

General principles

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

The need to keep distances for transfer of materials between plant/storage units to a minimum to reduce costs and risks;
The geographical limitations of the site;
Interaction with existing or planned facilities on site such as existing roadways, drainage and utilities routings;
Interaction with other plants on site;
The need for plant operability and maintainability;
The need to locate hazardous materials facilities as far as possible from site boundaries and people living in the local neighbourhood;
The need to prevent confinement where release of flammable substances may occur;
The need to provide access for emergency services;
The need to provide emergency escape routes for on-site personnel;
The need to provide acceptable working conditions for operators.

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

Prevent, limit and/or mitigate escalation of adjacent events (domino);
Ensure safety within on-site occupied buildings;
Control access of unauthorised personnel;
Facilitate access for emergency services.

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:

Intensification to reduce inventories;
Substitution of hazardous substances by less hazardous alternatives;
Attenuation to reduce hazardous process conditions ie temperature, pressure;
Simpler systems/processes to reduce potential loss of containment or possibility of errors causing a hazardous event;
Fail-safe design e.g. valve position on failure.

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.

Fire

A fire can spread in four ways:

Direct burning (including running liquid fires);
Convection;
Radiation;
Conduction.

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 ie 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

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

arranging separation distances such that damage to adjacent plants will not occur even in the worst case;
provision of barriers e.g. blast walls, location in strong buildings;
protecting plant against damage e.g. provision of thicker walls on vessels;
directing explosion relief vents away from vulnerable areas e.g. other plants or buildings, roadways near site boundaries.

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:

Locating all high-volume storage of flammable / toxic material well outside process areas;
Locating hazardous plant away from main roadways through the site;
Fitting remote-actuated isolation valves where high inventories of hazardous materials may be released into vulnerable areas;
Provision of ditches, dykes, embankments, sloping terrain to contain and control releases and limit the safety and environmental effects;
Siting of plants within buildings as secondary containment;
Siting of plants in the open air to ensure rapid dispersion of minor releases of flammable gases and vapours and thus prevent concentrations building up which may lead to flash fires and explosions;
Hazardous area classification for flammable gases, vapours and dusts to designate areas where ignition sources should be eliminated.

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

'Process plant hazard and control building design: An approach to categorisation', Chemical Industries Association, 1990.
CIA Guidance for the location and design of occupied building on chemical manufacturing sites, CIA/CISHEC, 1998.
BS 5908 : 1990 Code of practice for fire precautions in the chemical and allied industries, British Standards Institution.
Section 5, Paragraph 21 provides guidance on the minimum distance that the building can be placed from the site boundary. For some specific substances, HSE guidance notes or industry codes of practice are available, giving separation distances such as those from plant to site boundaries.
Section 10, Paragraph 54.3 provides guidance on methods to reduce any flammable gas concentrations below the lower limit, including the use of fixed water sprays or monitors positioned in such a way as to aid the dispersion of the gas into the atmosphere.
HS(G)176 The storage of flammable liquids in tanks, HSE, 1998.
Paragraphs 46 to 55 provide guidance on the siting of tanks.
HS(G)50 The storage of flammable liquids in fixed tanks (up to 10000 cu. m in total capacity), HSE, 1990.
Superseded by HS(G)176, Paragraph 12 provides guidance on the siting of tanks.
HS(G)51 Storage of flammable liquids in containers, HSE, 1990.
HS(G)52 The storage of flammable liquids in fixed tanks (exceeding 10000 cu. m in total capacity), HSE, 1991.
Superseded by HS(G)176.
HS(G)28 Safety advice for bulk chlorine installations, HSE, 1999.
Paragraphs 21-30 provide guidance on siting of bulk chlorine installations.
HS(G)30 Storage of anhydrous ammonia under pressure in the UK : spherical and cylindrical vessels, HSE,1986.
Paragraph 155-160 provide guidance on siting of vessels for receiving tanker deliveries of anhydrous ammonia.
LPGA COP 1 Bulk LPG storage at fixed installations. Part 1 : Design, installation and operation of vessels located above ground, LP Gas Association, Revised Edition July 1998 (includes Amendment 1, January 1999).
Supersedes HS(G)34 Storage of LPG at fixed installations.
Part 1 gives guidance on plant layout.
HS(G)34 Storage of LPG at fixed installations, HSE, 1987.
Superseded by the above.
Paragraphs 15 to 36 give guidance on plant layout.