This Technical Measures Document refers to Plant Layout.
Other relevant Technical Measures Documents are:
The relevant Level 2 Criteria are 18.104.22.168
(29) b, 22.214.171.124 (29) g, h, i and 126.96.36.199
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
- 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.
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 i.e. temperature,
- 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
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:
- Direct burning (including running liquid fires);
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
- 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
- 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
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
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 /
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
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
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
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
- 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
Supersedes HS(G)34 Storage of LPG at fixed
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.
Further reading material
- ILO, PIACT, 'Major Hazard Control: A practical manual', 1988.
- The Environmental Protection Act (EPA), 1991.
- The Water Resources Act, 1991.
- The Water Industries Act, 1991.
- The Town and Country Planning Act, 1990.
- Mecklenburgh, J.C., 'Process Plant Layout', George Godwin, 1985.
- Kaess, D., Jr., ‘Guide to Trouble-free Plant Layout’, Chemical
Engineering, pp 122-134, June 1, 1970.
- Meissner, R.E. III and Shelton, D.C., ‘Plant Layout: Part 1 Minimizing
Problems in Plant Layout’, The Ralph M. Parsons Co., Chemical Engineering,
99, 4, p81, April 1992.
- Brandt, D., George, W., Hathaway, C. and McClintock, N., ‘Plant Layout:
Part 2 The Impact of Codes, Standards and Regulations’, Davy-McKee Corp.,
Chemical Engineering, 99, 4, p89, April 1992.
- Kirk-Othmer, 'Encyclopaedia of Chemical Technology', Vol. 18: Plant
Layout, pp23-43; Plant Location, pp44-59; Plant Safety, pp60-86, Wiley, New
- Kern, R., CE Refresher: Plant Layout, 12 Parts, Chemical Engineering, 23
May 1977 through 14 August 1978.
- Basusbacher, E. and Hunt, R., ‘Process Plant Layout and Piping
Design’, Auerbach Publishers, Boston, 1990.
- Burklin, C.R., ‘Safety Standards, Codes and Practices for Plant
Design’, Chemical Engineering, pp56-63, October 2, 1972.
- EPA / CEPP, 'Risk Management Program Guidance for Offsite Consequence
Analysis', RMP Series, United States Environmental Protection Agency /
Chemical Emergency Preparedness and Prevention Office, April 1999.
- Konz, S., ‘Work Design: Industrial Ergonomics’, New York: John Wiley
and Sons, Inc., 1983.
- Konz, S., ‘Facility Design’, New York: John Wiley and Sons, Inc.,
- Sule, D.R., ‘Manufacturing Facilities: Location, Planning, and
Design’, Boston, MA: PWS-KENT Publishing Co., 1988.
- Lees, F.P., Loss Prevention in the Process
Industries, Second Edition, 1996.
- Section 8 provides guidance on Hazard Identification;
- Section 10 provides guidance on Hazard Assessment including Domino
- Section 10 provides guidance on Plant Siting and Layout.
Case studies illustrating the importance of Plant Layout