This Technical Measures Document refers to codes and standards applicable to inerting of plant.
Related Technical Measures Documents include:
The relevant Level 2 Criterion is:
The partial or complete substitution of the air or flammable atmosphere by an inert gas is a very effective method of explosion prevention. Inerting is normally only considered when the flammable or explosive hazard cannot be eliminated by other means i.e. substitution of flammable material with non-flammable, adjustment of process conditions to ensure substances are below flammable limits. Typical uses are within storage tanks where a material may be above its flashpoint and within reactor systems when excursions into flammable atmospheres may occur. Inert gases are also used to transfer flammable liquids under pressure. Inerting is applicable to enclosed plant, since plant that is substantially open to atmosphere cannot be effectively inerted because the prevailing oxygen concentration is likely to vary.
A major risk associated with use of inerting is that of asphyxiation, particularly in confined spaces. In those events where people are required to enter a confined space, a formal management control system in the form of a Permit to Work should be in place so that appropriate precautions and control measures can be implemented. The Permit to Work system is covered separately.
Gases that can be used for inerting include:
The practice of inerting is also employed in explosion suppression systems, where typically a quick acting pressure switch responds to the initial comparatively slow increase in pressure due to initiation of explosion conditions. This in turn triggers injection of an explosion suppressant such as chlorobromomethane or carbon dioxide into the path of the advancing flame front. This technique can also be employed to provide protection to interconnected plant by inerting plant items downstream of the explosion.
In most inerting systems a slight positive pressure should be maintained within the enclosed plant to reduce the possibility of air ingress. Inert gases may be generated on site, or via bulk storage of cylinder facilities.
To produce an explosion, three key 'ingredients' are required simultaneously. These are:
The fuel must be within its range of flammability, i.e. at a concentration above the lower flammable limit, but below the upper flammable limit. This means that it is perfectly acceptable to have an environment with the flammable material above the upper flammable limit provided appropriate control protocols are in place since the potential for further dilution would bring the material within its flammable range.
If the fuel cannot be eliminated or minimised, steps must be implemented to eliminate or minimise the source of ignition (see the Technical Measures documentation on Hazardous Area Classification). The final measure that can be adopted involves reducing the oxygen levels necessary to sustain combustion. This can be achieved by pressurising / purging with an inert gas such as nitrogen.
The flammable limits for individual materials with air are readily available in standard references, however appropriate methods should be employed to determine the flammable limits for mixtures of materials. A typical method is cited in BS 5345 : Part 1: 1989, Appendix E. Where flammable dusts are handled in an atmosphere containing flammable gas or vapour, determination of flammable limits is difficult and use of inerting should be considered wherever possible.
In many applications, the maintenance of an inert atmosphere is a safety-critical measure, in the absence of which many potential hazards could be realised. Reliability of the supply of inert gas is therefore of vital importance, and the system should be regularly inspected and maintained.
Consideration should be given to the possibility of failure of the inert gas supply and the acceptable unavailability. This will involve calculations to determine the rate of leakage / replacement in all process conditions encountered to find the worst case that must be considered.
Back up of supplies with alarm systems to bring about operator intervention or automatic change-over should be provided as required to meet the required availability determined. Consideration should also be given to the reliability of the control systems employed for operation and change-over. Back-up facilities may be via alternative bulk storage or cylinder provision.
A major consideration when designing plant to be protected by inerting is the need for continuous monitoring of oxygen and flammable gas or vapour concentrations.
Inert gases are often used to purge tanks and vessels which normally contain flammable substances prior to maintenance, commissioning or decommissioning. The presence of inert atmospheres should always be taken into consideration during operational or maintenance activities since potential hazards could arise from:
Control systems based upon the use of ‘explosimeters’ and oxygen analysers should protect against asphyxiation if entering such areas. Maintenance activities should only be undertaken by suitably trained and authorised personnel, and controlled by a Permit to Work system.
Current guidance is listed below.
Lees, F.P., 'Loss Prevention in the Process Industries: Hazard Identification, Assessment and Control', Second Edition, 1996.