INTRODUCTION
Entry into a confined space can be extremely dangerous. A number of people are killed or seriously injured in them each year. Confined spaces are commonplace in chemical plants and in the process industry generally. This article discusses some of the hazards associated with confined spaces and how these can be managed safely. Please note that every situation is different and that advice is best sought from a safety specialist before attempting confined space entry.
A confined space can be defined as any poorly ventilated enclosed area. Some confined spaces are easy to identify – they include storage tanks / vessels, silos, chemical reactors and piping / ductwork. Other confined spaces are less obvious – e.g. trenches, sewers, basements and even rooms with inadequate ventilation. They all either contain or have the potential for containing atmospheres which are hazardous to health. They are usually unoccupied but occasionally need to be accessed for maintenance or other reasons.
DANGERS OF CONFINED SPACES
Working in confined spaces can be extremely dangerous, as their atmosphere is different from that of the open air. In particular:
1. The absence of oxygen
Air in the confined space could be low in oxygen. This could be caused by the presence of an asphyxiating gas (such as nitrogen or carbon dioxide), or the oxygen could have been consumed in a chemical reaction (e.g. reaction of groundwater on chalk or limestone which produces carbon dioxide and displaces air, or formation of rust consuming oxygen).
2. The presence of toxic gases
The atmosphere in confined spaces can become polluted with toxic gases. Toxic gases can be given off by residue solids or liquids.
In addition, confined spaces can be susceptible to other hazards. These include fires and explosions (if flammable gases are allowed to build up), risk of drowning and exposure to hot conditions.
PREPARATION FOR WORK IN CONFINED SPACES
Working in a confined space is not normal practice and is usually restricted to activities such as maintenance work. The level of hazard means that working in confined spaces should be carefully planned.
1. Avoid entering in the first place
The best approach is to avoid entry in the first place. Ideally this could be done by either eliminating the confined space or by conducting operations remotely. For example:
- Blockages could be cleared using remote devices such as air sparges.
- Internal inspection could be done using cameras.
- Cleaning can be carried out using water sprays.
- Samples can be obtained remotely.
Operation and maintenance of the plant should be considered during the plant design. Simple changes can often be made which avoids or reduces the need for confined space working.
2. Permits to Work
If confined space entry is required, it must employ a safe system of work. Often, this is done using a Permit to Work system. The Permit to Work is a document which records how confined space working is planned. It demonstrates that hazards are understood and managed correctly. Typically, a Permit to Work for confined space entry will include the following information:
- A description of the work to be done
- The working environment;
- The working materials and tools – including Personnel Protective Equipment (PPE);
- The suitability of those carrying out the task;
- Arrangements for emergency rescue.
The Permit to Work must be signed off before entry to the confined space by the person who is responsible for the work. They must ensure that the workers understand the activities to be done and the precautions to be taken. Typically, Permits to Work are time limited. If the work extends beyond its original duration, the permit to work will need to be formally extended or replaced with a new one. At the end of the job, the permit should be signed off as complete.
3. Establishing a Safe Working Atmosphere
Before entry to the confined space, it should be purged with air to establish a safe atmosphere. This is often done mechanically – such as by using a blower fan. The source of fresh air should be drawn away from any possible contaminants. Ventilation should never be carried out with pure oxygen, only with air.
The purge should ensure good mixing by keeping the air flow turbulent. Low air flows could leave pockets of inert gases, particularly if their densities are considerably different from that of air. Provided that turbulent flow is achieved, good air quality can be obtained after about 5 air changes. Ventilation rates can be improved by increasing the number of openings into the equipment.
Purging shall continue until tests confirm that the atmosphere is safe – i.e. the air should be fit to breathe and free from toxic / flammable vapours. Testing should be carried out by a competent person using a suitable gas detector which is correctly calibrated. Testing should be representative of the atmosphere and should check for pockets of poor air quality.
4. Other Factors to Consider
Any potential sources of inert or toxic gases must be removed. The equipment should be cleaned to remove contaminants and gas pipelines into the equipment should be isolated. Pipelines should be positively blinded or, ideally, the line should be disconnected. It is not acceptable to rely only on a closed valve for isolation (even if it is locked in position).
To prevent unauthorised entry, the confined space should be locked off and equipped with warning signs. All electrical and mechanical isolations should be in place and locked off.
The access to the confined space should be big enough to allow workers wearing all the necessary equipment to climb in and out easily.
CONFINED SPACE WORKING
1. General Precautions
Work in the confined space should be planned in advance and recorded on the Permit-to-Work. Changes to the scope of work need to be re-assessed and recorded on the Permit-to Work before continuing. Anyone entering a confined space must be suitably trained and should wear appropriate PPE (this should be identified on the Permit to work).
During the work, good air quality in the confined space must be maintained. This may need the continuous use of fans / air blowers and frequent monitoring the atmosphere –particularly local to the workface and in potential dead zones. In some situations, it may be advisable for workers to wear portable oxygen monitors.
Special precautions should be considered if using specialist equipment. Use of welding tools and grinders can contaminate the atmosphere. Use of electrical equipment or tools which generate sparks can be hazardous in atmospheres with potential flammable atmospheres.
2. Emergency Procedures
Owing to the hazards present during confined space working, it is important that procedures are in place for when things go wrong. Multiple fatalities have occurred when rescuers have entered the confined space and have then been overcome by the same conditions as the original occupants. Therefore, rescue procedures should be planned in advance and recorded on the permit to work.
Typically, someone remains on watch on the outside of the confined space, whilst work is in progress. They will be responsible for ensuring that any workers in the confined space are rescued, in an emergency. The rescuer should be well trained in what to do in an emergency, including in the use of rescue equipment and in resuscitation techniques.
Rescue equipment will often include lifelines and lifting equipment (even the strongest person is unable to lift an unconscious person on their own using only a rope), as well as a resuscitation kit (oxygen cylinder with pressure regulator). If the rescuer has to enter the confined space, they should be provided with full independent breathing apparatus. There should be an adequate means of communication between those inside and outside the confined space.
