L137: Safe Maintenance Procedures (DSEAR)

What is L137?

L137 is the HSE's Approved Code of Practice (ACOP) and guidance for safe maintenance, repair, and cleaning procedures under the Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR). It provides essential guidance on how to carry out maintenance work safely where dangerous substances are or may be present.

Maintenance activities often create the very conditions that allow fires and explosions to occur. Hot work introduces ignition sources, opening equipment releases flammable vapours, and routine procedures may be bypassed. L137 addresses these risks by setting out safe systems of work for maintenance operations.

The guidance applies to all maintenance, repair, and cleaning activities in areas where dangerous substances could create explosive atmospheres, including tanks, vessels, pipework, and process equipment.

Who Needs This Document?

L137 is essential for:

• Maintenance managers and engineers planning and supervising work
• Contractors working on client sites with dangerous substances
• Welders and fabricators carrying out hot work
• Tank cleaning operatives entering and cleaning vessels
• Process plant operators involved in maintenance activities
• Facilities managers coordinating maintenance work
• Petrochemical and chemical industries with process equipment
• Food and beverage manufacturers with alcohol or solvent processes
• Printing and coating operations using flammable inks
• Any workplace maintaining equipment that has contained dangerous substances

Key Topics Covered

The Maintenance Challenge

Maintenance creates unique risks with dangerous substances:

Why maintenance is high-risk:

• Safety interlocks may be bypassed
• Equipment is opened, releasing residues
• Hot work introduces ignition sources
• Normal operating safeguards are disabled
• Work may be in confined spaces
• Multiple trades working together
• Time pressure to complete work

Common maintenance incidents:

• Explosions during welding on tanks
• Fires when cutting into pipework
• Flash fires from residual vapours
• Ignition of cleaning solvent vapours
• Oxygen-enriched atmosphere fires

Planning Safe Maintenance

L137 emphasises thorough planning:

Pre-work assessment:

• What dangerous substances are or were present?
• What residues might remain?
• What ignition sources will maintenance introduce?
• How will the area be made safe?
• What sequence of work is needed?

Risk assessment:

• Specific assessment for the maintenance task
• Consider all phases of work
• Identify control measures required
• Emergency arrangements

Method statements:

• Detailed sequence of operations
• Safety precautions at each step
• Equipment and PPE required
• Responsibilities clearly defined

Isolation and Preparation

Making equipment safe before maintenance:

Process isolation:

• Physical isolation from process materials
• Blank flanges (spades) to prevent flow
• Double block and bleed where appropriate
• Verification of isolation effectiveness

Electrical isolation:

• Lockout/tagout procedures
• Isolation at source
• Personal locks and keys
• Testing to confirm isolation

Draining and purging:

• Remove liquid contents
• Purge with inert gas (nitrogen) where appropriate
• Ventilate to remove vapours
• Test atmosphere before work

Cleaning:

• Remove deposits and residues
• Steam cleaning or solvent washing
• Allow adequate drying time
• Dispose of waste safely

Atmospheric Testing

Testing is critical before and during work:

When to test:

• Before opening equipment
• Before entry to confined spaces
• Before hot work commences
• Continuously during hot work
• After any work interruption

What to test for:

• Flammable gas/vapour concentration
• Oxygen levels
• Toxic substances where relevant

Testing requirements:

• Use calibrated instruments
• Test at multiple locations
• Record results
• Competent personnel

Safe levels:

• Below 10% of Lower Explosive Limit (LEL) for hot work
• 19.5-23% oxygen for normal atmospheres
• Specific limits for toxic substances

Permit-to-Work Systems

L137 strongly recommends permits for high-risk work:

When permits are needed:

• Hot work in hazardous areas
• Entry to confined spaces
• Breaking into pressurised systems
• Work on electrical systems
• Any high-risk maintenance activity

Permit system requirements:

• Formal written authorisation
• Clear description of work and location
• Hazards identified and precautions specified
• Time limits and validity period
• Sign-on and sign-off procedures
• Handback and cancellation

Key permit elements:

• Isolation verification
• Atmospheric test results
• Fire precautions
• PPE requirements
• Emergency arrangements
• Authorising signatures

Hot Work Procedures

Hot work is a major cause of incidents:

Hot work includes:

• Welding (arc, gas, MIG, TIG)
• Cutting (thermal, plasma)
• Grinding and disc cutting
• Brazing and soldering
• Any spark or flame-producing work

Control measures:

• Hot work permits
• Atmospheric testing
• Removal of flammable materials
• Fire resistant screens and blankets
• Fire extinguishers immediately available
• Fire watch during and after work
• Continuous monitoring

Fire watch requirements:

• Trained person with extinguisher
• Present throughout hot work
• Continue for minimum 30-60 minutes after
• Longer if risk of smouldering

Confined Space Entry

Maintenance often involves confined spaces:

Confined spaces include:

• Storage tanks and vessels
• Silos and hoppers
• Pipework and ducts
• Pits and chambers
• Any enclosed space with limited access

Additional hazards:

• Oxygen depletion
• Toxic atmosphere build-up
• Engulfment by contents
• Difficulty of rescue

Control measures:

• Entry only if essential
• Permit to enter
• Atmospheric testing
• Continuous monitoring
• Rescue arrangements
• Trained entry team
• Standby person at entry point

Contractor Management

Many maintenance activities involve contractors:

Employer duties:

• Provide information on hazards
• Ensure contractors are competent
• Coordinate work activities
• Monitor contractor work

Contractor duties:

• Follow site safety rules
• Use permit systems correctly
• Report hazards and incidents
• Work to agreed methods

Coordination requirements:

• Pre-work meetings
• Clear communication
• Supervision arrangements
• Emergency procedures understood

Emergency Procedures

Maintenance must include emergency planning:

Before work starts:

• Emergency procedures understood
• Escape routes identified
• Rescue equipment available
• Communication systems in place
• Emergency services notified where appropriate

During work:

• Immediate evacuation capability
• Fire fighting equipment to hand
• First aid arrangements
• Rescue capability for confined spaces

Legal Status

L137 is an Approved Code of Practice under the Health and Safety at Work etc Act 1974. Following its guidance provides the clearest route to compliance with DSEAR requirements for maintenance work. Failure to follow ACOP provisions can be cited as evidence of non-compliance in enforcement proceedings.

Why It Matters

Maintenance is disproportionately dangerous:

The Scale of the Problem

Incidents during maintenance are common:

• Many workplace explosions occur during maintenance
• Hot work is a leading cause of industrial fires
• Confined space entry causes multiple deaths annually
• Tank cleaning and repair work is particularly hazardous

Common Failure Causes

• Inadequate isolation
• Failure to test atmosphere
• Residues not removed
• Hot work without proper precautions
• Bypassing safety systems
• Poor communication between trades
• Time pressure compromising safety

Consequences of Incidents

For workers:

• Death from explosion or fire
• Severe burns
• Asphyxiation in confined spaces
• Toxic exposure
• Long-term health effects

For employers:

• Criminal prosecution and imprisonment
• Significant fines
• Civil compensation claims
• Business disruption
• Loss of skilled workers
• Reputation damage

Benefits of Following L137

Safety benefits:

• Prevention of fires and explosions
• Protection of maintenance workers
• Safe working environment

Business benefits:

• Legal compliance demonstrated
• Reduced incident rates
• Fewer work stoppages
• Lower insurance costs
• Better contractor relationships

Permit-to-Work Checklist

Key Compliance Steps

1. Assess all maintenance activities - identify where dangerous substances may be present
2. Develop safe systems of work - detailed procedures for each activity type
3. Implement permit systems - for hot work, confined space, and other high-risk work
4. Ensure competence - training for all involved in maintenance
5. Provide equipment - gas testing, isolation, PPE, fire fighting
6. Manage contractors - information, coordination, supervision
7. Establish emergency procedures - rescue, fire response, first aid
8. Monitor and review - audit systems, learn from incidents

Further Resources

• Dangerous Substances and Explosive Atmospheres Regulations 2002↗
• HSE DSEAR web pages↗
• HSG253 - Safe maintenance, repair and cleaning procedures↗
• HSG250 - Guidance on permit-to-work systems↗
• INDG370 - Fire and explosion: how safe is your workplace?↗

This page summarises the ACOP L137. For full legal compliance, obtain and read the complete document from HSE. This summary is not a substitute for professional advice or the full ACOP text.