WELDING ON CONTAINERS


Produced by:
Industrial Health and Safety Program,
Ontario Ministry of Labour
Alert #I12/0395
ISSN 1195-5228


HAZARD SUMMARY:

In two recent incidents, explosions during welding on fuel tanks 
killed three workers.  In the first, two workers welding a 150-
gallon diesel fuel tank were fatally injured.  In the second, a 
500-gallon gasoline tank exploded during welding, killing a 
worker.  These accidents would not have happened if proper 
cleaning, purging and testing procedures had been followed.

After the first incident, it was discovered that the diesel fuel 
in the tank was contaminated with gasoline.  As little as two per 
cent of gasoline in diesel fuel can create an explosive mixture 
in a closed container, with the flashpoint (the temperature at 
which a spark or other ignition source will cause an explosion) 
falling below the temperature inside the container.

REQUIRED PRECAUTIONS:

For any hot work such as welding or cutting on a container that 
may have contained flammable or combustible material, the 
following minimum precautions must be taken:

-  The container's internal layout must be determined to make 
   sure that fittings such as baffles will not interfere with 
   cleaning or purging. 
-  The container must be drained and cleaned using appropriate 
   methods.
-  To determine whether draining and cleaning has made the 
   container safe, its interior must be tested with a combustible 
   gas detector both before hot work begins and periodically 
   during the work. 

However, some containers cannot be drained and cleaned well 
enough to make them safe.  Such containers may be made safe by 
purging and inerting with an inert gas, but only if these 
precautions are taken:

-  Recognized procedures and proper equipment must be used.
-  The oxygen level inside the contianer must be monitored with 
   an oxygen analyser and maintained at essentially zero for the 
   duration of the work.
-  Workers must be made aware of the limitations of the inerting 
   process.

NEVER ASSUME A CONTAINER IS CLEAN OR SAFE.  MAKE SURE THAT IT IS 
MADE SAFE AND THAT ITS SAFETY IS VERIFIED BY TESTING BEFORE ANY 
HOT WORK BEGINS.  NOT FOLLOWING THIS RULE IS LIKELY TO KILL YOU.

The Regulations for Industrial Establishments (R.R.O 1990, Reg. 
851, s.78) require that where repairs or alterations are to be 
made on a drum, tank, pipeline or other container, it must be 
drained and cleaned or otherwise made free from any explosive, 
flammable or harmful substance.

See Engineering Data Sheet 4-14, "Welding and Other Hot Work on 
Containers" for more information (follows).  For assistance, 
contact your nearest Ministry of Labour office. 

=================================================================

                   ENGINEERING DATA SHEET 4-14
             WELDING AND OTHER HOT WORK ON CONTAINERS

1.  INTRODUCTION

    Hot work on containers, including welding, grinding and 
    cutting, is one of the most dangerous operations in industry. 

    Containers that have or may have contained a fuel or any 
    other flammable, combustible or high-flashpoint material are 
    potentially lethal, as are containers that may have an 
    interior coating that becomes hazardous when heated.  
    Hydrogen gas generated by interior corrosion and explosive 
    dusts (for example, sugar, starch and coal dusts) may also 
    create hazardous situtations.  In these cases, all that is 
    needed to cause a container to explode is the wrong 
    combination of temperature and oxygen, and an ingnition 
    source. 

    Many explosions and several recent fatalities have been 
    caused by the use of inappropriate procedures in cleaning, 
    purging and testing containers in preparation for hot work. 
    
    Workers have to be made aware that hot work on a container is 
    being performed on what is essentially an unexploded bomb and 
    that they must treat the process with suitable respect. 

    The basic rule that must be obeyed before performing hot work 
    on a container is: 

        NEVER ASSUME THAT A CONTAINER IS CLEAN OR SAFE.  MAKE 
        SURE THAT IT IS MADE SAFE, AND THAT THIS IS VERIFIED BY 
        TESTING. 

2.  LEGISLATION

    The Regulations for Industrial Establishments (R.R.O. 851/90, 
    made under the Occupational Health and Safety Act) address 
    repairs on containers that have contained flammable, 
    explosive or harmful substances.  The regulations state 
    (s. 78): 

        ". . . where repairs or alterations are to be made on a 
        drum, tank, pipeline or other container, the drum, tank, 
        pipeline or other container shall, 

        a) have internal pressures adjusted to atmospheric before 
           any fastening is removed; 
        b) be drained and cleaned or otherwise rendered free from 
           any explosive, flammable or harmful substance; and 
        c) not be refilled while there is any risk of vaporising 
           or igniting the substance that is being placed in the 
           drum, tank, pipeline or other container. 

    The Ontario Fire Code also addresses welding on containers 
    (5.17.3.3 (1)): 

        "Welding or cutting of metal containers shall not be 
        undertaken until the containers and compartments within 
        such containers have been cleaned of flammable and 
        combustible materials and checked with an explosion 
        meter." 

3.  CODES AND STANDARDS

    A number of universally accepted codes, standards and 
    guidelines address the hazards of hot work, including welding 
    and cutting, on containers that have contained hazardous 
    substances.  These publications explain in detail the safety 
    procedures to be followed before such work is carried out.  
    The following section is a brief summary of the information 
    in the standards referenced in section 5.  Since it is an 
    outline only, workers intending to perform hot work should 
    review all relevant standards in detail. 

4.  MAKING A CONTAINER SAFE FOR HOT WORK

    Listed below are the steps that must be taken before any hot 
    work is performed on a container that has held a flammable or 
    combustible liquid. 

    Note that any testing carried out using a combustible gas 
    detector, oxygen analyser or carbon monoxide analyser MUST be 
    carried out by a person who has been thoroughly trained in 
    the use of such instruments and who is aware of the 
    requirements and limitations of such testing. 

    All containers that may require entry of workers for any 
    purpose are considered to be confined spaces under the 
    Occupational Health and Safety Act and Regulations, and all 
    applicable requirements must be satisfied before entry.  See 
    Engineering Data Sheet 7-01 "Entry Into Tanks and Other 
    Confined Spaces". 

    4.1  IDENTIFY CONTENTS AND CONTAINER CONSTRUCTION 

         The contents of the container should be identified 
         before any work is carried out.  Remember, however, that 
         the contents may have been contaminated (e.g. with 
         gasoline in diesel fuel) and that container labels may 
         be misleading or incorrect.  If it is not absolutely 
         clear what the contents are, it must be assumed that a 
         potentially explosive material may be present, and the 
         procedures outlined below must be followed. 

         It is also important to determine the internal 
         configuration of the container and to identify any 
         fittings (e.g., baffles or tubes) that may interfere 
         with draining, cleaning or purging.  Any areas inside 
         the container that might trap explosive mixtures must be 
         identified.  All internal fittings should be removed 
         where possible. 

         If there is any possibility that the container may have 
         contained any flammable, combustible or high flashpoint 
         material, or if the container may have an interior 
         lining or coating, precautions as set out in 4.2 through 
         4.5 MUST be taken. 

    4.2  DRAIN CONTENTS
    
         The contents of the container must be drained 
         completely, either by opening the drain connection or by 
         other means if a drain connection is not available.

    4.3  CLEAN CONTAINER
    
         The container must be cleaned using the appropriate 
         method indicated in the referenced standards.  Cleaning 
         methods include: water displacement (for water soluble 
         substances); hot chemicals, using an appropriate hot, 
         strong caustic solution such as sodium triphosphate 
         solution; low pressure steam in conjunction with hot 
         soda ash solution; or a combination of the above 
         methods.  Before a chemical detergent is used, verify 
         with the manufacturer that it is appropriate for the 
         intended task. 

    4.4  TEST ATMOSPHERE
    
         The interior of the container must be tested with a 
         properly calibrated combustible gas detector to make 
         sure that the cleaning operation has made the container 
         safe.  (A combustible gas detector reads the percentage 
         of the lower explosive limit of the vapours present in 
         the container.)  If the reading on the detector 
         indicates that flammable vapours are present, i.e. if 
         the needle moves, the cleaning operation must be 
         repeated.  The testing procedure must be carried out 
         immediately before beginning hot work and periodically 
         during it. 

    4.5  INERT GAS PURGING
    
         In some situations it may be impossible to remove all of 
         the potentially hazardous liquid or liquids inside a 
         container.  Liquid residues may be trapped behind heavy 
         scale or rust or between a vessel's liner and shell and 
         so may not be easily detected.  A second possibility is 
         that a vessel may have an interior coating or lining 
         that will produce flammable vapours when heated. 

         When examination after cleaning indicates that either of 
         these conditions exists or may exist, other precautions 
         must be taken before beginning hot work.  These may 
         include, as a minimum, maintaining an inert atmosphere 
         inside the container with an inert gas. 

         Extreme care should be taken when using an inert gas 
         such as carbon dioxide or nitrogen to purge a container 
         and maintain a safe atmosphere inside it.  The workers 
         must be familiar with the limitations of the inerting 
         method and the characteristics of the inert gas being 
         used.  The oxygen content inside the container must be 
         maintained at essentially zero, i.e. as close to zero as 
         possible, during the entire period that the work is in 
         progress. 

         Performing inert gas purging without thorough knowledge 
         of the process or proper equipment is extremely 
         hazardous.  Inadequate knowledge or equipment is likely 
         to create a false sense of security, leading to serious 
         injury or death. 

         The steps to be followed in the inert gas purging 
         procedure are as follows: 

         1.  Close all container openings except fill and vent 
             connections. 

         2.  Introduce the inert gas into the container through a 
             hose extending to near the bottom of the container.  
             All metal parts of the filling equipment must be 
             bonded to the container. 

         3.  When inerting with carbon dioxide, low pressure must 
             be used to avoid the generation of static 
             electricity.  Portable carbon dioxide extinguishers 
             must never be used for inerting because they may 
             discharge static electricty. 

         4.  Maintain the flow of inerting gas throughout the hot 
             work. 

         5.  The contents of the container must be tested before
             beginning hot work and during it.  Its oxygen 
             content can be measured by an oxygen analyser.  When 
             carbon dioxide is used for inerting, oxygen can be 
             measured indirectly with a carbon dioxide indicator.  
             Each type of instrument must be suitable for its 
             purpose.  An oxygen analyser must be capable of 
             reading zero per center oxygen, and a carbon dioxide 
             analyzer must be capable of reading 100 per cent 
             carbon dioxide.  The accuracy of the instrument to 
             be used needs to be determined and taken into 
             account. 

         The concentration of flammable vapours present in the 
         container can be measured with a combustible gas 
         detector.  Note, however, that combustible gas detectors 
         of the catalytic filament type may not give a true 
         reading when the oxygen content of the atmospher tested 
         is less than about 10 per cent.  Without special 
         adaptation, detectors of this type cannot reliably 
         determine whether the inerting process has been 
         successful in eleiminating any explosvie mixture. 


5.  REFERENCES

    a)  ANSI/AWS F4.1  Recommended Safe Practices for the 
                       Preparation for Welding and Cutting of 
                       Containers That Have Held Hazardous 
                       Substances. 

    b)  NFPA 51B       Standard for Fire Prevention in Use of 
                       Cutting and Welding Processes.

    c)  NFPA 327       Standard Procedures for Cleaning or 
                       Safeguarding Small Tanks and Containers.

    d)  IAPA           Welding on Containers or in Confined 
                       Spaces.

    e)  CSA W117.2     Safety in Welding, Cutting and Allied 
                       Processes.

    f)  NFPA           Fire Protection Handbook, 16th Edition.


Produced by:
Professional and Specialized 
 Services and the Occupational
 Health and Safety Branch, 
Ministry of Labour
ISSN 1201-1444