Containers+-+welding+on 


 * 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 percent 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 container must be monitored with an oxygen analyzer 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 IT’S 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.

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 * 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 situations. In these cases, all that is needed to cause a container to explode is the wrong combination of temperature and oxygen, and an ignition 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 vaporizing 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 analyzer or carbon monoxide analyzer 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 vapors present in the container.) If the reading on the detector indicates that flammable vapors 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.

FILLING TREATMENT (alternative to inert gas purging)
It is desirable to fill the container with water during welding or cutting as a supplement to any of the cleaning methods (see fig. 2-8). Where this added precaution is taken, place the container so that it can be kept find to within a few inches of the point where the work is to be done. Make sure the space above the water level is vented so the heated air can escape from the container.

http://www.arcraftplasma.com/welding/weldingdata/containersafety.htm




 * 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 vapors 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 electricity.
 * 4) Maintain the flow of inerting gas throughout the hot work.
 * 5) The contents of the container must be tested before beginning hot work. During the operation the oxygen content should be measured using an oxygen analyzer. 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 analyzer 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 vapors 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 atmosphere 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 eliminating any explosive 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**

http://www.ccohs.ca/otherhsinfo/alerts/alert107.tx