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*                             ALERT                                 *
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*  Produced by:  Ontario Ministry of Labour                         *
*  Provided by:  Canadian Centre for Occupational Health and Safety *
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INFO ALERT:  C1/0793

DATE:  07/93


*** HAZARD SUMMARY:  AIR PRESSURE IN WATER PIPE SYSTEMS ***  

During accident investigations, Ministry of Labour field staff have
found that, after a water pipe system has been pressurized to test
connection leaks or for low pressure air testing, the pressure in the
end pipe often remains dangerously high.  Acting on the end pipe cap
or on the thrust block, this pressure is usually counterbalanced by
the pressure of the soil backfill until the excavation is re-excavated
to connect the end pipe to new pipes.  Unless the pressure is
relieved, the pipe cap or the thrust block can become a high velocity
projectile, potentially endangering workers in or near the excavation.


*** LOCATION(S) AND SECTOR(S): ***

Excavations for water pipe connections.


*** SUGGESTED PRECAUTIONS: ***

To avoid a potential hazard to workers, all constructors should take
the following precautions:

1)   When existing water pipes are to be connected to a new
     installation and there is no pressure relief valve on the system,
     the thrust block should be temporarily braced as soon as the soil
     backfill is removed from the excavation.  (This work may be done
     with heavy machinery, for example, by using a backhoe bucket.) 
     The pipe cap should be loosened from behind, and the worker
     should never be in front of the cap.  A written procedure for
     this operation should be on the project.  The workers must be
     aware of the potential hazard and trained to work in accordance
     with the procedure.

2)   It is recommended that pressure relief valves or similar devices
     be incorporated into the design of new underground piping
     systems.


The constructor must ensure that work in the excavation is done in
accordance with clause 48(1)(a) of Ontario Regulation 231/91, the
Regulation for Construction Projects.

Produced by the Construction Health and Safety Program, Occupational
Health and Safety Branch, Ministry of Labour