Alert: Air Pressure in Water Pipe Systems

• ISSN: 1195-5228
• Issued: December 1993
• Content last reviewed: June 2009

Hazard Summary:

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 pipe should be tapped following approved pipe tapping procedures. At no time should pipe cap screws be loosened under pressure allowing the pipe cap to "blow-off." 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 this 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.