January 23, 2014
Last month, two contract workers for the city of Astoria were injured while they were performing pressure tests on a manhole as part of the city's combined sewer overflow project. An air-filled plug placed over the manhole while the tests were being performed blew out, striking one worker in the face and knocking down the worker.
In this case, the hazard was the back pressure on the sewer plug that caused it to explode with enough force to send it 30 feet into the air. Even low back pressure can create enormous pressure on a plug. For example, a 36-inch plug with a back pressure of five pounds per square inch, is restraining 5,000 pounds of force. The force can cause the violent expulsion of an improperly installed plug, which can injure or kill a person in the plug's "danger zone."
The danger zone is the area where the pipeline plug, plug fragments, or the substance restrained by the plug may be ejected if the plug fails. The danger zone expands outwardly in a cone shape from the plugged opening. The danger zone increases as the inflation pressure is increased, as back pressure increases, and from possible ricochet effects from nearby pipes, walls, or other objects.
Pneumatic pipe plugs – often used for pressure testing – are also used for many other tasks that require workers to temporarily stop a pipe's flow. Workers who use pneumatic plugs without taking the time to understand how to select, use, and install them are putting themselves and their co-workers at risk.
Before choosing an appropriate pneumatic plug, workers must know:
When installing and using a pneumatic plug, workers must:
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