July 15, 2014
Last December, a contract worker for the city of Astoria was seriously injured while he was performing a negative pressure test on a new manhole as part of the city's combined sewer overflow project. A steel test cap that he had placed on top of the manhole and secured with an inflatable bladder blew off, striking him in the face. This issue of the Construction Depot describes the events leading up to the accident and why it happened.
Two workers - a lead worker and his helper - had been assigned to vacuum test a newly-installed manhole at Eighth Street and Harrison Avenue. Neither worker had done this type of work before. After he picked up a truck and trailer with testing equipment, the lead worker called the project superintendent and asked for instructions on how to conduct the test.
Following confined space procedures, the lead worker entered the manhole and inserted inflatable plugs in the incoming and outgoing sewer pipes and inflated the plugs with air to 25 to 30 psi. Using a spray bottle, he covered the inside of the manhole with a soap solution, then climbed out of the manhole, placed a steel cap over it, and inflated the cap’s bladder to 25 to 30 psi as the superintendent told him to do.
The two workers connected a vacuum pump to a fitting on the steel cap and drew a negative pressure in the manhole. They tried to achieve a vacuum of 10 inHg, but bubbles from the soap solution indicated a leak at a section joint near the top of the manhole.
They removed the cap and the lead worker repaired the leak with quick-set grout. However, he did not realize the inflatable plugs he had placed in the incoming and outgoing sewer lines were causing fluids in the line to back up in the pipe and in a manhole above the one being tested.
After the grout had set up, the workers applied more soap solution around the inside surface of the manhole, placed the cap over it, and inflated the bladder with air to 25 to 30 psi. They reattached the vacuum pump to the fitting on the cap and began drawing a negative pressure inside the manhole again.
Their plan was to draw the pressure down to 10 inHG, shut off the vacuum pressure at a valve on the cap, and then watch the gauge to be sure that the negative pressure would hold for at least 10 seconds (the time required for a successful test). What they did not know was that the plug the lead worker had placed in the sewer’s incoming pipe had dislodged, allowing the backed-up fluid to rapidly enter and pressurize the manhole.
As the lead worker was drawing a vacuum, the cap suddenly blew off the top of the manhole and struck him in the face. He was taken by ambulance to a hospital in Astoria, and then flown to a trauma center in Portland for intensive care.
The city’s topography (which is generally higher than the location of the manhole) increased the backpressure on the sewer line, and the time necessary for the grout to cure after the first test allowed fluid to buildup in the line. The result was a substantial increase in the pressure on the plugs, in addition to the vacuum created by the pressure test.
Oregon OSHA’s investigation of the accident determined that the workers were not properly instructed how to vacuum test manholes and safely perform their work. Also, the workers did not brace the pipe plugs in the sewer line, although the city of Astoria’s written specifications for vacuum testing required it. Bracing the plugs might have prevented the accident.
Many injuries involving pneumatic pipe plugs happen when workers are standing in the plug’s danger zone. The danger zone is the area where a 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 and increases as the inflation pressure and back pressure on the plug increase.
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