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Vacuum Collapse of a Tank


November 1, 2003
By Pulp & Paper Canada

When planning maintenance jobs either by our own workforce or by contractors, it is important to consider all aspects of the work to be performed and the potential impact it may have on current operat…

When planning maintenance jobs either by our own workforce or by contractors, it is important to consider all aspects of the work to be performed and the potential impact it may have on current operations.

The following is an incident in another industry but from which much can be learned. It fortunately caused only material damages, but had the potential of injuring seriously both maintenance and operation personnel since it ruptured the bottom of the vessel.

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The vacuum collapse of tank 9208 was caused by vacuum created in the tank. The vacuum situation was caused by the vacuum relief device for the tank being covered with plastic, which prevented the vacuum relief device from operating when material was removed from the tank. The transfer of material from the tank was done because of the need to add water to tank 9209 to allow circulation of the material in tank 9209 and the “Lock Out Tag Out”(LOTO) procedure for tank 9208 allowed the transfer of material in/out of tank 9208.

The painting contractor had put a sheet of plastic on the vacuum relief device to prevent it from being sprayed during the tank painting. The contractor had assumed that the tank was out of service, in spite of daily permit sign-off discussions, due to the requirements of the LOTO procedure. A contributing cause involved the contractor not recognizing the importance of the vacuum relief device. Even though the owner had informed the general contractor not to cover the tank vents; the contractor employee who trained the painting contractor failed to properly inform them not cover vent devices.

Neither the owner nor the general contractor had identified the vents individually during the pre-job walkthrough. This caused the assumption by the painting contractor that no critical vents were associated with that job.

A contributing factor involves the nitrogen make-up to the tank that was disconnected for the LOTO procedure to eliminate the potential nitrogen asphyxiation hazard for workers inside the painting enclosure around the tank. This precaution was necessary because the design of the vacuum relief device was sized to protect the tank from vacuum collapse without nitrogen makeup. If the nitrogen supply would have been connected, it may have prevented the vacuum collapse of the tank.

Recommendations:

1) Develop detailed maintenance/contractor procedures requiring physical identification of tank vents during walk-through and any other devices that cannot be impeded or covered during sand blasting/painting or by other maintenance activities. The procedure should address any changes made to the painting enclosure or changes in operational status of the equipment while painting is on-going.

2) Owner must do a daily visual inspection and verbal discussion of the painting job scope with all personnel involved in the job to ensure that all changes are understood, reviewed and appropriately addressed. The daily permit sign-off discussion at the job site may not be detailed enough to capture all the painting contractor work process plans for that day (i.e. any revisions to enclosure, covering any devices, if tank in service-any tank operating conditions.)

Herb G. Tessier is a senior consultant with DuPont Safety Resources with over 27 years experience in supervisory, technical and management with DuPont Canada.

Pulp & Paper Canada is pleased to publish articles on safety from industry experts such as John E. Little, Herb Tessier and Martin Lesperance, as well as encouraging submissions from contributors. Please e-mail anyao@pulpandpapercanada.com#text2#


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