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The CSB has been investigating an accident at the Kleen Energy plant, which occurred during a planned work activity to clean debris from natural gas pipes at the plant.

To remove the debris, workers used natural gas at a high pressure of approximately 650lb/in2.

The high velocity of the natural-gas flow was intended to remove any debris in the new piping.

At predetermined locations, this gas was vented to the atmosphere through open pipe ends that were located less than 20ft off the ground.

These vents were adjacent to the main power-generation building and along the south wall.

This cleaning practice is known within the natural gas power industry as a ‘gas blow’.

Industry personnel have indicated to CSB investigators that gas blows are a common practice during the commissioning of new or modified gas pipes at their facilities.

CSB investigators have reviewed gas utility records for the morning of the accident.

These records, together with written pipe-cleaning procedures and witness testimony, confirm that the gas blows occurred intermittently over the course of the morning.

At the same time that gas blows were underway, there were potential ignition sources present in the surrounding area, including inside the power-plant building.

There were many construction-related activities underway inside the building.

Determining the exact ignition source is not a major focus of the investigation at this point.

In most industrial worksites, ignition sources are abundant and efforts at accident prevention focus on avoiding or controlling the release of flammable gas or vapour.

Initial calculations by CSB investigators reveal that approximately 400,000ft3 of gas was released to the atmosphere near the building in the final 10min before the blast.

This gas was released into a congested area next to the power block building.

This congested area likely slowed the dispersion of the gas.

The gas built up above the lower explosive limit of approximately four per cent in air and was ignited by an undetermined ignition source.

A major focus of the CSB investigation is to determine what regulations, codes and good practices might apply to these gas blows.

To this point, no specific codes have been identified, but the CSB cautions natural-gas power plants and other industries against the venting of high-pressure natural gas in or near work sites.

This practice, although common, is inherently unsafe.

The CSB is investigating possible alternatives to this practice, including the use of air, steam, nitrogen, or water, or the use of combustion devices to safely destroy the gas.

Combustion devices such as flares can safely burn up flammable gas or vapour, preventing the possibility of an explosion.

Recommending safer alternatives will be a primary focus of the CSB investigation.

The NFPA panel responsible for the fuel gas code has voted to move forward with the CSB’s recommendations to make purging practices safer at work sites across America.

These provisions will apply at hundreds of thousands of facilities, once fully adopted.

The type of purging described in that code is different from the gas blows used in the power industry and power plants remain exempt from the national fuel gas code.

However, gas purging as defined in the code has certain similarities to gas blows, in that gas is applied at one end of a pipe and gas is intentionally vented at the other end to the atmosphere.

Companies must ensure that flammable gases are not vented into close proximity with ignition sources and workers.

The CSB investigation will focus on determining what permanent changes in standards or practices are needed to prevent future accidents involving gas blows.

US Chemical Safety Board

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