Day lodge air handling unit explodes, damages property and equipment

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Recently, Technical Safety BC concluded an investigation on an explosion in a propane-powered air handling unit located in a West Vancouver lodge. The incident, which resulted in property and equipment damage, was attributed to a number of reasons including: poor combustion, incorrect setup of the pre-purge timer, and failure of the controller along with lack of maintenance.

There were a number of factors leading up to the incident and the investigating safety officer points out that all of them did not happen in one day. “They (factors) evolved over time and at different rates of failure that finally affected each other, collectively contributing to the incident.”

Investigations revealed that the pre-purge timer was not set up to the manufacturer’s installation instructions (it was set to 15 secs instead of the recommended time of 6 minutes, 30 seconds. This gave insufficient time for the burner blower to clear the combustion chamber of gases.

Lack of maintenance led to the loosening of the linkage set screw between the actuator and the air damper. This led to detrimental effects - a reduction in the amount of air the burner blower could deliver when the pre-purge cycle was activated, which in turn affected the gas and air ratio for a strong and clean flame.

The above problem was accentuated by a tightened nut on the air dampener, which is used for single stage burners. This was caused by friction on the damper that further reduced the movement of the air damper.

The resulting weak and dirty flame burnt the flame sensor wire to the point that it intermittently sent false flame signals to the controller and produced soot within the heat exchanger. It restricted the air flow within the heat exchanger and further reduced the effectiveness of the burner blower to clear the combustion chamber of gases.

On the day of the incident, the equipment was switched on and the controller started its pre-purge sequence. However, due to the pre-purge timer setting and restricted movement of the fan damper there wasn’t enough air to clear the heat exchanger of all the unwanted gases. When the controller went to ignite the pilot, it received intermittent false flame signals from the faulty flame sensor. Instead of the controller stopping the process (until someone physically resets the controller), it ignited the pilot again. The resulting propane buildup caused an explosion inside the heat exchanger.

The explosion distorted the heat exchanger outwards. It also forcibly removed part of the flue and draft hood from the air handling unit and disconnected portions of the heat exchanger and pushed them out of their original position.

The building maintenance manager for the site confirmed that a maintenance schedule was not setup or documented.

This incident reaffirms the need for owners to employ certified individuals to install and maintain regulated equipment. Further, a regular maintenance schedule can go a long way towards avoiding major incidents that can cause serious damage and/or injury.

 To learn more, review our incident investigation summary: Air handling unit explodes in mechanical room of day lodge.

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