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Hard Light Off Results in Small Explosion in Boiler

Incident Investigation

Hard Light Off Results in Small Explosion in Boiler

April 22, 2019

Boiler, PV and Refrigeration

Reference Number:

II-842548-2019 (#11949)

Location: Delta BC
Regulated industry sector: Gas - Natural gas system


Qty injuries: 0
Injury description: N/A
Injury rating: None

Damage description: Burner, access door, and venting detached from boiler. Boiler is a total loss.
Damage rating: Major

Incident rating: Major

Incident Overview

While a service technician was working on a boiler that had been experiencing issues with lighting off or establishing a main burner flame. The boiler had a hard light off, meaning the main burner flame did not establish smoothly, this can be the equivalent of a small explosion in the boiler, and is caused by delayed ignition of the main burner flame. This hard light off blew off the burner, access door, and venting on the rear of the appliance. (Figure 1-2)


Site, System and Components

This is a forced draft appliance which relies on a fan on the inlet of the appliance to push and mix an air fuel mixture through the appliance, where the air fuel mixture is burnt and the products of combustion are then vented outdoors.

Due to their nature, forced draft appliances (especially appliances with inputs over 409,000 btu’s) employ a specific start up sequence to ensure safe light off of the appliance before operation of the appliance is handed off to the appliance’s firing rate controls in what is referred to as the “run cycle”. The start-up sequence is controlled by the appliance flame safeguard, which ensures the appliance is safe to light off, initiates and stages the light off sequence, and once the appliance enters the run cycle; monitors the flame to ensure that if the flame is lost the appliance does not operate.

Flame safeguards are designed so that any unexpected conditions during the operation of the appliance, or loss of flame, result in a lock out of the appliance.
When an appliance locks out all ignition and fuel sources are extinguished, the boiler attempts to purge any gas that was introduced into the combustion chamber. It then shuts down and will not re-activate until somebody resets the flame safeguard.

The light off sequence generally follows the following sequence:

  • First the boiler receives a call for heat from a sensor in the system.
  • Next the flame safeguard preforms a safe start check where it checks to ensure that all operating conditions are as required, and that there is no flame present in the appliance.
  • The safeguard then activates the appliance fan and waits for air flow to trigger a pressure switch to confirm there is airflow through the appliance. Once airflow is confirmed the appliance drives the input control to “high fire” where an end switch within the input control makes to prove the input control is in “high fire” position.
  • The flame safeguard then begins the pre-purge sequence, at this point air is passed through the boiler to exhaust any gas that may have accumulated in the burner chamber while the appliance was in standby mode. Code requires that at least four air changes happen within the appliance at a firing rate of not less than 60% of the maximum firing rate.
  • After the pre purge has completed, the appliance drives the input controller down to its “low fire” position for what is known as a “low fire start”, this is done so that a main flame can be established without too much turbulence in the air, and ensures a smooth light off (imagine lighting a lighter in the wind, vs no wind).
  • When the boiler drives to low fire start the input control motor of the appliance drives the combustion air damper to a position where it is nearly closed, limiting the air flow that is going through the boiler, once the boiler reaches this position an end switch is made in the input control motor and the boiler attempts to ignite.
  • Once the flame is established and the flame safeguard senses the flame, the safeguard will drive the input controller up to its required firing position, supervise the flame to ensure that it is stable, and allow the boiler to enter the “run cycle”.

This boiler uses direct spark ignition, this means that there is no pilot flame, and the main flame is established using a spark ignition rod, which when energized causes a spark to continuously jump from the rod to the burner head, creating a source of ignition.

The flame safeguard was a mechanical type flame safeguard, as opposed to some which are entirely microprocessor based. This means that a rotating cam within the flame safeguard opens and closes sets of contacts which activate the various functions of the boiler through the pre-purge, ignition, and run cycles. At the top of this cam is a dial indicator which shows what stage of operation the boiler is in, and is a handy way to identify at what stage a lockout is happening.

Outside of the flame safeguard’s control there are other factors which could cause a hard light off within the boiler. One example would be, if the supply gas pressure is too high there could be an incorrect gas-air ratio at the burner head which could make ignition difficult. Another example would be if the end switch in the input control motor was incorrectly adjusted and the low fire start was occurring before the burner had completely driven to low fire start position, in which case the boiler would be attempting to ignite while the velocity of air and gas across the burner head is too high (again imagining the lighter in the wind) allowing for some air-gas mixture to accumulate in the boiler before it ignites.

Both of these setting are generally set to manufacturers specifications when the boiler is first commissioned and only ever adjusted to bring the appliance back to these settings should something go out of adjustment.

The venting system employed a barometric draft control device, which operates similar to a draft hood but instead uses a weighted damper to allow dilution air into the draft system so that it does not pull too hard of a draft which could pull the flame off the burner head.
The ignition and operation sequence of this type of appliance is complex. The intent of the system it to use a series of redundant safeties to ensure safe and proper ignition and operation of the appliance to prevent the type of failure that occurred in this situation. Alteration or by-passing any of the safety devices or start/run sequence components can cause the equipment to operate unpredictably and in some cases fail causing damage to the equipment and potentially the surrounding area.

It is very important to ensure that only qualified and knowledgeable personnel operate and maintain this type of equipment due to the complexity of the systems and potential hazards associated with failure.

Failure Scenario(s)

The heating contractor had been experiencing nuisance lock-outs of the appliance for roughly a week leading up to the incident. Several technicians had attended site and worked on the appliance depending on who was available when the appliance locked out. During an after hour’s call a service technician adjusted the flame sensor and ignition rod, replaced some weights that appeared to have fallen off the barometric damper, and reset the appliance. He said it appeared to be operating fine when he left site, however he was called back to site the next day for another lockout. At this point he again adjusted the flame sensor and spark rod, and reset the appliance. He monitored the appliance through three cycles, however on the fourth cycle the appliance had a very hard light off which damaged the appliance, and venting system.

Facts and Evidence

Site Observations

  • When I attended site the service contractor had already cleaned up most of the damage, and altered the scene during the repair process. (Figure 3)
  • The interior of the boiler was clean and showed no signs of sooting or incomplete combustion that would indicate the boiler was having ignition or flame issues prior to the incident. (Figure 4)
  • The burner head for the boiler was examined and determined to be free of cracks or defects which may have caused poor mixing and possibly a hard light off. (Figure 5)
  • The safety shut off valves for the appliance were leak tested and holding tight, disassembly of the safety shut off valves found no damage to the sealing surfaces.
  • The input controller appeared in good shape, and an interview with the service technician indicated that no adjustments were made to the end switch cams.
  • The dial for the flame safeguard indicated that it had entered the trial for ignition phase when the hard light off happened, so a spark would have been present in the boiler at the time of failure. (Figures 6-8)
  • The flame safeguard was examined and found to have a pitted contact on the ignition contact. However, testing of the flame safeguard contacts determined that this would have only affected when the ignition spark would have come on, and would have had no effect on when the gas valves would have opened. Therefore reducing the possibility that the flame safeguard was at fault. (Figures 9-11)
  • The weights having been knocked off the barometric damper would indicate that other hard light offs may have occurred during the lock outs during the previous weeks.

Interview With Local Subject Matter Expert

  • An interview with the local subject matter expert for this boiler indicated that if there had been adjustments to the gas supply pressure or low fire start end switch, hard light offs could result in this piece of equipment. However the boiler was removed from site before I was able to determine if adjustments were made to the appliance gas pressure regulator.

Interview With Service Technician

  • Interviews with the service technician who was on site indicated that adjustments were made to the spark and flame rods to adjust them within manufacturers specs.
  • When asked if any safeties were bypassed during his troubleshooting process, the technician told me that none were observed by him.
  • When asked if any adjustments were made to the low fire start cam on the input control motor, he told me none were observed by him.
  • When asked if any adjustments were made to the supply pressure regulator, he told me that none were observed by him.
  • He also told me that he was not the only technician to make adjustments to the boiler while it was having problems.

Causes and Contributing Factors

  • It is highly probable that a hard light off resulted in an explosion within the combustion chamber and the detachment of the burner, access door, and appliance venting from the boiler.
  • It is possible that adjustments were made to the boiler by someone other than the technician who was on site that may have caused the hard light off.
  • Something may have also failed within the flame safeguard system that allowed the boiler to light off before the “low fire start” was achieved causing gas to build up within the boiler and ignite.

For full report, including supporting images, download the PDF.

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