Health and Safety
Preventing a dust collector inlet explosion
By John Bachynski
June 19, 2018 - Dust collectors containing combustible material are required to have an explosion backflow preventor installed on the main inlet duct to prevent an explosion occurring inside the dust collector from travelling back into the building.
By John Bachynski
In industry, dust collectors account for 42 per cent of explosions, which makes them a high risk for potential injury and damage to a facility where inlet duct explosion protection is not installed. In a recent survey for wood industry dust collectors, over 75 per cent of dust collectors did not have inlet explosion protection. This poses a considerable risk to the facilities and workers. In most cases, the owner/operator of the facility is unaware of the risk as the installed systems were not engineered properly and/or inspections of the installation did not identify the safety deficiency. Regardless, the installation of an inlet duct explosion backflow preventor is critical to the safety of the facility and personnel and required by the NFPA (National Fire Protection Association), which is part of the building code of Canada.
[This article is part of our 2018 Dust Safety Week coverage. Find more articles here.]
Before considering a purchase of an inlet dust explosion protection device it is essential to provide the explosion characteristics of the dust being handled and the explosion safety features of the dust collector. The key explosion characteristics include:
- Kst (Deflagration index): power of the dust explosion
- Pmax: the potential maximum pressure which the dust can reach without protection
- Pred: The reduced pressure expected in the dust collector based on the explosion protection system installed ie. Pressure relief vents
It is essential that a chosen duct inlet explosion protection system has sufficient strength to withstand the pred that is being achieved at the dust collector. For round dust collectors the pred can be 8 psi, which would require the inlet explosion protection valve to be rated for 8 psi. For example, if the inlet duct explosion protection valve rating is for only 4 psi, it would fail if the pred of the dust collector is 8 psi. Conversely if the pred of a square dust collector was 2 psi, the 4 psi valve would be OK. This is why it is essential to know the pred of the dust collector and ensure the inlet duct explosion protection valve is at equal or greater pressure rating. Also to be considered is the ducting strength between the dust collector and the inlet explosion protection valve. If the pred is 8 psi and the duct strength is only 6 psi, the ducting would fail with a resulting unpredictable fire ball.
The main types of inlet duct explosion protection are passive or active.
The most common passive type of inlet duct explosion protection is a flap type, valve which is held open by the suction of air flow towards the dust collector. When an explosion occurs in the dust collector, a leading pressure wave travelling ahead of the fire ball inside the duct, causes the flap to shut and a lock mechanism prevents the flap from re-opening. Each manufacturer has a specified minimum distance from the dust collector in order for the valve to shut properly. Current sizes for passive protection are up to 50 inches (very expensive and heavy) and each size must be third-party tested and be supplied with the testing certification. This ensures that the valve has sufficient strength to withstand an explosion at the dust collector pred. A common mistake in selecting an inlet duct explosion protection valve is to not correctly specify the pred. As the valves designed for higher pressure are more expensive, it is always a temptation to purchase a lower cost valve, however if the pressure rating does not meet the dust collector pred, the installation of the lower pressure rating valve can become a major hazard as it would likely blow apart during an explosion with the potential of flame and steel being projectiles. In the case of larger inlet ducts, exceeding 50 inches, it may be required to split the line into two smaller lines, install two smaller NFPA-approved explosion protection valves and then re-connect the line back to the original size. Another consideration is the cost to have an access platform/ladder installed so that the explosion protection valve can be maintained and inspected on a regular basis. Before this option an active protection system such as chemical suppression could be considered.
Active isolation by chemical suppression is the installation of a charged canister of inert dust, which, upon pressure detection, explodes the inert dust into the ducting and extinguishes the fireball thereby eliminating the pressure rise. This technology works similar to air bags in a car, where the activation of the impact sensor explodes a gas into the airbag before the occupant hits the windshield. On larger ducts where inlet explosion protection and access platforms can exceed $50,000, chemical suppression can be a viable option. Before proceeding with this option, be sure to investigate the regular inspection requirements and costs. NFPA requires that quarterly inspections are required until such time that the systems can be proven to be working effectively, at which time the inspection interval can be increased to once every two years.
Regardless of which inlet duct explosion protection system is chosen, NFPA requires the correct design and installation by engineers and contractors who have experience with the associated hazards. Using cookie cutter or one-size-fits-all approach can be both ineffective and also have potential fatal consequences when done incorrectly.
John E. Bachynski, P.Eng, is the president of EPM Consulting Ltd.