Dangers of the Unseen: Understanding Ductwork Accumulation

Everything seems fine. Your systems are running smoothly, nothing looks broken or out of the ordinary. But lurking beyond eyesight, things like combustible dusts and condensates could be building up within your system. And when that happens, safety is jeopardized and employees’ lives are endangered. From flash fires to explosions, several deadly but preventable incidents have occurred across multiple industries due to dust accumulation (for a list of examples, check out this article from Nilfisk). Understanding the combination and characteristics of materials moving through your system and the necessary conveying velocities can help mitigate these risks and help you meet NFPA standards and OSHA regulations.


Several actions can prevent dust accumulation incidents from happening:

  • Performing a hazard analysis or study of the system
  • Increasing the design duct velocities
  • Testing and validating the system before commissioning
  • Gathering historical data and testing the materials to understand the necessary requirements, including conveying velocities, for the highest level of safety

Ductwork accumulation occurs based on insufficient conveying velocities in the ductwork system. Typically, poor ductwork sizing, expansions without the provision of additional airflow, capped branch lines, and improper modifications to the ductwork system can cause insufficient conveying velocities. In many cases, our team has seen instances where a machine center will have a small connection stub that is connected to an expansion fitting. For example, a table saw might have a two-inch diameter connection and then a fitting is added to increase that diameter to a larger, more readily available size such as a four-inch diameter. The increase in diameter reduces the ductwork velocity of the air stream through the downstream fittings. This reduction in airflow can cause a loss of momentum that leads to the dust falling out of suspension and settling in the bottom of the ductwork. In some cases, we have seen two-inch diameter connections expanded multiple times up to a six-inch diameter line downstream of the connection to the machine. In every case, a simple inspection of the ductwork showed that material had packed and plugged the ductwork allowing no airflow to the connection point and reduced conveying velocities downstream of the plugged ductwork. These situations can be hazardous and should be corrected immediately to meet NFPA and OHSA compliance standards.

Accumulations due to low ductwork velocities are typically seen at expansions, elbows, and in the worst case, through the entire ductwork system. Common ways of combatting these situations are to size the system’s blower to provide adequate airflow and pressure to keep the ductwork velocities at acceptable levels, or, if possible, increase the speed of the existing blower. Typically, the latter is not possible due to the size and class of the blower selected for the original design. You can also change the connection size or hood to allow consistent velocities without the need for an expansion fitting. Ductwork branch lines that are no longer needed should remain open and the airflow intake should be metered into the ductwork system to ensure the acceptable ductwork velocities are maintained. For larger ductwork modifications, consult an experienced engineer to obtain proper and compliant ductwork modification for the section or area in need of adjustment.

If your team needs help modifying or identifying issues with your dust collection system, call our team. We’re experienced, licensed, and ready to assist.

Chief Engineering, PLLC