Choosing Between Passive and Active Explosion Protection Solutions

These conditions include:

Fuel in the form of finely-divided combustible dust

An ignition source which can range from a welding spark to a lightning strike

An oxidizing agent, which is typically the oxygen in the air

Confinement that causes a pressure buildup during the incipient stages of the explosion

Dispersion of the dust into the airstream.

Process equipment most typically associated with dust explosions includes dust collection, particle size reduction equipment, dryers, blending and mixing, storage hoppers or silos, and mechanical conveying such as bucket elevators. These vessels can have dust in suspended form, either during normal operations or in an upset condition. Once the particulate is suspended into a dust cloud, all it takes is an ignition source to initiate the deflagration.

Ignition control, proper housekeeping of residual dust and continuous safety training of plant personnel are all critical to help prevent an explosion from occurring under normal operating conditions. Unfortunately, abnormal conditions that result in an explosion can occur in any process line. This is why standards such as NFPA 61 (Agricultural and Food Processing Facilities), NFPA 654 (Combustible Particulate Solids) and NFPA 652 (Fundamentals of Combustible Dust) require the use of explosion mitigation techniques for vessels subjected to an explosion threat. NFPA 68 (Deflagration Venting) and NFPA 69 (Explosion Prevention Systems) lists different mitigation methods that can be employed to deal with an explosion threat. The most common techniques include explosion venting, explosion suppression, and explosion isolation. These protection methods may be achieved through passive, active, or in the case of isolation, or both.

Passive Protection - Explosion Venting

Explosion venting is a low-cost and simple passive explosion protection method. It involves having a weak panel on the vessel, which can provide pressure relief during an explosion. However, it does not extinguish the flame and may lead to post-explosion fires. There is also a risk of fire propagation through interconnected ducts, causing additional explosions or fires. Despite these limitations,

Explosion venting can prevent catastrophic damage and loss of life. When considering this method, the following factors should be taken into account:

• Potential fire propagation
• Dust toxicity
• Fireball direction
• Thermal effects
• Reaction forces

By evaluating these factors, facility owners and managers can determine if explosion venting is a suitable dust explosion mitigation solution.

Explosion Relief – Indoor Vessels

For indoor explosion relief systems, venting the duct to the outdoors is crucial to prevent hazardous gas and dust buildup. This, however, can have an impact on system design, as flame ejection may be more intense, requiring a larger vent area or a stronger vessel. NFPA 68 provides guidelines for the design of safe and effective explosion

relief systems, so facility managers should consult them as well as qualified professionals when determining the best indoor vessel application solution.

Flameless Explosion Vents

Flameless explosion vents allow indoor venting without ducts, although NFPA 68 limits volume and quantity per vessel. They suppress the deflagration but hot air, combustion products and steam may still eject from the vessel, so a safe zone may be necessary to protect personnel and equipment. There are two types of flameless explosion vents:

• Spring: Spring-type flameless vents use a spring mechanism to control pressure and flame release, and they are reusable and cost-effective for frequent explosions. They can work well with high PRED vessels and may require no replacement parts. However, suitability depends on the application and requires case-by-case evaluation.

• Rupture: Rupture-type flameless vents use a predetermined rupture point to release pressure and flame. There are two main types: center fold and perimeter opening. They are used in low-frequency explosion scenarios where a reusable vent is not needed.

Safe zone size and location depend on venting process characteristics and potential hazards. Venting systems should meet relevant standards and regulations like NFPA 68 for safety and effectiveness.

Active Protection - Explosion Suppression

Active explosion suppression rapidly detects and suppresses explosions using a dry chemical agent while also isolating connected ducts. The fast response time reduces the risk of damage to equipment and life safety. Components include Pressure Detection, Control Unit, and Explosion Suppression Extinguisher, which work together to sense pressure changes and rapidly discharge the extinguisher to suppress the deflagration and isolate the explosion.

Passive Explosion Isolation – Flap Valves

Flap isolation valves, or flap valves, isolate and contain dust explosion pressure and flames. Tested and certified to specific Kst ranges, they prevent explosions from spreading throughout ducting systems. When activated by the pressure wave generated by an explosion, the flap valve redirects the pressure and flames into a separate chamber or vessel.

Proper installation, including a locking mechanism, and adherence to ducting requirements and ATEX standards for safety are crucial. However, they are not recommended for use in gas or hybrid applications.

Passive Isolation Valve – Clean Air

This float-style passive isolation valve is used for clean air applications, such as dust collector exhaust ducts and mill air intake ducts with light dust loading. During a deflagration, the pressures generated push an internal poppet inside the valve to the closed position, creating a mechanical barrier

to prevent the spread of flames and pressure from a dust explosion. These valves are available in sizes DN100 to DN600 and require no external power source, making them easy to install and operate. They are not suitable for heavy dust concentrations.

Passive Isolation - Air Inlet Isolation

IsoDisc TM is a certified passive device that isolates and contains pressure and flames resulting from dust explosions. With an explosion direction opposite to the air flow, it is designed for clean air service applications, such as for makeup air intake ducts associated with milling operations.

These devices are available in DN50 to DN400 pipe connection diameters. IsoDisc TM conforms to EN15089 standards and is an effective and versatile solution for industrial facilities looking to protect against deflagration flame ejection through these paths.

Passive Isolation - Rotary Valve

Rotary valves designed to meet NFPA 69 standards are often used on the discharge of process vessels to prevent dust explosion propagation to equipment downstream.

They require certified PRED, a metal body with at least six vanes, less than 0.008 inches clearance between the flights and the internal housing, and interlocking with other safety systems

Once installed, rotary valves create a mechanical barrier against pressure and flames. Although these devices are effective for isolating explosions, they should not be relied upon exclusively. Instead, they should be treated as part of a comprehensive safety program to protect personnel and equipment.

Active Explosion Isolation

Active explosion isolation, when designed to meet NFPA 69 standards, quickly detects and isolates pressure and flames from a dust explosion. It has three components: detection (pressure or optical), control, and isolation device (fast- acting mechanical valve). The isolation device prevents the explosion from spreading, and some systems also include a suppressor. High-risk areas such as processing facilities use active explosion isolation to minimize the risk of damage and injury.

Active Explosion Isolation Barrier - Mechanical Active mechanical barriers, which are used on strengthened pipes or heavy-gauge ducts, provide a barrier for flame and pressure in the event of a dust explosion. They are frequently applied in high-value processes or equipment, such as pharmaceutical applications, to prevent material contamination resulting from dust explosions. Actuated float valves and actuated pinch valves are other types of active mechanical barriers that quickly shut off the flow of combustible dust or fuel to prevent the spread of the explosion.

Which is Better: Active or Passive?

Choosing between active and passive explosion protection systems requires careful consideration of various factors such as the nature of the process, the likelihood of an explosion, and the potential consequences.

While active systems such as suppression and isolation systems offer a rapid response to contain and mitigate an explosion, they require regular maintenance and may not be suitable for certain processes. On the other hand, passive systems such as explosion venting offer a more cost- effective and low-maintenance solution but may not be as effective in controlling the spread of an explosion.

Factors to consider when choosing between Active and Passive Protection include, but are not limited to:

• Safe Area for flame ejection will impact use of standard explosion vents
• Thermal ejection risk, as thermal ejection distance will exceed flame ejection
• Toxicity/Environmental, as some material cannot safely be released to the outside or safely exposed to personnel
• Costs – both initial and on-going
• Material buildup risk (functionality, hygienic), especially important for passive devices that cannot tolerate higher dust loading, and for food and pharm applications
• Post-event impact – how long after successful explosion control will it take to rest protection means and get process back in operation
• Explosivity characteristics of dust (Kst, Pmax), as different protection techniques will have varying limits of tested applicability
• Vessel strength, especially where available vent area or an associated vent duct may result in higher PRED values than an active suppression system

• Duct size (isolation), as passive isolation barriers are restricted to certain sizes, depending on the product and the manufacturer
• Dust loading (isolation), as this is a common restriction for passive isolation but not a consideration for active isolation.

When deciding between passive and active protection, it is suggested to:

• Evaluate each application for protection of primary vessel and stopping propagation through interconnections to other vessels.
• Determine if use of protection measure enhances safety for personnel, or does it create a danger zone to be managed
• What is the overall cost of ownership, both initial investment and lifetime maintenance
• How will operational downtime be affected by protection measure, including in a post- deflagration situation
• Does the proposed solution have independent third-party approvals for the intended use of this product, to satisfy the Authority Having Jurisdiction?

Ultimately, the decision to choose between active and passive systems should be based on a thorough risk assessment and consultation with experts in the field to ensure the safety of personnel and facilities.

---

Rob Markle
Central Region Sales Manager, IEP Technologies

As the Central Region Sales Manager for IEP Technologies, Rob Markle brings 39 years of experience in industrial fire and explosion protection to the table. He has spent the last 22 years with IEP Technologies where he is responsible for the sales, application, and design of Explosion Protection solutions in compliance with NFPA Codes and OSHA Directive.

Rob is a valued member of the NFPA with extensive knowledge of the explosion protection requirements for various industry-specific processing facilities that handle combustible vapors and particulate solids. He also serves as a volunteer for a task group on NFPA 30B and as an alternate for the NFPA Technical Committee.