Guidelines for choosing an explosion protection system for ...

1 Copyright, CSC Publishing, Powder and Bulk EngineeringCombustible dust hazards with the potential tocause an explosion are a fact of life in bulk solidsplants. To mitigate the effects of a dust explosion inyour plant, you need to correctly select an explosionprotection system for your dust collector and otherenclosed dust handling equipment. Part I of thistwo-part article describes major explosion protec-tion methods and the regulatory standards thatcover them. Part II in November will explain how tochoose a system with the most suitable and cost-ef-fective method (or combination of methods) foryour application.

2 While the Guidelines focus onchoosing an explosion protection system for a dustcollector, they can also be applied to choosing a sys-tem for other enclosed equipment and the ductworkbetween equipment. In the past decade, the bulk solids industry has seen theintroduction of many new explosion protection tech-nologies with advanced capabilities and smarter fea-tures. High-profile dust explosions in recent years havealso increased OSHA s emphasis on mitigating com-bustible dust factors make it more impor-tant than ever for bulk solids plant owners and processengineers like you to know which explosion protectionstandards and technologies apply to your combustible dusthazards and process requirements so you can select a suit-able protection method (or methods).

3 In a bulk solids plant, dust collectors are the equipmentmost commonly protected from dust explosions. Beforewe discuss how to choose the most suitable and cost-effec-tive explosion protection system for your collector, let slook at the basics of venting, isolation, and suppression ex-plosion protection kinds of explosion venting explosion relief vent-ing and flameless venting are available for dust collec-tors and other enclosed equipment that handles dust. (Alsobe aware that according to NFPA 654: Standard for thePrevention of Fire and Dust Explosions from the Manufac-turing, Processing and Handling of Combustible Particu-late Solids,2equipment with venting must also be isolatedto prevent flame from propagating between connectedequipment; find information on isolation methods in thenext section.)

4 explosion relief venting. explosion relief venting, one ofthe most widely used methods for mitigating dust explo-sions, requires one (or more) explosion relief vent installedon the wall of a process vessel. The vent consists of a mem-brane that s constructed of a material weaker than the ves-sel wall; examples are shown in Figure 1a. During a dustexplosion s incipient (beginning) stage, the vent rupturesand directs the explosion s overpressure, flame, burnt andunburnt material, and other combustion by-products awayfrom the vessel to a safe location, as shown in Figure explosion relief vent is designed to ensure that the ex-plosion s pressure rise doesn t exceed the vessel s pressureshock resistance.

5 These vents are designed according toprocedures in NFPA 68: Standard on explosion Protec-tion by Deflagration provides anequation for estimating the vented fireball s size; from thisinformation, you can calculate the safe distance requiredin front of the vented vessel to protect workers, equipment,and the building structure from the ejecting venting. A relatively new technology, flame-less venting protects indoor equipment from dust explo-sions by combining an explosion relief vent s weakmembrane with a mesh trap that arrests flame and retainsparticles. Examples are shown in Figure 1b. Like an explo-sion relief vent, the flameless vent s membrane, installedon the process vessel, ruptures during a deflagration.

6 Butunlike with a relief vent, the deflagration s overpressure,flame, and material discharge through the membrane intoGuidelines for choosing an explosionprotection system for your dust collector Part IEmre Ergun Fenwal protection appeared inPBEO ctober2011 Copyright, CSC Publishing, Powder and Bulk Engineeringthe mesh trap, which prevents the flame and material fromdischarging into the surrounding area. Instead, the flame-less vent discharges onlyhot gas and overpressure. Asafety perimeter must be established around the flamelessvent to protect workers from this discharge. Like the explosion relief vent, the flameless vent is designedaccording to procedures in NFPA 68.

7 To ensure that theflameless vent can successfully protect the vessel, the ratioof room volume to vessel volume (that is, the ratio of the vol-ume of the room in which the vented vessel is located to thevessel s volume) must also be kept below the flameless ventmanufacturer s recommendations. Since any dust or dirtblocking the openings in the mesh trap would compromisethe vent s operation, the mesh must also be regularlycleaned to ensure that the surface is free of dust or dirt at vent manufacturers offer low-inertia, fire-re-sistant fabric covers to help keep the mesh surface isolation devices prevent a deflagration in aprocess vessel from propagating through a connectionsuch as a duct, chute, or conveyor to other equipment,where it could cause subsequent explosions.

8 The deviceswork by mitigating the flame propagation and pressurepiling between connected isolation devicecan be active or passive. An active device has detectioncomponents, including one (or more) explosion pressuredetector and flame detector, and controls; the detectors de-tect explosion pressure or a flame and send a signal to thecontrols to rapidly deploy the device. A passive device isself-actuated by the airflow from a deflagration. When toprovide an isolation device between connected processequipment is described in Subsection in NFPA isolation devices are typically chemical or me-chanical.

9 A chemical isolation device,as shown in Figure3a, is an active device that works by rapidly discharging achemical extinguishing agent, such as sodium bicarbonate,into connecting ductwork to mitigate flame mechanical isolation devicecan be either an active de-vice, such as a high-speed gate valve, or a passive device,such as a flap valve, as shown in Figure 3b. Millisecondsafter the active high-speed gate valve s detectors sense ex-plosion pressure or flame, the controls rapidly deploy amechanical barrier closing the valve s gate acrossthe connecting passive valve, which canhave a flap or float, is self-actuated by the airflow from adeflagration so it requires no detectors or controls.

10 This de-vice is typically used to isolate nuisance-dust-handlingequipment with relatively low dust suppression systems are often installed in appli-cations where it s not possible to safely vent an explosionaway from process equipment or where the process mate-rial is toxic or harmful to workers and the system detects an incipient dust explosion very soonafter ignition and discharges enough chemical suppressant(a chemical extinguishing agent) quickly enough into thedeveloping fireball in the equipment to extinguish allflame before a destructive overpressure (For adetailed illustration of the basic steps in suppressing a de-flagration, see reference 8.)