Protected Enclosure Pressure Resistance Requirements: The Big UnknownProtected Enclosure Pressure Resistance Requirements: The Big Unknown
January 21, 2020
Many particulate solids handled in industry (e.g. chemicals, polymers, food, wood/paper, metals, etc.) are, or can generate, combustible dusts. Dust collection systems are used to minimize the release and subsequent accumulation of hazardous combustible dust. While dust collection systems can help protect the workplace environment, these systems can introduce a risk by concentrating the most hazardous dust particles within an enclosure. A dust collection system may provide four of the five dust explosion pentagon conditions (i.e. fuel, air, containment and dispersion), with a competent ignition source being the only element missing.
NFPA standards provide requirements for the design of dust collection systems with respect to fire and explosion hazards. Explosion protection systems include both passive and active systems. An example of a passive system is deflagration venting coupled with a passive flap-style isolation device on a connected duct to prevent flame propagation back into the workplace. An active system could incorporate a chemical suppression system, triggered by pressure rise, which rapidly discharges an inert powder into the equipment and connected duct(s) to quench the developing fireball. Since dust collection systems are commonplace, the correct design and protection of these systems is of primary importance to protect personnel against fire and explosion hazards. These requirements are generally well understood and addressed by specialists who have a strong technical background in this area. Conversely, collection system users may not be able to easily translate these requirements into practical solutions, depending on their knowledge of combustible dust safety.
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Based on hundreds of Dust Hazard Analysis (DHA) studies performed across a range of industries, it is apparent that many explosion protection designs have weaknesses. Critical systems (e.g. deflagration isolation between connected pieces of equipment) or components (e.g. detection system that can trigger deflagration isolation) required by relevant NFPA standards are frequently not incorporated into the overall system or are improperly implemented. One such issue, which is the focus of this article, is the pressure resistance of the dust collection system components. The following items are also often either missing or deficient:
* Specific material explosibility data: Explosibility data (Kst, Pmax) for the dust being handled is required for the proper design of explosion protection systems and to ensure appropriate performance. Such data are often not available.
* Equipment design basis information: Application limits are not provided by the vendor to show that the explosion protection system is designed for the dust being handled. The dust explosibility parameters are of critical importance for the correct design of the explosion protection system components. This information is often not provided and, in a number of cases, the installation clearly violates the design basis (e.g. insufficient vent area, isolation device not located sufficiently far from the collector to be able to isolate, etc.).
* Restricted occupancy zone around vented equipment: Restricted occupancy (i.e. “exclusion”) zones are required in order to minimize the likelihood personnel would be present in the vicinity of a deflagration vent. Such zones are required to limit potential injury from the associated thermal and blast hazards. Furthermore, critical control panels (e.g. those for emergency shutdown of a dust collector) are sometimes located within this hazard region, which can place operators at risk (e.g. if they are interacting with the controls to mitigate a fire in the collection system, which could transition into a deflagration without warning). It is common to encounter equipment with explosion vents without a restricted occupancy zone.
Protected Enclosure Pressure Resistance: Why it Matters
Figure 1 illustrates the pressure history inside a collector, both with and without venting. During a vented deflagration, venting panels start to open at a predetermined pressure (Pstat). The pressure within the enclosure continues typically continues rise after the vents open; the resulting reduced deflagration pressure, which is the maximum expected internal pressure in the enclosure (Pred), can be substantially higher than Pstat. Pred can range from a few psig to several atmospheres depending on the specifics of the application (e.g. enclosure volume and geometry, vent area, vent opening mechanism, dust parameters). Figure 2 shows the reduced overpressure Pred with the suppression of a dust deflagration. The protected equipment therefore needs to withstand the reduced deflagration pressure (i.e. Pred) with some margin of safety.
In many cases, the explosion protection equipment vendor delegates the responsibility to determine the pressure resistance of the dust collection system components (e.g. collector, ducts, connections, etc.) to the owner/operator. This requirement may be embedded in the proposal and/or design drawing and, in many cases, the operator may not even be aware that they have accepted this responsibility. An explosion protection equipment vendor may specify a system that places a significant pressure resistance requirement on the protected equipment (e.g. a Pred of 5 psig), which most heritage equipment is not designed to withstand. The pressure resistance of heritage systems is frequently unknown.
See Figure 1
See Figure 2
Pressure Resistance Requirements in NFPA 68 and 69 Standards
The design of explosion protection systems is governed by the National Fire Protection’s Standard on Explosion Prevention Systems, NFPA 69.1 The life safety objectives of NFPA 69 are to prevent enclosure rupture and/or minimize injury in adjacent areas.2 The General Prescriptive Requirements of NFPA 69 specify that, for enclosures protected by deflagration venting, suppression, isolation, or containment:3
“The enclosure strength (Pes) of the protected equipment shall be determined and all pertinent calculations or test information, acceptable to the authority having jurisdiction, shall be documented and certified by a licensed professional engineer”
Furthermore, this portion of NFPA 69 also specifies that the explosion pressure developed accounting for the protection system employed (Pred) shall not exceed 2/3 of the ultimate strength for the enclosure, provided deformation of the equipment can be tolerated, and shall not exceed 2/3 of the yield strength of the enclosure, where deformation cannot be tolerated. The NFPA Standard on Explosion Protection by Deflagration Venting (i.e. NFPA 68)4 provides the same guidance.5 The yield and/or ultimate enclosure strength must therefore be defined in order to design or qualify an explosion protection system under the prescriptive requirements of both NFPA 68 and 69.
For systems protected by explosion suppression, a design input specifically identified in NFPA 69 is the “pressure resistance of protected enclosures”.6 This section of the code identifies that the owner or operator is responsible for providing this information.7 NFPA 69 states that “the pressure resistance of the protected enclosure shall not be less than the maximum suppressed deflagration pressure.8 When the pressure resistance of the vessel is not available from the manufacturer, the owner or operator should determine this pressure resistance by calculation based on conditions of the actual enclosure. Note, however, if the owner or operator chooses to use generic values for typical construction, this could result in enclosure failure.”9
If a deflagration isolation system is to be used for explosion protection, Chapter 11 of NFPA 69 states that “the piping, ducts and enclosures protected by an isolation system shall be designed to withstand estimated pressures.”10 An isolation system design input specifically identified in this section of NFPA 69 is the “design strength” of the protected equipment.11 This section of the code identifies that the owner or operator is responsible for providing this information.12
The Performance-Based Design Option of NFPA 69 specifies that “Prevention and control systems shall limit the reduced pressure (Pred) within an enclosure” to meet the standard’s life safety objectives,13 (i.e. prevent enclosure rupture and/or structural failure). Hence, even for a performance-based design, the enclosure capacity must be defined in order to show that the life safety goals have been met.
The explanatory material9 in NFPA 69 identifies that FM Data Sheet 7-7614 is one source of generic equipment pressure resistance information frequently used in designing explosion protection systems (this is the only information source identified by NFPA 69). However, regardless of the availability of generic pressure resistance estimates, it should be very clearly understood that the NFPA 69 standard:
* Identifies that the specification of the pressure resistance of the protected enclosure is the responsibility of the owner/operator
* Directs the owner/operator to determine this pressure resistance by calculation based on the actual enclosure conditions
* Identifies that the use of “generic values for typical construction” could result in enclosure failure.
Hence, in addition to avoiding the inherent uncertainty associated with a generic pressure resistance value and the associated potential for catastrophic failure, determining the enclosure-specific pressure resistance may allow design conservatism to be reduced. This may also allow increased operational flexibility for an existing system (e.g. the use of materials with more severe combustibility characteristics, reduction in required vent area, etc.).
Example: Catastrophic Failure of a Suppressed Dust Collector
Figure 3 shows the result of an analysis performed by BakerRisk that determined that the roof-to-shell seam of a baghouse would fail under the suppressed load incurred from the activation of a chemical suppression system. The suppression system had been designed based on generic pressure resistance guidelines. Even an inadvertent (i.e. false positive) activation of the system could have incurred personnel injury and loss of an asset.
See Figure 3
As discussed, one important aspect of an effective DHA should be to identify potential issues with explosion protection system designs that otherwise may not be captured. In many cases, the issues may exist due to heritage equipment being repurposed, or simply due to the lack of understanding of the hazards incurred by the operations and processes. The relevant NFPA standards provide thorough guidance that a DHA team can use to identify such issues. One requirement that may not be obvious, and a common deficiency with systems protected by explosion protection, is to define the equipment pressure resistance. The pressure resistance of an enclosure (e.g. dust collector, cyclone, etc.) and connecting equipment (e.g. pipes) protected by an explosion venting, suppression and/or isolation system must be defined in order to ensure that the protection system design meets its intent (i.e. prevents catastrophic failure). The NFPA 69 standard explicitly identifies that the determination of the enclosure pressure resistance is the responsibility of the facility owner or operator. Moreover, it provides an explicit caution that the use of “generic values for typical construction”, such as those available in FM Data Sheet 7-76, may result in the failure of the protected enclosure. Last, and of lesser importance, the development of an enclosure-specific pressure resistance may allow a less conservative design basis and/or increased operational flexibility for an existing system.
Philip Parsons, principal consultant, BakerRisk, has experience with design, analytical techniques, and hands-on testing. His work has consisted of performing siting studies and explosion hazard analysis projects in which he has predicted internal and external blast loads from deflagrations and detonations. Parsons serves on both the NFPA 652 and NFPA 654 committees.
Jérôme Taveau, senior consultant, BakerRisk, has more than 18 years of experience in process safety management for major hazards industries. He is a recognized expert in the field of industrial explosions who has authored more than 20 peer-reviewed journal articles and given more than 70 presentations in 15 countries. Taveau has served in NFPA 61, 68, 69, 652, 654, and correlating committees on combustible dusts. For more information, call 210-824-5960 or visit www.bakerrisk.com.
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1 National Fire Protection Association (2019) Standard on Explosion Prevention Systems (NFPA 69), Quincy, MA
2 NFPA 69, 2019 Edition, Section 4.2
3 NFPA 69, 2019 Edition, Section 6.3.4
4 National Fire Protection Association (2018) Standard on Explosion Protection by Deflagration Venting (NFPA 68), Quincy, MA
5 NFPA 68, 2018 Edition, Section 6.3.1
6 NFPA 69, 2019 Edition, Section 10.4.3.2 (4)
7 NFPA 69, 2019 Edition, Section 10.4.3.3
8 NFPA 69, 2019 Edition, Section 10.2.3
9 NFPA 69, 2019 Edition, Section A.10.2.3
10 NFPA 69, 2019 Edition, Section 11.1.6
11 NFPA 69, 2019 Edition, Section 126.96.36.199 (3)
12 NFPA 69, 2019 Edition, Section 188.8.131.52
13 NFPA 69, 2019 Edition, Section 5.2.2
14 Factory Mutual Global (2017) “Prevention and Mitigation of Combustible Dust Explosion and Fire,” FM Data Sheet 7-76, May 2008 Edition, April 2017 Interim Revision
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