New Explosion-Relief Standard Affects Furnaces

NFPA 86 “Standard for Ovens and Furnaces,” which has been applicable to industrial-heating equipment for decades, specifies explosion-relief guidance that differs from that given in the new NFPA 68 standard. Consequently, furnace makers and users alike should carefully evaluate the provisions of both of these consensus safety standards in order to determine their best approach for compliance.

NFPA 68 Applicability

Explosion venting has been a common feature of ovens, furnaces and other equipment for decades. The new NFPA 68 standard does not, however, specifically address whether a particular piece of equipment is required to install deflagration venting. It only addresses how such venting should be designed and maintained if its installation is required by other standards. It directs readers to NFPA 30, 33, 35, 654 and other standards to assess whether a particular piece of equipment must incorporate explosion relief. Examples of non-heated equipment types that may need to comply with NFPA 68 are baghouses, storage bins, spray booths and fluidized-bed processors. Interestingly, NFPA 68 does not reference NFPA 86 – “Standard on Ovens and Furnaces,” ostensibly because of the inconsistencies between the two.

NFPA 86 Applicability

The scope of NFPA 86 explicitly includes “…ovens, dryers, furnaces, thermal oxidizers and any other heated enclosure used for processing of materials.” As such, its requirements are applicable to a large number of industrial-heating-system types, with a few notable exceptions (steam boilers, solid-fueled furnaces and listed ovens smaller than 400,000 BTU/h). The NFPA 86 standard also states that “the terms ovens, dryers and furnaces are used interchangeably and also apply to other heated enclosures used for processing of materials.”

Importantly, NFPA 86 contains many provisions that serve to prevent explosions in the first place, and furnace makers and users should be mindful about adhering to all such relevant safeguards. Should the preventative measures fail, however, the purpose of explosion venting is to relieve the internally generated combustion pressure in a manner that causes no injuries and little or no property damage.

As with most NFPA standards, older equipment built before a standard took effect is not required to meet new requirements unless its configuration or operation has been modified subsequent to the manufacture date and after the adoption of the newer standard. This is the so-called “grandfather” clause, and it represents the state-of-the-art at the time the equipment was built and commissioned.

The Dilemma

Typically, oven manufacturers and operators have relied on the requirements of NFPA 86 for sizing explosion vents, even though its guidance differed substantially from the sizing formulas given in NFPA 68. Now that NFPA 68 has become a standard and contains mandatory language, oven makers and users are faced with a dilemma. For new or altered ovens in 2007 and beyond, users must now choose whether to comply with the formula given in NFPA 86, which is based on “industry experience”[1] or the formulas given in NFPA 68, which are based on engineering application of scientific research and testing.[2,3,4]

Fortunately for oven users, compliance with both NFPA 68 and NFPA 86 is possible if the more conservative of the two standards (i.e. the one requiring greater vent area) is followed. The evaluation of which standard is more conservative, however, depends greatly on oven size and configuration. This article offers some simple tools that can help identify which standard is more conservative for a particular oven.

NFPA 86 Methodology

NFPA 86 (2007) presents the simplest formula for explosion relief in an oven or furnace. As will be shown later, this method computes the more conservative amount of vent area for some furnaces, but not all.

Explosion relief shall be designed as a ratio of relief area to oven volume.
The minimum design shall be at least 1 ft²of relief area for each 15 ft³of oven volume.

NFPA 68 Methodology

NFPA 68 (2007) provides specific instructions for sizing deflagration vents to protect enclosures where the combustible material may be gases, vapors, mists or dusts and where the enclosure may be elongated or compact, low-strength or high-strength. The mass of the vent device, the presence of ductwork downstream of the vent and corrections for partial volume deflagrations are also addressed. The basic formula for vent area (A_v) depends on three factors – the enclosure surface area (A_s), a combustion rate or “venting parameter” (C) and the maximum allowable pressure or “reduced pressure” obtained during the vented deflagration (P_red).

A_v= C•(A_s)/_√P_red

The reduced pressure (P_red) and enclosure surface area (A_s) are functions of the mechanical design of a particular enclosure, whereas the venting parameter (C) is a function of the fundamental burning velocity of the fuel present.

The formulas are developed such that As incorporates all the internal surfaces of the enclosure where the combustible constituents may accumulate. In indirect-fired ovens, two separate computations of As may be necessary – one for the chamber where the hot combustion products flow (and where fuel could accumulate in the case of a burner upset) and one for the work chamber (assuming volatiles from the product can exceed 25% LEL).

Similarly, P_redis a function of the enclosure’s mechanical strength, which is information that the oven manufacturer should be able to provide as a specification of their product. In terms of NFPA 68, P_redis defined in terms of the “ultimate strength” of the enclosure, which is the pressure that results in failure of the weakest structural component of that enclosure. Because it is the pressure that will actually be developed during a vented deflagration, P_redshould be chosen based on the ability of the enclosure to withstand such pressure (plus a safety margin) without structural failure.

Comparison of Methods

A parametric analysis comparing the methodologies of NFPA 68 versus NFPA 86 is shown in Fig. 1. The plot shows vent area per unit volume as a function of oven length for a default oven and three variations, according to the NFPA 68 methodology. The default oven was selected to be 10-feet wide by 10-feet high with lengths varying from 10-1,000 feet. The effects of “frontal area,” “enclosure strength” and “slenderness” (height-to-width aspect ratio) are shown parametrically in Fig. 1 by the indicated variations in FrontArea, Pred and AR, respectively. A horizontal line is inserted into the plot for reference to the NFPA 86 methodology – currently 1 ft²/15 ft³. In order for an oven or furnace to comply with the NFPA 86 explosion-relief sizing guidance, it would need to incorporate vent area equal to or greater than the 1 ft²/15 ft³threshold. Horizontal lines are shown for one vent-area ratio that exceeds the NFPA 86 guidance (1 ft²/7 ft³) and for one that fails to meet it (1 ft²/25 ft³).

The first observation we see is that required vent-area ratio decreases with oven length for the NFPA 68 computations, so that a 10x10x10-feet cube-shaped oven needs more vent area per unit volume than a 10x10x100-feet box-shaped oven. The figure also indicates that a crossover point exists for the default oven at a length of approximately 20 feet. For longer ovens, therefore, the NFPA 86 standard becomes slightly more conservative than the NFPA 68 standard. For very long ovens, the NFPA 68 curves reach an asymptotic value where additional length (and volume) requires a proportional amount of additional vent area.

For the smaller-area oven (FrontArea=25 ft²), we see that the NFPA 68 computation requires almost twice the vent area per unit volume as the default oven, and it also requires substantially more area than the NFPA 86 requirement. Conversely, for ovens with larger frontal area (all other factors being equal), the NFPA 86 vent-area computation is more conservative than the NFPA 68 area.

Considering the lower-strength enclosure (P_red=20 inchWC), we see that NFPA 68 computes nearly 50% more vent area per unit volume than the default oven, clearly much more conservative than NFPA 86 method. It is noteworthy that enclosure strength plays no role in the NFPA 86 computation[5] even though it is intuitive that a weaker enclosure should be equipped with more vent area than a stronger enclosure of the same size.

Finally, we see that the “slenderness” of the enclosure has a net positive effect on the vent area requirement in the NFPA 68 formula. For two ovens with the same frontal area (100 ft²), the one with the taller aspect ratio (4 to 1) requires about 20% more vent area than the equal-dimensioned oven. While direct-fired ovens typically are not built with such a degree of slenderness, this example is illustrative of some indirect-fired ovens where the combustion gases flow in channels with aspect ratios of 10 or more. Again, the NFPA 86 formula has no means of accounting for a slender enclosure, which has the same volume as a non-slender enclosure, but presents substantially more surface area upon which the internal-pressure forces can act.

Roof-Panel Venting

One popular means of providing explosion relief for long furnaces is to incorporate “floating” roof panels that are displaced upward under the influence of excessive internal pressure and thereby provide a path for combustion gases to escape. NFPA 68 expressly allows this practice[6] as long as it is implemented consistently with personnel safeguarding and other protections. Figure 2 shows that such designs can theoretically provide sufficient NFPA 68 venting for some of the furnaces analyzed above, albeit with significant “lift” of the panel required (3-foot lift in the example shown). Proper restraint of wall or roof panels used for explosion relief necessitates that the panel not become a projectile hazard. Tethers that are too short may restrict the vent area, however, so they must be sized appropriately to provide sufficient vent area around the perimeter of the panel.

As seen in Fig. 2, the use of the designated roof panels to provide venting easily meets NFPA 68 guidance in two of the parametric furnace designs shown over the range of lengths plotted. However, the NFPA 68 guidance cannot be met by roof-panel venting for the slender or low-strength examples for virtually all of the lengths plotted.

Both weight and inertia must be considered as forces opposing the upward displacement of the roof panel for any practical furnace design. Appendix G in the NFPA 68 standard provides a method for upward adjustment of the vent size as a function of increased vent-panel mass.

Readers are cautioned that the enclosure strength (and desired P_red) must be determined independently for each enclosure. Enclosure strength is a function of many parameters, including wall material, size, thickness, attachment and reinforcement. Readers should not assume that an enclosure-strength calculation performed for one furnace is applicable to another unless they are truly identical. NFPA 68 and NFPA 86 both provide cautions about the location of explosion vents so that personnel are not put at undue risk.

Conclusion

The recent publication of NFPA 68 (April 2007), with its more scientific approach to explosion relief, creates a challenge and an opportunity for furnace manufacturers and operators. We have identified differences between the sizing methodologies of NFPA 68 and NFPA 86, and we have also shown that it is possible to adhere to both standards by selecting the methodology that gives the larger (more conservative) vent area for each specific enclosure. A thorough review of the new NFPA 68 standard is recommended for all interested readers.IH

For more information:Dr. Rick Martin is a senior managing engineer and Dr. Erik Christiansen is a managing engineer in the Thermal Sciences practice of Exponent Inc., 5401 McConnell Ave., Los Angeles, CA 90066; tel: 310-754-2720; fax: 310-754-2799; e-mail: rmartin@exponent.com; web: www.expo nent.com

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: combustion pressure, explosion vents, floating roof panels

SIDEBAR: NPFA 68 vs. 86

Example Problem:Consider a heat-treating furnace fired by natural gas with internal dimensions of 10 feet wide by 8 feet high by 16 feet long. The only combustible hazard is the fuel itself. The furnace is constructed of lightweight materials and lined with refractory ceramic-fiber modules. The furnace manufacturer specified that the maximum internal pressure should not exceed 15 inchWC. Would a floating roof design be adequate to vent this furnace according to NPFA 86 guidelines? Would such a design also adhere to NPFA 68 guidelines? If so, how long should the restraint tethers be? Which of the standards gives the more conservative result?

Answer:The volume is 1280 ft³, the roof area is 160 ft²and the combined internal surface area is 736 ft². The combustion-rate parameter for methane is 0.16 psi^0.5, and the reduced pressure is 0.54 psig (15 inchWC), per the manufacturer’s specification. We compute the nominal ratio of roof area to furnace volume to be 1 ft²/8 ft³so that roof venting could theoretically be used to vent this furnace in adherence to NFPA 86 (which requires a ratio of 1 ft²/15 ft³or greater – 85 ft²in this case). Coincidentally, the NFPA 68 formula (A_v=C·A_s/P_red^0.5) gives a result of 160 ft², which is exactly equal to the roof area. In order to meet NFPA 68 vent-sizing guidelines, the tethers for the roof panels would need to be at least 37 inches long (3.08 ft lift · 52 ft perimeter = 160 ft²vent area). The NFPA 68 formula gives the more conservative solution to this problem.