DISCLAIMER: Dr. Martin is not a representative of NFPA. He is a committee volunteer, and his opinions may or may not reflect the official position of NFPA.
This blog continues the questions from the previous blog posting.
Q:Does NFPA 86 require two Safety Shutoff Valves (SSVs) in series, or does it require the so-called “double-block and vent” arrangement?
A:NFPA 85, the Boiler and Combustion Systems Hazards Code, requires a “double-block and vent” on each main and pilot burner and has done so since the 1970s or sooner. Since the 1980s, however, NFPA 86 has only required two block valves in series. NPFA 86 is silent on the use of an automatic vent (or “bleed”) valve between the pair of block valves, although a manual test valve is required.
Q:Is “proof-of-closure” required on pilot-burner SSVs?
A:Possibly. A better question is “What size burner does not require proof-of-closure on its SSVs?” NFPA 86 does not differentiate between burners with different intended functions (i.e. pilot versus main). However, it does impose fewer requirements on smaller burners because the risk is lower. When the firing rate of a burner system exceeds 400,000 BTU/hour, at least one of the SSVs must be “proved closed” prior to each ignition attempt. Many pilot burner systems are smaller than this threshold.
Q:Does an existing oven that is relocated have to meet the new standards?
A:This question is addressed in the "Retroactivity" and "Applicability" portions of the standard. In theory, if an oven is simply relocated, without any alterations or extensions to the existing equipment or process, the earlier requirements apply and subsequent changes do not. However, such “grandfathering” may be superseded in cases where the pre-existing situation presents an unacceptable degree of risk.
Q:Does NFPA 87 supersede NFPA 86 for fired-heaters in refineries?
A:NFPA 87 is a new Recommended Practice for Fluid Heaters. Although portions of NFPA 86 could have been applied to fluid heaters in the past if agreed to by a user and manufacturer, NFPA 86 by itself was neither sufficient for nor applicable to fluid heaters. NFPA 86 now explicitly excludes refinery fired-heaters just as NFPA 85 has done for many years.
Q:Please explain the new requirement for explosion relief.
A:NFPA 86 has required explosion relief on most ovens and furnaces for 40 years or more. The 2011 Edition will permit oven manufacturers to compute the required vent area in one of two ways: (a) the traditional guidance in NFPA 86, which calls for 1 ft2of vent area per every 15 ft3of furnace volume, or (b) the computational methods given in NFPA 68 – Standard on Explosion Prevention by Deflagration Venting (2007).
Q:Are smokehouse ovens, paint-booth ovens, coffee-bean roasters, textile dryers, laminator ovens, etc. exempt from NFPA explosion-relief requirements?
A:Ovens of light construction are not exempt from the requirement for explosion relief in NFPA 86. Furnaces of “heavy-wall” construction (e.g., vacuum furnaces) are listed as exceptions to the explosion-relief requirement because it is usually impractical to install explosion relief in such equipment, and furthermore, the vessel itself is more capable (albeit seldom fully capable) of containing the pressure buildup from a deflagration than light-construction ovens. Exceptions are itemized in NFPA 86 for low-oxygen atmosphere ovens, thermal oxidizers and the work chambers of indirect-fired ovens.