The National Fire Protection Association has released the latest revision of its Standard for Ovens and Furnaces, NFPA 86. This is the default U.S. safety standard for industrial heat processing equipment. If no overriding standards apply, this is the one most equipment manufacturers follow. The standard also contains a great deal of material that applies to the users of ovens and furnaces - it's not just a design and construction specification.
The 1999 edition doesn't contain too many major changes to the sections covering burners and their controls. Here is a sampling of some of the more important ones. The standard's paragraph numbers are shown in parentheses at the end of each item.
Drip legs or sediment traps are now required on gas supply lines. Equipment manufacturers and installers have been installing drip legs for years and many local codes require them, but this is the first time they've been part of the standard. (4-2.4.4)
Requirements for regulator and pressure or flow switch vent lines have been formalized. Past editions of NFPA 86 have permitted regulator and pressure or flow switch vent lines to be manifolded together, as long as a diaphragm rupture of one of the vented devices can't backpressure the others. The new edition gets more specific about size, requiring the vent manifold to be no smaller than the area of the largest vent line, plus 50% of the additional vent line area. (4-188.8.131.52)
Standard requires explosion resistance of nonmetallic radiant tubes to be tested. Radiant tube systems have always enjoyed exemption from certain safety requirements because they are considered explosion-resistant and, therefore, capable of containing the accidental ignition of the gas and air they contain. Concerns have risen that ceramic radiant tubes, being more brittle than metal, might break under these conditions. NFPA 86 now requires that the explosion resistance of nonmetallic tubes be determined by test. Details on how to conduct the test are contained in the appendix. (4-2.7.4)
Descriptions of many safety system features are more specific. One new paragraph clarifies a point that has long been assumed but not explicitly stated: Safety circuitry contacts for safety interlocks and excess temperature limit controllers must be arranged in series ahead of the safety shutoff valve. Interposing relays are permitted if certain conditions are met. (5-2.7)
Standard has been clarified to explicitly allow testing of safety devices. The 1995 version of the standard forbid bypassing of safety devices, which some people apparently took to include bypassing for the purpose of running safety checks. The latest version retains the prohibition, but specifically allows safety device testing and maintenance. Built-in test mechanisms, however, must be interlocked to prevent system operation during the test mode, unless the device's listing permits it. (5-2.10)
Standard requirements for programmable control systems that supervise safety-related functions have been expanded and clarified. Several years ago, serious concerns arose about the use of programmable controllers to supervise safety-related functions in addition to all the other process control activities. The 1995 edition set down a number of specific characteristics a programmable control system must have, and what it would be permitted or not permitted to do, to be considered for use as a safety controller. The 1999 edition tightens up this section, now called "Combustion Safety Functions," by replacing many of these specifics with more general, functional definitions. It says programmable controllers "specifically listed for combustion safety service shall be permitted where applied in accordance with the listing requirements and manufacturer's instructions."
Conversely, a programmable controller not listed for combustion safety service is only permitted to monitor safety interlocks or provide burner control functions if its use does not interfere with, or prevent, the operation of the interlocks. In addition, controller contacts wired in series with the safety interlocks must be isolated. (5-3.2.1 and 5-3.2.2)
Additional set of conditions added for burner system restarts.5-4.1.5 adds a new set of conditions that, if satisfied, allow a burner system to be restarted without purging following a shutdown or flame failure. These conditions, all of which must be met, are:
- Each burner and pilot is supervised by a combustion safeguard.
- Each burner system is equipped with safety shutoff valves.
- At least one burner remains operating in the common combustion chamber of the burner to be reignited.
The logic underlying this is that as long as one burner continues to operate, it will ignite any combustible gases that may have entered the chamber during the previous operating cycle. Consequently, there should be no dangerous accumulation of gases when the relight attempt is made.
There's more to NFPA 86 than these paraphrased summaries. If you don't have a copy, call NFPA at (800) 344-3555.
Read Dick Bennett's followups on NFPA 86 by using the link(s) at the bottom of the page.