ONLINE EXCLUSIVE: Are Your Kilns In Compliance? Part 3: General Requirements for Combustion Systems on Tunnel Kilns
NFPA 86, Standard for Ovens and Furnaces, 2003 Edition is the current version of the standard addressing the safety aspects of the design, construction, and operation of gas- and oil-fired furnaces. Published by the NFPA (an international non-profit membership organization founded in 1986 as the National Fire Protection Association), the standard's stated purpose is to provide "the requirements for prevention of fire and explosion hazards associated with the processing of materials in ovens, furnaces and related equipment. The heat processing of materials involves serious fire and explosion hazards that can endanger the furnace, the building, or personnel."
Part 1 of this series (June 2004) addressed the general requirements for fuel gas safety shutoff valves, associated manual valves and leak test provisions, while Part 2 (August 2004) highlighted the general provisions for combustion system interlocks, including flame supervision.
Although these requirements apply to all gas- and oil-fired kilns, the most commonly used kilns in the ceramic industry are tunnel kilns. Since tunnel kilns have additional combustion system requirements, it is imperative that operators understand the requirements that are specific to this kiln type. Table 1 summarizes the combustion system requirements for a typical tunnel kiln.
Kiln ShutdownIt is important to follow the correct shutdown procedures whenever the kiln is shut down, even for short durations. The operator must ensure that the fuel flow to the kiln is stopped simultaneously with the shutdown by immediately closing the main manual fuel isolation valve(s). NFPA 86, 2003 requires that the operation and leak tightness of the main safety shutoff valves (SSOVs) be tested and documented at the prescribed intervals (at least annually). North American Manufacturing recommends that the SSOVs be replaced periodically to ensure proper operation.
Whenever the possibility exists that there is an indeterminate (possibly combustible) gas mixture in the kiln, the kiln must be allowed to cool to a temperature that will permit a safe re-purge.
Depending on the kiln design, other critical shutdown procedures may need to be followed for safe shutdown of the kiln. Operators should refer to and follow the kiln manufacturer's shutdown procedures to ensure the highest possible level of safety.
Kiln StartupOnce all the required equipment is correctly installed and commissioned, proper procedures must be routinely followed for lighting (introducing fuel into) the tunnel kiln. The lighting considerations provided here are intended to help the user better understand the many conditions that come into play whenever lighting a kiln-they are not meant to replace the kiln manufacturer's specific operating instructions.
Two possible lighting conditions are cold start and hot restart.
A cold start refers to starting the tunnel kiln when it is being commissioned or after it has been shut down for a period of time sufficient for any fired zone in the kiln to fall below 1400ºF. In the latter case, the operator must first ensure that the correct shutdown procedure (as specified by the kiln manufacturer) has been followed. To pre-start the kiln, the operator should follow the manufacturer's startup procedure. An example of a basic cold startup procedure is as follows:
- Visually check to ensure that all the manual valves in the combustion air supply lines between the blower[s] and the burners are in the open position for purge.
- Visually check to ensure that all the manual valves in the burner or zone fuel supply lines are in the closed positions, and verify that the manual and automatic SSOVs are closed.
- Start the exhaust, cooling and recirculation fans and combustion air blowers in the sequence specified in the manufacturer's start up procedure. Note: Depending on the location and arrangement of burners equipped with electronic flame supervision, the startup procedure may require specific kiln draft flow conditions to be established during the preheat interval.
- Open the manual (equipment isolation) valve in the main fuel supply to the kiln to permit the low (gas or oil) pressure switch to energize.
It is not uncommon at this time to open (at least partially) the doors on the kiln to provide pressure relief in the unlikely event that some pockets of combustible gases were not completely removed during the purge cycle. The doors can remain open during the trial for ignition. (Ref: A.188.8.131.52)
Once the purge is complete, the operator can open the main SSOVs and begin lighting the burners in those sections that are equipped with flame supervision. Raise the temperatures in these sections as indicated in the kiln manufacturer's start up instructions, following the established procedures to progressively elevate the kiln temperatures (zone by zone) above 1400ºF. A properly designed and listed 1400ºF bypass controller should be used as a permissive to bring the fuel to unsupervised burners. The operator should visually inspect the unsupervised burners for proper operation before proceeding to light the next zone.
Note: If the kiln is not equipped with compliant SSOVs and controls as described here, the kiln manufacturer or the authority having jurisdiction (AHJ) in your region should be consulted for specific instructions.
A hot relight is required when a brief interruption of the electrical power supply causes the tunnel kiln to shut down. Even though the shut down may have occurred unexpectedly, the operator should follow the manufacturer's shutdown procedure.
If it is determined that the main fuel was immediately shut off and there is certainty that there is no accumulated fuel in the kiln, the blowers and fans can be sequentially re-started according to the kiln manufacturer's start up procedure. An example of a basic hot relight procedure is as follows:
- Visually check to ensure that all the manual valves in the burner or zone fuel supply lines are in the closed positions. Verify that the manual and automatic SSOVs are closed.
- Pre-position the zone temperature control combustion air valves to the low-fire positions.
- A properly applied 1400ºF bypass controller can be used to avoid a repurge, as per NFPA 86, 2003 Ref. 184.108.40.206, Exception 1. However, it is the opinion of North American Manufacturing that all fired zones must be suitably proven to be above 1400ºF to apply this exception.
- With the purge complete, open the main SSOVs and start lighting the burners in the sections that are equipped with electronic flame supervision. Raise the temperatures in these sections as indicated in the kiln manufacturer's start up instructions, following the established procedures to progressively elevate the kiln temperatures (zone by zone) above 1400ºF. A properly designed and listed 1400ºF bypass controller should be used as a permissive to bring on the fuel to unsupervised burners. The operator should visually inspect the unsupervised burners for proper operation before proceeding to the next zone.
For a complete copy of NFPA 86, Standard for Ovens and Furnaces, 2003 Edition, contact NFPA at 1 Batterymarch Park, Quincy, MA 02169-7471; (617) 770-3000; fax (617) 770- 0700; or visit http://www.nfpa.org.
Authors' NoteThis series of articles is intended to summarize key points of the standard, limited to the requirements for combustion systems, and should not be viewed as a comprehensive summary of all NFPA requirements for kiln applications. This series is not intended to relieve any user or company from taking it upon themselves to gain a thorough understanding of NFPA Codes and Standards and the requirements for compliance of their own operation. As such, the authors and The North American Mfg. Co., Ltd. disclaim liability for any personal injury, property or other damages of any nature whatsoever, whether special, indirect, consequential or compensatory, directly or indirectly resulting from the publication, use of, or reliance on this article. We recommend that you purchase a copy of the current NFPA 86 standard and continue to work to ensure that all of your heat processing equipment is in compliance.
TABLE 1. An overview of tunnel kiln combustion system requirements.Common Interlocks
- Electronic flame supervision for all burners operating in a chamber below 1400ºF.
- A 1400ºF Bypass Controller may be used to permit the introduction of fuel to burners without flame supervision when the chamber is above 1400ºF.
- Interlocking of auxiliary contacts of motor starters into the safety circuitry for all equipment required for combustion of the fuel.
- Pressure switches for all fans required for combustion of the fuel.
- Flow interlocks on all flows required for purge proving. Means of minimum-required-flow proving in the exhaust system, if the exhaust system acts to induce air required for combustion of the fuel.
- Permanent and ready means for leak tightness testing for all fuel gas safety shutoff valves (main and pilot).
- User supplied regularly scheduled inspection, testing, and maintenance.
- User supplied and documented safety inspection and testing at least annually.
- Manufacturer supplied and user maintained operating instructions for startup, shutdown, emergencies, and procedures for inspection, testing, and maintenance.
- Excess temperature limit controller(s).
Fuel Gas Interlocks
- High gas pressure switch(s)
- Low gas pressure switch(s)
- Fuel gas safety shut off valves to requirements of section 7.7
Fuel Oil Interlocks
- High oil pressure switch
- Low oil pressure switch
- Fuel oil temperature switches for preheated oil applications.
- Low atomizing air pressure switch(s) "located downstream from all valves and other obstructions that can shut off or cause excessive pressure drop of atomization media".
- Oil safety shutoff valves to requirements of section 7.7
- Proof of "through air" flow from cooling zone to firing zone(s) exhaust where primary combustion air supply to burners is below the flow required to ensure complete combustion of the fuel (or a safe level of incomplete combustion of the fuel in some circumstances).