My first three columns summarized the key points in the requirements of safety shutoff valves in combustion systems. This fourth and final column in this series adds the general requirements for combustion system interlocks. Please note that it is not a comprehensive summary of all NFPA requirements for ovens and furnaces.

In general, the requirements are that:

• Safety devices must be selected, installed, used and maintained according to the manufacturer's recommendations. (Ref. 4.3 & 7.2.6)
  
• All safety devices must be inspected and tested at regularly scheduled intervals at least annually. (Ref. 7.2.5 & 7.2.5.2 & Chapter 14)
  
• Operators must be trained in the use of all safety equipment. (Ref. 4.2)

For fuel interlocks, high and low fuel pressure switches are required. The high fuel pressure switch must be located downstream of the final pressure-reducing regulator. Settings must be according to the operating limits of the burner system. (Ref. 7.8)

Under NFPA 86, a minimum combustion air pressure or flow interlock is required (Ref. 7.6), and a combustion air blower motor starter interlock is required. (Ref. 7.6.3)

The standard states the following for safety ventilation interlocks:

• There must be an exhaust and/or recirculation fan airflow interlock. (Ref. 7.6.1)
  
• All fans essential to operation must have interlocks to prove flow. (Ref. 7.5)
  
• Class A ovens have specific requirements for safety ventilation. (Ref 9.2)

Other NFPA 86 requirements are:

• A minimum purge airflow interlock is required and must be proved throughout the entire purge cycle. (Ref. 7.4.1)
  
• An excess temperature limit controller, or interlock, is required. (Ref. 7.16)
  
• Flame supervision is required for each pilot and main burner. (Ref. 7.9.2)
  
• Flame supervision may be bypassed once the combustion chamber is suitably proved to be above 1,400^oF (760^oC) by a listed 1,400^oF (760^oC) bypass controller. (Ref. 7.9.1 Exception No. 1)
  
• Flame supervision may be omitted if the burner system is suitably equipped with safety shutoff valves and if the valves are prevented from operating until the combustion chamber is proven to be above 1,400^oF (760^oC) by a listed 1,400^oF (760^oC) bypass controller. (Ref. 7.9.1 Exception No. 3)

NFPA 86 states: Safety devices shall be applied and installed in accordance with the manufacturer's instructions. This is an important criterion in all aspects of safety equipment and application in the standard's Chapter 7, including the compliant application of electronic flame supervisory equipment.

Commonly used non-self-checking ultraviolet (UV) flame detectors are subject to failure in either the “on” or “off” state, making the required “safe start check” logic very important. (Ref. 7.9.1) Some manufacturers of non-self-checking UV flame-detection systems post limits to their recommended hours of continuous service without a subsequent “safe start check.” In addition, FM Global (Factory Mutual) Property Loss Data Sheet 6-9 does not list a specific recommendation or operating interval limitation for the use of non-self-checking UV systems. GE GAP.4.2.0, (formerly Industrial Risk Insurers IRInformation IM4.2.0), which aligns with NFPA 86, states under 7.9.2: For furnaces that have burners that continuously operate for more than 24 hours, use only self-checking ultra-violet (UV) flame scanners….

Flame-rod and self-checking UV systems offer alternatives to non self-checking UV systems.

And finally, here are the requirements for bypass controllers (Ref. 7.17):

• A properly applied 1,400^oF (760^oC) bypass controller can be used to activate unsupervised burners. (Ref. 7.9.1 Exception No. 3)
  
• A properly applied 1,400^oF (760^oC) bypass controller can be used to permit a burner re-ignition without a re-purge. (Ref. 7.4.1.5 Exception (1)).

Given the complexity and diversity of combustion applications, this article 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 for their own operation. As such, the author and 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.