Reducing Your Oven's Hazards, Part 2
This series of columns is intended to summarize key points of the requirements for safety-shutoff valves in combustion systems. It is not a comprehensive summary of all NFPA requirements for oven and furnace applications.
In this Part 2, I will conclude with the summary of general safety-shutoff valve requirements:
- Safety-shutoff valves must be selected and rated for the design upstream and downstream, or backpressure and ambient conditions. (Ref: 188.8.131.52 & 184.108.40.206 & 7.2.6)
- Safety-shutoff valves must be rated for the pressure upstream of the pressure-reducing regulator feeding the safety-shutoff valves. (Ref: 220.127.116.11)
- A closed-position indicator switch is not accepted as a proof-of-closure switch. (Ref: 18.104.22.168 and 22.214.171.124)
- Valve-proving systems are permitted but do not relieve the user from the requirement to perform a leak-tightness test of all safety-shutoff valves at least annually. (Ref: 126.96.36.199). Annex material A.188.8.131.52 and Table A.184.108.40.206 provide excellent guidance in conducting leak tightness test of safety-shutoff valves.
- For multi-burner systems, an alternate safety-shutoff valve arrangement (see figure) can be used as per the exception following 220.127.116.11.
The example illustrated in the figure identifies a single safety-shutoff valve equipped with a proof-of-closure switch at each individually operated burner, along with a common safety-shutoff valve equipped with a proof-of-closure switch. If stringent lower explosive limits (LEL) cannot be satisfied by the application parameters, this special safety-shutoff valve arrangement requires the use of additional safety-shutoff valve supervisory logic. This logic monitors the operation of all proof-of-closure switches and is designed such that if an individual burner proof-of-closure switch is not proved closed whenever there is a loss of flame signal, then the common safety-shutoff valve (and all safety-shutoff valves downstream) must close.
It is important to recognize that implementation calls for listed safety-shutoff valves with proof-of-closure switch, not closed-position indication or auxiliary switches. NFPA 86 defines a proof-of-closure switch as “a non-field-adjustable switch installed in a safety-shutoff valve by its manufacturer that activates only after the valve is fully closed.”
NFPA 86 also discusses manual valves by noting that safety-shutoff valves must be supplemented with suitable manual valves and be protected with a sediment trap such as a filter/strainer and drip leg. (Ref: 18.104.22.168 and Figure 22.214.171.124.2) The manual (equipment-isolation) valve must be supplied and located remotely so that it can be accessed and turned off in an emergency. (Ref: 126.96.36.199 & 188.8.131.52.1) Manual valves must have affixed handles that must indicate the open and closed positions. (Ref: 184.108.40.206.2 & 220.127.116.11.3) Lubricated valves must be lubricated at least annually. (Ref: 18.104.22.168.5) In addition to the requirement for having the equipment-isolation valve, NFPA 86 requires that manual fuel valves be closed whenever a furnace or oven is shutdown. (Ref: 7.2.2)
NFPA 86 requires the use of electronic flame supervision. In addition to understanding the compliant application of safety-shutoff valves, it is important to also correctly apply electronic flame supervision, ventilation interlocks, purge logic and interlocks, fuel and air interlocks, excess temperature limit controller and 1,400oF bypass controller to achieve a compliant combustion system.
To purchase a copy of NFPA 86 and continue to work to ensure that all of your heat processing equipment is in compliance, contact NFPA at (617) 770-3000 or www.nfpa.org.
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 understand of NFPA codes and standards and the requirements for compliance of 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.