These questions and more were clarified as part of the many changes introduced in the 2003 edition of NFPA 86 Standard for Ovens and Furnaces, which provides the latest requirements for ovens and dryers. These are the types of combustion heating equipment used in the finishing, chemicals, plastics, pharmaceuticals, food, electronics, textiles, packaging or pulp, paper and converting industries.
Although the 1999 edition of NFPA 86 stated that “where the gas burner capacity exceeds 400,000 BTU/hr, at least one of the safety shutoff valves shall be proved closed,” (Ref 5-7.2.2, 1999 Edition), it did not define “proved closed” for safety shutoff valves.
However, the current 2003 edition has clarified fuel-gas safety-shutoff valve requirements and states that “where the main or pilot fuel-gas burner system capacity exceeds 400,000 BTU/hr, at least one of the safety-shutoff valves between each burner and the fuel supply shall be proved closed and interlocked with the pre-ignition purge interval.” (Ref 126.96.36.199)
NFPA 86 further expanded the proved-closed requirement. Section 188.8.131.52.1 provides options for the requirement, stating that proved-closed shall be accomplished by either of the following means:
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 (Ref 184.108.40.206). Because auxiliary and closed-position indicator switches activate before a valve is fully closed, they do not satisfy the proved-closed requirement. (Ref 220.127.116.11.2)
A valve-proving system is the second option to accomplish the proved-closed requirement. A valve-proving system checks for actual gas leakage in both safety-shutoff valves before startup.
In general, most gas-train applications still utilize the proof-of-closure switch vs. the valve-proving system due to its lower cost. The valve-proving system may add additional piping and wiring to a typical gas train.
Maybe the most important aspect of safety-shutoff valves is that valve-seat leakage testing and valve-proving systems shall be performed in accordance with the manufacturer's instructions at least annually. (Ref. Section 18.104.22.168). PH
Given the complexity and diversity of combustion applications, this article is not intended to relieve any user and/or company from taking it upon themselves to gain a thorough understanding of NFPA codes and standards, and the requirements for compliance of the user and/or company's own operation. As such, the author and Siemens Building Technologies disclaim liability for any personal injury or 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.