Furnace Safety Q&A (part 2)
Questions resume where we left off last month.
Q: Does NFPA 86 require two Safety Shutoff Valves (SSVs) in series, or does it require the so-called “double-block and vent” arrangement?
A: NFPA 85, the Boiler and Combustion Systems Hazards Code, requires a “double-block and vent” on each main and pilot burner and has done so since the 1970s or sooner. Since the 1980s, however, NFPA 86 has only required two block valves in series. NPFA 86 is silent on the use of an automatic vent (or “bleed”) valve between the pair of block valves, although a manual test valve is required.
Q: Is “proof-of-closure” required on pilot-burner SSVs?
A: Possibly. A better question is “What size burner does not require proof-of-closure on its SSVs?” NFPA 86 does not differentiate between burners with different intended functions (i.e. pilot versus main). However, it does impose fewer requirements on smaller burners because the risk is lower. When the firing rate of a burner system exceeds 400,000 BTU/hour, at least one of the SSVs must be “proved closed” prior to each ignition attempt. Many pilot burner systems are smaller than this threshold.
Q: Will conduit seals be necessary at pressure switches even though the pressure-switch housings are vented to atmosphere?
A: Section 501.15 of the National Electrical Code (NFPA 70) provides guidance for seals in conduit and cable systems. The new NFPA 86 requirement states: “Where seal leakage or diaphragm failure in a device can result in the flow of flammable gas or flammable liquid through a conduit or cable to an electrical ignition source, a conduit seal or a cable type that is sealed shall be installed.” Thus, if a housing vent eliminates the possibility of gas or liquid flowing through a conduit or cable, then conduit seals are not required.
Q: The requirements for purge, safety ventilation, combustion safeguards and safety shutdowns are confusing. Can you clarify?
A: Purge is sometimes confused with safety ventilation, but the concepts are distinct and addressed separately in NFPA 86. Requirements for safety ventilation specify a minimum airflow to ensure that flammable vapors evolved from the work being heated in the oven never build up and approach explosive concentrations. Requirements for purge address the need to remove flammable gases and vapors that may have entered the combustion chamber prior to lighting the burners.
Combustion safeguards and safety shutdowns are also related but distinct concepts. A safety shutdown occurs when a safety device initiates action (e.g., opens a switch) that causes the furnace operation to stop by shutting off all fuel and ignition energy. Low fuel-gas pressure, high furnace temperature and insufficient airflow represent potentially unsafe conditions that can be monitored with a safety device. A combustion safety is one form of safety device that senses the presence or absence of flames and de-energizes the appropriate fuel SSVs in the event of flame failure. Flame failure may require a complete safety shutdown of the entire furnace or might only require closing a single SSV that supplies the affected burner.
Q: Does the new standard permit the use of a Safety-Integrity-Level (SIL) rated PLC instead of a hard-wired Burner Management System (BMS)?
A: There are two fundamentally different paths to compliance with NFPA 86 safety requirements: performance-based and prescriptive.
Most users will find it easiest to implement the prescriptive requirements for furnace safeguarding as detailed in Chapters 6 and 8 of NFPA 86. For logic systems, there are now four solutions to choose from: (a) a “hardwired” system of safety interlocks, often using a listed BMS, that initiates a safety shutdown upon activation of any interlock; (b) a Programmable Logic Controller (PLC) listed for combustion safety service; (c) a “general-purpose PLC” modified to incorporate a prescriptive list of self-checking features; or (d) a safety-rated PLC that automatically adheres to all or most of the self-checking features enumerated but is not necessary listed for such service.
Knowledgeable users may elect to implement a performance-based solution to furnace safeguarding instead of adhering to all of the prescriptive requirements. However, alternate systems must be approved by the Authority Having Jurisdiction (AHJ). For such a system, AHJ review should be based on an overall risk analysis, the safety integrity levels selected and a verification of each instrument’s safety integrity level. With AHJ approval, a performance-based system can meet the equivalency provision of NFPA 86 without line-by-line adherence to the prescriptive requirements in the standard. ANSI/ISA 84.00.01, Application of Safety Instrumented Systems for the Process Industries, provides requirements and methods for selecting and installing safety-rated PLCs and related sensors. IH