Qualified personnel must determine safe temperatures, purging, interlocks, and emergency procedures for flammable atmospheres using the equipment manual and applicable safety requirements. See the NFPA 86 standard page.
Controlled atmospheres such as endothermic gas, nitrogen-methanol, and dissociated ammonia contain H₂, CO, and NH₃, and present fire, poisoning, and asphyxiation hazards. The flammability limit of H₂ in air is about 4%–75%, and that of CO is about 12.5%–74%. The core principles of safety are: when the furnace temperature is lower than the safe temperature, no combustible gas should be introduced (or be fully purged with nitrogen first), keep a flame curtain at the furnace door, interlock the gas source and furnace conditions, be equipped with flammable gas and CO alarms, and formulate operating procedures in accordance with NFPA 86 and other specifications and equipment instructions.
- Danger: flammable explosion, CO and NH₃ poisoning, nitrogen asphyxiation
- flammability limit: H₂ about 4%–75%, CO about 12.5%–74%
- Principle: Purge first if the temperature is lower than the safe temperature, and do not mix combustible gas and air in the cold furnace
- Facilities: flame curtain, pilot flame, emergency nitrogen purge, gas alarm, interlock cut-off
- Procedures: Standard operations and drills for starting up and shutting down furnaces, power and gas outages, and leaks
Main hazards
| Danger | Source | Consequence |
|---|---|---|
| Explosion | Combustible gases (H₂, CO, CH₄, methanol vapor) and air are mixed within the flammability limit and encounter fire sources | Furnace damage, casualties |
| fire | Gas leakage, quenching oil | Equipment and plant losses |
| Poisoning | CO (colorless and odorless), NH₃ | acute poisoning |
| Asphyxiation | Nitrogen replaces oxygen in a confined space | hypoxic asphyxia |
Flammability limit reference
| Gas | Flammability limit in air (volume fraction, approx.) |
|---|---|
| H₂ | 4%–75% |
| CO | 12.5%–74% |
| CH₄ | 5%–15% |
| NH₃ | 15%–28% |
Endothermic gas contains about 40% H₂ and 20% CO and can easily fall into the explosion range when mixed with air.
Core safety principles
1. Safety temperature principle Combustible gas can only be introduced directly when the temperature of the furnace is high enough (to ensure that the incoming combustible gas ignites immediately and does not accumulate, industry standards are often around 760 °C as a reference); below this temperature, it must be fully purged with inert gas (nitrogen) to reduce the oxygen content to a safe level before introducing combustible gas. Specific temperature and purge requirements are based on applicable codes such as NFPA 86 and equipment instructions.
2. Purge - Purge with nitrogen before introducing combustible gas and after decommissioning; - The purge volume is usually measured in several times of the furnace volume and is confirmed by oxygen content measurement; - Pipelines also need to be replaced before maintenance.
3. flame curtain and everlasting flame Keep a flame curtain at the furnace door and exit to ensure that the escaping combustible gas is safely burned; the igniter or open flame must be reliable.
4. Interlocking - It is prohibited to introduce combustible gas when the furnace temperature is lower than the safe temperature; - Automatically cut off and start nitrogen purge when there is a power outage, nitrogen outage or abnormal gas pressure; - Flame extinguishing detection; - Combustible gas and CO alarm linkage.
5. Emergency nitrogen It should be ensured that there is sufficient nitrogen reserve for emergency purging in case of power outage.
Daily checklist
- Nitrogen storage and pressure;
- Working status of flame curtain and igniter;
- Gas alarm function test;
- Periodic testing of interlocking functions;
- Leak inspection of pipelines, valves and flanges;
- Quenching oil level and temperature;
- Operator personal protection and training records.
Emergency points
- Gas leakage: cut off the gas source, ventilate, evacuate, and use a detector to confirm;
- Power outage: confirm that automatic cut-off and nitrogen purge have been started;
- CO alarm: evacuate immediately and ventilate, and enter only after confirming the cause;
- Regularly practice emergency plans.
Frequently asked questions
What safety devices are required between the generator and furnace?
Common ones include anti-backfire devices, shut-off valves, pressure monitoring, diffuse ignition devices and interlocks with furnace safety temperatures.
Can the heat treatment furnace be purged directly with air?
Purging with air when there is still combustible gas in the furnace will form an explosive mixture and is absolutely prohibited. It must be replaced with an inert gas such as nitrogen first.
How often should interlock testing be done?
Carry out in accordance with equipment instructions, applicable specifications and enterprise safety management systems, and keep test records.