How can we avoid an explosion?
When we start operating our equipment (ceramic kiln burners, ovens and furnaces), all of the requirements to start combustion need to be present. This is why it’s very important to control the ignition sequence and ensure the presence of the flame.
How to monitor the flame?
Firstly, we should remember that combustion is the rapid oxidation (burn-off) of a combustible. This chemical reaction has properties that are characterized as:
- Visible phenomenon
- Infrared emissions (heat)
- Ultraviolet emissions
- Ability to conduct electricity
We can use flame detectors to look out for the flame. These instruments detect using either the physical or electrical properties of flame. They work as security accessories; they send a signal to cut off the supply of combustible in the event a flame has not been detected.
The flame detectors generally available on the market tend to fall into two groups: infrared and ultraviolet or ionization probes. They operate with either flame electrodes (rod detector type) or via UV-tube.
Ultraviolet detectors are sensitive to ultraviolet radiation (UV) but they are completely insensitive to:
- Infrared radiation
- The reflection of hot refractories
- Visible radiation
The energy released by the electrons stays in the electromagnetic spectrum. The most interesting wavelengths are infrared, visible light and ultraviolet.
Flame electrodes (rod detectors/ionization probes) are based on the capacity of the ionized gases in the flame and the AC. Things to note are:
- The earth in the system, usually in the burner, must have at least four times the area of the proportion of the detector rod in the flame
- They are used only for gas flames
- Excessive temperature on the rod can cause failure of the system
What tools we shouldn’t use for detecting the presence of flame?
- Photocell flame detectors (cadmium sulfide) – they are easily ‘cheated’ by luminosity sources.
- Preferred use for oil applications
- Photocells of lead sulphide (infrared), just for circuit to detect the blink of the flame.
Why don’t we use infrared?
Infrared is used in some circumstances, but we need to consider that a furnace or kiln can rise up to 2,000°F (1,090°C). At this temperature the material around the flame gives out enough infrared radiation to cheat the detector. Infrared radiation is not the same as presence of flame.
Why not visible light?
Flames are never detected using their visible emissions; some flames are very difficult to see, especially natural gas flames. When the surrounding material starts irradiation, that radiation can be enough to cheat a simple detector as if it is watching the flame. Visible light is not the same as presence of flame.
Why use ultraviolet?
We prefer to use ultraviolet because UV is a unique property of the flames. The surrounding materials do not issue UV radiation. Without UV radiation, there isn’t a flame.
Why use electricity?
Flame detection by electricity is frequently used because conduction is a unique property of the flame. Crude gas and air do not transfer electricity. Without electricity, there isn’t flame.
How does a UV detector work?
A UV detector is a gas-filled tube that contains a filament; in the presence of UV light, there is a momentary electron flow (transfer of electricity to the filament). The detector can be seen to be working correctly when an orange light is shining in presence of UV radiation.
How does an electric detector work?
An electric flame detector (or rod flame detector) is a metal rod inserted into the flame, in which electricity is applied. When there is a flame present, the circuit is closed and the electricity can flow to the body of the burner, which is also made of metal acting as an earth in the system. This is more a question of detecting absence of flame.
Risk scenario – normal startup
We can start up the equipment in five steps:
1. Supply airflow
- Check that the air pressure is above the minimum limit
- Check that the gas pressure is above the minimum limit
- Check that the gas pressure is below the maximum limit
2. Purge time
- Guarantee at least 4 to 5 air changes inside the chamber. Accumulated combustibles in the chamber can cause an explosion
- Start up the pilot; open the pilot solenoid and energize the ignition
3. Pilot testing startup
- Verify the pilot and shut off the ignition power
- If the pilot does not start within 10 seconds, cut the energy from the solenoid pilot
4. Gas security valves (safety shutoff valves/SSOVs)
- Energize the circuit to allow opening of the security valves on manual or automatic
- Close ventilation valve
- Shut off the pilot
- During the next 10 seconds, the pilot valve must de-energize to interrupt the pilot
5. Test that the main flame is on
- Release the control so the system can start the temperature control
Typical startup of burners
An explosion can cause several types of damage, such as:
- Injuries or fatalities
- Damage to the equipment and facilities
- Cessation of production
We must remember, there are people and companies that cannot recover after an explosion.
1. You must always invest in security equipment/combustion safeguard systems.
2. Maintenance of equipment is vital.
- Include periodic check-ups of the security circuits
3. Train your team!
- Give them precise information about how the equipment should operate and what the maintenance requirements are
- Institute formal training programs
- Use simulators
- Organize field training
- Identify risks
4. You should always ask your kiln or industrial furnaces manufacturers for the following:
- Wiring and piping drawings
- Startup procedures
- Shutoff procedures
- Emergency procedures
- Maintenance procedures
5. If for some reason there is not a manufacturer’s procedure, you must develop detailed written procedures that include inspection, testing and maintenance. They should meet both normal conditions and emergencies.
6. If you have different types of operation, you should have one procedure for each operation type.
7. You should fix both the time and frequency of the inspections, testing and maintenance, and the corrective actions that should be implemented.
8. When pressure is not adequate it should be calibrated.
9. Keep a reference log of all test results.
10. Pressure and explosion relief devices should be inspected as recommended by the manufacturer.
11. The safety valve inspection should be carried out as recommended by the manufacturer. They have a life cycle! They should be replaced before exceeding the maximum number of open and closing cycles. The lifetime of the valves can be determined by:
- Real amount of open and closing times during a cycle of the valve.
- Estimating the time needed to approach 90% of the lifetime, based on normal cycles.
12. Closing valves should be manually lubricated as recommended by the manufacturer.
13. Emergency close valves should be activated as recommended by the manufacturer.
14. Oxygen pipes and components should be inspected in conformity with CGAG-4.1 – Cleaning Equipment for Oxygen Service.
15. Test/calibrate the temperature indicator to see if it is reading correctly.
16. If any temperature equipment or flow used for safety is replaced, the control point should be verified.
17. Install a shutoff valve far away from the oven, kiln or furnace. In case of emergency, it should be accessible for closing.
All NUTEC Bickley’s kilns and furnaces are equipped with flame controls and flame detectors. We supply an operations manual in which you can find all relevant information for keeping your kiln or furnace working at maximum levels of quality and safety. We design all of our equipment in the U.S. according to NFPA 86. In Europe we comply with EN 746-2.
In NUTEC Bickley we are metal furnace manufacturers, we provide a wide variety of models for metal industry: Box Furnaces, Car Bottom Furnaces, Rotary Hearth Furnaces, Steel Tempering Furnaces and more.
Experts in process for metals: annealing furnaces, aluminium aging ovens, quenching systems and more.