Myriad industrial process heating applications employ combustion systems, which are associated with a number of hazards. Despite these hazards, however, it is possible to keep industrial combustion systems operating safely and without event.
This article provides 10 tips related to combustion safeguards to help keep your personnel and plant safe from harm.
1. Understand What Flame Detection Is
Flame supervision may be defined as the detection of the presence or absence of flame. If a flame is present during the intended combustion period, the supervisory system will allow a fuel flow to feed combustion. If the absence of flame is detected, the fuel valves are de-energized.
This basic definition does not consider the hazard potential during startup or ignition, however. A dangerous combustible mixture within a furnace or oven consists of the accumulation of combustibles (gas) mixed with air, in proportions that will result in rapid or uncontrolled combustion (an explosion). The magnitude of the explosion depends on the quantity of gas and the air-to-fuel ratio at the moment of ignition.
2. Be Aware of Flame-Monitoring Devices
Sensors used for monitoring flames are either flame rods or ultraviolet (UV) scanners. Heat-actuated sensors such as thermocouples and filled capillary bulbs only may be used on small burners (those below 150,000 BTU/hr).
Industrial burners must be protected by a safeguard that reacts to flame failure within a time interval not to exceed 4 sec. Infrared detectors may be used, but they have several limitations:
- They are sensitive to hot refractory.
- They are temperature and vibration sensitive.
- They require special shielded or twisted pair wire.
- They should have wire runs kept under 50'.
Essentially, a flame rod is a stainless steel wire that intersects the flame. When an electric potential is applied to a flame rod, current flows from the rod to ground. Electrically, the flame is a diode and resistor in series. Often, this is called the flame-rectification principal.
Flame rods are low cost and provide a steady, reliable signal. Limitations include their 600°F (316°C) ambient temperature limit (the temperature at which the dielectric of the ceramic breaks down). Flame rods also have a 2200°F (1205°C) flame temperature limit (the temperature at which the stainless steel rod begins to droop). Flame rods should not be used with soot-producing fuels such as oil.
UV scanners can sense both gas and oil flames. They also can detect ignition spark, however, and are not immune to gamma rays, solar UV, welding sparks or UV-producing lighting products. In addition, they should be protected from excessive ambient heat. A heat-seal assembly with either a plain quartz lens or a magnifying quartz lens, a nonmetallic fiber insulator or purge air may have to be used.
Flame signal strength for both types of sensors may be checked with an analog microamp test meter.
A typical arrangement of a gas-fired piping and valve layout is shown.
3. Remember that Flame Safety Starts with Purging
The sequence for flame safety starts with purging the furnace or oven. Purge time should allow for four air changes.
Fuel valves can — and do — leak gas. The purpose of purging is to remove combustible gases from the combustion chamber before introducing an ignition source. The four air changes in the combustion chamber are based on a worst-case scenario that includes having a burner chamber that is completely filled with gas.
Once airflow for purge is verified, the proof-of-valve closure is confirmed and safety limits are proven, then the purge timer — which may or may not be integral to the combustion safeguard — determines the period of time required to evacuate the combustion chamber.
If a system is using UV scanners and runs continuously for more than 24 hours, a UV sensor with two UV tubes in one assembly can be used to monitor one burner flame. Each UV tube is powered during a different time cycle to eliminate interference between the two UV tubes.
4. Initiate Ignition Safely
When purge is complete, the ignition cycle then can be started. Some burners must be set at a low firing rate. The flame safeguard provides power to the pilot fuel valves (two pilot valves usually are required) and the ignition transformer, which in turn steps up voltage (usually to 6,000 V) to fire a spark plug or sparking electrode.
The pilot must be established and proven within 15 sec, or the flame safeguard will de-energize the pilot valves and ignition source. If a pilot is not used, the main gas valves are energized from the pilot valve’s terminal on the combustion safeguard. This is called a direct-ignited burner.
In the case where the burner fails to ignite, the cause of the failure to light-off should be determined. Re-purge and manual intervention — either reset or manual start — should occur before another ignition trial is attempted. If there is one purge timer for multiple burners with individual safeguards, re-purge may not be required.
5. If the Pilot Lights Successfully, Fire the Main Burner
After the pilot is ignited, the main valves are powered to allow fuel to the main burner, where the pilot flame provides ignition. Usually, two main valves are used to provide better protection against possible valve leakage. When the main valves are fully open, the pilot valve may be de-energized so that the flame sensor is only monitoring the main flame.
Upon a loss of flame, the combustion safeguard de-energizes the fuel valves in less than 4 sec. While the main fuel valves are slow opening, they are fast closing. Pilot valves and main valves should have proof of closure if the applicable burner exceeds 400,000 BTU/hr.
6. Operating with a Multiple Burner System
If a furnace or oven has a multiple burner combustion system with only one valve train, a multi-burner combustion safeguard should be used. This ensures that if one burner fails, they all go out.
Proper combustion safeguards can help keep personnel and plant safe from combustion events. A signal strength meter, flame safeguard control, UV scanner, ignition transformer and ignitor are shown.
7. Maintain Your Safety Limits
Typical safety limits that must be proven and maintained in a series electrical circuit with the combustion safeguard are:
- Low and high pressure gas switches.
- High temperature limit.
If any of these permissives fail — opening the circuit even for a fraction of a second — the fuel valves are de-energized. For this reason, tools are available for system maintenance and troubleshooting.
8. Perform Timely Inspections
It is recommended that the combustion safeguard and flame sensor are inspected weekly (for continuous operation) or monthly (for daily operation). The best way to test is to simulate flame failure by closing an upstream gas cock. The fuel shut-off valves should snap shut within a few seconds.
If a system is using UV scanners and runs continuously (more than 24 hours), the above sequence should be performed on a weekly schedule. An alternative is to use a shutter-type, self-check system or a dual/redundant self-check combustion system.
Some UV sensors have two UV tubes in one assembly to monitor one burner flame. Each UV tube is powered during a different time cycle to eliminate interference between the two UV tubes.
9. Test for Fuel Valve Leaks
A fuel valve leak test should be performed once per year. Likewise, high gas pressure switches, low gas pressure switches, combustion air pressure switches and high temperature limit switches should be tested once per year.
10. Understand Code Requirements
Whether the equipment is operating above or below 1400°F (760°C), all ovens and furnaces are subject to NFPA 86 standards and safety recommendations. A furnace operating above 1,400°F (760°C) does not require combustion safeguards when above 1,400°F, but it does at any time the temperature is below 1,400°F. (For further and more detailed information, NFPA 86, the standard for furnace and ovens should be referenced.)