Click here for a printable version of this article.

June 2001: Heating Systems

Heating equipment such as boilers, heaters, ovens, kilns or process furnaces are found in all businesses and industries. The equipment can be utilized simply for building heat or be a key element in the manufacturing process. Because most of these systems operate automatically, you need to rely on control and safety devices to ensure a safe startup and shutdown procedure.

According to FM Global, over the past 10 years, more than 150 combustion explosions have been reported to FM. These losses represented $259 million, and in most cases, the underlying cause was failed or missing safety devices.

KEY COMPONENTS

Delivering fuel to the combustion chamber in the correct proportions for efficient and safe burning is the main goal of a fuel control system. The combustion safeguard system is usually an electronic box located near the combustion chamber. It is the "brains" of the fuel control system and is fed control data by the components listed below.

Fuel is usually fed to the heating system through a gas or fuel train, which is a piped supply that contains a series of safety and control devices. Key components in the fuel train are as follows

Regulator: Fuel pressure is usually well above that required by the heating system. A regulator is located at the beginning of the fuel train. The desired operating pressure is set and the regulator automatically opens or closes to maintain that pressure regardless of flow rate.

Low Pressure Switch: This switch monitors gas pressure as the source. If the pressure falls below a pre-determined setting, perhaps due to a leak, the low-pressure switch shuts down the entire system. These switches are usually required to be manually reset which will help make the operator aware of what caused it to operate. The low-pressure switch is usually located before the Safety Shutoff Valve.

Safety Shutoff Valve (SSOV): The SSOV is both a control valve and a safety device. As a control valve, it opens and closes to start and stop the fuel flow. As a safety device, it shuts down fuel flow if a monitored device, such as a low-pressure switch, goes out of range.

High Pressure Switch: This switch operates like a low-pressure switch. If the regulator fails and pressure increases above the pre-determined setting, the high-pressure switch instructs the SSOV to shut down the fuel flow.

Modulating Valve: This valve controls the quantity of fuel entering the combustion chamber based on the control system demand. As a safety feature at start-up, there is a low fire switch, which ensures fuel flow is at a minimum during startup.

Combustion Air Fan: To burn fuel efficiently and safely, air is needed in proportional amounts as the fuel is being introduced into the combustion chamber. Additional air is also required to help combustion gases make it up the exhaust stack. Dampers, fan speed or both can regulate airflow. The combustion safeguard system monitors airflow so it can shut down the fuel flow in the event of lost air. Usually, there is also an exhaust gas temperature switch monitoring exhaust temperature. If the temperature varies beyond a pre-set range, the system shuts down.

Low Water Cutoff: In boilers, if the water level is to low and combustion continues, the metal tubes inside the boiler will overheat and collapse. A low water cutoff will sense a low water condition and shut down the system.

Purge Timer: At start up, it is necessary to purge the combustion chamber of all unburned gases. This is done by running the fan long enough to purge at least four or five volumes of the chamber. This is controlled by a timer built into the combustion safeguard system. If the purge cycle is not long enough, the system will not fire.

Flame Sensor: This sensor monitors the presence of a pilot or main flame. If ignition does not occur immediately, unburned fuel will accumulate in the chamber. Delayed ignition could result in an explosion. If a pilot flame does not appear within 10 to 15 seconds, the combustion safeguard system will abort the startup.

The above safety devices apply to gas or oil fired units. Because oil is a liquid rather than a vapor, additional interlocks are necessary to assure oil is at the proper atomization temperature and pressure.

All safety control devices must be maintained regularly to ensure they are in proper operating condition. Manufactures usually issue guidelines with respect to a maintenance and testing schedule. Ideally, competent personnel who are familiar with the equipment should conduct the tests. Records of the tests should be maintained for trend analysis and follow up. Recommended testing frequencies are as follows:

MONTHLY:

- Flame detector system
- Fan and airflow interlocks
- Check SSOV's for leakage<
- Low-fire interlock
- High steam pressure or temperature interlock
- Fuel pressure and temperature for oil
- High and low pressure interlocks for gas

SEMIANNUALLY OR ANNUALLY

- Proper flame color and shape and exhaust color
- Air flow or pressure switches
- Damper high and low fire interlocks
- Flame scanners and other safety controls
- Piping, hoses, wiring and electrical connections of interlocks
- Instrument calibration

AS NEEDED

- Disassemble and clean oil atomizers
- Clean strainers

The above schedule is a minimum guideline. Based on operational environments, the testing schedule frequencies may have to be increased.

It is important to remember that even the most up to date combustion safeguard system is composed of electro-mechanical devices that demand periodic inspection and testing. They are no different than any other industrial loss control systems such as automatic sprinklers, fire pumps or fire extinguishers. When an emergency condition presents itself, having these systems in proper working order may save your facility and prevent extended business interruption.