For safe operation of any fuel gas fired steam boiler:
Combustion is monitored and controlled to avoid creating an explosion inside the combustion chamber.
Boiler steam pressure and water level are monitored and controlled high pressure or low water level could destroy the boiler.
Boiler feed water is kept clean and the boiler is inspected periodically to avoid internal corrosion or fouling, which could destroy the boiler.
Guidelines for safe boiler design, control, maintenance and operator training are provided by NFPA 85 "Boiler and Combustion Systems Hazards Code". For a complete copy of NFPA 85 see www.nfpa.org.
The main items for combustion safety are these:
|
Item |
Safety |
Reason |
| Air Purge (startup) | Before lighting the boiler burner, the air blower is operated long enough to displace 4 times the boiler combustion chamber volume. | If natural gas and air are inside the boiler when the burner is lit, the boiler can explode. |
| High Natural Gas Pressure | A pressure switch in the natural gas line stops the burner if the gas pressure gets too high. | If the natural gas pressure control valve fails, excessive gas can flow to the burner. If too much natural gas flows to a combustor, the burner could go out or the flame could be large enough to cause overheating. If the flame goes out and natural gas continues to flow, a combustible mixture could fill the boiler combustion chamber (explosion hazard). |
| Low Natural Gas Pressure | A pressure switch in the natural gas line stops the burner if the gas pressure gets too low. | If the natural gas pressure falls too low, the burner could go out. If natural gas is still flowing, a combustible mixture could fill the boiler combustion chamber (explosion hazard). |
| Low Flame Strength | A flame detector stops the burner if the flame becomes weak or goes out. | If the flame goes out, natural gas flowing through the burner could cause an explosive mixture to fill the boiler combustion chamber (explosion hazard). |
| Air Blower Not Running | A switch stops the burner if the air blower is not running. | If the air blower is not running, the burner cannot operate. |
| Low Combustion Air Flow | A pressure switch in the burner air plenum stops the burner if air pressure gets to low (meaning the air flow is too low). | If the air flow becomes low, the flame may become weak or go out even though natural gas is still flowing (explosion hazard). |
The main items for boiler safety are these:
|
Item |
Safety |
Reason |
| High Steam Pressure | A pressure switch in the steam chamber stops the burner if the pressure gets too high. | If the burner continues firing even when steam flow is low, the boiler pressure can build and the vessel can explode. |
| Low Water Level | A level switch stops the burner if the boiler water level gets too low. | If the water levels gets low enough that part of the boiler runs dry, that part can overheat, weakening it and possibly resulting in an explosion. |
Special Hardware to Assure Safety:
|
Item |
Safety |
Reason |
| Natural Gas Pressure Regulator | An automatic pressure regulating valve steps natural gas supply pressure down to the level needed by the burner. | If the natural gas supply line pressure rises too much, the burner may over fire, putting out the flame or overheating the boiler. |
| Natural Gas Drip Leg | A drip leg (low point drain) located near the Natural Gas Pressure Regulator traps debris in the natural gas supply. | If solid or liquid debris enters the natural gas supply valve train, operation can be affected. |
| Natural Gas Block & Vent Valves | Two automatic block valves with a vent valve between them are used to stop natural gas from reaching the burner when it is off. The block valves have position switches so the control system knows when they are close. | A single automatic block valve in good condition would be enough to stop natural gas flow, but a second automatic block valve provides insurance. A vent valve between the block valves pipes any leakage gas to atmosphere when the block valves are closed. |
| Linked Natural Gas and Combustion Air Valves | The flow control valves for natural gas and combustion air are linked mechanically as one is opened or closed, the other does the same. | If too much or too little combustion air is supplied with the natural gas, the flame can go out. |
| "Self-Checking" Flame Detector | The flame detector has a mechanical shutter which blocks the detector tubes view of the flame periodically. If the detector tube still sees flame when blocked, the detector has failed and the burner shuts off. | If a flame detector tube fails it must be replaced. Sometimes these tubes can fail in the "flame OK" position if this happens the burner could go out and the detector would not know. NFPA guidelines require self-checking detectors when the burner operates more than 24 hours without shutdown. |
| Boiler Steam Pressure Emergency Relief Valves | Two special relief valves vent steam from the boiler if the pressure reaches dangerous levels. | If the pressure controller and the high pressure switch shutdown circuit fail, these valves prevent excessive steam pressure from destroying the boiler. |
| Water Level Gauge Glass | A special gauge lets the operator view the boiler water level directly. | If the level control instruments malfunction, the operator can still monitor water level and take appropriate action. |
| Low Fire To Light the Main Flame | The control system drives the burner main natural gas control valve to "low" flow when the main flame is to be lighted. | The pilot flame is much smaller than the main flame and may be unable to light the main flame if fuel gas is flowing at maximum rate. |
| Interrupted Pilot Burner | The small pilot burner turns off as soon as the main burner is lit. | A single flame detector is
used in this system. It must be located to see both the small pilot flame
and the large main flame. If the pilot operates all the time, and a problem
with the main flame caused it to go out, the system would not detect the
flame loss. At high burner firing rates, the pilot flame may be too small to
reignite the main flame.
Notes: With a separate flame detector for both the main flame and the pilot flame, the pilot can remain on all the time. Also, with a large pilot flame (usually at least 10% of the main flame) reignition of the main flame is very likely and the pilot can remain on even with a single flame detector in use. |