by Joe Farsetta, Certified Master Inspector® and Certified Commercial Inspector

The fuel or gas train is a series of components – some working independently, and others working in tandem with alternate components – that connect the burner of a gas-fired boiler, which help to ensure the commercial steam boiler’s safe operation. Boiler explosions can happen in one of two ways: pressure failure, or a failure on the combustion side (furnace), so it’s important for the commercial inspector to know what components may be installed as part of the gas fuel train, along with their function.

Some Basics

The following is a list of components commonly found in a fuel gas train:

  • The safety shutoff valve is a valve component (sometimes as two units in series) that will automatically shut down the gas supply to the boiler. Large commercial boilers typically have an auto-shutoff that reacts and shuts the gas down within one second. Older boilers may be equipped with dial safety shutoffs. Sometimes these were called block-and-bleed.
  • The manual shut-off valve shuts off the gas supply to the boiler itself.
  • The gas pressure regulator helps regulate the gas pressure.
  • The low-pressure gas switch detects when the gas pressure is too low to allow the burner to operate properly.
  • When combustible gas leaks beyond a safety valve or other device, the gas is captured and safely vented to the atmosphere by the vent valve in order to avoid accidental combustion. Vent valves work in tandem with safety shutoff valves. In fact, whenever a safety shutoff is closed, the vent valve automatically opens. If any gas sneaks by the safety shutoff, the vent allows it to vent to the atmosphere.
  • The high-pressure shutoff switch detects high-pressure conditions in the gas line and shuts it down. It acts as a fail-safe if the gas regulator fails.
  • The steam boiler may also be equipped with test valves, which are used to test the safety shutoffs. Be sure to ask the person in charge of boiler operations whether the safety shutoffs are tested monthly, as it may be a requirement to do so. Evaluating any testing procedures and available data can help the inspector determine how well the boiler is functioning, and how much attention is paid to its overall safety and functionality.



Main Shut Off


How to Inspect a Typical Fuel Train

  • At the gas feed itself is located the drip leg or dirt leg, which is also seen at a residential installation and performs the same function. Some commercial steam plants also utilize strainers on the line, whose purpose is similar to that of the drip leg: to catch liquid and sediment. Strainers are not very common, however.
  • Next in line would be the manual shutoff valve.
  • Next may be the gas regulator. It may be equipped with a vent fitting on top, and a pipe going to the exterior of the building. A momentary over-feed of gas pressure at ignition can occur. Therefore, the inspector should find out if there are electronic gas shutoffs located after the regulator or before it. The burner housing and plates should be examined for signs of burn marks, which can indicate flame rollouts.
  • Next should be the low-pressure switch. As discussed, this device helps regulate gas pressure. If the gas pressure drops below the predetermined minimum, the burner will shut down. If the natural gas pressure falls too low and the burner goes out, un-combusted gas would still flow. Once it fills the combustion chamber, if there is an accidental ignition or spark, this could result in a gas explosion.
  • There may be several manual or automatic shutoff valves in the fuel train, but they are unrelated to the high-pressure shutoff. So, one may find the high-pressure shutoff switch immediately adjacent to the burner assembly. When it senses high gas pressure (maybe from a defective or runaway regulator), it will immediately shut down the burner. It will then require a manual reset. A high gas-pressure situation requires the immediate attention of the boiler operator in order to avoid an explosion. As critical as this device is, not all boilers are equipped with one. Typically, only larger steam plants (2.5 million BTU and higher) will have them. If gas pressure is runaway, excessive gas will flow to the burner. Although it is a combustible gas, a momentary gust can cause the burner to run away, resulting in an overheating condition. Or, the opposite can happen, where the momentary gust can extinguish the flame. If this happens, the gas continues to flow into the burner chamber, and an accidental ignition can result in a gas explosion.
  • Returning to the location of the low-pressure unit, examine what may be found next. (Remember that not all systems are configured identically, so this list is for general reference only.)
  • The safety shutoff valves may be the next items in line. There may be a set of two valves, with one acting as the primary and the second as the fail-safe. The inspector may see threaded caps installed in these devices in order to perform testing. These were sometimes referred to as block-and-vent valves. In this specific configuration, which might be found in older systems, the two automatic valves were installed with a vent valve between them. Newer configurations may not have the vent valve installed. Ask the boiler plant operator if regular testing of the shutoffs is performed.
  • A modulating or firing rate valve may be installed. It is typically seen on continuous-duty boilers, which fire 24/7. It helps control the combustion mixture to the burner flame.

Additional Components

  • The low-flame strength 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. This serves a different function than the low-pressure shutoff. It is the “fire watchdog.”
  • The low-combustion air-flow switch is a pressure switch in the burner air plenum that is specifically designed to shut down the burner if the air pressure drops. Remember that combustion air is just as essential as the fuel in a burner operation.
  • The system may also be equipped with linked gas and combustion air valves. These valves may be physically linked to allow one to operate in one direction, and the other to operate simultaneously in another direction. As an example, one valve may open; when this happens, the connected (or linked) valve may close at the same rate. The purpose is to help regulate the fuel and combustion air mixture at the burner.

The safe operation of any fuel/gas-fired steam boiler requires the combustion to be monitored and controlled in order to avoid accidentally creating an explosion inside the combustion chamber. This directly involves the fuel train.


The commercial inspector should familiarize him/herself with the size and type of system they are inspecting, along with the various components installed along the gas fuel train. Ask questions and look for evidence of deferred maintenance. Examination of scheduled maintenance operations, inspections, repairs, and testing is critical.


More commercial inspector resources:

Steam Boiler: Types and Designs
Commercial Steam Boilers: A Primer
Steam Boiler Mechanics and Their Application in Commercial and Industrial Settings