Gas Trains 101

gas trains fuel train gas supply commercial boiler burner

A gas train (also called a fuel train) is the assembly of piping, valves, and safety devices that safely deliver natural gas to a commercial boiler burner at the correct pressure and flow.

Below is a breakdown of the common components, listed in the typical order of gas flow, and what each one does.

Term of the WeekGas Trains

Manual Gas Shutoff Valve
Gas Strainer
Gas Pressure Regulator
High Gas Pressure Switch
Low Gas Pressure Switch
Safety Shutoff Valves
Vent Valve
Valve Proving System
Firing Rate Valve / Modulating Gas Valve
Burner Management System
Big Takeaway

Simplified version of a typical gas train flow order

Gas Supply ? Manual Shutoff ? Strainer ? Regulator ? High/Low Pressure Switches ? Safety Shutoff Valves ? Modulating Valve ? Burner

Manual Shutoff Valve

What it is: A quarter-turn ball valve or plug valve

Function:

Allows a technician to manually isolate the boiler from the gas supply.
Required by code at the appliance.
Used for servicing or emergency shutdown.

Gas Strainer (Sediment Trap / Drip Leg)

What it is: A strainer or drip leg installed upstream.

Function:

Removes dirt, pipe scale, and debris from the gas.
Prevents contaminants from damaging regulators and valves.
Protects sensitive safety components downstream.

Gas Pressure Regulator

Common manufacturers include Maxitrol, Belgas, Sensus and Pietro-Fiorentini

What it is: A spring-loaded diaphragm regulator

Function:

Reduces incoming gas pressure (which may be 2–5 psi or higher) down to the burner’s required pressure (often 3.5"–14" w.c. for natural gas).
Maintains constant outlet pressure even if inlet pressure fluctuates.
Think of it as a “pressure stabilizer” for the burner.

High Gas Pressure Switch

Common manufacturers: include Honeywell, Kromschroder and Dungs

Function:

Monitors gas pressure downstream of the regulator.
If pressure rises above a safe setpoint, it opens the safety circuit and shuts down the burner.
Protects against regulator failure or overpressure.

Low Gas Pressure Switch

High and low pressure switches are safety interlocks tied into the burner management system.

Function:

Monitors for insufficient gas pressure.
If pressure drops below the safe minimum, it shuts down the burner.
Prevents flame instability or incomplete combustion.

Safety Shutoff Valves (SSOVs)

Often manufactured by: Dungs, Honeywell, or Kromschroder.

What they are: Electrically actuated normally-closed gas valves.

Function:

Open only when the burner control proves safe startup conditions.
Close immediately upon flame failure or safety fault.
Typically installed in double-block configuration (two valves in series).

This is the primary fuel safety device in the gas train.

Vent Valve (Proof-of-Closure Systems)

Often part of: SSOV’s valve assembly

Function:

Installed between two safety shutoff valves.
Vents any trapped gas to atmosphere when valves are closed.
Used in systems requiring proof of closure or valve leak testing.

Common in larger commercial and industrial boilers.

Valve Proving System (VPS)

Also known as : VPS

Function:

Automatically tests the tightness of the safety shutoff valves before startup.
Ensures no gas is leaking past a closed valve.
Required on many high-input boilers by code.

Firing Rate Valve / Modulating Gas Valve

What it is: A motorized control valve tied to the burner modulation system.

Function:

Adjusts gas flow based on firing demand.
Works in coordination with the combustion air damper.
Allows the boiler to operate at low fire to high fire smoothly.

In fully modulating boilers, this valve is controlled by the burner management system.

Burner Management System (BMS)

Examples include: Honeywell Flame Safeguard controls and Fireye burner controls

Function:

Controls startup sequence (pre-purge, ignition, pilot, main flame).
Monitors flame signal.
Supervises all safety devices (pressure switches, limit switches, valve proving).
Immediately shuts down gas flow if flame is lost.

This is the “brain” of the burner.