In burner trim and isolation service for gaseous fuels, the main gas valve in a fuel train is typically of which valve type (selected for good throttling and tight shutoff)?

Introduction / Context:
Fuel trains for gaseous burners include regulators, safety shutoff valves, and a main control valve. Valve selection balances throttling characteristics, pressure drop, leakage class, and actuator compatibility. Knowing the typical choice helps in reading P&IDs and troubleshooting combustion systems.

Given Data / Assumptions:

• The “main gas valve” is intended to modulate fuel flow for firing rate control and to close reliably under trip conditions (often paired with dedicated safety shutoff valves).
• Stable control around setpoints and good rangeability are important.
• The fluid is a clean gaseous fuel.

Concept / Approach:
Globe valves offer superior throttling characteristics due to their linear flow path change with stem position and well-defined flow coefficients. They provide finer control compared with on/off style gate valves and offer better modulation than basic butterfly valves at low openings. For many burner systems, the main control valve (modulating valve) is a globe valve equipped with a positioner/actuator, ensuring smooth firing-rate changes and stable pressure at the burner manifold.

Step-by-Step Solution:
Identify control objective: precise, repeatable modulation.Match valve characteristic: globe valves are preferred for accurate throttling.Conclude “globe” is the typical main gas control valve type.

Verification / Alternative check:
Combustion equipment specifications frequently call for globe control valves or specialized control ball valves; training schematics show globe valves on the modulating leg with safety shutoff valves upstream/downstream.

Why Other Options Are Wrong:
Gate: Good for isolation, poor for throttling—tends to chatter and erode seats.Butterfly: Economical and compact but less precise at low openings; commonly used for air, not always as the main fuel control.None of these: Incorrect because globe is a standard choice.

Common Pitfalls:

• Assuming any valve with an actuator is suitable for precision control.
• Overlooking leakage class and safety certifications required in fuel service.

Final Answer:
globe