Fuel gas lines: Globe vs. sonic valve

A prime consideration for fuel gas flow control valve is valve type. The current standard convention is to utilize the globe valve as it offers a high level of control. However, the globe valve has a tortuous path for the gas to flow through, and as result, the pressure loss between valve inlet and outlet is high. Recent advancements in angle-bodied valves which operate at sonic conditions (where the velocity through the valve reaches Mach number of 1.0) have spurred the heavy duty power industry to evaluate implementing this technology as an alternative option to traditional globe valves.

This article contains excerpts from the paper, "Advanced Class Gas Turbine Fuel Gas Control" presented at Power Gen International 2017 by Anthony Bravato and Christopher Bethel of Mitsubishi Hitachi Power Systems.

In general, the ‘sonic’ angle valve is appealing given that it can offer control capability similar to a globe valve with a reduced pressure loss due to the less complex flow path; the gas flows from the inlet to the throttle point (between the plug and seat) and then exits the valve and enters the outlet pipe without further change in direction. This simplified outlet flow path provides valve designers the opportunity to make a more gradual transition from the higher-velocity / lower-pressure condition at the throttle point to the lower-velocity / higher-pressure condition of the valve outlet. With these geometry options, a valve designer can further reduce pressure drop over the valve.

The angle bodied valve additionally gives the valve designer the ability to shape the valve outlet in a way that resembles a converging-diverging nozzle with desirable pressure recovery characteristics. A traditional globe valve will have a more sudden expansion which is more analogous to a sharp-edged orifice.

There may be cases where the gas pipeline pressure is insufficient and fuel gas compression would typically be needed, but by implementing a high recovery flow control valve set, the entire fuel gas supply train pressure drop is reduced enough to avoid inclusion of gas compression.

While reducing pressure drop of the fuel gas control valves should always be a high priority, a primary benefit of using a sonic angle valve in a fuel gas application for large frame gas turbine application is the fact that the normal operating condition is with flow choked. This mode of operation ensures that, as long as the ratio of inlet and outlet pressure is sufficient to create choked condition, the mass flow is constant for a given valve opening. Another way to explain this phenomenon is that after reaching the critical pressure ratio further decrease of the downstream pressure does not result in an increase of flow.