Gas flow and gas-hammer analysis

Compressible-flow models of fuel-gas and process-gas systems, including the transients that follow a turbine trip or a fast valve closure.

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Power station and cooling tower in a hazy landscape

Gas does not behave like water

Gas density changes with pressure and temperature, so velocity and pressure drop change along every run of pipe. Liquid methods and rules of thumb miss that, and they say nothing about what happens in the seconds after a trip.

  • One turbine tripping could disturb the fuel supply to the units still running
  • Relief valve set points need checking against real surge pressures
  • Delivery pressure at the far users is lower than expected
  • An addition or reroute changes how the system shares its flow

What the analysis covers

Steady-state gas systems are modeled in AFT Arrow and gas transients in AFT xStream.

  • Compressible flow

    Pressure drop, velocity and delivery pressure through the network for each operating case.

  • Gas transients

    Trips and fast valve closures modeled as events, with peak pressures traced through the system.

  • Relief and trip interactions

    Whether a surge reaches relief valve set points or the trip settings of equipment that should keep running.

  • Applications

    Gas-turbine fuel-gas systems, process-gas lines and plant utility gas networks.

Refinery column and structure under a cloudy sky
Power plant exterior with stacks and red piping

Fuel-gas surge at an overseas power plant

A gas transient study of the fuel-gas system serving two gas turbines modeled a turbine trip as a sudden valve closure. Maximum surge pressures from the tripped unit would not trip the other operating turbine, and would not lift any relief valves.

Work performed by our team members before FSPG, shown as project experience.

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Adding a unit to a shared gas header?

Check how a trip on one unit reaches the others before the tie-in is made, while the fix is still a setting or a valve rather than a rebuild.

Further reading: Check for a Bumpy Ride, LNG Industry, April 2021

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Gas flow questions

The compressible-gas counterpart of waterhammer. A sudden valve closure or a turbine trip sends pressure waves through the gas system. Peak pressures are usually lower than in liquid systems, but they can still trip other equipment or lift relief valves.

Gas density changes with pressure and temperature, so velocity and pressure drop change along the line. Compressible-flow models account for that change; methods written for liquids do not.

Yes. Plants often have both, for example a cooling-water network and a fuel-gas header feeding the same units, and they can be scoped and reported together.

Describe the gas system, its users and the event you are concerned about.

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