Thermodynamics of Gas Piping Systems

Thermodynamics of Gas Piping Systems

Colorado Springs, Colo., USA -- November 9, 2020 --  The November issue of Chemical Engineering Magazine includes a features article written by Walt Prentice, Applied Flow Technology Business Application Engineer. The article discusses how the goal of the engineer should be to match mathematical models as close to reality as is reasonable and in as timely a manner as possible.

The best way to accomplish this is to use modeling software that does not rely on such simplifications as incompressible solution methods or adiabatic or isothermal flow. Those may be useful for first-pass hand calculations but should not be relied upon for in-depth analysis. Compressible flow has many more complications than discussed here, such as sonic choking, so considering those effects only makes analysis harder. Do not sacrifice model quality for calculation ease. 

From the article:

Every engineer uses assumptions because every analysis has an overabundance of variables to consider. Often, there are just too many to analyze and simplifications are made to eliminate variables or create more relationships between them. The decision of what assumptions to make rests on the engineer’s shoulders. Even beyond making a problem solvable, engineers build assumptions into their analyses to make the math easier and faster. Although a problem may be technically solvable, there is value in simplifying the problem further. After all, exact answers do not exist in engineering.

When analyzing something as complex as gas flow through pipes, the math becomes daunting to say the least. Hydraulic modeling of liquid and gas systems are worlds apart, simply because gases have variable density, while liquids do not. (This may not sound like a huge difference, but open a compressible flow textbook, and you will quickly change your mind.)

With all the intricate considerations of compressible flow, heat transfer complicates the situation exponentially. Yet, without analyzing heat transfer, the system model may deviate far from reality. Flow behavior, among other things, relies heavily on temperature. This leaves engineers with a dilemma and often pushes them to simplify their operating conditions with assumptions to make the math easier. Two common assumptions in gas-handling processes are (1) adiabatic flow and (2) isothermal flow. The derivations, implications and outcomes of each assumption need to be understood before an engineer puts confidence in them.

Includes sections on:

  • System Boundaries
  • Adiabatic Flow
  • Changes to the Real System
  • Value of Modeling Software


   Read the Online Article    

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