Outgassing versus flashing

Volume 11

Issue 2

The Emerson severe service team provides global customers with Fisher® severe service control valve solutions. Whether it is severe service applications for the power, hydrocarbon, chemical or pulp and paper industry, these technical experts deliver sound solutions to address critical applications for aerodynamic noise, cavitation and out-gassing issues, as well asparticulate erosion. Please visit our website or contact your local Emerson Process Management sales office for more information on how the severe service team can help you.

Outgassing Versus Flashing - What are the Differences?
While outgassing and flashing may seem similar to many process engineers, the driving forces and the effects on a control valve are quite different. Flashing occurs when thepressure of a fluid falls below its vapor pressure. At this point, the fluid begins to change from a liquid to a vapor, both of which have the same chemical makeup. The vapor pressure is a function of the fluid temperature and, therefore, flashing is a function of both the pressure and temperature of the fluid. For the fluid to flash, heat must transfer from the liquid during the vaporizationprocess and this requires time. Outgassing results in a liquid and gas mixture as well, but the driving force is strictly due to the fluid pressure being below the saturation pressure of a gas dissolved in a liquid. Once the fluid pressure is below that saturation point the gas comes out of solution and the liquid and gas have a different chemical makeup. The best illustration of outgassing is whatoccurs when a bottle of champagne is opened. Immediately, the pressure inside the bottle drops to atmospheric, which is below the saturation pressure of the dissolved carbon dioxide. The same holds true in a control valve. A fluid with the required heat transfer takes a certain amount of time to flash whereas outgassing requires no heat transfer and occurs much faster. Downstream of the valve thetwo phenomena can appear to be similar, but at the throttling point the difference can be dramatic. Flashing is well understood behavior and valve sizing for flashing is covered by standard liquid sizing calculations. Outgassing, which can occur in many combinations of liquids and gases, does not follow a specified thermodynamic path and special valve sizing routines

are required to handle it.Likewise, valve and trim styles as well as material selected for flashing applications may not be suitable for outgassing. So how can you tell if your application may have an outgassing or flashing problem? Two indicators are the liquid and gas exiting the valve have different molecular weights and the valve is providing liquid-level control for a phase separation process. Your local Emerson Salesor Local Business Partner office can help obtain the best solution to your outgassing or flashing problems.

Fisher® Control Valves Help Minimize Flashing Damage
Flashing can result in significant erosion damage to a control valve and adjacent piping. There is no erosion coefficient, no industry standards, and no scientific means for predicting the intensity of flashing damage. However,Emerson understands that flashing damage potential is a function of many factors which include flow rate, fluid velocity, pressure drop, corrosive nature of fluid, body geometry, and valve body and trim material. Flashing cannot be prevented by the control valve as flashing phenomenon is a direct result of application conditions and fluid properties. However, selecting a control valve designed withspecial consideration of flashing can avoid or minimize the flashing damage and then extend valve service life. Emerson engineers worldwide are trained to recognize flashing problems and address them in the most effective manner. When flashing is encountered, control valves can be selected just as they are for erosive fluids. Emerson offers several valve styles and options that are best suited for...