If you’re already well steeped in the knowledge of outgassing and flashing of gases in your process fluid flows, then this post is not for you. If you’re like me and may have heard the terms but never really stopped to understand the differences, then hopefully this post will help.
I saw a recent Severe Service Journal article, Outgassing Versus Flashing – What are the Differences?, from Emerson’s Fisher Severe Service control valve team. Both conditions involve the separation of gas from the liquid fluid flow. Before I dive into what they are and their differences, let’s start with what kind of trouble they can cause process manufacturers.
Flashing gas 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.
…cause problems in a control valve because of the sudden increase in flow volume once the gas comes out of solution. Because the gas has a much larger volume than the liquid, the flow area needs to be larger or else the velocity of flow will increase. A restricted flow area can create a stream of high velocity liquid droplets with the potential to be erosive to any surface they impinge upon.
Another problem is the release of energy that the valve and piping are subjected to and the potential for high vibration levels.
Erosion, increased flow velocity, and high vibration are all things that can have an impact on process availability, product quality, and operations and maintenance budgets.
So what exactly are they and what are the differences? Flashing gas is a function of the pressure and temperature of the flowing fluid. It occurs when:
…the pressure 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 vaporization process and this requires time.
Think of outgassing as what happens when you open a soft drink can and the sudden change in pressure causes bubbles of carbon dioxide to come out of the liquid. 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.
With regard to control valve applications:
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.
Two indicators to look for if your process is having a flashing or outgassing problem:
…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.
The Severe Service Journal article goes on to describe alternative Fisher valves and associated trims based upon the fluid dynamics of your process. There are also quite a number of folks across the globe that can assist in the sizing and selection process.