C-P Systems
What Is Dissolved Gas? | Process Engineering Glossary
What Is Dissolved Gas?
Dissolved gas refers to gas molecules present within a liquid phase at a molecular level, rather than existing as a separate bubble or vapor phase. The amount of gas that can dissolve in a liquid at equilibrium depends on temperature, pressure, and the specific gas-liquid pair involved, generally described by Henry’s Law for reasonably dilute conditions, with dissolved gas concentration typically increasing with pressure and decreasing with temperature.
Dissolved gas can significantly affect process behavior in ways that aren’t always immediately obvious, including corrosion rates, cavitation risk in pumps, and unexpected gas release when a liquid stream experiences a pressure or temperature change.
Applications of Dissolved Gas
Corrosion Rate Management
Dissolved oxygen and carbon dioxide are significant contributors to corrosion in many aqueous systems, making dissolved gas management, often through deaeration or chemical scavenging, an important corrosion control strategy.
Pump Cavitation Risk Assessment
Dissolved gas coming out of solution as pressure drops locally within a pump can contribute to cavitation-like damage, making dissolved gas content a relevant consideration in pump suction system design.
Degassing and Deaeration Process Design
Processes requiring low dissolved gas content, whether for corrosion control or product quality reasons, rely on dedicated degassing or deaeration equipment designed to reduce dissolved gas to target levels.
Benefits of Knowing Dissolved Gas
Explains otherwise puzzling corrosion and cavitation issues. Understanding dissolved gas behavior helps explain corrosion or pump performance issues that might otherwise seem unrelated to an obvious process cause.
Informs appropriate degassing system design. Recognizing dissolved gas requirements supports designing degassing or deaeration equipment appropriately sized for the actual dissolved gas reduction needed.
Supports better understanding of pressure and temperature effects. Understanding how dissolved gas solubility changes with pressure and temperature helps predict where and when gas release might unexpectedly occur in a process.
Limitations to Consider
Henry’s Law approximation has limits. The simple Henry’s Law relationship used to estimate dissolved gas solubility is most accurate at low concentrations and can become less reliable at higher pressures or gas concentrations.
Measuring dissolved gas accurately can be challenging. Accurately measuring dissolved gas concentration, particularly at low levels relevant to corrosion control, requires specialized instrumentation and careful sampling technique.
Achieving very low dissolved gas targets can require significant equipment investment. Meeting stringent dissolved gas specifications, such as those needed for certain corrosion-sensitive services, can require substantial degassing equipment and energy investment.
Dissolved Gas FAQ
How does dissolved gas relate to partial pressure concepts?
Dissolved gas solubility is directly related to the gas’s partial pressure above the liquid, forming the basis for Henry’s Law-based dissolved gas estimation, relevant also to dehumidification calculations.
Why is dissolved gas relevant to crevice corrosion and cathodic protection?
Dissolved oxygen contributes to corrosion mechanisms including crevice corrosion, making dissolved gas management relevant alongside cathodic protection as part of a comprehensive corrosion control strategy, particularly for storage tank and piping systems with a dead leg.
How does dissolved gas relate to a can pump’s cavitation risk?
Dissolved gas coming out of solution at reduced pressure within a pump, including a can pump, can contribute to cavitation-like damage, an important consideration alongside standard fluid mechanics-based suction system design.
Outbound Links
About C-P Systems
SETTING THE STANDARD FOR CHEMICAL ENGINEERING FIRMS EVERYWHERE
Through unmatched professionalism, knowledge and experience, we set the industry bar for chemical engineering firms. With decades of chemical plant engineering and piping design experience, our team of licensed engineers can handle any project scope.