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What Is Dissolved Gas? | Process Engineering Glossary
What Is Dissolved Gas?
In piping engineering and process engineering, dissolved gas is gas held in solution within a liquid at a given pressure and temperature. When pressure drops or temperature rises, the gas comes out of solution and forms free bubbles. This release drives critical design decisions across pump selection, pipeline hydraulics, separator sizing, and relief system engineering.
Applications of Dissolved Gas Management
Produced Water Treatment
Produced water from oil and gas wells carries dissolved hydrocarbons and hydrogen sulphide. These dissolved gases create hazardous vapour concentrations when the produced water is handled at atmospheric pressure in open tanks and treatment vessels. Closed degassing systems with controlled venting to a safe disposal point manage this dissolved gas release safely. In addition, the recovered gas often has sufficient heating value for use as fuel.
Boiler Feed Water Deaeration
Dissolved oxygen and carbon dioxide in boiler feed water cause corrosion throughout the steam and condensate system. Thermal deaeration heats the feed water to its boiling point at the deaerator operating pressure, driving dissolved gases from solution by reducing their partial pressure to near zero in the steam space above the liquid. The liberated gases vent to atmosphere or to a low-pressure steam header.
Carbonated Liquid Handling
In food and beverage processing, dissolved carbon dioxide in pressurised liquids must be maintained in solution until filling. Pump and pipeline design for carbonated beverages therefore keeps the pressure above the carbon dioxide saturation pressure throughout all handling stages. Any pressure drop below saturation causes unwanted foaming and product loss.
Water Hammer Prevention
Gas pockets released from dissolved gas in pipelines can cause water hammer when they collapse. A liquid slug accelerating into a gas pocket and then colliding with a closed valve or a full pipe section creates an impulsive pressure spike. Managing dissolved gas release through proper venting, maintaining system pressure above bubble point, and avoiding rapid valve closure all reduce the risk of water hammer from dissolved gas effects.
Benefits of Understanding Dissolved Gas
Correct Hydraulic Design
Recognising where dissolved gas releases along a pipeline allows the engineer to apply two-phase flow methods in the correct sections and single-phase methods elsewhere. This produces accurate pressure drop and pump sizing calculations rather than optimistic single-phase estimates that lead to undersized pumping systems.
Equipment Protection
Identifying dissolved gas release at pump inlets, control valves, and orifice plates allows the engineer to redesign these locations to avoid gas cavitation. As a result, equipment operates within its design envelope and avoids the vibration, erosion, and performance loss that dissolved gas cavitation causes.
Safe Relief System Design
Accounting for dissolved gas flash loads in relief calculations ensures relief devices are sized for the actual worst-case event. This protects vessels from overpressure during depressurisation scenarios that involve both boiling and dissolved gas evolution simultaneously.
Limitations to Consider
Metastable Supersaturation
Dissolved gas does not always release exactly at the theoretical saturation pressure. Metastable supersaturation means the gas can remain dissolved at pressures below the saturation value until suitable nucleation sites trigger release. This uncertainty makes precise prediction of where gas release begins along a pipeline or through a valve difficult without experimental data for the specific fluid.
Composition Variability
The dissolved gas content of process liquids varies with upstream operating conditions, feed composition, and seasonal temperature changes. A liquid that is well below its bubble point at summer ambient temperatures may approach or reach its bubble point in winter. Engineers must size systems for the worst-case dissolved gas content rather than the average condition.
Analytical Complexity
Calculating the bubble point and dissolved gas content of complex mixtures such as crude oil, produced water, and multicomponent chemical streams requires thermodynamic equation-of-state calculations. Simple hand methods are insufficiently accurate for these fluids. Process simulation software with appropriate fluid property models is necessary to produce reliable dissolved gas data for detailed engineering design
Dissolved Gas FAQ
What is dissolved gas in piping engineering? Dissolved gas is gas held in solution within a liquid at a given pressure and temperature. When the pressure drops below the saturation pressure defined by Henry’s Law, the gas comes out of solution as free bubbles. This release creates two-phase flow conditions in pipelines, reduces centrifugal pump performance, adds to separator gas loads, and increases relief valve sizing requirements. Managing dissolved gas correctly is essential for safe and reliable plant operation.
How does dissolved gas affect centrifugal pump performance? As liquid passes through the impeller eye of a centrifugal pump, the local pressure drops sharply. If this pressure falls below the dissolved gas saturation pressure, gas bubbles form at the impeller inlet. These bubbles reduce the effective liquid throughput, cause erratic performance, and can produce vibration and mechanical damage. Engineers prevent this by maintaining adequate net positive suction head above the saturation pressure and by degassing the liquid upstream of critical pump inlets.
What is the difference between dissolved gas release and vapour cavitation? Vapour cavitation occurs when liquid pressure drops below the vapour pressure and the liquid itself vaporises. Dissolved gas release occurs when pressure drops below the saturation pressure of a dissolved gas species, which is often higher than the liquid vapour pressure. Both produce bubbles at the pump or valve inlet, but dissolved gas cavitation is generally less mechanically damaging because the bubbles re-dissolve slowly as pressure recovers rather than collapsing violently. However, both mechanisms reduce pump efficiency and should be avoided through proper system design.
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