The need for interface level measurement arises whenever immiscible liquids–those incapable of mixing—reside within the same vessel. The lighter material rises to the top and the heavier material settles at the bottom. In oil production, for example, water or steam is used to extract oil from a well. Well fluids then route to production separators where they settle into their primary constituent parts as a water-hydrocarbon interface. Water may also be used as a transport medium or a cleaning agent and forms an interface with an allied material which is later extracted.
Interfaces are most commonly found in the diverse separation processes that are essential to every industry. Separation recovers additives, catalysts or solvents, extracts impurities, and routes media into different processing channels.
Principal Interface Applications
Here are a few of the most common industries and processes where interfaces are found:
Petroleum and Gas
• LPG Dehydrators
• Heater Treaters
• Crude Desalters
• Free-water Knock-out
• Crude Dewatering
• Acid Settling Tanks
• Alkylation Tanks
• Coking Drums
Water & Wastewater
- Settlement Tanks
• Sludge Thickeners
• Filtration Systems
• Final Effluent Monitoring
- Liquid Oxygen and Nitrogen production
- Digester Vessels
- Extractors & Separators
- Grease Traps
- Pulp and Paper
- Mining and Quarrying
- Food and Beverage
- Chemical Plants
- Storage Facilities
Though our emphasis is on liquid/liquid interface, interfaces also form between liquid and solids, liquid and foam, or liquid and a gas—such as gases (other than air) that are used in tank blanketing.
Immiscible liquids meet along an interface layer where they undergo some amount of emulsification. This emulsion layer (also called a rag layer) may form a narrow and precise boundary; but more frequently it is a broader gradient of mixed liquids—or liquids mixed with particles that form a slurry. Generally, the thicker the emulsion layer, the greater will be the measurement challenge. Knowing the position of a process interface is necessary for maintaining product quality and operations efficiency. The interface is measured and controlled by precision level switches and transmitters. Though at least 20 different types of liquid level measurement devices are in service today, only a very few are suitable for accurate and reliable interface level measurement.
Level Measurement Technologies
Here are three of the most reliable technologies for interface level measurementThe information presented describes the technologies as they pertain to Magnetrol® level instrumentation:
Displacer Controllers and Transmitters- Modulevel®
Measurement Principle: Movement of the interface level along the length of the displacer causes the precision range spring to extend or compress. This causes the movement of the core within a linear variable differential transformer in the Digital E3 Electronic MODULEVEL resulting in a digital or analog output. In the Pneumatic MODULEVEL, this causes the movement of a magnetic ball which guides the magnet carriage resulting in a pneumatic output change.
Interface Measurement: This technology is widely used for interface service because it is unaffected by emulsions and will accurately track the middle of the emulsion layer.
Thermal Dispersion- Thermatel®
Measurement Principle: Switches using thermal dispersion technology detect heat transfer which reduces the temperature difference between the switch’s two sensors; one sensor is for reference and the other is heated to a temperature above the process temperature. The temperature difference is greatest in air, then decreases when cooling occurs due to a change in media. The electronics compare the electrical signal from the sensor against the set point and provide a relay actuation.
Interface Measurement: The THERMATEL TD1/TD2 and TG1 switches have been designed and engineered for level, flow or interface detection. When used as an interface detection switch, the set point can be adjusted to detect the difference in media between two fluids that have different thermal conductivity. Water has a very high thermal conductivity while organic materials (oil) have a much lower thermal conductivity. THERMATEL detects the difference in media due to the temperature difference which will be greater in the organic layer than in the oil layer.
Guided Wave Radar- Eclipse®
Measurement Principle: ECLIPSE is based on Time Domain Reflectometry. TDR transmits pulses of electromagnetic energy down the wave guide, or probe. When a pulse reaches a liquid surface that has a higher dielectric constant than the air in which it is traveling (dielectric constant of 1), the pulse is reflected. Ultra high-speed timing circuitry provides an accurate measure of liquid level. Even after the pulse is reflected from the upper surface, some of the energy continues along the length of the probe through the upper liquid. The pulse is again reflected when it reaches the higher dielectric lower liquid.
Interface Measurement: The dielectric constant (ε) of the interface media is critically important for GWR. As shown in the illustration at right, the upper dielectric should be between 1.4 and 5, and the lower dielectric should be greater than 15. The typical oil and water interface application shows the upper, nonconductive oil layer being 2, and the lower, very conductive water layer being 80. ECLIPSE measurement is suitable where the interface is clean and distinct and the depth of the emulsion layer is shallow.
MAGNETROL has produced a special applications brochure featuring these and more interface level measurement technologies, with an overview of technology specs, process capabilities, and transmitter options. Download the brochure today and learn more about the benefits and applications of each technology.