M. Thomas and K. DROUGHTON, Endress+Hauser, Houston, Texas
There are several reasons to quantify and qualify the material properties of liquids in vessels and pipes. Numerous applications are characterized by process media with unique properties, leading to many distinct measurement techniques.
Most transporters and processors of hydrocarbon compounds are familiar with several different level measurement technologies, including but not limited to radar, ultrasonic, differential pressure, capacitive and guided wave radar (GWR). However, most are unfamiliar with using radiometric technology to measure level or maintain a density profile in a vessel.
This article will examine how radiometric—or gamma—measurement and density profiling works, when to use it, and describe why it is considered the most underutilized measurement technique in demanding petrochemical and refinery applications.
Areas of application. Noninvasive radiometric systems are useful for point and continuous level, density and interface measurement in liquids, solids, suspensions and sludges (FIG. 1). Radiometric sensors—also known as detectors—maintain measurement integrity even in the presence of extreme process conditions (such as extreme pressures and temperatures), along with corrosive, toxic and abrasive media. Detectors can be sensitive to background and extraneous radiation within a facility, such as from non-destructive x-ray materials testing; however, newer, patented designs address this issue, as explained later.
Gamma level measurement is frequently employed where other measuring principles fail due to harsh conditions, peculiar process media characteristics or vessel geometric limitations. For example, emulsion and multi-layer interfaces are frequently encountered in refinery and petrochemical processes, and the GWR and capacitive techniques conventionally used to measure vessel interface and total level are not reliable for some multiphase layer applications (FIG. 2).
Radiometric measurement is noninvasive in relation to process media, with instrumentation installed outside a vessel or pipe to measure level or create a density profile through the vessel walls. This provides a high degree of availability, although gamma instrumentation’s higher cost relative to other level measurement technologies must be considered.
Radiometric measurement principals and components. To safely and accurately emit and measure gamma radiation, several components are necessary.
Gamma source. Industrial radiometric measurement systems utilize a cesium or cobalt isotope to emit gamma radiation, which is attenuated as it passes through materials. Two types are used:
As each of these radioactive isotopes decay, they emit beta radiation in the form of particles, and gamma radiation as electromagnetic waves. To shield beta radiation, the source is encapsulated by a double-walled stainless-steel housing (FIG. 3). The gamma source is then installed in a container, also known as a source holder, protecting it against mechanical and chemical impacts (FIG. 4). By enclosing isotopes in a protective housing and source container, instrumentation adheres to required classifications of radioactive sources with C66646, according to ISO 2919 and the U.S. Nuclear Regulatory Commission.
Safe source containers. The source container is made from high-density lead, providing a shield for the gamma radiation inside a steel enclosure. The radiation is only allowed to emerge at a narrow angle through a slit, which can be switched open or closed (FIG. 5). The gamma source container’s radiation is emitted with minimal attenuation in one direction only and is completely damped in all other directions, providing safety in the surrounding operating vicinity.
Source containers are available in different sizes to provide optimal shielding in relation to the activity of the radioactive isotope. Special process conditions require specific adjustments to source containers depending on the application. For example, source containers housing radioactive isotopes in a double-walled protection pipe inserted into a vessel are commonly used in applications such as oil and gas separation, which is discussed later in this article.
Compact transmitter. The gamma radiation is emitted through the vessel or pipe containing the process medium and is detected on the opposite side by a compact transmitter (FIG. 6). This transmitter consists of a sensing unit called a scintillator to capture the gamma radiation, and an electronic unit to analyze and transmit the measured values to control and monitoring systems. These compact transmitters can be adapted to many tasks by using different materials and measuring ranges for the scintillator.
The radiation is attenuated based on the process material’s density, thickness and concentration. As the gamma wavelengths intercept the detector’s scintillator, they are converted to particles of light, or flashes. Each flash is then transmitted through the scintillator to the photomultiplier (FIG. 7).
In the photocathode, the flash is converted into a small electric charge, then amplified to a current pulse in the photomultiplier where it is processed as a measurement signal. As the process medium’s level or density increases, it absorbs more gamma energy so less makes it to the detector. Simply put, the measurement span is inversely proportional to the amount of energy received.
Gamma modulator. The gamma modulator is the last critical element in most radiometric measuring systems.
Nondestructive material tests performed on nearby equipment, such as weld radiography using gamma radiation or other radiating media, can result in inaccurate measurement within the compact transmitter. To distinguish between external noise and energy from the measurement system’s source material, the gamma modulator applies a unique differentiator to the measurement system’s source material.
The modulator is comprised of two rotating absorption rods located between the source container’s emission channel and the transmitter, creating a unique modulated on/off energy signal. This generates modulated gamma radiation on a fixed frequency, which the detector can distinguish from external radiation (FIG. 8). Therefore, accurate measurement is possible even in the presence of interfering external radiation, significantly increasing production availability.
Gamma use cases. The following use cases demonstrate scenarios where radiometric technology provides greater control and minimizes downtime vs. conventional measurement methods.
Measuring interface in separators and desalters. When measuring heavy crude, water, emulsion and sand in a separator, gamma is often the only viable level measurement technique because of its ability to accurately stratify the contents of a vessel using density profiling.
Separators are frequently employed to split mixtures of gas, oil, water and sand that are extracted from oil fields. Heavy substances (e.g., sediments) sink to the bottom, while light substances (e.g., methane) rise and are removed from the top of the tank (FIG. 9).
An emulsion, or rag, layer forms between the oil and water, and is minimized by the addition of demulsifiers. The process proceeds optimally when a defined interface layer occurs between the oil and the water.
If the thickness of the interface layer is not reliably monitored, water may pass over the weir in the separator, which can cause problems in downstream oil production processes. If the water surface subsides to a low level, oil may be withdrawn at the bottom of the separator along with water, reducing revenue potential and risking pollution to the environment.
The position of the interface layer can be determined by the density profile within the separator. This is accomplished by mounting a gamma source container within a double-walled protection pipe, and then inserting the source into the protection pipe with a rod or cable. Detectors are arranged in horizontal groupings along the outside of the vessel relative to the perceived elevation of the oil-water-emulsion interface—typically within a span stretching below the top point of the internal weir—and near the water bottom and sand heal elevations (FIG. 10).
This low-maintenance external measurement technique continuously monitors the changes in liquid density, providing interface layer information without using any mechanical components. This information enables plant personnel to maximize process throughput while optimizing the use of team resources and material operational expenses, as well as minimizing downtime.
Fluidizing solids in a bed reactor. Some processes use fluidized-bed reactors in the production of polyethylene or polypropylene. In these reactors, solid particles are fluidized by gas flowing upwards, which generates close contact between the fluidized product, solid particles and the fluidizing medium (gas). This close contact increases the reaction rate.
The fluidized product in the reactor does not form a defined surface, but the density profile of the solids content—the fluidized bed—must still be determined for process control and optimization. Precise positioning of several compact transmitters provides the density measurement of the fluidized bed in different reactor zones (FIG. 11). A solids profile is then derived from these density values, which shows the desired product properties in a targeted fashion.
Delayed coking. Petroleum coke is produced in coke drums. The residues from the vacuum distillation are heated to approximately 500°C (932°F) and transferred to the active coke drum, where they are cracked by hot gas. During the coke drum-filling operation, gas is continually withdrawn, and significant foam formation occurs on the product surface (FIG. 12).
Antifoaming agents are added as certain levels are reached to keep the foam layer as thin as possible, but optimal additive quantities require constant knowledge of the surface foam position. Radiometric level measurement technology is the only reliable option for this application.
The continuous foam level position signal directly controls the spray of an antifoaming agent. Additionally, radiometric point level can be used for overfill protection to keep from filling above the maximum allowed level, preventing both foam and product from entering the gas vent and causing downtime.
Steadfast when others falter. Gamma technology provides unrivaled accuracy for measuring and profiling complex fluids and interfaces. It is often only considered as an option when other more cost-efficient solutions fail, but many production managers are unaware of its viability in industry.
With proper design and installation, radiometric instrumentation is safe and among the most durable of all measurement technologies. When other methods fail to provide the necessary accuracy and reliability, radiometric instrumentation often provides a solution. HP
NOTES
a Endress+Hauser’s Levelflex FMP51/52/54
b Endress+Hauser’s Levelflex FMP55
c Endress+Hauser’s Liquicap FM151/52
d Endress+Hauser’s Gammapilot FMG50
MARK THOMAS is the Oil and Gas Industry Manager for Endress+Hauser USA. He is responsible for business development and company growth in the oil and gas industry. As part of this role, he is the U.S. representative on the global SIG (Strategic Industry Group), which helps develop the long-term vision, brand, product direction and education of the company on industry direction. Thomas earned a BA degree in 2003 from Texas Tech University and achieved an MBA from AUI in 2008.
KYLE DROUGHTON is a Product Business Manager for Level and Radiometric Products at Endress+Hauser, covering the U.S. Gulf Coast. He partners with customers and manufacturing representatives to solve challenging applications, while developing the level and radiometric product lines to better suit customers to meet current and future challenges. Droughton is a U.S. representative for the Global Gamma Network, which serves as a global product development forum and best practice experience exchange. He graduated from Texas A&M University in 2014 with a BS degree in industrial distribution.