and K. DROUGHTON, Endress+Hauser, Houston, Texas
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.
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.
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.
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).
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
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.
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
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).
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.
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.
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.
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).
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.
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.
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).
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.
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
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.
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
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
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
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.