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Home / Articles / 2016 / Detection of oil in or on water
The properties of oil and water are sufficiently different to make distinguishing them relatively easy. Here we explore some of the state-of-the-art
sensors used for oil in and on water detection, including interfaces.
By Béla Lipták
Jul 22, 2016
Béla Lipták, PE, control consultant, is also editor of the Instrument Engineers’ Handbook, and is seeking new co-authors for the for coming new
edition of that multi-volume work. He can be reached at liptakbela@aol.com.
The properties of oil and water are sufficiently different to make distinguishing them relatively easy. This article describes some of the state-of-the-art
sensors used for oil in and on water detection, including interfaces.
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2. “Water dump
control can
use
radiofrequency
interface
detectors for
“free water
knockout.”
Outfalls from ships and municipal or industrial waste treatment plants must be monitored because if the emissions contain hydrocarbons they’ll float on the water’s
surface and form a barrier, which will prevent the oxygenation of the water and cause fish to suffocate. On-off, oil-on-water detectors are capable of measuring even a
few drops of petroleum floating on the surface, so these alarm devices are used downstream of plant outfalls and at oil loading and unloading stations of tankers and
trucks.
Laser nephelometers measure light scattered by particles contained in all oils. They operate by focusing a laser beam onto the surface of the water. A second,
receiving lens refocuses the reflected (scattered) light onto a photocell. When there’s no oil on the water’s surface, only a minimum amount of reflection occurs. When
floating oil is present, reflected light intensity increases substantially due to light scattering caused by particles in the oil. The measurement is based on the differential
between the outputs of the measurement photocell and a reference photocell, which measures the output of the light source itself.
The sensing head is usually mounted on pontoons or floats in the water. An s-shaped baffle can be provided to direct the flowing
water past the sensing head. Accurate detection requires regular, conscientious maintenance for continuous and reliable
performance. The capacitance approach used for monitoring of oil film thickness on the water, which I will discuss next, requires less
maintenance, but is limited to detecting larger quantities of floating oil.
This device requires maintenance, as do all optical measurements because the windows must be kept clean. If an air column is
provided between the water surface and the window, that column will reduce fouling caused by splashing. Therefore, these sensors
are mounted above the water. This instrument must also have optical filters to eliminate effects of sunlight or other stray light
sources.
Oil slick thickness can be detected by capacitance sensors that detect the thickness of the oil layer by generating a DC signal for
transmission. The generated signal is in proportion to the inverse of the measured capacitance because total capacitance between
the plates drops as the thickness of the low dielectric constant oil rises (Figure 1).
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3. Figure 1: Parallel-plate capacitor detects the thickness of an oil slick layer on water.
The dielectric constant of water (80) is so high relative to that of oil (1.9 to 2.1) that the capacitance contribution of water can be neglected and the oil thickness can be
calculated (estimated) based only on that of the oil (κoil) as shown in the equation below:
toil = κoilA/C
where:
A = effective area of one capacitor plate
C = measured capacitance
toil = oil thickness
κoil = dielectric constant of oil
Water discharged into lakes or rivers should not contain oil because it contributes to the biological oxygen demand (BOD) of the discharged water and can also be
toxic to the aquatic biota or to the fish themselves.
Ultraviolet radiation can be used to detect the oil contamination of water. When UV radiation is sent through an oil-contaminated water sample at a peak intensity of
365 nm, visible radiation (400-800 nm) is emitted. The intensity of this radiation can be measured by a photocell. The intensity of this emitted radiation increases as the
concentration of oil (the fluorescent substance) rises.
The most common configuration is to pass the process sample through the sensing head in an up-flow direction (Figure 2). The head is equipped with two windows
that are set at right angles to each other to minimize the intensity of direct radiation from the source striking the photocell, and also to reduce the effect of multiple
scattering of the visible radiation. Optical filters at the incident and at the emergent windows are used (not shown) to reduce this effect to a negligible level.
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4. Figure 2: Laser induced fluorescence detector, using UV light and optical fibers. (Image courtesy of ProAnalysis)
The UV analyzer used in this system is a single-beam, dual-wavelength analyzer. This is superior to single-wavelength designs because it’s able to compensate for
variations in sample sediment content, turbidity and algae concentration, and also for window coatings.
The capacitance of water is much higher (its dielectric constant is about 80) than that of oil (about 2), so measuring the dielectric constant is a convenient way to tell
them apart. In addition to conventional capacitance probes, special dual-concentric designs are also available to detect the interface between water and oil in tanks. In
addition, flow-through sensors are also available for in-pipeline applications.
The flow-through version of the dual-concentric electrode consists of two concentric pipes that are insulated from each other, thereby forming the capacitor through
which the process stream flows. The unit is available both for switching or for transmitting applications, and can be used for both oil-in-water and water-in-oil
measurement.
Radio-frequency (microwave) sensors use the fact that shortwave RF energy is absorbed much more efficiently by water than by oil. A radio-wave detector produces
waves of a constant amount of energy. The more this energy is absorbed by the process fluid (the more water is in the mixture), the lower will be the voltage at the
detector. The advantages of this design (relative to capacitance sensors) include wider range (0% to 100%), lower sensitivity to buildup, insensitivity to temperature
and salinity variations, and suitability for higher-temperature operations (up to 450 °F or 232 °C).
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5. Figure 3: RF probe used for water dump control on oil separators. (Courtesy of Agar Corp.)
Water dump control can use RF interface detectors for “free water knockout” (Figure 3). The probe is installed horizontally at one-third of the diameter of the separator
vessel, and is set to open the water dump valve when the emulsion concentration drops below 20% oil. This way, the emulsion (rag layer) will build up above the
probe. These instruments can detect water concentration within about 5%.
Download the 2016 State of Technology Report on Level Instrumentation
Rag layer profiler portable tank profilers are also available, using RF principle of operation. Here, the tape-supported radio frequency element is gradually lowered into
a tank, which can be up to 100 feet (30 m) tall. As the sensor is lowered, it measures both the location of the interface (within an error of 0.12 in. or 3 mm), and it also
measures the emulsion concentration throughout the tank height (from 0% to 100% within an error of 1%).
The following is a comment from David Williams, Senior Applications Engineer, Business Development, VEGA Americas.
I read your article in Control magazine on “Detection of oil in water” and like the information that you provided. I was unaware of the laser nephelometers and it was
a great learning opportunity for me. I am very familiar with the oil-water applications such as desalters, free water knockout (FWKO), treaters, and settler and the
many ways different ways of trying to measure it.
Here at VEGA, since we are a RADAR and nuclear instrument manufacture, we have tried guided wave radar to some success depending on the amount of
emulsion that might be generated inside the vessel. We have also applied nuclear continuous level by placing a source or sources inside the vessel and measuring
the interface, again success depends on the densities of the fluids and the amount of emulsion. With these devices not being reliable enough for critical applications
we developed density profiler called the Multiple Density Array (MDA). I don’t know if you have heard or seen what we are doing with this or not but I wanted to bring
it to your attention. We place multiple small sources inside the vessel and measure a density in the horizontal plane. A simple measurement, really. We just stack a
few density points on top of each other to build a density profile of the fluids inside the vessel as they separate.
We have had great success with the MDA in desalters and HF settlers in refineries, FWKO and treaters in the SAGD operations in Canada, and desalters and
separators on off shore platforms. I have attached some information that you might find interesting. If you have any questions please feel free to contact me.
Thank you and best regards.
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