Water in Oil Meter and Oil in Water Meter Analysers
Sunil P Agarwal
Senior General Manager
(Instrumentation & Controls)
TATA Consulting Engineers

Abhinav Prasad
Manager
(Instrumentation & Controls)
TATA Consulting Engineers

Many process industries are unaware of the importance & benefits of measuring hydrocarbon and water content. The technical document covers two such analysers which are Water in Oil Meter and Oil in Water Meter. This document would help the reader to understand and select a suitable analyser for their process industry based upon the principle of operation, components , mounting, application and benefits of these analysers

Measurement of water in oil and oil in water are very important for any hydrocarbon industry. There are varieties of analysers available in market. Selecting right type of an analyser considering accuracy and reliability requirements, measurement principles and their applicability/limitations, mounting requirement, cost, etc., for a particular application is a very complex task. The performance of the analyser may not be satisfactory if selection is not carried out properly considering all these criteria. This paper highlights all major features of 'water in oil' and 'oil in water' analysers to facilitate right selection of analyser for a particular application.

In earlier days, sampling was the only method for measuring hydrocarbon in water. These samples were taken to laboratory for analysis. This caused delays in analysis and was less accurate. This had caused loss of production , efficiency, inaccuracy in process, deteriorated quality, etc.

Water in Oil Meter (WiOM) and Oil in Water Meter (OiWM) have addressed these issues to a large extent and have become important pieces of instrumentation in hydrocarbon industry. These analysers use online measurement techniques which provide continuous and reliable data to the plant operator.

Introduction of Highway Addressable Remote Transducer (HART) and other digital protocols in these analysers have improved the speed of transmission of measured signals to the control system(DCS & PLC) and this has opened the gate for more automation in the processes. These meters have helped the oil industry to use crude oil efficiently and reduce oil loss from the system.

A WiOM (also known as water cut meters) measures the content of water in crude oil, other hydrocarbons and low dielectric liquids. The measurement unit of WiOM is in percentage of water. WiOMs are used where accurate and drift-free determination of water content is crucial.

An OiWM measures the concentration of oil or other hydrocarbons or dissolved hydrogen sulphide in water. The measurement unit of OiWM is either in parts per million of oil or in milligram/litre.

Principle of Operations in WiOM and OiWM

Principle of operation of WiOM : There are two operating principles for measuring water in oil. These principles are being used due to distinct dielectric properties of water and oil.
  • Microwave resonance:
    The permittivity of oil and water mixture is measured. The measured permittivity of oil and water mixture is then compared with dry oil permittivity and water permittivity. The water molecules have positive charged side as two hydrogen atoms and negative charged side as one oxygen atom. When the oil and water mixture passes through the microwave, the water molecules align themselves continuously with microwave field. This causes the microwave propagation to slow down. On the other hand, hydrocarbon molecules do not respond to changes in microwave field. Due to the symmetrical structure of hydrocarbon molecules, the propagation effect of microwaves is insignificant.

    This principle uses Bruggeman equation for calculating the percentage of water. Microwave resonance can be used to measure different ranges from 0-1% to 0-100% water. The accuracy may vary from +0.05% for the range of 0-1% water and +5% for the range of 1-15% water. To measure upper limit for 15 -100% water, WiOM requires density calculation giving accuracy around +5% of reading. For the measurement of full range from 0-100% water, the WiOM compensates the salt content in water by measuring the conductivity. The accuracy of full range WiOM is around +1% of water.
  • Capacitance type:
    The sensor used in this method is capacitance type which measures the dielectric of water in oil and water mixture. Capacitance type can be used to measure ranges from 0-25%. The accuracy may vary between +0.01% to +0.2%
  • Optical & Density measurements were old principles which were used in laboratory for analysis.
Principle of operation of OiWM - There are three operating principles for measuring oil in water. These principles typically use ultra violet (UV) light as a source and a detector which measures the wavelength of the light reaching it. The detected wavelength is compared with the chart already stored in the memory of the microcontroller.
  • Ultra Violet (UV) Fluorescence:
    This principle uses the unique fluorescence property of crude oil when UV light passes through the water in a pipe. The fluorescence of the UV light is received on the UV detector. The change in UV light due to fluorescence of oil is measured.

    The advantage of using UV Fluorescence is that the detector is tuned only to measure the fluorescence of crude oil. The wavelength of any other impurities in the water is not measured by the detector. This principle is mainly used in offshore and onshore oil industries having the measurement range of 0-20,000 ppm and accuracy of approximately +1%.
  • UV Absorption/Photometer:
    Similar to UV fluorescence, UV light is used as the source. The emitted light is then passed through water in the pipe. The oil or hydrocarbon in water absorbs/emits UV light. The change in UV light received at the detector due to absorption/emission of UV light is measured.

    UV absorption has the measurement range of 0-1,000 ppm and accuracy of approximately +1-2% as other impurities also absorb UV light.

    This principle can be used in any industries involving hydrocarbons such as Breweries and Oil & Gas industries in the downstream of the process such as cooling water system, waste water treatment, oily water system, etc.
  • UV Persulfate/Thermal Oxidation:
    This principle is used mainly in regulation of Waste Water, Chemical Oxygen Demand (COD) and Biochemical/Biological Oxygen Demand (BOD) where Total Organic Carbon (TOC) is measured. In this, the sample is extracted at periodic interval from the pipe. The sample is then passed over a reactor. The organic and inorganic carbon is oxidized by dissolving all carbon bonds at 1200oC in the ceramic oven and converted into carbon dioxide (CO2). The converted CO2 is then measured. The organic and inorganic carbon is oxidized using carrier gas. The supply of the carrier gas may use filtered ambient air.


    Figure-1: WiOM with static mixer

    UV Persulfate/Thermal Oxidation has the measurement range of 0-20000 ppm and accuracy of approximately +2%.

    UV Persulfate/Thermal Oxidation can handle high salt concentrations of approximately 10 g/l.

    Due to slow measurement principle (response time of 3 minutes), batch measurement and lower accuracy, UV Persulfate/Thermal Oxidation is not used in any application where continuous measurement of oil or hydrocarbon is required.
Components and Mounting
Components and Mounting of WiOM : The Water cut meter (Refer Figure-1) comes with a pipe spool piece with a transmitter and a receiver or probes and temperature transmitter for temperature compensation. These are installed on the pipe spool. The pipe spool comes with flanges which can be fitted directly in the process line or in the bypass line. The microcontroller/micro-processor based electronics is housed in analyser or a control panel. All wires from the transmitter, receiver and temperature transmitter are connected to the analyser or control panel.

The transmitter is mounted directly on the pipe spool whereas control panel is mounted in the field with suitable supports.

Built-in static mixers are sometimes provided in water cut meters for conditioning of oil and water mixture.

Components and Mounting of OiWM :
There are two methods of mounting OiWM as mentioned below:
  • In-line probe type:
    The probe of the in-line OiWM analyser (Refer Figure-2) is inserted in the process line. The probe length may vary with process line size. The inline probes are retractable type and can also be used in high pressure lines with special accessories.


    Figure 2. In-line type OiWM

    These analysers use fibre optic cable between the probe and the control panel. The light source used is laser. The laser light is converted to UV light in the inline probe. The measured wavelength of UV light is sent from inline probe to the micro-controller in the control panel using fibre optic cables. Unlike UV light, the laser light is used for its advantages of better life and less prone to energy loss when travelling through longer distance and bends.

    The micro-controller based control panel can be mounted away from the pipeline. The size of the control panel may vary depending upon hazardous and safe area. The control panel in hazardous area is smaller in size due to which there is restriction in the number of probes handled by a controller (usually 1 to 2 probes). The control panel in safe area can be of larger sizes. More than one(approximately 4 nos) controller can be mounted in each control panel. Large size panels can be floor-mounted. Sometimes external cooling is also required for the control panel.

    The micro-controller analyses the measured signals and indicates the concentration of hydrocarbon in water.

The advantages of using inline probe type are:
  • Continuous and immediate measurement
  • No loss of fluid
  • Reduces piping
  • Low maintenance
  • Requires less space
  • Works at high pressure and temperature of up to 35 bar (g) and 200oC , respectively
Bypass type:
As the name suggests, a bypass line (Refer Figure-3) of smaller size is tapped off from the process line. The size of bypass line depends upon the pressure which the analyser can handle. If the pressure is very low then separate pump is used to carry the sample up to the analyser.


Figure 3. By-pass type OiWM

The analyser and the electronics of the micro-controller are housed inside a control panel (Refer Figure-3). The control panel can handle one bypass line. A suitable structural arrangement for mounting the control panel, by -pass line and pump is required.

The probe and the control panel are mounted in the by-pass line. Since, the analyser probe and the controller are housed in the same control panel, external cable is not required between the probe and the analyser.

The light source used is either laser or UV. The laser light is converted to UV light in the probe. The measured wavelength of UV light is sent from the probe to the microcontroller for analysis of measured signal which indicates oil concentration in water.

The outlet of the bypass line is either connected back to the process line or is drained through the drain line. If the pressure in the process line is higher, then another set of pumps are used to inject the fluid from the by -pass line back to the process.

This arrangement of OiWM has the advantage of low initial cost which is due to:
  • Small fibre optic cable between the controller and analyser probes
  • Since the analyser probes are not in pressurized process line, the length of the probes are shorter and are easily retractable
Other advantages of using by-pass type OiWM are:
  • Continuous and immediate measurements
  • Works at high pressure and temperature of up to 20 bar (g) and 120oC, respectively

Figure 3. By-pass type OiWM

  • Sample/Extractive type:
    Like any other sampling technique, Sample/Extractive type OiWM (Refer Figure-4) also requires hose pipes and pumps for bringing samples. The ceramic oven, reactor, tubes, pumps, filters, controller, etc, are mounted inside an analyser panel. The analyser panel can handle multiple samples. This arrangement has the advantage of low initial cost as the complete arrangement comes in a single analyser panel. The hose pipes and tubes need to be installed from the sampling point up to the panel. The complete set up may require maintenance and a trained operator to handle the analyser.
Applications
Some of the applications of WiOM:
  • For the testing of the production separator
  • Fiscal quality and quantity measurement
  • During loading and unloading in terminals and pipelines
  • Custody transfer
Some of the applications of OiWM:
  • Onshore and Offshore oil platforms
  • After crude separators, degassers, etc.
  • In water drain lines from tanks and equipment for oil detection
  • In waste water treatment plants, effluent treatment plants (ETP), oily water system (OWS), boilers, cooling water and process heat exchangers.
Benefits
Benefits of measuring water content in crude oil in water using WiOM:
  • Measurement of oil and water production simultaneously in the mixture of oil and water
  • Indication of effect of water and oil separation treatment which maximizes production and decreases retention time
  • Automatic rerouting of oil back to the process for retreatment
  • Detection of undesirable condition such as contamination
  • Detection of interface during dewatering of storage tank
Benefits of measuring oil content in water using OiWM:
  • Improvement in productivity of water treatment systems Improvement of health and safety conditions by reducing oil level in water
  • Reduction of hazardous impact to environment
  • Improvement in cost of operation by minimizing the use of chemicals and maintenance frequency
  • Improvement in cost of operation by minimizing the use of chemicals and maintenance frequency
  • Automatic rerouting of water back to the filters for retreatment
  • Detection of leakages
It is very important to understand the selection criteria of Water in Oil Meter and Oil in Water Meter analysers while preparing the specifications. These analysers are costly. In order to achieve required performance, it is very important to critically consider all parameters while selecting analysers for a particular application.