Reprinted from HYDROCARBON ENGINEERING APRIL 2004

The Rosemount 3300 series radar based liquid leveland interface measurement transmitter uses guidedwave radar (GWR) technology. GWR technology com-

bined with advanced signal processing makes possible toplevel and liquid-liquid interface measurements with the samesimple transducer, which is an inert metal probe suspendedin the process media (Figure 1). Low power microwave pulsesare guided by the probe, and a part of the energy in the pulseis reflected back whenever there is a change of dielectricconstant in the process media surrounding the probe. The 2-wire loop powered electronic transmitter unit computes thedifferent surface levels from the time delay of the reflectedsignals, using advanced signal processing in the micro-processor based electronics. The Rosemount 3300 is thefirst 2-wire GWR allowing simultaneous interface and levelmeasurements in tanks up to 20 m (66 ft). It has a very userfriendly set up software and service tool, making configura-tion of even these rather difficult applications easy.

The significant advantage in oil industry applications isthat the Rosemount 3300 with the probe penetrating the bulkof the liquid enables measurements to be made below thetop liquid or foam surface, which normally provides the onlyreflection and information that conventional radar systemscan monitor. So with the transmitter, measurement of inter-face level in separator tanks or waste sumps is possible, anda true bulk surface level measurement is made in boiling,

turbulent or frothy liquids. Where both surface level and inter-face position are required, Rosemount multivariable technol-ogy uses HART digital communications to superimpose thesecond measurement information onto the 4-20 mA signal,and this is simply separated as needed to provide a secondanalogue output using a HART Tri-Loop or similar interface.

Oil separation from injection waterAt Merit Energy in Wyoming, salt water is injected into theground to displace and recover oil reserves. The oil andwater mixture is delivered to a large capacity processingtank, at a rate of up to 15 000 bpd (Figure 2). Level and inter-face measurement is crucial, to know the tank capacityremaining, and to control the variable ratio of oil and saltwater streams flowing out of the tank. From here the oil isdelivered to a separator, which removes the last traces ofwater before delivery by pipeline to a refinery. The salt waterfrom the main processing tank is filtered and re-injected intothe ground.

Any oil carried over into the re-injection water filters pro-duces a blockage, resulting in the need to replace all the fil-ters. While the expected life of the filter is only one week, theprevious measurement system, which used a non-contactingradar surface level measurement, a pressure transmitter,and complex calculations to deduce the interface level, had

Anna Olander, Emerson Process Management,Sweden, explores guided wave radar technology in oil water interface and otherhydrocarbon level measurement applications.

Figure 1. The Rosemount3300 series GWR.

Figure 2. Receiving tank forreturned oil/water mixture.

Figure 3. Rosemount 3300 level and interfacetransmitter installed on a tank roof.

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failed to prevent regular oil contamination, leading to signifi-cant replacement costs. The filters were replaced on

average three times a week, at a total cost of US$ 85 800 perannum.

Installation of a Rosemount 3300 GWR to measure bothsurface level and interface position, has made it possible toskim the oil off the tank with a much higher degree of confi-dence than before. Being a large tank, the radar probe styleused in this case was a 9 m (30 ft) long flexible twin lead, withthe transmitter positioned on the tank roof, in the positionused by the previous radar transmitter (Figure 3). Filterreplacement has been reduced to near the optimum of onceper week, reducing costs to US$ 28 600 per annum. Thisgives a yearly saving of approximately US$ 57 000, and anequipment payback of less than three weeks.

Further efficiency improvements have been made in thatthe oil skimmed off and fed to the separator has far lesswater entrained than before, making the final separator workmore efficiently, reducing the maintenance needed and thevolume of waste water requiring disposal.

Waste and oil sump monitoringIn Gothenburg, Sweden, the Preem Refinery installed aRosemount 3300 GWR level and interface measurementtransmitter on a waste sump. The sump is used to collectand separate oil and water from all the refinery waste. Theprobe, in this case of a rigid twin lead design, operates effi-ciently directly in the sump (Figure 4). The loop powered sys-tem uses HART digital communications to transmit both leveland interface data back to the control room.

The unit has been in operation in the Preem refinery fornearly two years and has provided accurate and reliablelevel and interface information for sump level control, elimi-nating level upsets and oil overflows (Figure 5).

Refinery separator vesselsOften refinery separator vessels use a displacer to measurethe interface level between oil and water. However, displac-ers are affected by density changes in the oil, which cancause inaccurate and unreliable readings. For example oneUS Midwest refinery had separators that received oil frommany different sources, and the properties of the oil changedfrequently. The displacer responded to the variations in den-sity, which led to regular carry over of oil into the wastewatertreatment stream, putting a significant load onto the treat-ment system, and wasting the potentially saleable oil.

A 3300 GWR probe was installed simply in the displacerchamber, using the same flange type as the original displacerunit, and the same two wire loop cabling (Figure 6). Themeasurement required is just the interface level, so aRosemount 3301 transmitter could be used since the cham-ber is constantly flooded. The refinery engineers wereimpressed with the simplicity of the installation, and achieveda much more reliable interface measurement output,because GWR measurement is not affected by densitychanges.

An added benefit was that the displacer unit had neededfrequent adjustment and cleaning to remove sticky depositsclinging to the sides, occasionally obstructing movement.With the new GWR probe assembly, which has no movingparts, maintenance has been reduced to occasional steamcleaning of the probe, simplifying the service procedure andminimising the necessary downtime.

Liquid propane measurementLiquid propane is used as a refrigerant in various hydrocar-bon processing and chemical plants: a standard process

Reprinted from HYDROCARBON ENGINEERING APRIL 2004

Figure 4. Waste water sump separation system.

Figure 5. Refinery waste water collection sump.

Figure 6. Rosemount 3300 is a simple replacementoption for displacer systems on separators.

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problem is monitoring the level of propane liquid in the accu-mulator tanks, to control the recirculation process andensure the propane supply is adequate to feed the coolersrequested by process operators (Figure 7). Due to surfaceturbulence in this application, and the variations of tempera-ture and density of the low dielectric constant liquid, levelmeasurement here has historically been unreliable.

The Rosemount 3300 GWR was felt to be potentially amore reliable technology for this application, because it hadproved tolerant of liquid density changes and surface turbu-lence in other similar process vessels. One of the standarddesigns of this probe is coaxial in construction, concentratingthe measurement into a specific defined area, almost like astilling well. This increases the signal to noise ratio of the sur-face signal, important when working with such turbulent andlow dielectric fluids.

Initial tests were carried out at Chevron Phillips in Texas,and used a probe length of 1.5 m (5 ft) installed through aflange on an existing process connection to the side of thetank. The propane level was detected immediately, and theRosemount 3301 used here has now made accurate mea-surement for over one year (Figure 8). According to ChevronPhillips, the Rosemount radar solution had enabled the con-trol system to work effectively, as per design, for the first timeever.

Further applicationsThe GWR techniques embodied in the Rosemount 3300series have demonstrated significant advantages comparedto other level measurement equipment, enabling measure-ments of both liquid surface and interface level. The addi-tional sensitivity provides reliable measurement of turbulent,boiling liquid surfaces, and has also improved the measure-ment reliability for surfaces of low dielectric constant hydro-carbon liquids, which have been difficult to measure usingany technique in the past.

Engineers have now accepted that the Rosemount 3300GWR can achieve reliable interface level measurement per-formance in these applications, and deliver reliable resultsover a simple loop connection in a digital format, allowingplant control systems to develop further than before. So theyextend the challenges further. New applications continue topush the boundaries for understanding where the data nowavailable from GWR transmitters can improve efficiencies in

hydrocarbon engineering. The principle has already beensuccessfully applied to ‘accurate’ and reliable level measure-ment within distillation columns. ‘Accuracy’ here is difficult todefine, since the measurement is not possible using othertechniques, and the true ‘surface’ is not really identifiablevisually because of foam and turbulence. Nevertheless, themeasurement is of interest, and has been providing a para-meter that is consistent and has offered new process controlcapabilities. The applications of Rosemount GWR technologyin hydrocarbon processing continue to extend the bound-aries of this level measurement technology.

Reprinted from HYDROCARBON ENGINEERING APRIL 2004

Figure 7. Refrigeration system using propanerecirculation.

Figure 8. Rosemount 3300 measuring surface levelof turbulent propane liquid.

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