The challenges of accurate interface measurement

Magnetrol recognises that interface measurement is a key concern for oil and gas and petrochemical industries as well as many others. With that in mind, Magnetrol has developed a white paper addressing the challenges and considerations around interface measurement and currently available technologies for process optimisation

Figure 1: Multiphase level often includes hydrocarbon at top, emulsion (rag layer) at middle and water at bottom.
Figure 1: Multiphase level often includes hydrocarbon at top, emulsion (rag layer) at middle and water at bottom.

Interface, or multiphase level measurements exist throughout the oil and gas streams as well as petrochemical. While level measurement technologies have come a long way in effectively measuring liquids and solids, multiphase level measurement continues to be the biggest challenge and opportunity that exists today to which there is no perfect technology. However, experience has shown that process optimisation and increased uptime can still be achie-ved in many separator applications through reliable, best-in-class, level technology.

The basics of interface

In the oil and gas and petrochemical industries, the need for reliable interface measurement arises whenever immiscible liquids, those incapable of mixing, reside within the same vessel. The lighter medium rises to the top and the heavier settles at the bottom. In oil production, for example, water, or steam is used to extract oil from a well. Well fluids then route to production separators where they settle into their primary constituents as a hydrocarbon over water interface.

Interfaces can form between liquids and solids, liquid and foam, and liquid and gas; but the emphasis here will be concentrated on liquid/liquid interface (often with a vapour space above the top/lighter liquid). Immiscible liquids meet along an interface layer where they undergo some amount of emulsification. This emulsion layer (also referred to as a ‘rag’ layer) may form a narrow, distinct boundary, but more frequently it is a broader gradient of the mixed liquids. Generally, the thicker the emulsion layer, the greater the measurement challenge.

While monitoring the top, or total level, is critical for safety and overfill prevention, knowing the level of an interface is necessary for maintaining product quality and operations efficiency. If there is water in oil that is not separated effectively (water carryover), then, this can induce processing problems, equipment failures and unplanned shutdowns. If there is oil in water (oil extraction), then, there can be production loss, environmental fines, penalties and forced shutdowns.

Of all the level switches and transmitters available, only a handful are suitable for reliable interface measurement. The leading interface measurement technologies include guided wave radar (GWR), buoyancy-based displacers and magnetostrictive, RF capacitance, nuclear/gamma radiation and thermal dispersion. Ideally, the technology utilised for interface applications does not have to differ from other level instruments installed at the facility in order to maintain familiarity with users. Standardising on a technology helps reduce training, installation and commissioning, maintenance and downtime. Of course, all of these have an associated cost.

Current level technolog ies utilised for interface measurement

There is no perfect, one-size-fits-all technology for interface applications. Outside of considering reliability and price points, familiarity often plays a pivotal role in determining the level measurement solution. This is particularly true for established technologies such as differential pressure (DP) and displacer-based products.

Figure 2: Guided wave radar (GWR) with signal reflections down probe.

DP is still the most widely used level measurement technology as seen in the Control Market Intelligence Report in March 2017, where over 40% of instrumentation users/respondents advised that they prefer and use DP in approximately one-third, or more of their applications as a percent of all instruments. However, DP is not a preferred technology for interface measurement. Extensive calibration is required along with assumptions that density and total level are constant. Utilising this technology typically results in one inferred interface measurement near the middle of the emulsion layer as opposed to both total level and interface measurement. Variation in the thickness of the emulsion layer affects density, and can therefore induce significant inaccuracy.

Referencing the same Control Market Intelligence Report, the second most preferred technology as a percent of all instruments and applications is GWR. Over 25% of respondents preferred GWR in approximately one-third of their applications. The ability to use GWR for total level (potential overfill prevention) and interface applications greatly increases user familiarity, allowing the technology to be applied correctly while decreasing training and commissioning time. GWR may also have limitations for interface but these are often mitigated with demulsifiers, or increasing process temperature to assist the separation of  heavier oils.

Magnetostrictive technology is also used for interface measurement. It is based upon buoyancy principles, therefore specific gravity-related drawbacks exist, but it has advantages particularly in applications with large, or swelling emulsion layers. Consideration must be taken for solids build-up, such as paraffin, or asphaltene adhesion, due to moving parts.

Other interface technologies, such as displacers (mechanical) and RF capacitance, are preferred by only 12.6% and 8.2% of respondents, respectively, in one-third of their applications.

Heavy oils may present major inaccuracies when coating probes, or building up on floats, which can also increase maintenance intervals. However, there is a comfort level with these technologies for oil and gas sectors in particular.

Interface Level Technology Comparison table in the Magnetrol interface whitepaper displays a condensed look at the primary technologies used in interface, along with their strengths and limitations.

A figure is also included to highlight the importance of addressing density, or API gravity, for technology consideration. High specific gravity (low API) heavy crude oils impact the emulsion layer and can potentially add to the maintenance requirements.

Field experience for process optimisation and increased uptime

In the oil and gas and petrochemical industries, there are numerous interface applications that potentially produce an emulsion layer. Having a reliable level measurement will help optimise processes while increasing uptime. The following are applications and case studies, highlighting the challenges faced for level technologies and the importance of this measurement.

Figure 3: Direct-insertion magnetostrictive transmitter measuring emulsion layer.

It should be noted that no matter the technology, optimal installation conditions will assist in maximising device performance. For instance, when inlet crude oil from a well enters a separator, retention time may be the most important factor to allow for the desired instrumentation performance, and therefore, process optimisation.

In other words, if the feed comes into a horizontal separator, the optimal installation location of the level measurement device is further away from the inlet (closer to the weir) where separation of the crude and water becomes more uniform.

Demulsifiers assist with emulsion breakdown but can be reduced (estimated $1,500-$2,000 per tonne) when working in concert with reliable interface level measurement.

When device performance is maximised, a tighter control of the top of the emulsion layer is possible. The top of the emulsion is an indicator of water present in oil. With the primary goal of the separator to remove water from the oil, the level measurement can now allow operation closer, or further away from the weir to optimise separator efficiency and retention time.

If the separator-type is primarily for water storage, with a thin layer of oil on top, then tighter interface control will also provide a more accurate representation of how much water (only) is present in the vessel. This allows improved truck utilisation, ensuring full truckloads during water extraction from storage vessels.

This ideal installation may not always be possible on a retrofit, but ideally instrumentation location is taken into account during separator design.

What is important to consider in any application, regardless of whether it is interface, or total level, is what can occur during upset conditions, or start-up and shutdown.

Most devices may work fine in normal interface operation. However, reliable measurement is required in those upset cases as well – when only one liquid exists (only water, or only oil); when chamber is flooded with only oil and water (no gas phase exists); and multiphase oil, water and gas, including overfill prevention.

The first industry that comes to mind when discussing interface is upstream oil and gas, or exploration and production. The initial challenges begin at the wellhead separators and resonate through the remaining hydrocarbon streams. Aside from this initial separation, an increasingly influential interface measurement for unconventional plays utilising hydraulic fracturing is at saltwater disposal facilities.

These types of interface challenges exist through midstream tank farms and storage terminals, into downstream boots and desalters at refineries, and even petrochemical quench towers in the quench settlers/quench water separation drums.

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