Jun-2024

Emulsion in the field the genesis of TDR multiphase level measurement

Multiphase level measurements exist throughout the process industries and are particularly relevant in the Oil & Gas and Petrochemical sectors due to the value derived from effective water and hydrocarbon separation.

Ametek

Article Summary

While level instrumentation has come a long way in measuring liquids of all varieties, multiphase level measurement is often considered the greatest challenge and opportunity that exists today. This is evidenced by over half of separator failure modes being attributed to level instrumentation per Offshore Reliability Data (OREDA, 2002).

Many technologies attempt to tackle multiphase measurement but all have direct and/or ancillary limitations:
Radiometric and nucleonic technologies can profile multiphase media conditions, potentially without intrusion into the process vessel; however, they come with high upfront costs, regulatory burdens and increased safety requirements.

Multi-technology approaches using Guided Wave Radar (GWR) and Capacitance may be able to provide total level and interface measurement, but there are still limitations within the technologies; such as capacitance calibration shifts, buildup concerns and emulsion thickness limitations.

Sensors measuring discrete segments (tomography) are limited in resolution by the dimensions of those segments along with other limitations based on the technology utilised (e.g., conductivity). Solids detection (i.e., sediment or sand) often requires separate instrumentation from the interface level transmitter.

Multi-probe arrays are available that measure oil/water percentages but require multiple points of entry into the process vessel and surrounding piping; often complicating maintenance, increasing costs and adding potential leak points.

Solids detection (i.e., sediment or sand) often requires separate instrumentation  from the interface level transmitter.
While this proves there has been considerable effort by instrumentation manufacturers to measure multiphase levels, only now is there a cost-effective TDR-based multiphase level detector capable of widespread adoption.

Emulsion challenges
When immiscible liquids reside in the same vessel, eventually the lighter liquid rises to the top and the heavier settles to the bottom. This is the case with oil and water, where effective separation is critical to the productivity of upstream wells, processing plants and refinery/petrochemical complexes.

GWR is a microwave radar device combining time domain reflectometry (TDR) and equivalent time sampling. The synthesis of technologies creates a high-speed GWR transmitter that is extremely effective at tracking total level and interface in separators; particularly when there is a relatively distinct boundary between the liquids. However, as the emulsion layer grows, GWR tends to measure near the top of the layer. Even a small amount of water in the oil (top of the emulsion layer) makes it conductive enough to produce an impedance change detectable by the transmitter. This leaves little remaining energy to be transmitted through the rest of the emulsion layer.

With the growth of TDR based level instrumentation, there is an emphasis on expanding the use of TDR into multiphase applications where mainly highpriced profilers or multi-probe arrays exist today.

Genesis of multiphase TDR
An innovative approach was required to take advantage of the strengths of traditional TDR-based transmitters while improving upon the design to compensate for thick emulsion layers and potential sediment levels.

There are inherent differences from a user standpoint between this new multiphase detector and traditional loop-powered TDR-based instrumentation; one of them being higher power consumption to accomplish up to four measurements.

As with other technologies that contact the process, the probes are a critical element to maximising the performance of Genesis. Probes will range from a large diameter coaxial to a completely new Pentarod design. The Pentarod is a fiveconductor probe with four reference rods surrounding a PFA coated active center rod.

The concentrated signal yields coaxial-like performance; yet it has an ‘open’ design that is less susceptible to measurement errors due to media buildup or bridging. The PFA coated center rods, aside from improved resistance to heavy coating, allow the pulse to travel in water with less absorption.

Genesis builds upon the proactive diagnostics found in GWR today in order to actively monitor buildup on the probe; enabling operators to streamline maintenance and reduce downtime. These buildup diagnostics capabilities include:

Furthering on these capabilities and of particular importance in upstream separators, an alarm can be configured when sand/sediment reaches an identified level on the probe to proactively eliminate unplanned shutdowns.

Due to the growing number of interface level transmitters and significance of this measurement, a technology breakdown is provided in Table 1 highlighting strengths, weaknesses and recommended uses in applications with thick/dynamic emulsion layers where a profile of the total level, top of emulsion, bottom of emulsion and sand/sediment is desirable.

Realising operational improvement
Liquid-liquid separation is fundamental throughout the Oil & Gas value chains. Level measurement is the primary method of tracking fluid interface and the following are a few core applications with thick/dynamic emulsion layers where enhanced multiphase measurement improves productivity, safety and ultimately profitability.

Desalters
At the early stages of a refinery, a desalter separates water and salts/ chlorides/sediment from crude oil to mitigate effects on downstream equipment – particularly corrosion caused by chlorides. If water is carried through to the distillation column, there is the potential for entrained water to flash to steam during heating, which can damage trays or other parts of the tower (crude unit). On the water/brine outlet, if oil gets into the wastewater stream, then it may result in fines or diminish the efficiency of the water treatment process as particulates plug screens or filters. It is imperative that desalters run at optimal levels relative to the electrostatic grid to maintain productivity and assist in balancing the inlet crude, outlet crude, outlet water/brine and chemicals for emulsion control. The key to bringing this balancing act together is controlling and optimizing the emulsion layer.

Electrostatic coalescers
With the same principal of operation as a desalter at a refinery, electrostatic coalescers are utilized upstream for dehydration and desalting; commonly found on floating production storage and offloading vessels (FPSOs).

Monitoring the separation of water, crude and the associated emulsion layer is of primary importance with emphasis again on preventing too much water from contacting the grid and oil from leaving with the effluent water. Proper removal of soluble salts/chlorides also prevents downstream pipes from rotting out.