Instrumentation Monthly Test | Measurement | Control
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. 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).
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. Oil and water that undergoes emulsification is widely seen as the most difficult type of interface to control. A thick or dynamic emulsion layer creates challenges for one of the most widely utilized level technologies: Guided Wave Radar (GWR).
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 high-priced 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.
A new TDR Multiphase Detector, aptly named Genesis®, was invented to dynamically measure thick emulsions and sediment levels. The measurement is accomplished by sending high frequency energy down the probe to detect upper level(s) while simultaneously sending energy back up the probe to detect various other levels that may be present.
This unique (and patented) combination of “Top-Down” and “Bottom-Up” measurements, along with sophisticated software algorithms, make it possible to measure total level, top of emulsion, bottom of emulsion and sediment through a single opening in the vessel.
As with other technologies that contact the process, the probes are a critical element to maximizing the performance of Genesis. Probes will range from a large diameter coaxial to a completely new Pentarod design. The Pentarod is a five-conductor probe with four reference rods surrounding a PFA coated active centre 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 centre
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:
- Location on the probe where buildup has occurred
- Relative measure of buildup on the probe; indicating that buildup could potentially, interfere with the actual level measurement
- Rate of buildup as a percent per time period
- A diagnostic warning when buildup may affect the level measurement
SEPARATORS
Most upstream liquid-liquid separators are designed to separate immiscible liquids by gravitational. force as the higher density liquid falls to the bottom while the lighter liquid rises to the top. These can be two-phase or three-phase separators in a variety of shapes.
As with any separation process, particularly upstream Oil & Gas, it is imperative to maximize profitability by recovering as much of the oil as possible while limiting oil removal mixed in with the water. Better visibility into the thickness of the emulsion layer helps accomplish this.
Another difficulty often found in upstream operations is sand or sediment accumulation at the bottom of separators. Oftentimes, a separate instrument is required to identify when sand reaches a certain level. A multiphase detector can reduce the necessary openings in the vessel by providing a sand measurement in addition to the various interface levels.
CONCLUSION
Measuring dynamic conditions in the most difficult types of separators is now achievable with the
Genesis TDR multiphase level transmitter. Genesis is a powerful, yet cost-effective, solution capable
of measuring total level, top of emulsion, bottom of emulsion and sediment from a single instrument.
If you have an application to discuss please email us at info@able.co.uk
