Accurate Level Measurements in Boiling Hydrocarbon Applications

by , | Aug 15, 2023 | Downstream Hydrocarbons, Level | 0 comments

The stated mission of our blog is to connect you with the experts here at Emerson from around the globe. Occasionally, I get copied on communications between a visitor to the blog and one of our experts. In the case of one I saw this morning. I wanted to share a generic version of the exchange and highlight some key points from a presentation shared.

The blog visitor asked:

I’m currently working in the Permian Basin for [company] and I’m having trouble with a radar that’s off. The process is liquid propane and I noticed it’s pretty bubbly in the sight glass. They are using [other supplier] radars but my question is, can we fix this problem with some 5300 series?

Emerson’s John Butler replied:

I can’t speak to the [other supplier] unit because I don’t know their capabilities that well, but this sounds like something we deal with fairly frequently and can usually solve.

This would be considered a boiling hydrocarbon application, which uses more of an advanced feature in the radar, called PEP (Probe End Projection). Attached is a presentation that was done back in 2010 on this subject at our Emerson Exchange.

Take a look at it and let me know if this sounds like what you are seeing.

The presentation John shared was titled Measuring Levels in Highly Dynamic Petroleum Processes. It highlighted some highly dynamic hydrocarbon processes such as boiling/vaporizing hydrocarbons, off-gassing hydrocarbons, and processes with rapid level changes.

Measurement technologies that can address these challenging applications include advanced guided wave radar for fast response, probe end projection to stabilize the level output, and special considerations for bridle design. Together, this approach can help to stabilize level output and run without process upsets.

Guided wave radar probe end projectionIn the presentation, John and the customer shared a case study of a gas plant’s receiver economizer processed propane at 55 psig and 10 degF at a 50% setpoint. The process was challenging to control due to 3 interacting loops, very rapid level changes, and boiling fluid upon pressure drops. Traditional guided wave radar level measurements occasionally lose the level and fail high, causing a process trip.

The solution was to use a Rosemount 5300 series guided wave radar (GWR) level transmitter. This technology is based on the TDR (Time Domain Reflectometry) technology which is also called “contacting radar” or “radar on a rope.” The level is measured by low-power nano-second microwave pulses guided down a probe suspended in the process media. When a radar pulse reaches media with a different dielectric constant, part of the energy is reflected back to the transmitter. The time difference between the transmitted reference pulse and the reflected pulse is converted into a distance value from which the level is calculated.

Using ultra-rapid three-nanosecond direct switch technology between transmitter and receiver gives a better signal-to-noise ratio, a better margin to handle disturbing factors as objects close to probe, coating, foam, vapor, and turbulence and makes it able to manage long measuring ranges and low dielectric (low reflective) media also with the single probe. Probe end projection is used to manage the lowest dielectrics on longer measuring ranges. The probe end is used when the transmitter sees thru the media to calculate the level.

Inaccurate level measurement with undersized bridleMicrowaves slow down in process fluids at a rate in relation to the fluid’s dielectric constant. Since hydrocarbons such as propane, in this case, have low dielectric constants, the receiver can see to the end of the probe, which is a fixed, known distance. As process fluid is added, the end of the probe will appear to be farther away since the time of flight is longer due to this increase (apparent distance).

With a known product dielectric (propane fluid – 1.6 ε0 at 0° C), the level can be calculated by comparing the known distance to the apparent distance. This results in better measurement availability.

Correct sizing of the bridle where the measurements occur is essential since gas bubbles rising in small-diameter bridles tend to lift the fluid level. In contrast, bubbles break out in large bridles without causing a level rise.

For these types of applications, it is important to insulate the bridles. The temperature in the bridle should be as close to the vessel temperature as possible. Since bridles have much less fluid mass than the vessels, they heat and cool faster with ambient temp changes. With different temperatures, bridles will show a different level than the vessel.

Visit the Rosemount 5300 Level Transmitter section on Emerson.com for more on how this transmitter provides great reliability and safety features for challenging liquid and solids level applications.

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The opinions expressed here are the personal opinions of the authors. Content published here is not read or approved by Emerson before it is posted and does not necessarily represent the views and opinions of Emerson.

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