Using Advanced Radar Technology Helps Improve Separation Process Performance

Related Vendors

DÜPERTHAL Sicherheitstechnik GmbH & Co. KG

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ProMinent GmbH

Emerson Automation Solutions

Emerson Process Management GmbH & Co.OHG

What to Do in Case of Emulsion Layers

The accuracy of interface level measurement can be influenced by the presence or absence of a distinct interface between the two liquids. Occasionally the two liquids do not separate precisely, and instead an emulsion layer (i.e., a mixture of the two products) forms between them. Typically, the thicker the emulsion layer, the more challenging it becomes to accurately measure the interface level. Depending on their composition, some emulsions may break down into two distinct layers if given time to settle. In these applications, installation of the GWR transmitter in a calm, still area of the vessel where the layers have a chance of breaking down may enable the interface measurement. The use of a stilling well can help to achieve better product separation and therefore more accurate interface level measurements.

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Thinner Top Layer Measurement

Although GWR transmitters are a proven and widely-applied solution in interface level measurement applications, the top liquid layer must be of a certain minimum thickness to enable the device to distinguish between the signals reflected from the two different liquids. This minimum thickness has typically varied between 50 and 200 millimetres, depending on the transmitter model and probe style being used. However, Rosemount GWR transmitters from Emerson provide functionality that enables the minimum detectable thickness of the upper liquid layer to be reduced to 25 mm.

Such a significant enhancement has been enabled by Emerson’s Peak in Peak interface algorithm. This allows the transmitter to detect signal peaks that are closer together without having to decrease its signal bandwidth, which would reduce its high sensitivity and its ability to overcome liquid disturbances.

Being able to detect a thinner top liquid layer helps to prevent unwanted product ingress and optimises the performance of the separation process, helping end users maximise operational efficiency and profitability. The ability to detect a thinner layer is especially beneficial in cases where there should be no second product in the vessel, and where the presence of a hydrocarbon on top of methanol, for example, is an indication that there is something seriously wrong with the process.

Measurement to the Top of the Tank

Accurate level and interface measurement right to the top of a tank is challenging for GWR transmitters, even when they are used with a large diameter coaxial probe. These probes provide the strongest return signal, have no upper dead band, and their accuracy is not affected by obstacles on the tank wall. However, high amplitude noise created as microwaves pass through the process seal between transmitter and probe has traditionally affected measurement accuracy at the top of the tank. By improving the design of its process seal, Emerson has eliminated this noise, thereby facilitating accurate and repeatable measurement to the top of the vessel and enabling users to optimise tank capacity and increase throughput and profitability.

Double Bounces? How to Increase Reliability

Another challenge to the measurement reliability of GWR technology in separator tanks is the so-called double bounce. This is when a radar signal bounces back and forth between the product surface and the tank roof (or another object within the tank) before being detected by the transmitter. Normally these signals have a low amplitude and are ignored by the transmitter, but sometimes the amplitude may be strong enough for the transmitter to interpret the double bounce as a surface echo. To increase measurement reliability, the latest GWR transmitters feature a factory-configured threshold setting that determines the minimum signal amplitude limits of key parameters such as surface echo, interface echo, reference peak and end-of-probe peak. For upstream oil and gas applications, an echo below the threshold is considered as oil, and an echo above the threshold is considered as water. This prevents double signal bounces from being mistaken as an interface level measurement, and enables more predictable behaviour, making the device a true plug-and-play option.

Proof-testing in Safety Applications

Regular proof-testing is an essential requirement for safety instrumented systems in separator tanks, to ensure the integrity of devices in safety-critical applications such as overfill prevention and dry pump protection. Traditionally, proof-testing has been performed with multiple technicians in the field and one in the control room, verifying the safety system reaction. This requires a considerable amount of time and effort, can pose safety risks to workers, can result in the process being taken offline, and can be prone to errors. However, technological advances in modern GWR devices enable proof-testing to be performed remotely, making the process much quicker, safer and more efficient.

The latest GWR transmitters can be remotely proof-tested with 94 % test coverage using verification reflector functionality. The verification reflector function uses an adjustable reference reflector fitted to a flexible single lead probe, at a desired height to generate a unique echo signature. The device constantly tracks the reflector echo to determine if the level is above or below the alarm limit. A test function built into the device software verifies that the GWR has been correctly configured and is correctly tracking the reflector echo. It also confirms that the alarm loop is working, with a high-level alarm being displayed in the control room. This test function can be accessed remotely using software packages, as well as locally using a hand-held device.

Testing can be performed in minutes from the control room during operation, thereby reducing downtime, minimising risk and maximising safety. Compared with traditional diagnostics, which only monitor the transmitter electronics, verification reflector functionality can also be used to diagnose problems affecting the upper parts of the probe inside the tank, such as product build-up, corrosion and other process-related conditions.

* The author is Marketing Engineer, Emerson Automation Solutions, Göteborg/Sverige.

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