A European refinery operates a distillation process to separate crude oil into its various fractions based on their boiling points. The process takes place in atmospheric and vacuum distillation units (ADUs and VDUs), where the crude oil is heated and partially vaporised. The vapour rises through the column, which is equipped with trays or packing to enhance separation. As the vapour ascends, it cools and the different hydrocarbon fractions condense at different levels according to their boiling ranges, while heavier fractions, such as gas oil and residue, are recovered lower down.

For safe and reliable operation, the flow of the different fractions extracted from the process must be controlled. The heavy bottom residue in particular can be difficult to handle. The medium tends to cause scaling and may contain solids, while operating temperatures can reach +420°C / +788°F. It even solidifies at lower temperatures.

Previously, the customer used a different flow measurement technology which was prone to clogging due to the solid particles in the heavy bottoms. When critical components of the meter failed, maintenance was not possible without shutting down the process. As a result, the customer had to accept reduced measurement functionality until the meter could be repaired during the next scheduled shutdown.

As measurement availability is critical, the company was looking for a replacement. The new flow instrumentation needed to provide stable and accurate flow measurement over the long term, as maintenance or replacement of the flowmeter is only possible during a shutdown for overhaul, which takes place every three to five years.

Process conditions

The customer opted for the OPTISONIC 4400 HT as a replacement. The ultrasonic flowmeter is designed for high-temperature liquids and has a long track record in refinery and petrochemical processing applications.

The flowmeter was supplied in a 12" size and, for this particular application, features redundant pairs of high-temperature ultrasonic transducers with sophisticated foil waveguide technology. If required, the ultrasonic transducer modules can even be replaced without interrupting the process. The flowmeter is also equipped with four signal converters in a redundant configuration, each utilising two pairs of ultrasonic transducers. The OPTISONIC 4400 HT was calibrated prior to delivery and comes with various approvals for use in hazardous areas.

The flowmeter plays a vital role in maintaining plant safety and process reliability. It provides the process control system and the safety system with the required flow information simultaneously.

Its dual-path design compensates for variable flow profiles, maintaining stable and accurate measurement performance. The OPTISONIC 4400 HT has no moving parts or other flow constrictions where material can accumulate. In the event of scaling, the ultrasonic signal can penetrate the deposits. There are also no pressure taps that could become clogged or diaphragm seals that could fail.

The ultrasonic flowmeter offers a wide turndown ratio, eliminating the need for recalibration when operating conditions change. Its measurement principle allows it to accommodate changes in pressure and temperature without requiring further adjustment over the specified operating range. Installation was also convenient for the customer, as the flowmeter can be configured for different process conditions and temperatures without requiring extensive specialist knowledge.

The meter design is versatile and adaptable to different requirements. It is available in a redundant configuration and can also be supplied as a SIL-certified version for safety-related applications. Digital communication options and extensive diagnostics (OPTICHECK technology built-in) round off the functionality of the device.

Heavy bottoms is just one of many demanding process streams KROHNE has successfully addressed in the refinery and petrochemical industries over the years. Other applications involving similarly challenging process fluids and demanding operating conditions include feed for cokers, steam crackers and facilities producing sustainable aviation fuel (SAF).