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Flow Measurement during Tanker Unloading

Flow Measurement during Tanker Unloading

The mineral oil that arrives via tankers on the north sea coast of Germany begins its journey through the facilities at the tanker pier. For the transfer of crude oil there are extremely efficient state-of-the-art delivery platforms, called jetties.

At this particular pier, there are three jetties with a total transfer capacity (turnover rate) of 40.000 cubic metres per hour. This capacity is divided between two jetties with a 12.000 m³/h transfer rate each and one large jetty, which can deliver the crude oil at a rate of 16.000 m³/h. The tankers pump the oil ashore using the ship's on-board pumps via the jetties with their marine loading arms and into the terminals storage tanks, meaning that the actual rate of delivery depends on the capacity of the respective tanker's pumps.

For safety reasons, a specified feed rate per pipeline must not be exceeded and for this reason, the unloading process must be monitored with flowmeters. The ship pumps can also be controlled by measuring the unloading capacity so that the plants and process are run in the best possible way. Last but not least, flow measurements allow for the first recording of unloaded mineral oil quantities. The fiscally relevant quantity measurement is carried out by measuring the level in the tanks.

Until now, Pitot tubes and turbine meters have been used for flow measurement. These wetted measuring devices have their known weaknesses: They are subject to mechanical wear and tear and only have very low measuring dynamics, i.e. they only measure accurately over a limited range. For maintenance and repair work, the pipeline must be opened which means operation is temporarily unavailable. As part of a modernisation of the plant technology, the responsible engineers looked specifically for a better alternative.

Once again, non-invasive flow measurement with FLUXUS proves to be the ideal solution: Since the clamp-on ultrasonic transducers are simply mounted on the outside of the pipe, they are not subject to any wear and tear by the medium flowing inside. There is no need to open the pipe for installation and it can be done during ongoing operation. The acoustic measuring method is free of inertia and provides an excellent dynamic range which means high accuracy from the lowest to the highest flow velocities. As a result, start-up and shut-down operations can be recorded accurately.

After demonstrating the capabilities of FLEXIM’s clamp-on ultrasonic technology with a portable FLUXUS flowmeter, the tank terminal's process engineers decided to fit one of the three jetties with FLUXUS.
A particular difficulty of the measurement is that the tank pumps often convey air which considerably dampens ultrasonic signals in the mineral oil flow. This challenge was overcome by using ultrasonic transducers that work with Lamb waves. In order to guarantee maximum reliability, the measurement was carried out in two channels. Finally, in order to achieve the highest possible accuracy, the actual density and viscosity values of the unloaded mineral oil, which are available in the process control system, are fed in as a current signal via the transmitter’s freely configurable inputs.

Non-invasive flow measurement with FLUXUS also proved to be impressive in the long-term test. That’s why the other two jetties were later retrofitted with the same equipment. On one of the two jetties, refinery residues are also handled in addition to mineral oil. They are heated when unloaded and then pumped through a trace heated and insulated pipeline to the tank onshore. Given the fact that the medium is highly viscous, the advantages of non-invasive acoustic measurement from outside really come into play. The high temperature clamp-on transducers were integrated into the insulation. All measurements were field calibrated in comparison to the tank level measurements. With the help of precise and dynamic flow measurements achieved with FLUXUS, an additional safety system that quickly detects any leaks will be implemented.

Advantages

  • Reliable and accurate flow measurement from the pipe wall outside
  • No wear and tear, No risk of leakage, No pressure loss
  • No need to open pipelines, no process shut-down for installation
  • Highly cost effective when comparing to wetted flowmeters of this nominal pipe width
  • Considerably less time and effort needed for installation and possible maintenance in comparison to wetted flowmeters

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