Nuclear technology for level measuring in delayed coke drums.
The production of coke is a finely monitored process, which requires efficient managing to achieve increased throughput and avoid the risk of shutdown in the refinery.
Liquid hydrocarbon is converted to coke in a heating process, and the gaseous materials are removed from the top of the coke drum as it fills. Just ahead of the coke level a foam front develops, with a density greater than the gaseous material but not as great as the coke. Once the proper charge is achieved the coke is quenched with water and then drained. The coke is removed from the drum by drilling with a water jet (known as coke cutting).
Efficient foam control and careful monitoring of the levels of foam, gas and solids is required to prevent over-fill of the drum. In this process, interface density measurement is the only method that will produce the necessary control.
Over the years several methods of level control have been tried, including radar gauges, neutron back scatter and gamma point detectors. The latter method was introduced by Ohmart as far back as the late 1950s. This involved a series of measurement points located on the same side of the coke drum, which measured the coke and the foam. It was a good method but limited to fixed points and some sources were cut off and lost in the coke as it was being cut.
Then in 1999, Ohmart/Vega patented FibreFlex technology, which utilises a special fibre-optic bundle as the scintillator. This flexible, gamma-based level gauge can manage continuous level measurement and allows for long measurement ranges of up to 7 metres. The FibreFlex measures the top of the process, regardless of whether it is foam, liquid or coke and measures across the diameter of the drum not just at a localised point.
Nuclear continuous level measurement works by directing a narrow fan of radiation through the vessel to a detector on the other side. The FibreFlex uses small source size, which gives it favourable cost and safety advantages. Mounted on the outside of the coke drum, its flexibility and light weight makes it easy to install. No rigging is required and the shape can be adjusted to fit existing platforms.
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