David Williams - VEGA
The latest development in the safe and reliable operation of delayed cokers includes the use of continuous nuclear levels on the drums. This can be accomplished by adding the nuclear continuous level instruments alongside older Neutron Backscatter (NBS) devices or by simply replacing the NBS altogether. Both can and have been accomplished successfully.
When using a nuclear continuous level system in conjunction with the NBS devices, it is important to understand what each device is actually measuring. It is common to assume that since both are nuclear measurement devices, that they operate and measure the same, but this assumption is incorrect. The means by which they operate and what they actually measure are completely different.
The NBS uses neutron radiation to measure hydrogen density and infers a level by measuring the amount of hydrogen in front of the device since the radiation source and the detector are located in the same housing. The more hydrogen present equals a higher output, therefore it is assumed to measure liquid level.
Nuclear continuous level instruments use gamma radiation directed to the detectors from the opposite side of the drum. As the level inside the drum increases, it reduces the amount of radiation to the detectors and the detectors infer a high-level reading. One of the critical differences between the two nuclear devices is how they measure foam, especially during a potential foam-over. It is important for operators to understand what the foam level is doing in order to take corrective steps to prevent a foam-over. The purpose of this presentation is to explain the differences between NBS and nuclear continuous level instruments and ensure that operators of delayed cokers clearly understand how to interrupt the measurements when the situation calls for it.