David Williams - Vega Americas
The main way to measure outage in a Delayed Coker Unit has traditionally be done manually using a tape or plum bob. Over the years with advancement in de-heading valves as well as safety and environmental issue, obtaining the outage measurement has become more difficult. Many ways have been attempted including but not limited to Radar, Drill stem, etc. Customers have approached VEGA Americas for a solution using the coke drum level measurement system. We have successfully been able to measure the coke bed and verified it in relationship to the drill stem.
VEGA has learned many new things through our experience measuring foam levels in delayed Coker units. One of the most important items that we have learned is the direct measurement of coke fine carryover. This is measured by the vapor density gauge, mounted at or close to the top tangent line. The vapor density gauge measure the change in relative density of the hydrocarbon vapor leaving the drum. This change can be used to measure the coke fine carryover. Another thing the vapor density gauge can measure is coke buildup over time and can be used to determine when a “dome clean out” is needed.
Since the coke drum structure is typically the tallest unit in a refinery, the nuclear levels mounted on the drums (normally at the top of the drums) can be influenced by radiographic sources brought into the refinery for Non-destructive testing (NDT). These source can be quite large ( can be as high as 60Ci of Co-60). A 60Ci Co-60 source can put a field of 0.1mR/hr at 3000 feet. The field that the continuous level system typically is lower than 0.1mR/hr so the NDT source will drive the level measurement to 0%. To protect against this, especially during critical times of the drum cycle, VEGA has developed a means of detecting this radiation from NDT sources to alert the operator as well as protect the level measurement system.