When a large-scale refinery needed to make Safety Instrumented System (SIS) improvements on their coker furnace on-line spalling operation, the Rosemount 8800 Dual Vortex Flow Meter proved to be the perfect solution. Initially, the refinery had installed 12 vortex flow meters on each of the four feed lines for each of their three furnaces. The new SIS required additional flow measurement redundancy, but they needed to figure out how to utilize the existing setup without modifying their piping. Fortunately, the Rosemount 8800 Dual Vortex Flow Meter was available, providing two complete vortex meters with independent sensors and transmitters in a single meter body.
Delayed Coker Units and Troublesome Coke Deposits
Refineries use thermal cracking process units called delayed cokers to heat residual oil feeds after the vacuum reduced crude has gone through several previous processing units. The intended result is to crack heavy, long-chain hydrocarbon molecules into upgraded product streams like fuel-grade petroleum coke. Coke is dense, concentrated carbon. Delayed cokers heat heavy coke sediments formed during the previous refining steps at extremely high temperatures (875 –940°F / 470-505°C) to produce smaller gas and liquid products.
Ideally, the coke would remain in coke drums as it is cracked, moving through the furnace without building up residue in the furnace tubes. Unfortunately, coke formation occurs on the furnace tube walls, necessitating periodic decoking to clear it out. If left unchecked, the tubes would clog and eventually pose an extreme safety risk.
The On-Line Spalling Decoking Method
Spalling is the pitting and flaking of fragments off larger solids over time. On-line spalling is a decoking method that utilizes intense heat and high-velocity steam running through the furnace tubes of a delayed coker unit to quickly erode the coke residue from the walls. The objective is to maintain the furnace’s capacity and efficiency without constantly shutting down the process to clean the tubes.
There are only a few avenues operators have for decoking delayed coker units. On-line spalling is one of three methods refineries can use for this process. It’s a popular method because it allows the coker to continue operating while the furnace tubes are being cleared. There are also fewer environmental concerns with this method than steam air decoking. However, while these batch-continuous processes are efficient, operating at such high temperatures means significant safety and maintenance challenges.
Safety Instrumented System (SIS) for Coker Heaters
Our customer had three fired heaters in the coker with four feed lines each upon which this spalling procedure was performed. They were using a licensed on-line spalling process which needed to be completed at least once every two months.
Even in the simplest and most straightforward circumstances, measuring steam flow is not easy. Because vortex flow meters excel at steam applications, this was the flow meter of choice for the necessary SIS controls on the coker heaters. The customer had already installed 12 vortex flow meters on each of the four feed lines for their three furnaces, but additional meters were needed for increasing redundancy requirements on this critical process.
Dual Vortex Meters: Measurement Redundancy in a Compact Package
One of the biggest challenges for this customer was measuring the steam flow in the on-line spalling process because of the wide turndown requirements. The minimum steam flow was 600-1,000 pounds per hour. The maximum steam flow was 8,000-15,000 pounds per hour. The minimum flow rate was necessary to keep a positive steam flow in the tubes to prevent backflow into the steam system even when spalling wasn’t taking place. The maximum flow rate was due to the intensity necessary to break up the coke residue in the furnace tubes.
Only one flow meter technology could handle these requirements – vortex. But the customer still needed to add additional meters to the feed lines to satisfy the new SIS requirement for measurement redundancy. That’s where the Rosemount 8800 Dual Vortex Flow Meter came in to save the day. The customer could get two measurements for one by replacing their existing vortex flow meters with the Dual Vortex solution. Each Dual Vortex meter is constructed with two complete vortex meters with a single shedder bar for independent sensors allowing for 1oo2 (one out of two) voting. They are also capable of up to SIL 3 Certification.
With the Rosemount 8800 Dual Vortex Meter’s exceptional turndown range, easy-to-install configuration, and reliable gasket-free, non-clogging design, this refinery was able to replace their existing vortex solution without modifying their piping or sacrificing the efficiency of their current vortex meter setup. They saved time and money and improved safety with this sophisticated solution. To learn more, link.
You may also be interested in learning about the Rosemount 8800 Quad Vortex Flow Meter at this link.