E. BAUM, IMI, Cologne, Germany
Double disc isolation valve technology could hold the key to optimizing dangerous fluid catalytic cracking unit (FCCU) maintenance and repair processes at refineries. This article explores the key considerations behind refinery turnarounds and how they can be made safer and more efficient.
On February 18, 2015, an explosion occurred at a California (U.S.) refinery. At the center of this incident at the ExxonMobil Torrance facility was an electrostatic precipitator (ESP), a pollution control device on the air side of the FCCU (FIG. 1). During normal operation, the ESP would remove catalyst particles using charged plates that produce sparks—this process can be a potential ignition source.
On the day in question, an attempt to isolate equipment for repairs caused hydrocarbons to backflow through the process.1 The sparks in the precipitator then ignited the hydrocarbons, setting off an explosion that injured two workers and dispersed debris throughout the surrounding community.1
Lessons to learn. While only minor injuries were sustained during this accident, the potential threat to life was clear. Put simply, it is essential that plant managers analyze refinery FCCU shutdown and reactor isolation processes to determine how they can be made safer.
Indeed, the conditions in which unit turnarounds are carried out are undoubtedly hazardous. This process involves isolating the reactor from the refinery’s main column by removing a manual spacer measuring between 24 in. and 100 in., and installing a plate blind.
The plate blind location is at the reactor overhead vapor inlet to the main column, and when the overhead vapor line is parted to remove the spacer ring flange, personnel and equipment are exposed to a > 300°F (> 149°C) hydrocarbon-rich stream. While this can already make a flash fire possible, the situation could be made worse if oxygen is pulled up into the reactor vapor overhead line.
Although these conditions could lead to potential fatalities and extensive damage to piping and equipment, they must remain in place for up to 12 hr while the blind is being installed. Additionally, the parting of this connection on unit startup leaves personnel exposed to temperatures of 600°F (316°C), the level of heat required to bring the FCCU online and allow refractory dry-out processes.
Isolating the issue. During this time, the FCC reactor must be isolated from the fractionator column to ensure sufficient heat is available to raise temperatures in a controlled way if the overhead line remains open. Overall, these isolation efforts can take up to 36 hr, with the blinding process becoming part of the critical path for unit startup and shutdown. Maintenance and repair teams are undoubtedly at significant risk throughout this process.
Considering the risk, the key question is how the process can be made shorter while preventing incidents such as the Torrance refinery explosion from ever happening again. The installation of automated equipment is crucial to this process. Given that it is situated between the reactor and the main line, installing an automated isolation valve here could have a marked impact.
Specifically, doing so will allow for much safer conditions during fractionator blinding operations. By closing the valve, the plant reactor can be quickly isolated during planned turnarounds or unplanned shutdowns without allowing hydrocarbons to escape into the atmosphere.
Actuating the double disc isolation valve can also dramatically shorten FCCU turnarounds. If the valve is closed, the work dumping catalyst out of the reactor and washing of the main column can continue concurrently. As a result, plant teams can save around 24 hr that would have been spent carrying out these tasks separately and consecutively. In addition, keeping the valve in the closed position can enable bolt retorquing to take place while the reactor heats up.
When compared to traditional, long-standing blinding methods, which require plant stakeholders to wait until refractory curing is complete before rolling the blind, the possible efficiency gains are clear. Consequently, another 12 hr can be saved during the FCCU turnaround procedures—vital savings considering the importance of the unit remaining online to maximize plant yields.
Demonstrating detailed design. Given the clear importance of isolation valves in mitigating risk, focus should shift to what constitutes effective component design. Double disc through conduit (DDTC) gate valves are well-suited to these on/off applications, as their separate, independent shut-off discs and overall design provide multiple advantages.
Specifically, the valve’s internal split-wedge-ball arrangement allows for reliable operation and gives discs clearance to move. Its internal wedge-ball design also provides actively controllable and adjustable mechanical sealing, and its non-self-locking wedges ensure the discs are released from their seats without jamming at high or variable temperatures.
As such, DDTC gate valves allow for true double block and purge processes, ensuring full and effective double isolation during turnarounds. Plant maintenance teams can consequently enjoy peace-of-mind about safety when carrying out turnaround procedures, knowing the valve can be easily controlled when opening and closing.
Negating wear and tear. When specifying valves in this scenario, it is also key that any selected solution be appropriately durable. Conditions in the reactor overhead vapor line are extremely hazardous and must be considered during component design and specification.
Manufacturers have designed their DDTC gate valves for severe coking services. Production at the author’s company’s manufacturing plant in Düren, Germany, involves including corrosion and wear-resistant overlays on all seats to further bolster component resilience in hazardous environments. Similarly, the component’s seat surfaces are built to provide complete protection from process flows in both the opened or closed positions.
The valve’s internal wedge-ball arrangement should also give its discs clearance to move, further minimizing the seat-to-seat friction and wear. The valve’s design also includes external piping loads and minimizes the effect of seat deflection due to external loads.
In conclusion, though shutting and isolating the plant FCCU for turnaround processes is vital, it can place maintenance teams at risk. Existing manual processes such as removing spacer ring flanges and installing plate blinds can result in a flash fire or explosion, so more effective ways to isolate the refinery reactor from the main column must be explored. In these demanding circumstances, the use of well-designed, hard-wearing DDTC gate valves can enable plant stakeholders to reduce maintenance time while keeping refinery personnel safe. HP
LITERATURE CITED
Chemical Safety Board, “ExxonMobil Torrance Refinery explosion,” May 2017, online: https://www.csb.gov/exxonmobil-torrance-refinery-explosion-/
ELKE BAUM is the Service and Sales Leader for Application Delayed Coking/FCC at IMI Process Automation. With more than 20 yr of experience in high-temperature valve solutions, she has held leadership roles including Head of Aftermarket and Sales Director for North Europe and the CIS. Baum began her career as a Design Engineer at IMI Z&J.