Many of the units in a refinery operate under harsh conditions. Perhaps one of the harshest is the delayed coker unit. At the time of writing this post, Wikipedia defines this unit:
A delayed coker is a type of coker whose process consists of heating a residual oil feed to its thermal cracking temperature in a furnace with multiple parallel passes. This cracks the heavy, long chain hydrocarbon molecules of the residual oil into coker gas oil and petroleum coke.
In a Hydrocarbon Processing article, Improve coker efficiency with reliable valve automation [link to excerpt only, subscription required for full article], Calabrian Corp’s Benny Deters and Emerson’s Ross Wolkart describe the challenges this demanding process places on the valves, valve actuators and plant controls.
The authors highlight the severe operating conditions:
…including excessive heat, vibration, and corrosion… High inlet temperatures of the residual oil flowing from the fractionator through the transfer line into the coke drum exceed 800°F (425°C) at low pressures of 10 psig to 15 psig.
Over time, these drums fill with coke increasing the torque load on the actuators driving the valves. The coke removal process increases the severity of operation:
…there is extreme vibration. The high-pressure water lines used to drill out and cut the coke from the drum internals create pressures of up to 4,000psi. The steam and quench water piping used in the decoking process is susceptible to rapid expansion and temperature fluctuations of condensate and/or water, producing an often violent water hammer effect.
The final control and measurement instrumentation is complex for the drums:
Typically, there are eight to 10 valves for each drum in the coking process. These valves perform multiple services including recirculation, switching, quenching, washdown, steam hydrocarbon stripping, and drum steam reheating. They control flow in piping that transport steam, water, slurry, hydrocarbons and product, and they critical to operation.
The authors noted the problems on actuator performance from water hammer and vibration, coke dust, and corrosion. At two Louisiana refineries, a better solution was tried using a modified, multi-turn, electric actuator. It had industrial-grade epoxy coatings for corrosion resistance, marine-grade aluminum housing, and no microprocessor components unlike the previous actuators, which experienced vibration-induced connection failures. It:
…featured incorporated, reliable circuit boards with no termination wiring, and compact internal limit switches and relays that could withstand the high temperatures in a delayed coking unit.
The actuators were sized to provide a 2x safety factor to handle the increased torque caused by coking and pipe expansion/contraction. This solution helped the refinery to avoid premature failures, which:
…lead to extra costs for operator overtime, additional labor and safety risks for manual valve operation, replacement costs and potential refinery downtime.
The refineries using the selected actuators have been able to increase production, improve personnel safety and conduct regularly scheduled shutdowns, thereby significantly reducing maintenance costs.