Part 1 of this two-part series discusses the coking process and its valve and actuator requirements. Coking is a refinery unit operation that upgrades material called bottoms from the atmospheric or vacuum distillation column into higher-value products. As the name implies, the end product is petroleum coke, a coal-like material.
Types of coking
Two types of coking processes exist — delayed coking and fluid coking. Both are physical processes that occur at pressures slightly higher than atmospheric and at temperatures greater than 900oF (485oC) that thermally crack the feedstock into products, such as naphtha and distillate, leaving behind petroleum coke.
With delayed coking, two or more large reactors called coke drums are used to hold, or delay, the heated feedstock while the cracking takes place. While the lighter products are pumped back into the fractionator through the overhead vapor lines, coke is deposited in the coke drum as a solid. This solid coke builds up in the coke drum and is removed by hydraulically cutting the coke using water. To facilitate the removal of the coke, the hot feed is diverted from one coke drum to another, alternating the drums between coke removal and the cracking part of the process.
With fluid coking, the feedstock is charged to a heated reactor. The cracking takes place, and the formed coke is transferred to a heater as a fluidized solid where some of it is burned to provide the heat necessary for the cracking process. The remaining coke is collected to be sold.
Operations in any refinery’s delayed coking unit are extremely severe, perhaps the harshest of any refinery operation because of the batch nature of the process, daily swings from very high temperatures to ambient, high vibration on the deck as well as the significant potential for coke buildup in the equipment. Over time depending on steam purge system reliability and effectiveness, coke may build up in the valves and valve operating torques may increase accordingly. Torque increases put additional stress on these ball valves and on the electric actuator that operates them. Designing a robust valve and providing an ample safety factor when selecting these actuators is key in ensuring uninterrupted operation between scheduled turnarounds. As the operating drum fills with coke, torques on the valves’ trim tend to increase, putting additional stress on the transfer line ball valves and similarly added torque on the electric actuator.
During the coke removal process, there is a lot of vibration. The high-pressure water lines used to drill out and cut the coke from the drum internals create pressure in excess of 4,000 psi. The steam and quench water’s rapid expansion and the condensate’s temperature fluctuations often produce water hammer. In addition, corrosion in traces of sulfur combine with the unit’s washdown water and significant amounts of abrasive airborne dust are created. This dust covers all surfaces within the drum and its immediate surroundings.
The coke dust creates opportunities for corrosion. It also builds up in crevices, impeding functionality. Harsh, hot and potentially dangerous conditions require higher exotic alloys for extra protection and maintained functionality.
Each drum in the coking process requires 10 to 12 valves for recirculation, switching, quenching, stripping, washing and reheating steam. Valve sizes and pressure class have increased significantly over the last few years as end users look to increase throughput and yields. These severe and harsh conditions of the coker operation often have a detrimental effect on these valves and the actuators operating them. Making it more critical is the fact that the valves must work on a schedule of sequential strokes to divert in a precisely timed event.
Valves’ (operating) sequences are controlled through a distributed control system or stand-alone, dedicated programmable logic controller (older units still sometimes rely on relay-driven panels) and an actuator, which should be reliable. Valves and actuator units must have safety interlocks restricting their opening and closing through limit switches that provide positive position feedback to maintain safe process control. An inoperable valve actuator must be reinstated quickly so that the system can continue functioning. If the actuator fails, the valve should be capable of opening manually (normally this manual operation is strenuous and time consuming and, therefore, is not a preferred option).
In a coker application, while the valves can be changed out for maintenance, the actuators need to be reliable and require little maintenance. A short Mean Time Between Failure can be costly on such a demanding application.
Valves can fail for many reasons including the following:
- Water hammer
- Broken internals
- Dislodge components of actuation
- Disconnection of motors
- Penetration of coke dust in critical areas
The installation of a robust valve and actuator solution is required to provide extended service life results in critical severe services such as coking.
A valve solution
The importance of long-life, resilient coker valves cannot be understated. This is a batch process, and coker can become a potential bottleneck in continuous refining operations. If the critical coke drum filling, switching and decoking schedule is interrupted, the entire refinery production can be impacted. Refinery throughput can be disrupted, costing an end user millions of dollars per day in lost production.
The valve actuator, seemingly a small item in the total process, has a significant importance and a great impact on a delayed coking operation. Premature failure can lead to extra costs for operator overtime, additional manpower for manual valve operation (the safety aspects of manual operation), replacement costs and potential refinery downtime.
A strong, corrosion-resistant solution is a metal-seated ball valve with tungsten carbide or tougher material overlay. The actuator should be field-proven, ductile, iron-housed and powder-coated. It should have a compact size, a unique internal electric circuitry configuration and modified hard-wire operation, which could provide considerable cost savings and prevent shutdowns of this critical operation unit in a refinery.
Editor’s note: Look for Part 2 of this series in the September issue, which will discuss the trunnion-mounted ball valve design of the solution described in the "A valve solution" section.
Gobind Khiani is a professional engineer with more than 22 years of experience in the petroleum industry and is a member of the Flow Control Editorial Advisory Board. He has served as lead engineer in engineering and project management roles, for operating companies and engineering, procurement and construction companies. Khiani has a bachelor of science from University of Pune, India, and a master’s in engineering from the University of Calgary, Alberta. He can be reached at firstname.lastname@example.org, or connect with him on LinkedIn.