A refinerys delayed coker unit (Fig. 1) operates under what are perhaps the harshest conditions of any process in the plant. Equipment with moving parts, notably valves and the actuators that operate them (Fig. 2), are especially vulnerable to these severities. The following case history discusses conditions occurring in delayed coker operations at two refineries in Louisiana, US; the impacts on valve performance; and a solution that provides extended life, increased reliability and other benefits at the facilities.
| Fig. 1. Multiple valves and actuators control |
the coking/decoking process.
Fig. 2. Actuators are compact and versatile so as to be configured in a variety of mounting configurations.
Severe conditions in the coker
Severe operating conditions, including excessive heat, vibration and corrosion, exist in every refinerys delayed coker operation. 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.
As the operating drum fills with coke, torques on the valves wetted parts tend to increase, putting additional stress on the transfer line ball valve and added operational torque on the multi-turn electric valve actuators. During the coke removal process, 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 4,000 psi. 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. Corrosion is present as the cokes traces of sulfur combine with the units washdown water.
During the washdown phase, a significant amount of abrasive, airborne dust is created, covering all surfaces within the drum and its immediate surroundings. The coke dust not only creates challenges for corrosion protection, but also builds up in crevices, impeding instrument and equipment functionality. In addition to the harsh conditions present, the space available for performing maintenance is confined, hot and potentially dangerous for plant personnel.
Impact on valves and actuators
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 transports steam, water, slurry, hydrocarbons and product, and they are critical to the operation.
Controlled by a programmable logic command, the valves sequential event must be consistently and reliably executed by an actuator (Fig. 3). To maintain process control, valves have safety interlocks restricting their opening and closing through limit switches. An inoperable valve actuator must be reinstated quickly so that the system can continue functioning. If actuators fail, the valves must be opened or closed manually. This is a strenuous, time-consuming and potentially dangerous process for the unit operator, although it is necessary to keep the coking/decoking process in sequence and on schedule.
Fig. 3. An actuator executes sequential
events for coker valves.
Automated valve performance and mean time between failure (MTBF) are constant and costly problems for the refinery. While valves are not as susceptible to failure and can generally be serviced during periodically scheduled shutdowns, actuators have been prone to early, unplanned failures and the need for continual repair.
Coker actuator failure can occur almost immediately after installation, and normal life expectancy is less than one year. Experience has shown that actuator failure can be attributed to a variety of conditions that are fairly consistent in any coker application. Water hammer and vibration effects can break internal electrical connections and dislodge sensitive microprocessor components. Also, motors can become disconnected from the actuator housing through vibration. When inspected, these motors have been found precariously hanging from their wiresand nothing else.
Furthermore, coke dust fines regularly penetrate the actuator housing, causing inoperability of electronic components and on/off pushbuttons to become clogged and faulty. Corrosive elements erode aluminum actuator housings, wiring and even the external handwheel, rendering this sole backup device useless.
Maintenance costs can be excessive, accumulating with continual callout and overtime repair charges. Ongoing actuator problems have even necessitated the use of a full-time, dedicated troubleshooter whose sole function is to keep the valves operating as scheduled, or repaired in a timely manner.
A successful solution
In the case of two Louisiana refineries with delayed coker operations, the search for a robust and reliable actuator solution to provide extended service life resulted in the testing and selection of a modified, multi-turn, electric design that had provided exceptional service in other applications.
While not the newest version of the technology, this established actuator had been engineered to provide the necessary features for success in this challenging service. The selected unit used industrial-grade epoxy coatings on all external surfaces as standard construction to provide excellent corrosion resistance. Also standard was its marine-grade aluminum enclosure with stainless-steel captured bolting.
Unlike some newer and more sophisticated actuator designs, this solution had no microprocessor components. Rather, 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 actuator could be remotely managed from a hardwired push-button panel unaffected by the dust; yet, the control was within sight of the actuator to verify its proper operation. Internal control components were smaller, compact and lightweight to resist self-destruction from inertia and the momentum generated from vibration and water hammer effects. Its linear drive train and gearbox assembly were specifically configured to withstand high torque and thrust loads while maintaining alignment.
Sizing safety factors were also considered. For reliable operation, this actuator was sized to provide a two-times safety factor, which is highly recommended to account for the variable and generally higher torques needed as the process progresses and as piping expands and contracts, requiring higher torques not included in new torque values. The overall actuator footprint and weight were sufficient for operation in confined spaces and for accommodating a variety of horizontal and vertical valve installation positions.
After being tested and installed, the selected actuator provided the refineries with immediate relief from failure. The units have been in continuous service for more than six years in one refinery and for more than three years in the other. There have been virtually no failures, and the limited need for maintenance can be completed during scheduled plant shutdowns.
The importance of valves and the electric actuators that control the operation in a delayed coker unit cannot be minimized. As a batch process, the coker becomes the potential bottleneck in an otherwise continuous refining operation. If the critical coke drum filling, drum switching and decoking schedule is significantly interrupted, it can impact the entire refinery throughput, costing the company millions of dollars per day in lost production.
The valve actuator, seemingly a very small item in the total process, has a significant importance and a great impact on the delayed coking operation. Premature failure can lead to extra costs for operator overtime, additional labor and safety risks for manual valve operation, replacement costs and potential refinery downtime.
The robust, reliable solution was found to be a field-proven, ductile iron-housed, powder-coated actuator with external control in proximity of the valve. Its compact physical size and weight, unique internal electric circuitry configuration and modified hardwired operation have provided the refineries with considerable cost savings and have averted potential production outages.
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. HP
Benny Deters is the director of manufacturing at Calabrian Corp., a specialty chemical manufacturer in Port Neches, Texas. He is an electrical engineering graduate from McNeese State University, and he has more than 25 years of management experience in delayed coking and plant operations.
Ross Wolkart is the gas and pneumatic product manager for Emerson Process ManagementValve Automations EIM line. He is the companys refining and pipeline application specialist and a graduate of Southeastern Louisiana University. Mr. Wolkart has more than 16 years of sales and service experience and expertise in delayed coking operations.
Delayed coker facts
What is delayed coking?
A semi-batch thermal cracking process used to upgrade and recover residual liquid and gas streams
while leaving behind petroleum coke, which is usable as an industrial fuel source or can be further
refined for additional industrial purposes
Inlet temperature: 900°F (480°C)
Inlet pressure: 15 psig to 35 psig
Process temperature: 700°F (371°C)
Coke drum size range:
Height: 55 ft to 120 ft
Diameter: 15 ft to 30 ft
Online coke drum filling: 10 to 12 hours
Drum decoking process: 4 to 6 hours