Actuator Retrofit and Operational Optimization of Plug Valves in Delayed Coking Units

Abstract: The original design of a Sinopec coking unit employed a single electric actuator to drive the lifting two-way plug valve. However, the actuator had a complex structure, slow actuation, a high failure rate, and was difficult to maintain. The feed cut-off valve SPV101AB frequently suffered from coking and jamming, resulting in severe actuator wear and posing a serious risk of unplanned shutdowns. During the scheduled maintenance in 2017, the single electric actuator was replaced with a dual electric actuator. Although SPV101AB continued to experience jamming and positioning failures after six months of operation, the issue was resolved by removing coke buildup through valve cover disassembly and optimizing the operating procedures. Since then, the system has operated reliably for nearly a year without failure, leading to significant savings in both labor and material costs.

1. Overview

The 1.6 million tons per year coking unit at a Sinopec facility was completed and commissioned in 2010. Originally, the feed cut-off valve on the coke tower, the oil and gas cut-off valve at the tower top, and the tower top vent valve were all imported plug valves from Spain, each equipped with a single electric actuator. These actuators had complex mechanisms, experienced high failure rates, and were difficult to maintain. Since April 2011, issues with incomplete switching and repeated malfunctions have occurred, nearly causing multiple shutdowns of the unit.

2. Structure and Working Principle of the Lifting Plug Valve

The SPV101AB feed isolation valve of the 1.6 million t/a delayed coking unit, along with the SPV102AB oil and gas outlet isolation valve and the SPV103AB vent valve, are all imported plug valves operated by single electric actuators (see Figure 1 for process layout). Each plug valve comprises an actuator, valve position indicator, valve stem, sealing surface, valve plug, steam seal, and other components. The actuator is relatively complex, consisting of a gear transmission system, back cap gland, upper and lower seals, fixing ring, upper bearing, operating screw, intermediate threaded ring, and a threaded connection to the valve stem. The electric two-way lifting plug valve completes its switching operation through three sequential steps:

• Valve plug lifting – disengages the sealing surface to increase the clearance between the plug and valve body, thereby reducing rotational resistance;
• 90° rotation – rotates the plug;
• Lowering and resealing – re-engages the sealing surface to restore the seal.

The valve uses a reduction mechanism to drive the screw, and the actuator’s operation is controlled by a steel ball locking mechanism that sequentially completes the three steps. The overall structure is relatively complex. Moreover, the slideway housing the steel ball is susceptible to wear and deformation over prolonged use, which can cause incomplete valve actuation and complicate maintenance. With the adoption of a dual electric actuator, the valve body structure remains unchanged. Following the removal of the original single electric actuator, a new mounting bracket and upper valve stem are fabricated. Separate lifting and rotating electric actuators are installed and programmed to operate in a coordinated logical sequence.

Figure 1: Process flow diagram of coke tower feed and discharge

In the dual electric actuator system, each switching cycle proceeds as follows: the lifting actuator is remotely activated to raise the valve plug. Once it reaches the preset position, the rotating actuator engages to rotate the plug 90°. Finally, the lifting actuator reverses to lower the plug, completing the cycle when the plug reaches its limit position and the preset downward torque is achieved.

Advantages:
Compared to the single electric actuator, the dual actuator system features a simpler structure, a lower failure rate, and easier maintenance. The upper and lower limits of the valve plug are easier to adjust, facilitating troubleshooting. Additionally, the system supports convenient manual, local, and remote operation modes.

3. Modification Process

(1) After converting the actuator of the SPV-101AB feed isolation valve (Boya plug valve) from a single electric actuator to a dual electric actuator, a local operation cabinet was installed for each valve. The following modifications were carried out on-site:

• The local operation cabinet was powered by the original power supply for the single electric actuator, with an additional 220V control power supply added.
• The control feedback line of the original single electric actuator, which was connected to the logic operation panel, was rerouted to the local operation cabinet.
• The forward control feedback line from the original actuator was also connected to the local operation cabinet.
  Power and control cables were added from the local operation cabinet to both the lifting and rotating electric actuators.
• The opening and closing actions of the SPV-101A and SPV-101B dual electric valves are now controlled by the original logic operation panel. The control logic required by the original panel was maintained, ensuring that the safety interlock between SPV111 (the four-way valve) and SPV-101A and SPV-101B (the dual electric feed isolation valves) remains intact.

The operation and logic of SPV111, SPV-101A, and SPV-101B remain exactly the same as in the original setup, with the sequential control system feedback continuing to accurately reflect the valve positions. The control buttons on the local operation cabinet perform the same functions as those on the original logic operation panel and fully comply with the original control logic.

(2) After upgrading the actuators of the SPV-102AB oil and gas isolation valve and the SPV-103AB oil and gas vent valve from single electric to dual electric actuators, local operation cabinets were installed for each valve. The following modifications were carried out on-site:

• The original power supply for the single electric actuators was reused for the local operation cabinets.
• The feedback line from the sequential control system was rerouted to the local operation cabinet, and new power and control cables were installed from the cabinet to the lifting and rotating electric actuators.
• One-button open/close functionality was implemented via the local operation cabinet, and the sequential control system feedback remains fully aligned with the actual valve open/close status.

4. Operation Status and Post-Modification Analysis

After the modification, the valve switching time was reduced from 7 minutes to just 40 seconds, significantly improving operational efficiency. The oil and gas isolation valve SPV102AB and the oil and gas vent valve SPV103AB have operated reliably for two years without any failures. However, the feed isolation valve SPV101AB began exhibiting issues approximately six months after it was put into service. In the initial phase, normal operation was maintained by adjusting the torque and limit settings of the lifting actuator. Subsequently, SPV101AB underwent online inspection and repair in early 2019.

In July 2018, SPV101AB began failing to fully reseat, and bottom steam injection was also obstructed. By adjusting the downward torque, the steam lines were cleared, allowing normal feed switching operations to resume. By March 2019, there was no remaining room to adjust SPV101B’s torque and limit settings, and the sealing performance of the isolation valve had significantly deteriorated. Maintenance was performed on March 29. After repeated leak tests confirmed that the four-way valve was leak-tight, the cover of SPV101B was opened for inspection. It was found that the bottom of the valve body was heavily coked, with compacted coke deposits. The bottom steam injection line was completely blocked, and the sealing surface of the valve plug was covered with a coke layer approximately 2 mm thick.

On May 22, 2019, SPV101A failed to rotate after lifting. The lifting actuator’s display panel indicated that the valve plug generated lifting torque upon reaching 100% of its lifting stroke. This indicated that coking between the valve plug and the valve cover had caused it to jam, preventing rotation. On July 2, 2019, SPV101A was opened for inspection. After removing the cover, a large amount of soft coke was found between the valve plug and the upper bonnet. The sealing surface of the special valve was also coked, with localized deposits reaching nearly 2 mm in thickness. Additionally, the bottom of the valve body was severely coked, with deposits approximately 90 mm thick and compacted.

Cause Analysis:

• The medium flowing through the feed isolation valve SPV101AB is residual oil discharged from the heating furnace. This oil is high in temperature and viscosity, making it prone to coking.
• No visible scratches were found on the sealing surface, ruling out any sealing surface damage as the cause of coking in the valve body.
• After the actuator of SPV101AB was upgraded, the original lifting height was increased from 8 mm to 12 mm. (The original actuator had experienced several rotation jams.) Increasing the lifting height was intended to enlarge the gap between the valve core and the valve body to prevent jamming. Based on the cone ratio of the valve core, every 10 mm increase in lift results in approximately a 1 mm increase in the clearance between the valve core and the valve body. Therefore, a 4 mm increase in lifting height results in a 0.4 mm increase in the sealing surface gap. At the same time, the valve core’s lifting time—from the zero position to fully open—was reduced from 3 minutes with the single electric actuator to just 15 seconds with the dual electric actuator. This combination of increased lifting height and faster lifting speed prevented the steam injection from fully sealing the dead space on the valve surface at the same steam flow rate. Consequently, high-temperature residual oil entered the bottom of the valve body during the lifting process.
• Once coking occurred at the bottom of the valve body, the available space became restricted, reducing the steam injection flow. This further weakened the steam seal and accelerated coke buildup at the bottom.
• As coking at the bottom worsened, the valve could no longer fully close. To maintain normal production, the zero positions of the two plug valves were temporarily adjusted upward as an emergency measure.
• The coke on the valve core consisted of loose, granular deposits of varying sizes, indicating long-term accumulation.
• After the zero position was raised, a gap formed at the sealing surface, diminishing the effectiveness of steam injection in isolating the medium inside the valve passage. Over time, residual oil accumulated above the valve core during continued operation.
• With each production cycle, a small amount of residual oil entered the valve. This continued until May 2019, when the upper cavity became full and jamming occurred.

Optimization Measures:

Based on operational performance over the past year, the lifting height was reverted to the original 8 mm. Following this adjustment, the rotary actuator no longer experienced any jamming. The tower switching procedure was optimized to ensure that no residual oil remains in the valve channel during switching, effectively preventing coke formation by stopping residual oil from entering the valve body. Specifically, the purge time for the old tower’s feed isolation valve was extended after tower switching. During the switching process, the old tower’s feed isolation valve must remain fully open and stationary. Steam is injected through the short pipeline section between SPV101A/B and the four-way valve at over half of the maximum flow rate, purging both the short section and the valve channel for 8 to 10 minutes. Due to sequential control limitations, this purge duration is currently limited to 8–10 minutes. Since the new tower requires quenching oil, the old tower’s feed isolation valve must be closed before switching to quenching oil. Modifying the sequential control logic to extend the purge time is being considered for future implementation. After thoroughly purging the short section and valve channel, the valve is then closed. Lifting and rotating are then performed in a clean environment, which helps prevent residual oil from settling at the bottom or entering the top of the valve body.

Steam Injection Monitoring:
Monitoring of steam injection for the special valves has been intensified. The steam injection temperature of the feed isolation valve SPV101AB is now measured daily as well as during each tower switch. These temperature readings help evaluate potential coking at the bottom of the valve body, allowing for early detection and prompt corrective actions.

5. Conclusions and Suggestions

Before the transformation, the switching operation of the feed isolation valve did not consider the need for purging before actuation. The stroke time of the single electric actuator was relatively long, providing about 3 minutes of steam purging through the short pipeline section during the valve core’s lifting stroke. While this duration allowed some purging, inspections revealed that coking still occurred inside the valve body. After replacing the single electric actuator with a dual electric actuator, the valve’s stroke time was significantly reduced. As a result, the effective purge time was reduced to just 30 seconds—insufficient to meet the purging requirements. This reduction in purge time accelerated coking inside the feed isolation valve during operation. Subsequently, an optimized plan was implemented to extend the purge time before valve closure, ensuring more thorough cleaning of the valve passage and enabling the isolation valve to operate reliably and stably.

Overall, converting the actuator from a single electric type to a dual electric type significantly reduced the failure rate. After removing coke from the feed cut-off valve SPV101AB and applying operational optimizations, SPV101B has operated continuously for 24 months and SPV101A for 22 months—both without any coking and maintaining stable performance throughout. Equipment transformation is a complex process that should never be approached with a one-size-fits-all or copy-and-paste mindset. It requires a thorough analysis of operational parameters and their impact on equipment performance before implementation. Based on this analysis, appropriate measures should be developed and implemented prior to carrying out any modification work.

Delayed coking continues to be a crucial process for the deep conversion of residual oil and is widely used in major refineries. The typical configuration—one furnace with two coke towers or two furnaces with four towers—underscores the vital role of high-temperature plug valves in tower-switching operations. Coking failures in these valves are common, and ensuring long-term, stable operation of feed cut-off valves remains a significant challenge. This case provides valuable insights and serves as a reference model, demonstrating the effectiveness of transformation and optimization grounded in practical operational experience.