Authored by: Frank J. Martinelli
Throughout the years, Delayed Petroleum Coking Units have had their struggles keeping up with the ever-increasing demand and output of Petroleum Coke. Process cycle times have decreased in duration which in turn has led to more aggressive process cutting schedules. This increased frequency in cutting cycles has created the need for more frequent repairs & maintenance to coke chute wear plates associated with petroleum coke discharge from process drums. Wear plates can vary in thickness, metallurgical properties and plate anchorage. Often bars or rails are placed in the drop zone directly below the coke drum discharge, in an attempt protect the coke chute and integral wear plates from the coke unloading and cutting process.
When Coke is cut from the drum, the chutes and their protective plates can see temperatures well in excess of [1]200 oF, thermally shocking the embedded/welded steel plates. These temporary high temperature exposures create significant [2]thermal expansion to the protective steel wear plates. As the plates cool down between the coking process cutting cycles, the wear plates contract dimensionally at ambient temperature back to their original size. This phenomenon occurs after every cutting cycle causing the embedded/welded plates to fatigue resulting in crack formation at critical welds and or embedded concrete anchorages. This cyclic behavior creates a maintenance nightmare for operations personnel charged with maintaining these critical structures. During many de-cokes, squat outages, or scheduled turn-arounds these protective plates frequently require repair and/or replacement. It should be noted that this happens to each coke chute regardless of whether the protective steel plating is placed over, or embedded into cast-in-place concrete, precast concrete, or incorporated into a steel frame superstructure.
Over the past several years, the Industry has been tasked to develop a solution that would break the “repair and repair again cycle” regarding these important protective wear plates. After much research and development that included metallurgical studies of abrasion resistance, corrosion mitigation, constructability assessments, a “Floating Wear Plate” design was developed that addresses the shortcomings associated with historical flaws in wear plate design, installation and service. Through a collaborative effort involving process personnel, engineers and construction contracting professionals, the Floating Wear Plate design has been successfully installed on eight (8) coke drum chutes to date. Six (6) of the plates have been in service since 2012 with no visible wear damage to the plates. In the fall of 2017, during a 30 day turn around where all six (6) Coke drums were removed & replaced, all six of the Floating Wear Plates were removed and inspected thoroughly. New positioning chains were installed as an upgrade to replace the steel cables that secured the plates into position as part of the original design. The following Repair Case Study is included to exhibit the technological advancement of the Floating Wear Plates over conventional coke chute wear plates.
Repair Case History
In 2012 at a refinery in the Chicago suburbs, a six (6) drum Delayed Petroleum Coking Unit required significant repairs to the Coke Drum Support Structure. A decision by the refinery management team was made to complete long-term repairs to the Coke Drum Support Structure that would return it to its original operating capacity. One aspect of the comprehensive structural restoration program was repair to its embedded protective steel coke chute plates, and the underlying cast-in-place concrete. Considerations for the repair of these plates needed to be a long-term solution.
Culminating, after weeks of engineering, design, and constructability reviews, a plan was put in place that closely matched the needs of the refinery, providing a long-term solution to their wear plate maintenance problem. The refinery management, after an exhaustive vetting program, decided to implement the Floating Wear Plate design into the planned outage.
As part of the vetting process, various options were considered. One option was the removal and replacement (repair-in-kind A-36 steel) of the existing 1” thick embedded steel wear plates including repairs to the underlying structural concrete chutes. Issues associated with this option were thermal coefficient of expansion concerns (i.e., dimensional growth at elevated temperatures) to embedded A36[i] steel anchorages and short term service life that requires a significant outage window for future repair/replacement. A second option was removal and replacement (repair-in-kind 304L Stainless Steel) of the existing 1” thick embedded steel wear plates including repairs to the underlying structural concrete chutes. Issues associate with this option were thermal coefficient of expansion concerns (i.e., dimensional growth at elevated temperatures) to embedded 304L stainless steel anchorages relative to the structural concrete and short term service life that requires a significant outage window for future repair/replacement. A third option was Floating Wear Plates with repairs to the underlying concrete chutes. The positives of this option were no thermal growth coefficient concerns as the plate “floats” on the surface of the structural concrete, unrestrained freely allowing dimensional expansion/contraction and long term repair with simple & short duration replacement options when necessary.
A root cause failure analysis of existing embedded plates was key to understanding wear plate failure mechanisms. Engineering concerns requiring more in-depth consideration prior to finalizing the design included:
- Construction material selection for both the steel plates and the underlying concrete repairs
- Existing chute angle (53o slope)
- Petroleum coke density & weight
- Drag coefficient on the floating plates
- Access to complete the work
- Where and how to effectively suspend the plates
- Design of the floating plate attachments and brackets
- How to lift the plates into place
- How to easily remove and replace them when they reached the end of their service life.
As illustrated in the Repair Case History above, the wear plates were constructed from 1” thick 304L stainless steel. Each plate was sent from the manufacturer in 10’ wide by 20’ long pieces. The final plate size needed to be approx. 20’ wide by 40’ long to accommodate the coke current chute design. Fabrication requirements involved a full penetration [1]AWS D1.1 structural weld. Due to material transportation issues it was decided to weld the various plates together on site. A plan was put in place to perform the welds with little to no distortion.
The final design allows for the 1” thick 20’x40’ 304L A-36 steel plate to be hung by four (4) high tensile strength steel cables, attached to four (4) mounting brackets located on the back wall. Special brackets and anchorage systems were engineered to support the imposed loads including a 4 to 1 safety factor.
Once installed, each wear plate acts as an independent floating system. Thus, allowing the cut petroleum coke to contact the plate without causing thermal growth restraint cracking to the plates or the underlying concrete. Petroleum coke may at times build up on or under the plates however the coke pulverizes overtime and “works its way out” from below the plate and down the coke chute.
Plans were also put in place to repair the badly damaged concrete chutes in the drop zone of each coke drum. Access was the main consideration for these repairs due to the chutes
53o angle/slope. Custom motorized access platforms were developed to allow technicians to remove the existing concrete and perform the much-needed structural concrete repairs prior to the installation of the new Floating Wear Plates. The concrete materials used for repairs needed to be able to withstand the harsh aggressive environment of the Delayed Coking Process, but also have the capability to be put back into service where it could experience high temperature exposures in less than a week.
Once the rapid-setting concrete was placed, finished and hardened, each plate required an individualized engineered lifting and rigging plan to allow placement onto the newly repaired concrete substrate. The new wear plates were hoisted by crane onto the base of each chute, then pneumatic winches were used to slide the plates into position. After the plates were maneuvered into place, manlifts were then used to install the high tensile strength steel cables to the mounting brackets on both the wear plates and the back wall.
Each Floating Wear Plate and its associated hardware, chains, anchors, and brackets are subjected to a yearly inspection plan. Any worn items/pieces can easily be repaired or replaced during very short duration maintenance opportunities. To date, the plates remain in great shape with little to no abrasion damage. In 2017, the original wire cables that held the plates were switched out to high tensile strength steel chains due to corrosion issues at the end of each cable and its connection. This action is seen as an upgrade associated with long term monitoring & assessment of this innovative and important process enhancement.
Requiring an “out-of-the-box” solution to an industry-wide maintenance problem, Floating Wear Plates spotlight the collaborative effort possible when Owners, Engineers and Contractors work as a Team to improve a critical refining process maintenance issue.
[1] (Novy, 1972)
[2] (Coefficients of Linear Thermal Expansion, n.d.)
[3] (Barsoum & Khurshid, 2017)
