Scheduled turnarounds (TA’s) on CCR naphtha reforming units are infrequent with customers targeting every 5+ years, and frequently hitting their targets. That leaves little opportunity for many engineers and operators to participate in these events. So, it seemed appropriate to generate an article that shares what I’ve experienced during my career, while keeping each article focused on specific areas or equipment.
This article will focus on the internals inspection of an upflow stacked Rx on a UOP CCR PlatformingTM Unit. The target audience are those that haven’t had the opportunity to do much inspection on a unit they may operate as their primary job. Coupling a better understanding of equipment design and constraints will significantly improve one’s overall expertise.
During any worksite activity safety is an integral part of all that you do. Turnarounds will present some tasks that are unique and not routinely carried out i.e., vessel entry, etc. Safety will be woven into this article, but this is not an all-inclusive piece on safety. As I move forward with this discussion there will be an assumption that all pre-job safety assessments are complete, safety systems/equipment are in place and confirmed functional, and that any activity requiring specific Personal Protective Equipment (PPE) and backup personnel are available and in place.
Preparation for Vessel Entry
All catalyst on the Rx side has been unloaded and the atmosphere has been switched over to air (20.8 mol% O2) and the Rx is cool enough for safe entry. This article assumes only small amounts of spent catalyst are present, and largely in the knuckle of each Rx bottom head. Spent catalyst will normally have some Fe and S present which makes this catalyst potentially air pyrophoric. Moreover, this spent catalyst may still retain some residual aromatic hydrocarbons. Ultimately, the customer will determine what PPE is required for vessel entry, but most generally this vessel entry is done without supplied breathing air and the entrant is fitted with wearable H2S and O2 monitors.
- Ideally, the operations group has designated a representative to be present when the Rx is first opened to participate in initial observations. This will vary from site to site but provides another set of eyes and additional insight into what’s being observed.
- Inspectors generally carry out all dimensional checks and document their findings in a lengthy and very detailed report.
- Don’t bring anything into the Rx that you don’t need for your inspection, and make sure you have everything as you leave. Anything left behind may result in catalyst flow obstruction and potential shutdown.
Rx Side Inspection – Coverdeck: With the manway open, and from the outside, use your flashlight to check as much as you can before entry, and photograph it. This will be the only time you’ll be able to see it as it was originally found, and this information may be extremely valuable at a later date and cannot be recreated or recollected again with certainty.
The first thing I look at is the condition of the coverdeck. It’s not unusual for some catalyst to be present, but generally no more than a few pills here and there. Make sure all coverdeck sections are in place and secure.
Note: Older coverdecks were bolted like the one shown below, but most newer designs will have pie-shaped plates with wedge pins used to secure each section of the coverdeck. Each wedge pin must have a small chain attached to it so that it cannot fall into the Rx. Please make sure they’re all securely attached.
The photograph below is of an older coverdeck with cylindrical scallop risers. As you can see, the riser heights are different. This is a strong indicator of scallop damage beneath the coverdeck due to rapid transition temperatures and excessive catalyst bed pressure. Catalyst bed pressure is a discussion for another article.
This diagram below is used to identify scallop location and damage. From the coverdeck, write a number on the Rx wall behind each scallop so it can be worked with this diagram. The convention is that the scallop at the manway is #1 and working clockwise (top view) continue numbering each scallop around the circumference. The photo beneath the scallop diagram serves a couple of purposes. It shows that the wall behind the scallop is numbered, and shows a scallop riser dimension being taken. This information will be placed on the scallop diagram bellow i.e. scallop # and riser height from the top of the seal plate.
Scallop Seal Plates
These seal plates and risers are designed to create a controlled gap between the OD of the scallop riser and the ID of the seal plate. This gap allows the riser to move freely through the seal plate during thermal cycles and periods of axial growth and contraction, while keeping the gap small enough to mitigate catalyst losses through the gap onto the coverdeck.
Note: Over the years, additional gap and riser height were added to mitigate problems with scallop riser to seal plate binding. If you are having problems with this, you should contact your licensors technical service representative and request input on if your existing gap criteria or riser heights can be modified.
Make sure to assign a corresponding # to both the scallop riser and the scallop seal plate. They are a matched set. When “D” shaped risers are manufactured they are not machined, and therefore have some variability in their dimensions. This is why each seal plate is assigned to a specific riser. Be diligent about keeping these sets together if they are taken out of the Rx.
While you’re on the coverdeck, take a flashlight and look down each scallop. Older units may have a closed bottom on the scallop, but most of the newer designs have an open bottom. Make note of any visible damage or obstructions and add this information to the scallop diagram. Also, you should note if you see any catalyst in the scallop. If there is, use a tape reel to measure the outage. Once this inspection is complete, place a fitted cover on the top of each scallop riser to prevent anything from falling down the scallop while maintenance and inspection activities are on-going in the Rx. Ideally, these covers are the last thing removed from the Rx before the manway is closed and all work is complete.
Note: Do not use duct tape to cover the top of the scallop risers. Use a “single” fitted piece that cannot leave anything behind when removed.
Rx Effluent Outlet Elbow
The outlet elbow is an extension of the centerpipe that extends through the coverdeck and exits through the side of the Rx. It’s bolted at the base with a flange at the outlet. The bolts around the outlet flange are tough to reach, and my thoughts have always been… If it’s difficult for me to reach, then it was for the guys that had to tighten it. So, pay close attention to the tightness of these bolts. If there is a leak during normal operation it could easily go unnoticed, but cost you significant $$’s. The pressure on the coverdeck (Rx inlet) is higher than the outlet of the centerpipe, and if there is a small leak in this flange it would result in Rx bypassing.
Expansion Bellows and Vent Baskets
In many upflow Rx reactor designs there is a piece fitted to the base of the outlet elbow and top of the expansion bellows that has holes around it. I don’t have a good image of it, but if you’ve seen it, you’ll know what I’m explaining. This piece allows a small percentage of the incoming feed gas to be routed to the top of the catalyst bed beneath the coverdeck to help prevent catalyst fluidization.
The image left is from a downflow centerpipe, but the vent baskets shown still serve the purpose of highlighting their look and purpose. These vent baskets are an alternative design to the one described in the previous paragraph. The basket itself is typically made from punch plate, similar or the same as the scallops. Over time, these vent baskets can get fouled or plugged and will need to be wire brushed and cleaned.
Catalyst Transfer Pipes
Most licensors are good about sending design updates to all customers, but if you feel you haven’t received all of them, just ask as they are easily accessible to your service representative. All newer Rx designs have moved to 300 SS catalyst transfer pipes w/ Dur-O-Lok couplings, eliminating the need for flanges altogether. This change was made to mitigate concerns with MCC formation. If you have the older flanged design and it’s not SS, be sure to look carefully at all bolting and between the flanges to make sure that you don’t have any coke growth present. The tack welds made on each bolt to keep them secure is a prime initiation point for MCC formation. Also confirm that your gasketing is up-to-date by referencing any design updates. All low chrome steels should be avoided for use in the Rx’s.
Photograph provided by Becht
Sketches provided by Becht
Old Style Catalyst Xfer Pipe
Photograph provided by Becht
This article was focused on initial vessel entry and specifically what can be seen from the coverdeck. Moreover, how to document and preserve the original condition for discussions that will arise years from now. This is not intended to create duplication of work, as inspectors and maintenance have their jobs to do and that doesn’t change. This article was written to encourage others in operations to participate and hopefully provides some guidance on where to start.
There’s clearly much more to cover beneath the coverdeck that can be reviewed i.e. expansion bellows and shroud, centerpipe, centerpipe support plate, scallops, expander rings, scallop support ring and lugs, etc., and can be discussed in subsequent articles. What are your thoughts? Connect with us.