In part three of our series on corrosion under insulation (CUI), we take a closer look at planning CUI inspections and how this can also be seen within an RBI (risk-based inspection) context. CUI is not only a difficult phenomenon to detect, but also towards inspection planning for example within an RBI framework there are a number of challenges. To answer the question 'how to plan CUI inspections' we are pleased to meet Piet Van Dooren, Group Expert Static Equipment & Inspection from Borealis.
CUI within an RBI framework: how does Borealis approach this?
"Within an RBI approach, you typically start from probability of failure (POF) and consequence of failure (COF). Combining both factors gives a risk of failure based on which an inspection period is determined. In the case of CUI, you actually have to combine damage mechanisms on the inside and outside.
In terms of type of CUI inspections, we mainly perform visual inspections because we consider this the most reliable technique for inspecting large areas for CUI. We define the most sensitive areas to focus on:
- places where the cladding is missing or interrupted, e.g. at branches, supports, end of piping at flanges, … ;
- small bore piping, which is more difficult to insulate, and where the protective coating has often not been applied properly or not at all in the past.
For these critical locations, we are going to estimate a higher POF. Consequently, the inspection time based on RBI will typically be shorter.
Since scaffolding placement and zone accessibility typically accounts for 90% of the inspection cost, we will also include neighboring pipelines/inspection points at the same time with longer inspection times to make the whole thing more cost-effective.
However, we have noticed that sometimes we encounter CUI even in places where we don't expect it. Therefore, we decided to increase the number of visual inspections from only the sensitive points to also a larger part of the rest of the pipes. After all, it remains a phenomenon that is not easy to predict."
We understand that unexpected repairs have their impact on turnaround time. How do you deal with this?
"Unlike before, we are now replacing pipes more quickly on a preventive basis to reduce turnaround time. We then provide replacement pieces by the next turnaround for the pipes where we expect high risk of CUI. After all, if CUI is effectively identified, it takes less time, is less cumbersome, and is qualitatively better to replace pipes than to blast/paint them in situ. By replacing the pipes, we also know we're right on track for the next 20 years."
"No, we do this even if no degradation has yet been determined. The quality of the coating is no longer 'up-to-standard' according to current European standards for older plants. Moreover, coatings have improved over the years because CUI was still an unknown factor during the construction of earlier plants. One of the weaknesses are the field welds: you can prefabricate pipes to a certain length and weight but eventually they have to be welded in.
In the past, this was not immediately looked into and the weld was often not repainted afterwards (after all, it was hidden under the insulation). Back then, it was not correctly estimated what the consequences would be in 20 years or more. In addition, you see that many plants were founded for a lifespan of 20-25 years. After this period, you would actually have to dismantle and rebuild them, as with a car that is depreciated when the repair costs are higher than those of a new car. With chemical plants, however, lifetime extension is more common."
Are new units today typically designed for a longer life of 20-25 years?
"No, that is not feasible. Only in some cases parts of a plant are designed for a longer life. This can occur, for example, with static equipment (such as a high distillation column) that is difficult to replace."
How long has Borealis been applying RBI to CUI?
"Borealis has been one of the first in the industry to start doing this since 2010. We also collaborated on the CUI Guidelines (EFC Publication No. 55) from the European Federation for Corrosion (EFC). In that collaboration, you also see a number of other companies that were already involved at the time."
Do you have any insight into what it provides, in addition to safety, also economically to schedule your CUI inspections in this way?
"It's hard for me to cite figures, but an unplanned downtime in a profitable plant immediately goes toward thousands of dollars a day. That often far exceeds inspection budgets. In other words, it is much more efficient to spend money on targeted inspections on CUI than to be surprised by it. Risk-based inspection really delivers. Once you uncover the cost of a missed failure, interest in inspection quickly rises."
Do these inspections always require shutting down part of the plant?
"We are doing more on-stream inspections. These are inspections 'on the run', in other words without shutting down the process. It is then examined per pipe whether it is possible for safety reasons/production reasons to expose the piece of pipe. This is often the case for pipes below 90-100°C. Everything that can be done without having to stop the process is a bonus and relieves the (usually very intensive) turnarounds. Only 'temporary repairs' are possible; these are then replaced by a definitive repair during the next stop. The advantage of on-stream inspections is that, based on the observations, you can decide whether you can continue until the next turnaround. Eventual replacement of the part can then also be scheduled. This way you will not be taken by surprise during the turnaround.
This way of working has become more our philosophy over the years."
How do such repairs affect RBI intervals?
"Another important influence on the inspection intervals is what action we decide to take when we discover CUI during the inspections.
In some cases (when the corrosion is not severe and there is still enough wall thickness left) we sandblast on site and re-coat.
We do this especially for columns, larger distillation columns and reactors since you can't just replace them due to cost and technical feasibility.
The equipment is placed in a 'cocoon': scaffolds that are completely shrink-wrapped. In this way, a safe environment is created, but it is also possible to guarantee the quality of the sandblasting and coating. This is of the utmost importance, as it extends the CUI inspection interval of the equipment by ensuring quality work.
However, in the case of piping, we are more likely to opt for replacing a component. This is especially so with smaller diameter piping as there it is often easier to replace the part in question than to repair it."
What is Borealis' vision concerning isolation in the context of CUI?
"To reduce the risk of CUI, the first priority is to keep the moisture out as much as possible. A good finish to the 'weather cladding' (the protective covering around the insulation material) is essential for this. This aspect is receiving more and more attention, especially in new plants. For technical details in connection with the finish, the CINI handbook is followed.
In addition, the need for insulation is also more frequently questioned. In the past, insulation was often used for safety reasons, like personal protection. For example, because the pipe is too hot and one could get burned. However, there are other ways to avoid this without applying insulation."
Do you have your own inspectors who come to inspect those work sites and verify that the coatings, insulation and cladding are applied correctly?
"We mainly employ our own staff for inspection and quality control of coating, insulation and cladding. Specifically for coating inspections, people are currently being trained internally: we are paying more and more attention to this, both for works internally (on site), and for works externally (in a construction workshop.)"
Do you have any tips to prevent CUI?
"The corrosion rate is not related to the thickness of a pipe. Since smaller pipes typically have thinner wall thicknesses, they will therefore be perforated more quickly at the same corrosion rate. On top of that, these smaller pipes are typically harder to insulate and are often welded 'in the field' and these get less attention when painted. So I recommend paying extra attention to these and possibly replacing them preventively, which is easier to do than for larger pipes."
As illustrated above by Piet Van Dooren of Borealis, CUI offers certain challenges. However, with the right approach, there are certainly solutions to these. For example, setting budgets for effective inspection can save a lot of money.
Although it is important to identify the critical locations where an increased risk of CUI can be expected, one should be vigilant and extend inspections to other areas as well. After all, CUI is not easy to predict. Finally, to reduce turnaround time, preventive replacement of pipelines (especially smaller diameter pipelines) is becoming more and more common.
After this blog, there will also be a three-part series on RBI. If you'd like to learn more about this, keep an eye on our LinkedIn page! Do you have further questions on the CUI issue? Don't hesitate to contact us, our experts are happy to help you.
