Designing a Fit-for-Purpose Insulation System
There isn’t a "one-size-fits-all" insulation system. Therefore, insulation design should be more than just drawing up a specification. Pipeline engineers and equipment designers should make detailed insulation design. Based on a consequence of failure assessment, piping and/or equipment with a higher ranking should lead to the creation of insulation systems with the lowest susceptibility. In order to do so, the following criteria need to be considered when designing insulation systems:
- Choice of Insulation Materials
Insulation materials can be roughly subdivided into permeable (open-celled) and impermeable (closed-cell) materials. For systems below ambient conditions where surface condensation or icing is possible, closed-cell materials like PUR/PIR foam or cellular glass are often chosen, whereas for hot systems, mineral wool like stone or glass wool or expanded perlite are common products.
The choice also depends on local or historical grounds. For instance, Europe uses a lot of mineral wool for hot insulation, whereas in the United States, calcium silicate, perlite and cellular glass are more common. In specifications, it’s common practice not to refer to product names. Therefore, many insulation specifications refer to general technical requirements.
Important characteristics in relation to CUI include:
- Water absorption (ASTM C610 or ASTM C612)
- Leachable chlorides content (ASTM C871 or ASTM C795)
- Hydrophobic behavior
- Compressive strength (when foot traffic can be expected)
- Dimensional stability
- Insulation Cladding or Jacketing
The first step is to determined the purpose for cladding or jacketing. There are several design criteria such as:
- The need for a vapor barrier (below ambient service temperatures)
- The need for weather protection and UV resistance
- Mechanical resistance
- Accessibility for maintenance or inspections
Cladding/jacketing can be subdivide into metal and non-metal, each of which has specific characteristics and scope of applications. Although the above criteria determine the choice, this is also influenced by local available craftsmanship and practices. Another important part of cladding/jacketing is the use of caulking or sealants. The choice of whether all joints (longitudinal, circumferential as well as protrusions) shall be finished (or flashed) depends on how sheeting details are designed.
- Local Geographic Conditions and Plant Layout
Seaside environments are different from inland environments, and arctic conditions differ from tropical ones. Downwind drift from cooling towers or frequent fire deluge drills are other major factors to consider. In Europe, the result of these considerations is that many sites are classified in the highest corrosion class. It's also important to create enough distance between piping and equipment to allow proper insulation installation and enable maintenance and inspection in the future.
- Equipment, Piping and Tank Design Details
For pressure piping or equipment, standards and codes like ASME, API, BS, Lloyds are available. But details with regards to insulation design are often limited to things like clips or insulation support rings. In addition, some of these details can create potential ingress points. So, bridging the gap between mechanical design and insulation design is a giant leap forward in mitigating CUI. The following suggestions can significantly improve design when it comes to CUI:
- Collars on protruding tubes
- Vacuum rings that don’t retain water
- Lifting lugs that can easily be removed after installation
- Pipe support on high density (For further design details I refer to the EFC CUI guideline and the CINI Industrial Manual.)
Safety, health and environmental conditions are different depending on the country or region. So, it's important to verify the installation and application guidelines provided by the manufacturer and verify this with applicable legislation and rules. Health and safety requirements regarding things like fibers, dust and solvents can influence the choice.
Clients' own inspection procedures can require inspection plugs or access points for visual inspection or non-destructive evaluation (NDE). In addition, accessibility for maintenance purposes should be considered, for instance making it possible to easily change gaskets can determine insulation design for valves. It is recommended that clients' best practices be used and that lessons learned from the use-phase regarding insulation be applied.
QA/QC during erection is often left to the insulation contractors' organization. Recently I’ve seen many asset-owners investing in independent autonomous QA/QC departments who are also drawing up inspection and test programs (ITP) in which critical "hold" and "witness" points are checked. These steps are vital when commissioning and setting up an inspection and maintenance strategy.
- Life Cycle Costs (LCC) and Total Cost of Ownership
These are two terms that, to some of us, will sound like senior management gibberish, but in my experience, companies with a good working CUI mitigation strategy have a maintenance manger who’s able to convince the senior management about the link between "overall equipment efficiency" and a CUI inspection and maintenance policy. And since CEOs talk in terms of money, these figures become important. One thing all studies have shown: Thermal insulation has a return on investment (ROI) of less than two years.
It’s been reported under every type of insulation material and cladding and even in newer installations that installations that hold the least amount of water and dry most quickly result in the least amount of corrosion damage to equipment (NACE SP0198). As a logical consequence, impermeable, closed-cell insulation materials and vapor-tight barriers seem to be the best option. Even then, operational conditions (thermal expansion/contraction), foot traffic, and failing caulking at protrusions can still damage these systems and cause water ingress. Which brings me to my opinion that the following options should be considered.
- Option 1: Non-Contact System
Wet insulation that comes in contact with the substrate is the cause of all the above problems, so why not create a cavity between the insulation and substrate? This is called non-contact insulation. Although the idea is evident and being used by Statoil and Shell, some details need to be addressed. For instance, the spacers used to create this cavity must not create a crevice to allow crevice corrosion.
Depending on the service temperature, especially for vertical columns within this cavity, a vertical free hot air flow can create extra convection and consequently extra heat loss. However, this can easily be minimized by making compartments and can be compensated with a higher thickness.
- Option 2: An Aerated Insulation System
Moisture will always condense where the water vapor pressure rate is at its lowest and at the coldest spot. In thermal insulation, this is the cladding/jacketing. By creating an air cavity between the insulation material and cladding/jacketing, moisture has not only the possibility to freely condense, but also to find its way to the lowest point where it can escape through a drain hole.
Neither of these options are new, and they're implemented by companies like Shell, Statoil and Dow. Also it’s documented in the CINI Industrial Manual and standards like NORSOK, DIN and AGI-Q. Despite this, it's not widely known within the insulation branch. Although some big asset-owners are behind these systems, it’s important that independent testing give more scientific and reliable data that can lead to better standardization.
Source: Johan Sentjens, January 30, 2015 http://www.corrosionpedia.com/2/1372/corrosion/corrosion-under-insulation-the-challenge-and-need-for-insulation
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