One cannot manage change. One can only be ahead of it.
— Peter Drucker
Evaluating fitness-for-service of FRP and composite assets
Peter Drucker is famous as a management consultant, but his words ring true in the chemical industry. When industrial equipment is exposed to operating conditions like corrosion, mechanical damage, pressures and other forces, the equipment will change. Engineers put a lot of effort into anticipating these changes and designing for them. Even when the changes are expected, it is common for engineers to constantly monitor the changes so that they can be ahead of it and repair or replacement decisions can be made in a timely manner. In this entry, I want to talk a bit more about using the right information at the right time to make the right decision.
Fiberglass reinforced plastic (FRP) is used to contain a wide variety of chemicals where steels are not suitable and the cost of exotic metal alloys is prohibitive. Examples of chemicals where FRP is well suited include seawater, humid chlorine vapor, many brines, sulphuric acid below 70% concentration, hydrochloric acid, bleach, sulphur dioxide, sulphur trioxide, sodium bisulphite and many others. Because of its resistance to corrosion from most natural environments, it is also suitable for structural elements in civil structures such as bridges, manhole covers, and building cladding. A number of standards and codes that have been developed through consensus of experts in the field are used extensively for design. These consensus standards and codes (Codes) often prescribe widely different methodologies to determine design and construction requirements. It is important to note that as soon as the construction is completed and accepted, the design codes no longer apply directly to the structure.
Fitness-for-Service assessment is used for in-service equipment to determine whether the equipment is capable of continuing to operate for some time into the future. The principal question to be answered in Fitness-for-Service is: “Considering the degradation and damage observed in the asset, what is its remaining service life until the risk of failure exceeds an acceptable value?” Degradation of in-service equipment usually occurs as a result of damage mechanisms that become understood from study of the materials used in construction. In the case of steel equipment subject to general corrosion, Codes often apply a “corrosion allowance” and communicate it on the nameplate as guidance for Fitness-for-Service. For most metal equipment, there are reliable non-destructive methods for detection and quantification of damage. In many cases, this evaluation can be done without affecting operation or existing mechanical integrity – this is also known as non-intrusive inspection.
Completing a Fitness-for-Service assessment follows a systematic engineering approach. The approach includes evaluation of flaws, damage or aging that have changed the materials of the equipment in such a way that it cannot usually be evaluated using the original construction codes. For steel or metal assets, some guidance for this evaluation is available from specific consensus codes. In general, evaluation of metal equipment can involve techniques such as fracture mechanics and finite element analysis to assess the structural capacity and damage progression of the equipment for continued operation.
Worldwide, there are currently very few consensus standards that apply to Fitness-for-Service of FRP. FRP and other composite materials have very different damage mechanisms from metals, so it is not possible to translate the methods required for steel to FRP. Furthermore, although university engineering education now includes some discussion of composite materials, the applicable damage mechanisms are rarely included until Masters and PhD studies. This leaves practicing engineers and facility managers with few tools to understand the Fitness for Service and Mechanical Integrity of the FRP assets in their care. Furthermore, this lack of tools also imposes psychological barriers to FRP in many applications where it would be very cost effective.
The Fitness-for-Service evaluation method in most common use at this writing is an evaluation of chemical attack of the resin-rich inner layers – also known as the Corrosion Barrier. This evaluation provides conservative results, often at significant cost for inspection and maintenance. In addition, it is intrusive and usually requires human entry into confined space with the associated safety risks.
Effective reliability management of FRP assets – piping, vessels and civil structures – requires effective, non-destructive and non-intrusive evaluations that identify damage and degradation of FRP equipment in service. This evaluation must incorporate the actual damage mechanisms that have been observed in FRP so that reliability can be related directly to the results.
As early as the 1960s, non-destructive testing of FRP has been studied. NASA made significant gains using acousto-ultrasonic methods and discovered that changes in the strength of FRP can be detected non-destructively and non-intrusively. UTComp® has adapted and refined NASA’s work to use readily available instruments and easily-learned procedures. Further, UTComp® has been able to show through its experience that the existing structural capacity of FRP can be reported and related to the remaining service life of FRP. Total cost of this Fitness-for-Service approach has been shown to cost less than 10% of the cost of the conventional approach where only chemical attack of the inner surface is assessed.
A number of large and small owners of FRP equipment have adopted this approach, around the world. Read about their experience and cases here.
– Geoff Clarkson, P. Eng, FEC
UTComp Chief Technical Officer
Want to know more? Email Geoff at G.Clarkson@UTCOMP.com or call 519-620-0772.