Practical External Inspection of FRP Vessels

Introduction

For the purposes of this article, vessels include storage tanks, chemical reactors, scrubbers, and other containers for storage or processing of chemicals. Since usage of Fiber Reinforced Polymer (FRP) for vessels and piping in the chemical processing industry (CPI) started in the 1950s, many users have experienced reliability problems. In response to this, there has been significant work to establish standards and codes for FRP vessel design.^[1-5] They have contributed significantly to increased reliability of FRP equipment.

Even with these increases in reliability, some vessel failures still occur, some of which could have been prevented or mitigated by following a systematic external inspection program. This article provides a systematic process/program for these external inspections that serves to increase reliability. 

If corrective action is recommended, it can be identified at one or more of the stages in the process, including analysis of the cause, evaluation, and engineering of possible solutions, design, and execution. This work is usually unique to each situation and is beyond the scope of this article.

Behavior of FRP Material

Fiber reinforced polymers are used in many corrosive applications because the polymers provide superior corrosion protection to many metal alloys. Unlike metals, FRP used for most industrial applications is not ductile – it cannot be bent and it does not form ductile fatigue cracks. When overloaded, it behaves as a brittle material. FRP materials also undergo changes while they are in service from stresses and chemical attack. The standards and codes used to design FRP equipment recognize this and use design factors to increase the thickness of the FRP to beyond what is required for long term service.

It is also very important to note that the tensile strength of FRP can be tailored to match how the design stresses are applied. For example, if the only stress expected by the designer is hoop stress – such as for an open-top tank, then the FRP in the shell could be designed and built with very little strength in the vertical direction, since it wouldn’t be necessary. But if a cover is attached to the tank and the tank is overfilled so that liquid contacts the cover, the tank will now be under pressure and the shell may be over-stressed in the vertical direction where the hoop strength does not apply. An example of what can happen is shown in Figure 1, where a storage tank with a cover was overfilled and the tank shell failed because of the vertical stress that was much greater than the design. It is important to note that this failure occurred because the vessel was subjected to loads that were not included in the design requirements, even though there were no flaws detected in the material.