Case study: NDT inspection of FRP vessels for sodium hydroxide storage
UTComp and UltraAnalytix® licensee RPC Technologies Pty (RPC) collaborated to evaluate the condition of fiber-reinforced plastic (FRP) vessels at a water desalination facility that turns seawater into potable water for human consumption and irrigation across a wide region.
The client asked RPC to carry out a visual and ultrasonic inspection of two storage tanks to determine their suitability for continued service and their predicted remaining service life. The identical atmospheric vessels are used to store NaOH (sodium hydroxide, or caustic soda), a highly corrosive substance used in the desalination process. In particular, the client wanted to understand how different operating conditions affected the tanks: each is used to store different concentrations and volumes of NaOH, with one tank in full-time use while the other is used mainly as a backup.
In May 2021, a team from RPC — trained in ultrasonic inspection using UltraAnalytix technology and procedures — completed the inspection and forwarded the data to UTComp for analysis and reporting. UltraAnalytix data showed a stark difference in the condition of the mid-shell and lower-shell areas of both tanks; the Percentage of Design Stiffness (PDS) had deteriorated significantly in the lower shell, indicating damage to the laminate due to contact with the caustic soda. The project was the first UltraAnalytix inspection for this equipment. Pleased with the outcome, the client has asked RPC to develop an inspection program and schedule for approximately 30 FRP vessels at the desalination plant.
Storage vessels, process tanks, pipes and other assets made from FRP have been widely used for many years in a variety of industries, from chemical processing to manufacturing to utilities. However, non-destructive testing of FRP equipment is relatively new. UltraAnalytix by UTComp is the world’s only proven non-destructive and non-intrusive inspection method that ensures the safe performance and maximum lifespan of FRP and other industrial composite assets.
Australia-based RPC Technologies has a 40-year history of engineering excellence in the transportation, renewable energy, defence, infrastructure, water and wastewater sectors. Today, RPC is part of a global network of licensees who provide ultrasonic inspection of FRP and composite equipment using the UltraAnalytix system.
An inspection team from RPC Technologies visited the plant during a scheduled shutdown in May 2021 to assess the condition of two atmospheric FRP vessels used to store caustic soda, a corrosive substance used in the desalination process. The RPC team completed an external visual inspection and collected ultrasonic data following UltraAnalytix procedures.
The FRP vessels assessed by RPC are identical 70,000-litre tanks, approximately 11 years old and made by the same manufacturer using the same vinyl ester resins and techniques. Measuring 4.5 meters in diameter and 5.15 meters high, the tanks store caustic soda, which is used in the desalination process to change the pH of seawater to make it less corrosive and prevent fouling of the water treatment equipment. However, caustic soda can also damage fiberglass resins.
UltraAnalytix technology allows licensees to accurately measure FRP thickness, calculate the Percentage of Design Stiffness (PDS) and assess the condition and bonding of FRP materials for a fast, reliable way to assess the mechanical integrity of composite equipment.
The visual inspection focused on the condition of support structures as well as protective coatings and paint including:
- Hold-down anchors, bolts and clamps
- Access hatches and FRP flanges
- Ladder supports, nozzle connections, pipe and valve supports
- Signs of cracking and/or corrosion on the outer surface
RPC followed a “traffic light” system to describe the condition of the vessels:
- Green: nothing to report other than wear and tear; condition “as new” with a PDS value greater than 51%
- Amber: notable change in condition with a PDS value between 31% and 51%, which may require an engineering review
- Red: immediate action required, i.e., presence of leaks, deformation, cracking of the laminate structure or a PDS value of 30% or less
As both tanks are located indoors and protected from the effects of harsh weather and damaging UV rays, the external structures and coatings showed no visible signs of fatigue or failure. While all anchors and fittings were in good condition, some pipework, valves and nozzles were inadequately supported. Visual inspection found one of the tanks to be leaking from a damaged nozzle that had been repaired previously by another company.
UltraAnalytix readings were taken around the circumference of the tanks at approximately 500 mm above the base. Additional readings were taken around the lower shell at approximately 2 m above the base, including FRP flanges and reinforcements access hatches, nozzles and fittings.
When in use, the tanks are typically filled to less than half capacity before being emptied. As a result, lower parts of the tanks would be expected to show more signs of wear than the upper sections; however, damage to the lower shell area of both tanks has occurred more rapidly than expected. Several readings from the lower shell sections showed low PDS values and significant damage to the corrosion barrier.
Figure 1 below shows the basic configuration of the sodium hydroxide storage vessels:
Discussion, Key Findings and Analysis
UltraAnalytix data was collected from the mid-shell and lower-shell sections of the tank as well as six reinforcements (repads) located at nozzles, valves and inspection ports.
For this analysis, it was assumed that the tank was made with a uniform shell thickness at all levels. The mid-shell and lower-shell readings included some where the inner surface could not be detected. Where the inner surface could be detected, it was clear from both the shape of the reflected pulse from the inner surface and the apparent sonic velocity that damage to the corrosion barrier has occurred. The lower-shell readings also show evidence of damage to the resin up to a depth of about 50% of the shell thickness. The mid-shell readings also show evidence of damage to the resin but to a lesser extent than the lower shell.
Table 1 below shows the UltraAnalytix results for the sections evaluated in this inspection:
- Reinforcing Pad Condition and Bonding:
Results for this tank did not show any need for repairs or updates to the reinforcements.
Reinforcing pads (repads) are designed to increase the thickness of FRP in zones that must support additional loads or stress concentrations, such as pipe and nozzle connections and manway doors. If a reinforcement is less than 6 mm thick, an engineering review and repairs will be recommended because inadequate reinforcement is frequently the root cause of failures.
Bonding between repairs / reinforcements and vessel sections is determined as Percentage of Theoretical Bonding. These bonds may have as little as 50% of their theoretical shear strength. For this reason, UTComp uses the criterion that a bond exists when the percentage shown in the UltraAnalytix data is greater than 50%. If two or more consecutive points or more than 25% of the points are not bonded, immediate repair is recommended.
Figure 2 below shows a typical reinforcing pad area:
Based on the results, Tank 1 is suitable for continued service until the next recommended UltraAnalytix inspection in 2022.
- No defects or damage to the support structure were detected
- No defects or damage to the exterior of the tank were detected
- No defects or damage to the external components of the tank were detected
- Defects or damage to pipe and nozzle connections of the tank were detected
- Damage to the corrosion barrier was detected with significant change to the mid and lower tank sections compared to its “as new” condition
- Estimated remaining service life is five years
- Due to the low PDS of the laminate, the tank was given an “amber” rating by the RPC inspection team
Figure 3 below shows the estimated Remaining Service Life for Tank 1:
The RPC team recommended that the vessel be re-inspected with UltraAnalytix in one year to evaluate the rate of PDS loss and that an additional data set be taken higher up on the tank to determine the extent of the laminate damage. Other key recommendations were:
- Install supports under stainless steel valves
- Install support for stainless steel pipe connected to nozzle N5
Figure 4 below shows two areas where additional supports are recommended:
In addition to a visual inspection of the exterior, UltraAnalytix measurements were collected from three sections of the tank and four reinforcements located at nozzles, valves and inspection ports. Damage due to leaking was evident at nozzles N1 and N3 as well as a ground-level inspection port.
The mid-shell readings showed uniform thickness and the FRP to be in “like new” condition. The lower-shell readings included some where the inner surface could not be detected. Where the inner surface could be detected, it was clear from both the shape of the reflected pulse from the inner surface and the apparent sonic velocity that damage to the corrosion barrier has occurred. The lower shell readings also showed evidence of damage to the resin up to a depth of about 50% of the shell thickness.
At the inspection port, damage has occurred to the tank shell throughout its thickness. This could be the result of leakage of the seal bonds used to seal the penetration made in the shell for fitting the port. It is recommended that these seal bonds and all others be evaluated. For nozzles smaller than DN200mm, UTComp recommends that a penetrating-type installation be used.
Table 2 below shows the UltraAnalytix results for the sections evaluated in this inspection. The readings for the lower shell indicated clear evidence of corrosion barrier damage:
- Reinforcing Pad Condition and Bonding:
Although the reinforcement at nozzle N1 shows adequate thickness and Percentage of Theoretical Bonding, several readings from this area show significant damage to the corrosion barrier and reflections from the inner surface of the FRP were not detectable. Repairs are recommended.
Figure 5 and Table 3 below show the condition of the bonding and other statistics for this repad:
Based on the results, Tank 2 has damage that requires repairs before the next recommended UltraAnalytix inspection in 2022.
- Damage was detected at nozzle N1 and the lower shell requiring immediate remediation
- Damage was detected to the laminate at the ground-level inspection hatch
- Some flange sealing gaskets and nozzle connections showed evidence of previous leaking; however, no leaking was visible at the time of inspection
- No damage to the external components or support structures of the tank was detected
- Because the integrity of the tank requires remediation work to continue service, RPC inspectors gave it a “red” rating
Figure 6 below shows the estimated Remaining Service Life for Tank 2:
Figure 7 below shows residue of sodium hydroxide leaking through the laminate at nozzle N1. There was also evidence of previous leaking from flange sealing gaskets and nozzle connections.
Visual inspection and UltraAnalytix analysis of Tank 2 shows that the lower shell suffered a significant change in condition during 11 years of service. UTComp and RPC made several recommendations for immediate repairs and remediation that will allow the tank to remain in service including:
- Install stainless steel washers on all hold down bolts
- Replace gasket at leaking flanges
- Install supports under stainless steel valves
- Install supports for inlet pipework
- Remove secondary laminate at nozzle N1 to expose leak path and conduct repairs
- Determine the extent of damage to the laminate in the lower shell region and repair if necessary
- Check nozzle seal bonds for leaking
- Next UltraAnalytix evaluation in 2022
All inspection work and repairs were carried out during a scheduled three-week maintenance shutdown at the desalination plant. Using the UltraAnalytix system, the inspection team found significant damage to the corrosion barrier and shell of the tanks that could not have been detected by visual inspection alone or without destructive testing.
Most importantly, the data revealed a stark difference in the condition of the mid-shell and lower shell areas of the vessels. The Percentage of Design Stiffness (PDS) had deteriorated significantly in the lower shell, indicating damage to the laminate due to contact with the caustic soda. Some loss of thickness or stiffness was to be expected, but RPC’s ability to assess the advanced deterioration of the lower shell without destructive testing or entering the tank provided further validation of the UltraAnalytix system’s advantages over industry-standard methods. The data revealed serious damage that might have gone unnoticed for weeks or months before the equipment failed — with potentially serious consequences, especially given the facility’s location in an environmentally sensitive area.
UltraAnalytix provided the client with a fast, accurate, safe and cost-effective way to assess their FRP assets while avoiding unnecessary downtime and replacement costs. UltraAnalytix equips licensees like RPC with accurate, reliable tools to give clients the data they need to make informed decisions about the performance and lifespan of their FRP assets.