Fiberglass reinforced plastic (FRP) tanks are common storage tanks for a wide variety of hazardous chemicals from hydrochloric acid to sodium hydroxide both with pHs ranging from 2 to 14, respectively. The resin layer is the designed to protect the tank shell from corrosion, or chemical attack, that can thin or penetrate through the resin layer and attack the structural layer. Many FRP tanks that store hazardous products a part of a closed process and therefore can not be removed from service for inspection. In high purity processes, like the semi-conductor fabrication industry, it is impossible to remove the tank from service to insert a remotely operated camera into the tank for an internal inspection due to contamination. As a result, an in-service FRP tank inspection solution is required. While acoustic emission, strain gage measurement, pressure testing provide quantitative information on the tank structural condition, they are unable to provide data on the FRP liner thickness and if it has deteriorated over time. Liner assessment can be assessed using ultrasonic testing technology from the tank shell outside while in-service. TKS’ database of accurate wave velocities for different epoxy vinyl ester resins allows the user to assess liner thickness accurately and in some cases delineate changed in liner Barcol hardness.
FRP Tank Shell Transition
The through thickness of an FRP tank shell is a continuum of corrosion protection to structural reinforcement. Epoxy vinyl ester resins are the primary resins used for FRP tank and provide excellent corrosion-resistant properties and satisfy critical requirements across a broad range of performance requirements. Starting from the inside out, FRP tanks have a corrosion barrier that is 2.5 to 6.3 mm (100 to 250 mils) thick and are designed for contact with a specific chemical environment. The initial layer of the corrosion barrier usually is 0.3 to 0.8 mm (10 to 20 mils) thick and is 95% resin, reinforced by one or two surfacing veils. This layer is then backed with 2 to 6 mm (90 to 230 mils) of 75% resin, reinforced with chopped strand mat (powder binder only). Finally, the corrosion barrier is backed with a structural laminate that provides the strength and stiffness of the overall corrosion-resistant composite structure . All standard veils are suitable for most service environments. Hydrofluoric acid (HF) containing solutions require the use of synthetic or carbon veils. Typically, one veil layer results in a final thickness of approximately 0.3 mm.
Figure 1 Example FRP tank shell cross-section showing corrosion barrier, interior and structural layer.
Non-destructive Testing Options for In-service FRP Tanks
The most effective means for evaluating the corrosion protection liner of an FRP tank is to perform an internal inspection. Internal tank inspections require the tank to be emptied, cleaned and vented for inspector entry. A confined space entry (CSE) is required to support the inspection adding significant cost. Most FRP tanks are on a 5- or 10-year inspection cycle per local, state, federal or corporate policy, e.g. [2-5]. Barcol hardness testing is another NDT process that provides direct information on current condition of the liner. While the nominal Barcol hardness will vary by different epoxy vinyl ester resin products, the nominal is typically in the 25 to 40 range. An increase in FRP liner Barcol hardness may indicate that the liner has become brittle and prone to cracking. A decrease in liner hardness tends toward the liner becoming more susceptible to chemical attack. There is no correlation Barcol hardness to thickness, however.
Figure 2 FRP tank c-veil used to strength corrosion barrier layers.
FRP Tank Liner Thickness Measurement
Ultrasonic thickness testing (UTT) is used routinely for carbon steel, stainless steel, and aluminum tank measurements. These metal alloys lend themselves to ultrasonic thickness testing due to small grain structure, predictable attenuation of ultrasonic waves, and material isotropy. An FRP tank shell poses a challenge for ultrasonic thickness testing due to the transition from the inner layer consisting of up to 90% resin and 1-2 layers of polyester surface veils, an interior layer consisting of layers of laminated 1-1/2 ounce per square foot random strand mat with 70% resin and the structural layer. The latter for cylindrical tank wall is commonly filament wound at an angle determined by the manufacturer with unidirectional reinforcement interspersed with a thicker layer of 1-1/2 ounce per square foot chopped strand glass. The transition from the FRP tank interior layer to structural layer is generally not detectable with ultrasonic thickness testing technology. However, the transition from these two layers to the corrosion barrier is detectable.
Figure 3 1-1/2 ounce per square foot random strand mat used in FRP tank inner layer.
While results may vary between filament wound and chopped strand structural layer tanks, it is possible to detect liner thinning and in some cases hardness variations with ultrasonic waves. An example is provided below from and FRP tank shell liner on which 0.020” of liner thinning was observed. The ultrasonic thickness testing A-scan shown in the following figures show approximately 0.020” wall loss corresponding to the liner thinning. In this case the ultrasonic transducer was place on the tank OD while in-service and the measurement was recorded. The ultrasonic reflection measured by the red data gate is the transition from the structural to liner layer. The ultrasonic reflection measured by the blue data thank inside diameter, or end of the liner layer. It is important to select the correct material velocity for the tank tested. TKS’ database of proven velocities enables accurate measurements. In the absence of known velocities, relative changed in liner thickness may be estimated using an assumed velocity.
Figure 4: FRP shell cut out with corrosion barrier in-place.
Figure 5: FRP shell cut out with thinner corrosion barrier.
Figure 6: FRP Tank ultrasonic thickness data showing nominal corrosion barrier thickness.
Figure 7: FRP Tank ultrasonic thickness data showing thinned corrosion barrier.
1. Derakane™ epoxy vinyl ester resins chemical resistance guide — Resin selection guide for corrosion resistant FRP applications, www.ashland.com.
2. 6 NYCRR Part 598: Chemical Bulk Storage (CBS) Handling and Storage of Hazardous Substances – Section 598.7 Aboveground tank systems – inspection,
3. N.J.A.C. 7:1E, “Discharges of Petroleum and Other Hazardous Substances” (DPHS)
4. PADEP Chapter 245: Administration of the Storage Tan and Spill Prevention Program
5. West Virginia Department of Environmental Protection (WVDEP) Aboveground Storage Tank Act at W.Va. Code § 22-30-6.