Introduction
Fiberglass tanks are complex structures made from multiple layers of different materials to achieve a balance between strength, weight, and corrosion protection. Starting from the inside – out, a typical filament wound fiber reinforced plastic (FRP) tank consists of an inner surface, interior layer, structural layer, and outside layer. The inner surface is commonly 10-20 mils thick resin rich layer. The interior layer is in the 100 mil range and contains 20 to 30 % chopped glass for strength. As its name implies, the structural layer carries most of the product related stress and contains 60-70% continuous strand filament winding. The outside layer is a resin rich surface layer with added coating(s) for ultraviolet protection and corrosion resistance.
Figure 1. Fiberglass tank cross-section.
Quality Control Inspection
Quality control (QC) inspections will vary depending on end user requirements. A common requirement is a 3-stage inspection process. The first inspection is commonly performed at the completion of the corrosion barrier and before structural winding occurs. The second inspection is performed after the tank is removed from the mandrel and before any nozzles are attached. The third inspection is performed prior to shipment of the tank. These inspections are generally performed by an independent inspector with at least 5 years of experience with FRP vessels. The QC visual inspections are performed to ASTM and ASME standards [1-2]. Different quality levels are defined in these documents along with different repair thresholds. For example, a construction specification may define a Level 1 liner inspection with 10% repair allowed and Level 2 in the structure with 10% repair allowed.
After the tank has completed a successful hydrostatic test, some contracts require that the tank shall undergo a mechanical integrity test using Acoustical Emission Test (AE Test) in accordance with the latest version of ASTM E 1067 [4].
Internal and External Inspections
A thorough fiberglass tank inspection of interior and exterior surface is typically required every 5 years per state or tank owner guidelines. There are many different types of fiberglass tank defect with most presented in in ASTM and RTP standards. Inspector safety and is a very important consideration during the internal inspection. Proper fiberglass tank pre-inspection safety steps must be taken to assure that the tank is clean and free from any chemicals and air quality acceptable. The FRP tank interior is accessed through a ground level or roof manhole. Special attention is given to the FRP tank penetrations which may include erosion, corrosion, mechanical, and environmental related deterioration. Example fiberglass tank defects are shown in Figure 2.
Figure 2. Advanced cracking on fiberglass tank floor.
Fiberglass Tank Hardness Testing
Fiberglass tank surface hardness readings on the interior liner are recommended post manufacturing and in-service. FTPI 2007-1, Recommended Practice for the In-service Inspection of Aboveground Atmospheric Fiberglass Reinforced Plastic Tanks and Vessels [5] recommends 12 separate readings with the highest and lowest discarded. The average of the remaining values should be within 10% of the manufacturer’s specification. DEREKANE® resins manufactured by Ashland Inc. is a common liner product with FRP tank liner Barcol hardnesses in the 30-40 range.
The device is intended for handheld testing of hardness fiberglass tank liners. The liner hardness is tested by placing the indenter point and leg on level surface. The indenter is pressed down firmly on the surface and the peak reading is noted from the indicator. FRP tank hardness reading should be spaced at least 1/16th of an inch away from previous readings. A fiberglass tank Barcol hardness reader is shown in Figure 3.
Figure 3. Fiberglass tank Barcol Hardness reader.
Fiberglass Tank Strain Measurement
Many different loading conditions must be considered during fiberglass tank design including external and internal pressure, weight of the vessel, superimposed static loads, attachments, environmental and thermal. At 5-year intervals, it is recommended to acquire fiberglass tank hoop strain measurements to assess the load baring capacity of the tank. Strain gages are installed a different elevations while the tank is empty. The tank is then filled to capacity and the hoop strain is recorded. The maximum allowable hoop strain is 0.002 microstrain with values typically in the 30 to 120 microstrain range. As the tank ages, hoop strain does tend to increase due to the general aging of the structural layer. Example hoop strain data is shown in Figure 4.
Figure 4. Fiberglass tank hoop strain measurement.
Fiberglass Tank Acoustic Emission Testing
Acoustic emission testing of fiberglass reinforced plastic (FRP) tanks is performed post-fabrication and in-service. The tests are performed to ASTM E1067-07: Standard Practice for Acoustic Emission of Fiberglass Reinforced Plastic Resin (FRP) Tanks/Vessels. Acoustic Emission (AE) is used to inspect new and in-service FRP tanks for; resin cracking, fiber debonding, fiber pullout, fiber breakage, delamination, bond failure in assembled nozzles and manways.
FRP tanks are tested with acoustic emission by filling with water or product in incremental steps to 50%, 75%, 87% and 100% capacities. As the tank is filled any active flaws will emit acoustic emission in the 150 kHz range waves which are picked up by acoustic emission sensors. An example acoustic emission sensor is shown in Figure 5.
Figure 5. An acoustic emission sensor installed on a fiberglass tank.
Thermographic Inspection of Fiberglass Tanks
An infrared (IR) camera is a non-contact device that detects infrared energy (heat) and converts it into thermal image. As a fiberglass tank is filled with product during a hydrostatic or acoustic emission test, a thermal gradient is created through the FRP tank wall which consists of multiple layers, between which, delaminations may occur. Similarly, repair patches from fabrication or in-service fiberglass tank repairs may also be observed during the acoustic emission testing. An example fiberglass repair or delamination is shown in Figure 6. The area is outlined by a dark red area approximately 6” x 4” in area in the fiberglass tank thermographic image.
Figure 6. Thermographic image of a delamination or previously unknown repair in a fiberglass tank shell.
References
ASTM D2563-08: Standard Practice for Classifying Visual Defects in Glass-Reinforced Plastic Laminate Parts. ASTM D2583: Standard Test Method for Indentation Hardness of Rigid Plastics by Means of a Barcol Impresser.ASME RTP-1-2007, “Reinforced Thermoset Plastic Corrosion-Resistant EquipmentASTM’s E1067-07: Standard Practice for Acoustic Emission of Fiberglass Reinforced Plastic Resin (FRP) Tanks/Vessels.FTPI 2007-1 Recommended Practice for the In-service Inspection of Aboveground Atmospheric Fiberglass Reinforced Plastic Tanks and Vessels.