Case Study: FRP tank (hydrochloric acid service)

Using UltraAnalytix NDT to Monitor Vessel Condition and Restoration

This case study describes the application of UTComp’s UltraAnalytix NDT system to monitor the condition of a fiberglass-reinforced plastic (FRP) tank, recommend repairs to the tank and provide structural data on the repaired tank.

At this writing, the FRP tank has been in service for 16 years. It is used in hydrochloric acid (HCL) service with fill level between 0% and 100%. The fluid temperature is 95°C. Starting in 2010, the Owner of this vessel has used a reliability and asset management program for this tank. The program uses the combined results of frequent external inspections and annual internal, external and UTComp® System evaluation of the FRP.

Part of the tank exterior is shown in Figure 1. The tank is 4.5m high and 2.5m in diameter.

Industry: Chemical Processing

Figure 1: Exterior of tank

Since the original installation, the Owner has modified the operating conditions for the tank to minimize fugitive releases of vapour from the tank. This includes submerging the overflow in a seal pot and installing a combined pressure relief and vacuum breaker on the tank. The tank is not connected to a ventilation system. Because of these changes, the tank shell is now subjected to both internal and external (internal vacuum) pressures. Analysis has shown that the original shell design was suitable for both of the pressures.

UTComp inspections in 2010 and 2011 were used to determine the strength of the FRP in the tank, as well as the condition of the corrosion barrier (CB). The results of the strength evaluation are shown in Figure 2. This figure reports the Percentage of Design Strength (PDS) of the FRP in different sections of the tank. The actual strength of the FRP is then determined by applying the Percentage of Design Strength to the Design Strength of the FRP used in the vessel. The Design Strength can be obtained from engineering analysis, original design documents, manufacturer’s test results, or from lamination analysis of ignition loss testing. When the actual strength values are known, engineering analysis can be completed.

Figure 2: PDS history of FRP tank

In Figure 2, the red line at PDS of 40% shows the value of PDS at which UTComp® recommends engineering analysis of FRP equipment take place to determine two (2) main items:

  1. The Fitness for Service of the FRP under current operating conditions, and
  2. The PDS value at which the FRP will require repair or replacement. This is known as the Critical PDS™. Note that the shell at 3m and 4m elevation both showed PDS values below 40% after the 2011 evaluations.

Evaluation of the corrosion barrier in 2010 and 2011 showed that the resin was in generally good condition, but the resin-rich inner layer had significant damage from thermal cracking and separation from the anti-wicking layers. Replacement of the CB was recommended.

Engineering analysis of the tank shell was completed for the updated operating conditions. At the current PDS values, it was determined that shell damage was likely if the external pressure on the tank reached the setpoint value for the vacuum breaker.

Based on plant production needs, the Owner decided to make both the structural repairs and the CB replacement in 2011. Structural FRP was designed and added to the inside of the tank, then a new CB was applied. After the repair was complete, UTComp® System readings were again taken. The chart in Figure 3 shows the result after the repairs were complete.

After the repairs, it is possible to continue monitoring the vessel without interruption and to show the entire PDS record. The UTComp® System has been shown to reliably predict the need for repairs and to show the result of repairs on the structural capacity of FRP.

Figure 3: UTComp® system results after repair

Your NDT solution for FRP asset integrity management is here.

UltraAnalytix NDT makes FRP composite inspection as rigorous and reliable as for metallic assets. Get unparalleled Fitness for Service and Remaining Service Life analysis by ASNT-certified personnel to improve plant safety and operational efficiency and increase your Return on Assets (ROA).
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