Case Study: FRP Pipe

Non-destructive evaluation of FRP piping system at phosphoric acid plant

Project managers planning the expansion of a large phosphoric acid plant in the Middle East engaged UTComp to assess the condition of existing FRP pipeline systems at the facility.

The inspection team examined over five kilometres of side-by-side FRP piping, 100 mm to 300 mm in diameter, used to transport acidic solutions from various parts of the plant to a port storage facility where phosphoric acid is loaded onto ships for export. The inspection team evaluated a phosphoric acid emptying line and an export line, dividing each into 100-metre sections that were inspected and reported upon. The assessment revealed some corrosion barrier damage plus evidence of leaks and corrosion damage to clamps, joints, supports and other key components that needed repair. However, in general, the FRP piping systems were in good condition with a Remaining Service Life (RSL) exceeding six years in most sections of the facility.

Background

The chemical manufacturing plant featured in this case study has a daily production capacity of more than 1,300 tonnes of phosphoric acid. The plant uses a wet process in which crushed phosphate rock is mixed with sulphuric acid; the acid mixture is then concentrated and purified for export and use in a variety of applications such as fertilizers, food additives, cleaning products and more.

UltraAnalytix® NDT by UTComp is the world’s only proven non-destructive and non-intrusive assessment system that provides a reliable, repeatable and scalable method for inspecting in-service FRP pipelines and ensuring their safe performance and maximum lifespan. The patented UltraAnalytix system provides trained inspectors with the tools they need to assess Fitness for Service by efficiently gathering data to accurately calculate FRP thickness and determine corrosion barrier condition anywhere along a piping circuit, calculate the Percentage of Design Stiffness (PDS) and estimate Remaining Service Life (RSL).

Project summary

The assessment discussed in this case study involved examining two phosphoric acid pipelines — an export line and an emptying line — each about five kilometres long and designed for operating pressures of 1,000 kPa. Some sections of pipe had been in service since the late 1990s, while other sections were installed in 2009. Inspection of the piping followed UTComp standard practice: as each ultrasonic survey was completed, an external inspection was also completed to identify surface damage such as cracks; UV damage; gouges, blisters or other deformation; corrosion damage to supports; condition of flanges, nozzles, lugs and repads; disbonding or peeling, and more.

Individual inspection plans (IP) were completed for each 100-metre section of piping. Results were presented as Circuit Reports that included a description of condition monitoring locations (CMLs), the results of ultrasonic inspections at CMLs, minimum remaining service life (RSL), and a summary of the findings of the external inspection. 

Figure 1 below illustrates how the pipelines were divided into manageable sections, each requiring its own inspection plan.

Phosphoric Acid Emptying Line

Year of assessment: 2016

Remaining Service Life: see Table 4 below

General corrosion barrier condition: damage to 80% of CB thickness

Inspection Statistics

• Total number of IPs: 14
• Total number of CMLs: 242
• Total number of UltraAnalytix readings: 968

Overall Condition:

• Small loss of thickness has occurred
• Good strength retention / PDS

Discussion, Key Findings and Analysis

Assessing FRP Pipelines: Key Factors to Consider

Damage Mechanisms and Failure Modes For FRP

Fitness for Service (FFS) evaluation to prevent failures and optimize the lifespan of FRP piping is based upon striking a balance between the mechanical and chemical stresses applied on in-service equipment and its design capacity. Failure modes describe visible damage such as fibre cracking, debonding and delamination that can occur to in-service equipment over time; damage mechanisms are the underlying processes and changes in the FRP material that lead to the appearance of a failure mode.

There are three main damage mechanisms that occur in FRP:

1. Damage to the polymer matrix or resin
2. Damage to the reinforcement fibres
3. Damage to the interface between the matrix and the fibres.

These damage mechanisms affect all the components that combine to give FRP strength and stiffness. While it is in service, FRP undergoes a change in bending stiffness known as creep that weakens the material over time and can eventually lead to failure. 

Problems always start in the resin or polymer rather than the fibre reinforcement.  Generally, the fibre fails only after the polymer has failed — in fact, the fibre might not fail at all — and the damage may go undetected before a leak occurs. 

Therefore it’s important to identify polymer damage early on to avoid bigger problems in the future. The challenge is that initially, polymer damage is invisible to the naked eye. However, it is detectable using the UltraAnalytix system, which calculates changes in stiffness (PDS) as a proxy for several damage mechanisms.

Figure 2 below illustrates the relationship between damage mechanisms and failure modes.

Assessing FRP Pipelines

FRP pipeline systems have several distinct components that each require slightly different inspection analysis of ultrasonic readings taken by following UTComp procedures. The make-up of each piping system or circuit is often unique or different from other piping systems or circuits at the same facility. API 570 prescribes an approach to inspecting metallic and FRP piping based on Condition Monitoring Locations (CMLs) in a Piping System Inspection Plan. 

Note that of all the damage mechanisms listed in API 574, only Erosion relates to thickness loss. For chemical process services, API 574 neglects a significant source of thickness loss, which is oxidation of resin. Most other damage mechanisms provided within API documents reduce the strength of FRP and do not coincide with thickness loss.

Inspection plans required for each piping circuit

API 570 requires an Inspection Plan (IP) for each piping system or circuit to be inspected. IPs are created by the inspector using isometric drawings or photographs. Plans are created within UltraAnalytix software so that readings can be assigned immediately to specific CMLs. For each FRP piping system or circuit, the IP is intended to show specific CMLs so that the current condition of the piping and components can be tracked at known locations and piping condition can be determined.

There are generally seven (7) standard components to consider for FRP piping systems: fittings, butt and wrap joints, socket joints, flanges, spools, spools at joints (SAJ), and repads. Each of these is treated separately and included with the results found in the Piping System Report prepared for each Inspection Plan.

Often, FRP piping components are purchased as standardized products, and classified based on a pressure rating developed by following calculations and tests according to a standard specification, practice or guide. One example is ASTM D2992. Piping systems are often designed and installed such that the design pressure rating exceeds the actual operating conditions for the piping. Analysis and evaluation of piping systems must relate the current stiffness or strength of the piping system to the operating requirements. The UltraAnalytix system does this by calculating three main values:

• Percentage of design stiffness (PDS)
• Thickness of the FRP 
• Depth of damage to the corrosion barrier

Percentage of design stiffness is defined as:

PDS = Current flexural modulus
            Design flexural modulus

Generally, PDS values above 50 per cent indicate the material is in overall good condition and will be fit for service until the next scheduled inspection. Values between 40 and 50 percent indicate problem areas  that may require an engineering review, while PDS values below 40 per cent indicate a high risk of equipment failure requiring immediate action.

For example, Table 2 below shows data from UltraAnalytix readings taken at a butt and wrap joint located in the section of the Export Line covered by IP6.

Key Findings

Export Line Report 

Table 3 below summarizes the main conclusions and recommendations for the inspection plans (IP) representing each section of the phosphoric acid export line.

Emptying Line Report

Table 4 below summarizes the main conclusions and recommendations for the inspection plans (IP) representing each section of the phosphoric acid export line.

Conclusion

This FRP pipe case study demonstrates how the UltraAnalytix NDT system provides quantifiable knowledge about the condition of FRP at any age — new or in-service — by monitoring the changes that take place within the material’s structure. These changes can be used as the basis for repair and replacement planning. 

UltraAnalytix is the only NDT methodology capable of assessing the mechanical integrity of FRP pipes by determining the condition of the polymer. (See a comparison of FRP testing methods.) While conventional ultrasonic testing can be used to measure thickness and find cracks and other defects, UltraAnalytix does more. By providing a fast, reliable way to calculate changes in stiffness (PDS), FRP thickness and the depth of corrosion barrier damage, it quantifies the condition of the FRP material, calculates remaining service life, and provides fitness for service assessments that allow owners such as the phosphate company in this case study to make informed decisions about their FRP assets.