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.
Industry: Mineral and Chemical Processing
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.
The fast, reliable way to assess
Fitness for Service of FRP piping
While not generally considered a strong acid, in higher concentrations phosphoric acid may attack metals. The plant is located in a desert coastal area, where harsh environmental conditions can damage infrastructure through a combination of extreme heat, humidity fluctuations, sand and salt. Filament-wound, fibre-reinforced polymer (FRP) composite pipes have a distinct advantage in such applications. FRP has a long record of success safely transporting and storing materials such as corrosive acids where steel and other alloys are unsuitable or cost prohibitive. However, non-destructive testing (NDT) of FRP piping is relatively new. Conventional inspection approaches, including destructive testing and visual inspections that typically require confined space entry, are not well-suited to assessing pipeline networks due to the small diameter of the equipment and the vast amount of piping snaking through a typical facility. Regular visual external inspections are important but provide only limited insight.
A technician takes UltraAnalytix readings of a section of FRP piping used to transport phosphoric acid.
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.
Figure 1
Phosphoric Acid Export Line
Year of assessment: 2016
Remaining Service Life: See Table 3 below
General corrosion barrier condition: damage to 80% of CB thickness
Inspection Statistics
- Total number of Inspection Plans (IPs): 20
- Total number of CMLs: 381
- Total number of UltraAnalytix readings: 1,524
Overall condition:
- Most pipe supports and clamps showed general surface corrosion
- Damage and reduced PDS were localized in some areas of the piping
- General damage to the corrosion barrier with accompanying thickness loss had occurred.
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:
- Damage to the polymer matrix or resin
- Damage to the reinforcement fibres
- 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.
Assessment practices outlined in the code related to FRP take into account the following key factors:
- Codes and standards governing design, such as ASME B31.3 or EN 14692, are often limited to structural items that don’t consider corrosion resistance or evaluation of piping systems that are already in service
- Emphasis is placed on inspections of new installations carried out by inspectors with knowledge of FRP curing, fabrication and quality
- Variations in manufacturing methods and physical properties.
Table 4 of API 574 lists some damage mechanisms associated with FRP piping in service conditions. The damage mechanisms listed in API 574 are included in Table 1 (right) along with the relationship to the mechanisms listed in Figure 1 above and detection methods. When the damage can be detected without interrupting operation of the piping, the term “non-intrusive” is used. Where operation of the piping must be interrupted, or access to the inner surface is required, the term “Intrusive” is used.
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.
| Inspection Plan | RSL (years) | Conclusions | Recommendations |
| IP 1 | > 6 |
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| IP 2 | > 6 |
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| IP 3 | > 6 |
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| IP 4 |
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| IP 5 | > 6 |
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| IP 6 | > 6 |
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| IP 7 | > 6 |
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| IP 8 | > 6 |
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| IP 9 | > 6 |
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| IP 10 | > 6 |
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| IP 11 | > 6 |
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| IP 12 | > 6 | ||
| IP 13 | > 6 |
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| IP 14 | > 6 |
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| IP 15 | > 6 |
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| IP 16 | > 6 |
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| IP 17 | < 6 |
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| IP 18 | > 6 |
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| IP 19 | < 6 |
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| IP 20 | < 6 |
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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.
| Inspection Plan | RSL (years) | Conclusions | Recommendations |
| IP 1 | > 6 |
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| IP 2 | > 6 |
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| IP 3 | > 6 |
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| IP 4 | > 6 |
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| IP 5 | > 6 |
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| IP 6 | > 6 |
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| IP 7 | > 6 |
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| IP 8 | > 6 |
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| IP 9 | > 6 |
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| IP 10 | > 6 |
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| IP 11 | > 6 |
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| IP 12 | > 6 |
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| IP 13 | > 6 |
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| IP 14 | > 6 |
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Recommendations
While both FRP pipelines were in generally good overall condition, there were numerous examples of visible damage due to corrosion, leaks, cracking, inadequate supports, missing or damaged bolts and clamps, as well as some loss of FRP thickness and corrosion barrier damage detected ultrasonically. Typical problem areas included cracked rubber at expansion joints and cracked or mismatched flanges in need of repair and monitoring (there were ultrasound readings for flanges). For example, Figure 3 (right) shows a flange leak and missing bolts.
UTComp recommends an UltraAnalytix assessment every three years and after all significant process excursions and environmental events. UTComp also recommends that a qualified engineer be engaged for all recommended engineering activities including replacement, review, evaluation, design of repairs and repair inspection.
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.