Research Confirms UltraAnalytix NDT is a Reliable, Cost-Effective Inspection Tool for RO Pressure Vessels
OVERVIEW
UTComp and UltraAnalytix® licensee RPC Technologies Pty (RPC) conducted laboratory-scale testing of FRP composite reverse osmosis (RO) pressure vessels used in municipal and industrial water desalination plants.The goal was to use the UltraAnalytix non-destructive testing (NDT) system to assess the condition and performance of the vessels while filled and under pressure, and to understand the conditions that may lead to loss of containment.
The project included calculating remaining service life (RSL); developing a system for early detection of cracks and leaks; determining the most efficient locations for data collection, and validating UltraAnalytix results with destructive testing. The client — a global water, waste management and energy company — provided a total of eight vessels from two desalination facilities for testing. Following UltraAnalytix inspection protocols, the RPC team took external ultrasound readings of the vessels at 2-4 times their rated pressures and monitored them for loss of containment. Multiple readings were taken at several condition monitoring locations (CMLs) along the length of each vessel for a total of over 2,000 readings. Raw data was sent to UTComp for post-processing and analysis. Results confirmed that UltraAnalytix provides a reliable, cost-effective tool for lifecycle management of RO pressure vessels using the same methods developed by UTComp through testing thousands of FRP assets in a variety of settings worldwide.
Background
Many major cities in water-stressed regions such as Australia and the Middle East depend upon large-scale sea water desalination plants to provide potable water for human consumption, irrigation for green spaces and agriculture, and for industrial purposes. Among the various methods available for desalination, RO membrane technology is the most widely used and cost-effective way to turn sea water into potable water.
Tube-shaped RO pressure vessels contain polymer membranes that filter salt and other impurities from sea water.
In the technology’s early days, RO pressure vessels were made from steel and most innovation focused on developing better membrane materials for filtering the water. But steel is heavy and vulnerable to corrosion from salt water. By the late 1980s, filament wound FRP provided a lightweight, strong and corrosion-resistant alternative to steel housings for RO membranes. FRP also provided a consistent, smooth inner surface that steel can’t match for perfect seals at the vessel end-closures and between the membranes and vessel housing. Today, high-quality RO pressure vessels made from FRP composites are precisely engineered according to the Boiler and Pressure Vessel Code of the American Society of Mechanical Engineers (ASME). One of the key challenges was designing FRP housings that could withstand the high operating pressures of sea water desalination systems.
Sea water RO equipment works by pushing sea water under pressure through polymer membranes that trap salt and other impurities while allowing water molecules to pass through. Made from corrosion-resistant FRP composites, each tube-shaped pressure vessel usually contains one membrane element. RO pressure vessels come in a range of diameters and lengths – typically four to eight inches (100-200 mm) in diameter and up to 20-25 ft (6-8 m) long with side access ports located near the bell-shaped ends. Large-scale desalination facilities do their work in several stages:
- First, an intake system brings sea water into the desalination plant for pre-treatment and filtration to remove suspended solids including organics and other particles.
- The pre-treated water is then pumped through a series of RO vessels rated for pressures up to 1,000 psi or more. The RO pressure vessels are grouped in racks or trains containing dozens individual units, and there may be hundreds of vessels in a large plant (depending on the volume of water being processed).
- Water passes through the RO membranes, leaving dissolved salt and other impurities behind; the desalinated or purified water (called permeate) is sent for post-treatment and into the product stream while the leftover “impure” water (called concentrate or brine) goes through additional RO passes for further separation.
- The process repeats until the remaining brine is finally discharged back into the ocean.
Behaviour of FRP Materials Used to Make RO Pressure Vessels
Corrosion-resistant, lightweight and strong, reinforced thermosetting polymers are ideal materials for making RO pressure vessels. However, the material is also relatively inflexible and brittle. Damage from mechanical stress and/or chemical attack reduces the polymer’s elastic modulus (its ability to stretch) and tensile strength; as elastic modulus declines, tensile strength falls at an even faster rate, increasing the risk of failure. In fact, when the modulus reaches 40%, the material’s remaining tensile strength could be less than 10% of new.
In general:
- All FRP failures originate as a brittle fracture of the polymer.
- Leaks are almost always limited to the polymer.
- Reinforcement fails after the polymer has failed.
Project Summary
Conventional inspection approaches including destructive testing aren’t well suited to RO pressure vessels due to their small diameter and the sheer numbers of them in use at any one facility. Visual inspections of the exterior provide only limited insight. This project demonstrates how the UltraAnalytix system can meet the need for a reliable, repeatable and scalable method for inspecting RO pressure vessels.
For this project, RPC conducted UltraAnalytix inspections of eight RO pressure vessels sourced from two desalination plants.
The client provided RPC with a total of eight RO pressure vessels for testing from two water filtration plants: two vessels from the Facility A and six from the Facility B. Although made by two different manufacturers, the equipment was similar in terms of application and design. Testing took place in RPC’s composites engineering workshop, where the vessels were pressurized and monitored short-term and long-term for loss of containment.
UltraAnalytix technology accurately measures FRP thickness, calculates the Percentage of Design Stiffness (PDS) and evaluates the condition and bonding of FRP materials for a fast, reliable way to assess the mechanical integrity of composite equipment. PDS values below 40 per cent indicate that there is a high risk of failure within one year.
UltraAnalytix readings were taken at up to 20 CMLs along the length and circumference of each vessel. Destructive testing validated the UltraAnalytix results by examining the condition of the laminate in pieces cut from the bell-shaped ends and reinforcements, as well as ash testing on a vessel with a defective “O” ring sealing face.
General Results
Facility A RO Membrane Vessels (2 units tested)
- Vessel 1
- Pressurized to 4 x design, and then held at 3.2 x design
- Loss of containment occurred at 94 hours
- Destructive testing completed on both bell ends
- Vessel 2
- Pressurized to 2.4 x design for 1,796 hours
- Pressure increased to 2.8 x design for 2,375 hours
- Vessel has not reached loss of containment
Facility A Vessel 1 Short-term Test
After the RO vessel was pressurized, multiple readings were taken at each of 20 CMLs. Figure 1 below shows the location of the CMLs.
Figure 1: Data was collected from 20 condition monitoring locations (CMLs) on vessel 1.
Results:
- Red bars in the chart below show PDS < 40 per cent.
- The unit was already near failure when testing began.
- Loss of containment occurred after 94 hours at 100 bar / 1,450 psi (3.2 x rating).
- Damage was concentrated in end bells.
Figure 2: The red bars in the chart above show areas where PDS was below 40 per cent, indicating that the vessel was at high risk of failure within one year.
Facility A Vessel 2 Long-term Test
For the long-term test, the location of the CMLs was adjusted. Loss of containment has not yet been achieved for this vessel; however results indicate changes in the condition of the FRP over time, particularly at CMLs 2 and 12, may be cause for concern.
Figure 3: Data was collected from 13 CMLs compared to 20 CMLs on Vessel 1.
Results:
- There was a small change to the location of CMLs.
- Loss of containment has not been achieved.
- The change rate increased at pressures of 88 bar / 1,276 psi.
Figure 4: The chart above shows the PDS of Vessel 2 over time.
Facility A Vessels Destructive Testing
The team conducted destructive testing of Vessel 1 after the vessel failed. This involved cutting pieces from the bell-shaped ends and reinforcements at access ports.
- Results showed discrepancies in the laminate and manufacturing variations between each end of the vessels, with variations in the placement of the reinforcements .
- UltraAnalytix data results were validated by destructive testing.
- Analysis found signs of tensile cracking and shear plane delamination.
Figure 5: Sections of the bell end of the vessel were cut apart for analysis.
Figure 6: Profile view shows signs of damage to laminate taken from the hoop reinforcement.
Facility B RO Membrane Vessels (6 units tested)
Vessels provided from Facility B were made by a different manufacturer than the vessels from Facility A. Some could not be over-pressurized while others were new units that had not yet been in service, providing an opportunity to collect valuable baseline data.
- Vessel 1 (450 psi rating)
- New vessel that has not seen service
- Pressurized to 2.4 x design for 3,666 hoursHas not reached loss of containment
- Vessel 2 (300 psi)
- New vessel that has not seen service
- UltraAnalytix provided baseline data analysis
- Vessel 3 (450 psi)
- Baseline UltraAnalytix data analysis
- Unable to over-pressurize due to manufacturing fault
- Vessel 4 (450 psi)
- Baseline UltraAnalytix data analysis
- Vessel 5 (300 psi)
- Pressurized to 2.9 x design for 3,666 hours
- Has not reached loss of containment
- Vessel 6 (300 psi)
- Baseline UltraAnalytix data analysis
- Unable to over-pressurize due to manufacturing fault
Facility B Vessels Long-Term Testing
Figure 7: Data was collected from 11 CMLs.
Two vessels from Facility B were successfully over-pressurized and monitored for loss of containment over a five-month period. Neither vessel has failed or leaked to date.
- Vessel 1 pressurized at 75 bar / 1,088 psi for 3,666 hours.
- Vessel 5 pressurized at 60 bar / 870 psi for 3,666 hours.
Figure 8: Condition of the laminate remained steady after five months of monitoring.
Facility B Vessels Destructive Testing
- Ash testing was conducted on a vessel with a defect in the “O” ring sealing face.
- Results showed the laminate structure was consistent without additional reinforcements.
- Researchers are still waiting loss of containment for next destructive test.
Figure 9: Ash testing revealed consistent structure in the layers of FRP laminate cut from a defective “O” ring sealing face.
DISCUSSION, KEY FINDINGS AND ANALYSIS
This project illustrates one of the critical challenges involved in lifecycle management of FRP composite assets: damage almost always starts within the resin or polymer rather than the fiber reinforcing material. The fiber only fails after the resin/polymer has failed — in fact, the reinforcement might not fail at all — and the damage is often invisible before a leak occurs. However, FRP construction codes don’t address the possibility that the polymer will degrade over time, leaving engineers and owners to believe that the condition of the polymer will remain consistent throughout its lifespan.
In RO pressure vessels, leaks are most likely to start as a brittle fracture of the laminate in high-stress areas such as the access ports as well as the end plate. While FRP is highly resistant to saltwater corrosion, mechanical stress will eventually degrade the resin to the point that leaks will occur.
Key Findings
- Inspection of the RO pressure vessels resulted in several key learnings:
- The condition of any unit is independent and unrelated to the condition of any other unit.
- Vessel end laminates can be quite different due to manufacturing inconsistencies and stress concentrations.
- Laminate condition at ports is very similar to laminate condition in the bell.
- The condition of the bell inboard of the end cap will reach failure condition before a leak occurs. (see the red circled areas in Figure 10 below)
Figure 10: Areas circled in red show areas where FRP is likely to reach failure condition before leaks occur.
- Leaks at the ports are commonly due to failed FRP.
- UltraAnalytix analysis demonstrates the rate of polymer deterioration is slow enough to allow proactive replacement within a structured asset management program.
Figure 11: A typical desalination plant may have hundreds of RO pressure vessels arranged in “trains” of several dozen connected vessels.
Key Achievements
This research project achieved a number of goals that will advance the use of FRP composite materials for RO vessels and other applications:
- The UltraAnalytix system can be used to calculate the remaining service life (RSL) of RO pressure vessels.
- The researchers were able to assess the quality of laminate.
- A systematic approach was developed for early detection of any polymer deterioration.
- Conditions leading to vessel failure are now better understood and predictable.
- UltraAnalytix data results were validated by destructive testing.
- The team identified the most efficient locations for data collection: two small sections about 75 mm in length, located between the vessel access ports and end caps.
Recommendations
- New RO pressure vessels should be inspected with UltraAnalytix prior to commissioning.
- Inspect the areas circled in red below (8 readings, each) on each unit in operation at regular intervals – 3 years for normal operating pressures.
- Vessels should be removed from service when PDS is less than 40% — at which point RSL is approximately one year — before loss of containment.
Conclusion
A typical desalination facility may contain several hundred to a few thousand RO pressure vessels. The technology has been around for decades, so there are potentially tens of thousands of these units worldwide that may be nearing the end of their service life. However, while the ASME code covers how RO vessels must be manufactured, it currently offers little guidance for determining their fitness for service.
Plant owners need a reliable, non-destructive and cost-effective method for assessing the condition of their equipment and predicting when it is at risk of failure. This research project demonstrates the value of UltraAnalytix in providing a fast, reliable, and cost-effective method for determining the fitness for service of RO pressure vessels.
Regular inspections using the UltraAnalytix system could save the global desalination industry billions of dollars by reducing losses of a valuable resource (water) through leaks and other failures, improving reliability, increasing operational uptime and lowering capital replacement costs.