Case study: UltraAnalytix NDT shows scrubber has 16 years remaining service life
UTComp’s global network of UltraAnalytix® licensees includes Denmark-based Nordic Glasfiber. In 2019, Nordic Glasfiber assessed a hydrogen chloride (HCl) scrubber tank at a combined heat-power plant (CHP). The client, one of Denmark’s largest district heating companies, wanted to know the condition and remaining lifespan of the 20-year-old vessel and whether it was suitable for adding another stage to the HCl scrubbing process. The plant uses “wet” scrubber technology that involves spraying waste flue gases with a liquid reagent which removes toxins as the gas percolates through the scrubber tank. The gas goes through additional stages of scrubbing, condensers recover latent heat and the chemical components of the scrubbed gas are analysed and measured according to environmental standards before it is vented into the atmosphere. Adding a second stage to the HCl scrubber would increase the surface area of cleaning material and enable the system to remove more pollutants from the waste gas. The UltraAnalytix inspection concluded that the vessel was in overall good condition for its age and suitable for continued service.
Cogeneration or combined heat and power (CHP) has a long history in Denmark as an energy efficient and environmentally friendly way to produce electricity and heat. CHP technology makes it possible to efficiently burn a variety of fuels (including natural gas or coal, biomass and municipal waste) to generate electricity and use the excess heat, which would otherwise be wasted, as district heating for buildings. Today, more than 60 per cent of Danish district heating is cogenerated with electricity. There are about 670 CHP plants of various sizes across the country, including the one in this case study which burns 289,000 tonnes of waste annually to generate approximately 100 megajoules of district heating and 23 megawatts of electricity for the region.
Various types of scrubber systems are used in many industries to remove or neutralize toxins and particulates from gaseous emissions, especially acidic gases such as hydrochloric acid (HCl), hydrogen sulfide (H2S), chlorine (Cl) and sulfur dioxide (SO2). Like most plants in Denmark, the CHP facility in this study uses “wet” scrubber technology that involves pumping waste gas into a scrubber tank which bathes the gas in a liquid reagent (in this case, HCl). Pollutants are removed from the gas as it goes through various stages of scrubbing and measurement before it is vented into the atmosphere.
For the process to work, the hot gas must first be cooled to the dew point, from temperatures as high as 200C down to between 70C and 80C. The combination of high temperatures, high humidity and chlorine scavengers in the flue gas creates a harsh, corrosive environment that makes scrubber tanks made from FRP the preferred choice over steel alloy vessels.
Nordic Glasfiber has two decades of experience in the inspection, design, and manufacturing of fiberglass tanks, silos and scrubbers used in a variety of industries including the chemical, pharmaceutical, food, energy and waste processing sectors. Since 2018, the company has been a UTComp licensee providing non-destructive ultrasonic inspection of FRP and composite equipment using the UltraAnalytix NDT system.
Approximately 14 meters tall with a diameter of three meters, the FRP vessel discussed in this case study was manufactured in Germany and installed in 1999. Nordic Glasfiber completed the first UltraAnalytix inspection in April 2019 when the tank was approximately 20 years old — well within the normal lifespan for this type of equipment. The client wanted to know the condition of the FRP and whether the tank was suitable for adding another stage to the HCl scrubber. This would enable the scrubber to remove more pollutants from the waste gas by increasing the surface area of cleaning material in the tank.
A team from Nordic Glasfiber visited the plant in April 2019 to complete an external inspection and collect ultrasonic data from the HCl scrubber in accordance with UTComp training guidelines and standard practices. UltraAnalytix data was sent to UTComp for analysis and reporting. It was the first UltraAnalytix inspection for this equipment.
Ultrasonic measurements were taken from six sections of the FRP vessel and two reinforcements. UltraAnalytix technology allows licensees to accurately measure FRP thickness, calculate the Percentage of Design Stiffness (PDS) and assess the condition and bonding of FRP materials for a fast, reliable way to assess the mechanical integrity of composite equipment.
The external inspection focused on looking for signs of deformation or damage to the FRP shell, protective coatings and paint, as well as the condition of support structures including ladder supports, nozzle connections, pipe and valve supports.
The inspection found the tank to be in overall good condition although some minor damage to the corrosion barrier and recirculating pump was detected in the lower part of the vessel.
- No defects or damage to the support structures were detected.
- No defects or damage to the shell tank and external components were detected.
- No defects or damage to pipe and nozzle connections of the tank were detected.
- The FRP vessel is suitable for continued service to the next recommended UltraAnalytix inspection.
Ultrasonic measurements were taken from six sections of the FRP vessel and two reinforcements. Table 1 below shows the UltraAnalytix results for the sections evaluated in this inspection.
Reinforcing Pad Condition and Bonding
Reinforcements and repairs are applied to FRP tanks to add thickness in locations where stresses are concentrated or additional strength is required, such as pipe and nozzle connections and manway doors. If a reinforcement is less than 6 mm thick, an engineering review and repairs will be recommended because inadequate reinforcement is frequently the root cause of failures.
This UltraAnalytix inspection included data acquired from reinforcement pads (repads) located at the outlet duct and manway hatch.
Bonding between repairs / reinforcements and vessel sections is determined as Percentage of Theoretical Bonding. These bonds may have as little as 50% of their theoretical shear strength. For this reason, UTComp uses the criterion that a bond exists when the percentage shown in the UltraAnalytix data is greater than 50%. If two or more consecutive points or more than 25% of the points are not bonded, immediate repair is recommended.
Outlet Duct Connection Repad Bonding
The condition of the bonding around the outlet duct connection repad is shown in Figure 1 below. In the figure, the red line shows the Bonding Threshold at 50%. Any points within that circle are considered to be unbonded. Table 2 below contains the statistics for this evaluation.
Manway repad condition and bonding
The condition of the bonding around manway repad is shown in Figure 2 below. In the figure, the red line shows the Bonding Threshold at 50%. Any points within that circle are considered to be unbonded. Table 3 below contains the statistics for this evaluation.
The UltraAnalytix inspection concluded that the vessel was in overall good condition for its age and suitable for continued service. As expected for a nearly 20-year-old piece of equipment, there was some corrosion barrier damage, especially in the lower sections where the liquid HCl bath is sprayed into the vessel during scrubbing. These areas may need repairs down the line but no immediate remediation is required. In addition:
- More information was required to verify the location of mechanisms for pressure and vacuum relief.
- The next UltraAnalytix inspection is to take place in 2022, with a maximum interval of three years between future inspections.
UltraAnalytix provides licensees and clients with a fast, safe and reliable method for collecting the data they need to make informed decisions about the performance and lifespan of their FRP assets. This is especially important for utility companies like client discussed in this case study: CHP facilities must operate according to environmental standards and shifting regulatory regimes, and the plants themselves have a typical lifespan of 25 to 30 years. UltraAnalytix provided the asset owners with additional certainty and peace of mind that their FRP equipment will continue to perform as expected for as long as they need it, without the need for destructive testing or hazardous confined-space entry.
Using the UltraAnalytix system, Nordic Glasfiber provided the client with the data they needed to decide whether it is economical or necessary to make repairs or upgrade their equipment by adding a second stage to the scrubbing process in order to improve the plant’s environmental performance.