Further Tests to Determine a Proper Lining System for Process Vessels
Process vessels are commonly constructed from carbon steel, which corrodes under harsh chemical, heat and pressure environments. Here we discuss the international testing methods used to determine proper lining systems for each service environment.
The oil and gas industry, like all business sectors, has developed material pre-qualification tests and set pass/fail values based upon their many decades of operational experience and recognition of both the benefits and risks in selecting fit-for-service corrosion protection systems. Here we discuss test methods necessary to determine proper lining systems.
Low Temperature Thermal Shock
Lining materials, especially those in high temperature and pressure services, can be subjected to acute and rapid thermal changes when exposed to situations of explosive decompression. These conditions can lead to large thermal contractions of both the lining and substrate that may result in high stress of the lining, embrittlement, cracking, and premature failure.
Also, if there is any water permeation through the lining, the density of the trapped water will vary once the temperature reduces to sub-zero conditions, causing the trapped water to increase its volume, eventually leading to disbondment or actual break down of the lining system.
In order to assess whether or not a lining is resistant to the effects of rapid temperature reductions to sub-zero conditions, a sample can be subjected to low temperature thermal shock testing.
A lining will typically be applied to a grit-blasted mild steel panel, allowed to ambient cure, and then exposed to a temperature level equivalent to that of the high end of the thermal cycle to be employed. The sample is then controllably returned to ambient ensuring that the specimen is not exposed to any rapid changes in temperature. The sample is heated to the upper temperature of test, for instance 212°F (100°C). Once equilibrated at the higher temperature, it is instantly cooled down to the lowest temperature of test, for example, -130°F (-90°C).
This rapid thermal shock cycle is repeated an additional two times to subject the sample to a total of three rapid and extreme heating and cooling exposures. Upon completion of the test, the lining is inspected for any signs of failure such as cracking or delamination. (Learn more about delamination and disbondment in Solving Multilayer Coating Delamination Issues During Pipeline Installation.) If no signs of failure are noted, the lining system is deemed resistant to low temperature thermal shock within the tested temperature range.
Chemical Resistance of Surface Linings
For any lining, the ability to resist commonly encountered chemicals in the target industry is a requirement. In the oil and gas industry some of these chemicals include, but are not limited to:
- Crude oil hydrocarbon constituents, such as paraffins, olefins and aromatics
- Crude oil non-hydrocarbon constituents, such as sulfur, oxygen and nitrogen compounds
- Inorganic compounds, such as water, hydrocyanic acid, hydrogen sulfide, hydrogen chloride, sulfuric acid, hydrofluoric acid, carbon dioxide, alkalis and mineral salts
- Organic compounds, such as halocarbons, toluene, methanol, glycols, aldehydes, phenols, amines and nitriles
Chemicals interact in different ways with linings. Some will be particularly reactive and aggressive towards linings leading to rapid attack, whereas others will be benign, causing little or no attack. The resistance of linings to chemicals will ultimately be dependent upon the particular chemical’s concentration, physical state (solid, gas or liquid), temperature and immersion time, among others. (More information about the chemicals typically encountered in the oil and gas industry is explored in The Science behind Oil and Natural Gas Pipeline Corrosion and Coatings.)
A common manner to test the chemical resistance of linings is typically assessed via immersion testing in accordance with ISO 2812-1.1 This standard specifies general methods for determining the resistance of coating materials to the effects of other liquids rather than water. Blasted mild steel rods are coated with the coating system to be tested and immersed in the chemical at ambient or elevated temperature levels (Figure 1).
Figure 1: Chemical testing at ambient and high temperature levels
The coated rods are periodically reviewed for any sign of damage in the form of erosion, blistering, cracking or delamination (Figure 2).
Figure 2: Chemical resistance rods in various conditions
The test is typically run for 52 weeks. The observations coupled with the length of the coating's exposure to the chemical are used to assign a chemical resistance rating. As a result, at least one coating manufacturer has adopted a chemical resistance rating of Poor, Moderate, Good and/or Excellent (Table 1). An Excellent rating is required for a lining to be recommended as a vessel lining.
Table 1: Chemical Resistance Rating
Abrasion and Erosion Resistance
ASTM International defines erosion as “damage to a solid surface (generally involving progressive loss of material), caused by the relative motion between the surface and a contacting substance or substances.”2
Mechanical abrasion or wear is a significant problem in the oil and gas industry. Separators in an oil and gas platform, for instance, suffer from abrasive wear due to sand impacting the lower sections. Catalyst movement in fluid catalytic cracking units and cavitation problems in pumps are other examples of wear situations in refineries. (See an Introduction to Pipeline Corrosion and Coatings for more information.)
Abrasive wear in the oil and gas industry is classified into three types:
- Gouging Abrasion: This type of mechanical damage is likely to be found associated with high angle impacts with respect to the substrate.
- Grinding Abrasion: This type of mechanical damage pulverizes fragments of the abrasive substance that then becomes sandwiched between mating metal surfaces.
- Sliding Erosion: This type of mechanical damage is likely to be associated with low angle impacts with respect to the substrate, more of a low stress, scratching abrasive action.
As all of the aforementioned damages are of a mechanical nature, a lining system cannot completely stop the abrasive action and consequent deterioration from occurring, but it can help by slowing down the process. In a lined vessel where abrasion is expected as part of the normal operation, the life expectancy of the lining will be dictated by its resistance to the erosive mechanism.
Although laboratory tests can potentially provide considerable insight into the various factors that contribute to a lining’s resistance to erosion, most of them do not exactly replicate the mechanisms seen in the field. Furthermore, there are a great number of different tests but none is definite and data is generally not comparable among different testing protocols.
Notwithstanding the previous statement, it is the author’s opinion that a meaningful test program employing a number of abrasion mechanisms should allow for a decent approximation of field conditions. Some of these tests include Taber sliding resistance, slurry erosion resilience and impingement resistance:
- Taber sliding erosion resistance quantitatively measures the degree of abrasion produced by the Taber Abraser on a pre-weighed cast disc of the cured lining, in accordance with ASTM D4060.3 A cast specimen disc of the lining is placed on a turntable tray of a Taber abrader (Figure 3). The tray is filled with water to simulate the abrasion associated with the transport of fluids. It is then rotated perpendicularly against a pair of abrasive wheels under load. A circular track is worn in the specimen disc by the action of the abrasive wheels and erosion resistance is determined by volume loss per 1000 cycles.
- Slurry resilience testing can be carried out in accordance with a modified accelerated test version of ASTM G6.4 The apparatus (Figure 4) reproduces the environment experienced by linings exposed to highly abrasive slurries, where the entrained solids slide and cascade against the coated specimen rods resulting in both sliding and impact abrasion. The specimen rods are weighed, and the assembled drum is then rotated at 100 rpm for 100 hours exposing the coated rods to abrasive wear. Following the test cycle, the slurry abrader is disassembled and the coated rods washed, dried, and inspected for signs of excessive wear and failure. A measure of erosion resistance can be determined from the calculated volume loss. (Discover ways to address erosion corrosion in Erosion Corrosion: Coatings and Other Preventive Measures.)
- Impact erosion can be measured via a jet impingement test. This test is intended to simulate high-speed impingement at pipeline elbows, protrusions, bends, and corners. A cast specimen disc of cured epoxy lining is weighed and submerged in a jet impingement tank (Figure 5). A jet of salt water solution containing solids, e.g., quartz sand at 2,100 - 2,300 mg/L loading, is directed from a nozzle tip towards the submerged specimen disc at a given velocity, 20 m/s for instance. The angle of impingement can be varied, but will typically be set at 90° for maximum impact. The temperature of the fluid can be increased up to 158°F (70°C) for testing different operating conditions. This high velocity jet on the specimen is comparable to the impingement experienced below vessel jet wash systems. This test is normally carried out over 50 hours and the resulting volume lost to erosion is measured.
Figure 3: ASTM D4060 Taber Abrader Test
Figure 4: ASTM G6 Slurry Drum Abrasion Test
Figure 5: Jet Impingement Testing
The test methods described above are demonstrative of the numerous available tests for laboratories to benchmark lining materials. All the test data must be available for the asset owner to review. It is important when assessing materials from different manufacturers that before the results are compared, the test methods and data presentation are checked and confirmed to be equivalent.
Once the vessel owner has opted for the use of lining systems for internal protection of their asset, the choice of material must be confirmed with the material manufacturer to reduce the risk of incorrect material selection. For this purpose, original equipment design data and expected future operational conditions must be available for the lining manufacturer to review. At least one lining manufacturer utilizes a document referred to as an “Application Survey Report” that contains all the necessary information for selecting the right materials.
The data provided by the asset owner should include but are not limited to the following:
- Details about the asset owner: Asset owner’s name and contact details, offshore or onshore asset, asset location
- Equipment: Type or duty, e.g., separator, knock out drum, heat exchanger
- Dimensions: T-T, diameter, size and number of nozzles
- Process conditions: Design and operational parameters such as temperature and pressure, cleaning process (steam-out or chemical cleaning), process chemicals
- Substrate: Carbon steel or other, condition of the substrate
- Maintenance details, if not new build: Previous protection alternatives
The lining manufacturer can then review all of this information and recommend a fit-for-service lining system.
- ISO 2812-1:2007, “Paints and Varnishes -- Determination of Resistance to Liquids -- Part 1: Immersion in Liquids Other than Water (Geneva, Switzerland: ISO).
- ASTM G40-15, “Standard Terminology Relating to Wear and Erosion” (West Conshohocken, PA: ASTM International).
- ASTM D4060-14, “Standard Test Method for Abrasion Resistance of Organic Coatings by the Taber Abraser” (West Conshohocken, PA: ASTM International).
- ASTM G6–07(2013), “Standard Test Method for Abrasion Resistance of Pipeline Coatings” (West Conshohocken, PA: ASTM International).