High temperature crude oil testing of tank linings

Abstract

Different methods of testing tank linings with regards to high temperature crude oil have been discussed. The perhaps most suitable and realistic test method is the one described in the NACE TM-0174 standard. Two Novolac epoxy tank linings, one solvent borne and one solvent free, was exposed to acidic crude oil for 180 days at 160°C according to the NACE TM-0174 standard. The solvent borne tank lining was applied in 3 coats of 100 µm dry film thickness (DFT) and the solvent free tank lining was applied in one coat of 300 µm DFT. Both coatings passed the test without visible failure and with good adhesion test results (ISO 4624). In conclusion, today’s coating systems will be able to handle the ever increasing demand for high temperature crude oil exposure.

Introduction and current standards/test methods

High temperature crude oil testing can basically be done in two ways; static or cyclic. Static testing implies that the coated test panels are exposed to crude oil at a set temperature for a specific number of days. In cyclic testing one usually vary the temperature during the test period, often from the extreme cold to the extreme hot.

Static testing in itself can be done in different ways. Here the simplest way is to immerse coated steel panels in jars containing crude oil and placing these jars in heated cabinets. The panels can be half or fully immersed depending on whether one is interested in the gas phase exposure. When it comes to crude oil care must be taken as the low boiling components might cause a dangerous build-up of pressure. It goes without saying that this kind of testing cannot be done with the highest temperatures, at least from a safety viewpoint.

When using the 'jar method', the panels face the same temperature from all sides. In life size storage tanks the outside of the tank is exposed to the cold of the surrounding air. This temperature difference causes a cold-wall effect which accelerates the permeation of chemical constituents of crude oil into the coating.

And this brings us to the more realistic immersion test, based on the standard NACE TM-0174. This standard describes the use of glass cells where two panels are fixed to a tubular glass cell. The panels used are depicted in Figure 1. Figure 2 shows the test cell with its basic components. In this test only one side of each steel panel is coated and exposed to the heated fluid, leaving the uncoated side exposed to air in ambient conditions. This will cause the cold-wall effect and temperature flux in the steel which makes this the test of choice when it comes to best simulate real life conditions. In this test there will always be a part of the panel which is exposed to the gas phase in the cell. In some cases the exposure conditions in the gas phase are more aggressive than in the liquid phase, as the substances found here are smaller molecules which easier migrate into the paint film. Also, this area is more exposed to water vapours and water condensation, especially at oil temperatures of around or below 100°C. Another factor the coating has to withstand is the continuous stream of distilled water coming down from the condenser. Experience has shown that when problems (blistering) first occur, it occurs in the transition between gas and liquid, either just above or just below the liquid surface (or both).

Figure 1. NACE TM0174 test panel schematic. Quadratic panels of dimension 200 x 200 x 5 mm are usually being used. The holes in the panels are used to fix the two panels together with the glass cell (see also Figure 2). The panel on the right show what area of the panel is actually being exposed to the crude oil during the test period.

Suitable paint technology for high temperature crude oil exposure

Crude oil is not amongst the most aggressive chemicals / mixtures and most two-component epoxy coatings can easily handle ‘normal’ crude oil exposure at ambient temperatures. Other requirements are needed for more acidic crude oils at higher temperatures.

When both chemical resistance and heat resistance is required at once, such as is needed when handling an acidic crude oil at high temperatures, a Novolac epoxy based coating is the number one choice. The main technology base for a tank lining is epoxy with varying degrees of quality, ranging from ‘standard’ solid bisphenol-A based epoxy to higher functionality Novolac epoxy resins. For crude oil exposure at lower temperatures, and for crudes with low acidity, solid epoxy based tank linings might do the job. With higher temperatures however, above 90°C, a Novolac resin based tank lining should be used. These epoxy resins are typically cured with some kind of cycloaliphatic or benzyl amine, with at least two reactive amine hydrogens, in order to get sufficient crosslinking.

Another important factor to define is the DFT. For a solvent borne epoxy it is important to not apply too much in order to avoid trapped solvents which can cause problems later on. For a solvent borne epoxy tank lining the recommended DFT would be 2 x 125 µm (or 3 x 100 µm if one prefers to have three coats). For a solvent free epoxy tank lining there is not the same concern with regards to solvent entrapment. One needs only to apply a sufficiently thick coat to give protection. A DFT of 1 x 300 µm or 2 x 200 µm should give adequate protection.

Figure 2. Test cell fitted with coated steel panel. The glass cell is filled up so that the liquid level is just beneath the heater / thermometer intake.

Test results and discussions

Two coatings, with the technical composition described in section 2, were tested according to the standard NACE TM-0174 with a crude oil to which was added 2% water and 1 ml of sulphuric acid/litre crude oil. The solvent borne Novolac epoxy tank lining was applied in three coats of 100 µm DFT. The first coat was light red, the second coat was yellow and the third coat was light grey. The solvent free Novolac epoxy tank lining was applied in one coat of 300 µm DFT (grey colour, seen where the Teflon ring was in Figure 4).

The coated panels were cured for 7 days at 23°C and then exposed to the mentioned crude oil mixture for 180 days at 160°C. The surrounding temperature varied between 18°C and 22°C. After 180 days the cells were dismantled and pull-off adhesion was tested according to ISO 4624 (Paints and varnishes – Pull-off test for adhesion).

Both panels, after exposure and pull-off testing, can be seen in Figures 3 and 4.

For the solvent borne tank lining it can clearly be seen from the pull-off holidays that the break is cohesive and in the first (light red) coat. For the solvent free tank lining the adhesion breaks are mostly cohesive except for some adhesive failure between coating and the epoxy adhesive in the gas phase.

Both coatings show some discolouration, with the solvent free type being worse than the solvent borne. Colour retention, however, is not considered a critical factor in this regard. Some discolouration is to be expected after half a year’s exposure to crude oil at 160°C.

Figure 3. A solvent borne novolac epoxy tank lining applied in three coats of 100 µm. The coating was exposed to acidic crude oil at 160°C.

Figure 4. A solvent free novolac epoxy tank lining applied in one coat of 300 µm. The coating was exposed to acidic crude oil at 160°C.

Read the article online at: https://www.hydrocarbonengineering.com/tanks-terminals/04102019/high-temperature-crude-oil-testing-of-tank-linings/