Protecting sulphur storage tanks

Paul Boughton

Juan Lopez Galera and Anna Michael outline the merits of high-temperature linings for sulphur storage tanks

Molten sulphur is present in an ever-growing range of industries and liquid sulphur storage tanks are used worldwide in crude oil refineries and natural gas plants to store liquid sulphur in very large volumes. Sulphur storage tanks are most commonly utilised as part of the gas treating system in sour crude oil refineries and gas sweetening facilities to temporarily store liquid sulphur produced in the sulphur recovery plant. These tanks are usually field erected and most commonly constructed of carbon steel.

Even though in recent years there has been major progress with regards to the mechanical design of sulphur storage tanks, they are still plagued with corrosion issues and internal corrosion is considered to be the main cause of longevity and safety issues. Unlike external corrosion that can be easily identified, internal corrosion is out of sight and can therefore go unnoticed, causing catastrophic consequences. As a result of internal corrosion, sulphur storage tank service life has been reported to be as low as five years, although general storage tanks demonstrate a life of 30 years.

Sulphur storage tank failures not only lead to loss of revenue and increased costs through downtime and replacement, they can also have a critical human health and environmental impact.

Corrosive environment

The corrosion mechanisms vary according to the design and service conditions, but the most common cause for internal corrosion is the deposition of solid sulphur on the interior surfaces of the tank together with the presence of liquid water. The combination of these two components creates the phenomenon of wet sulphur corrosion that can cause severe attack to the carbon steel, especially when the hydrogen sulphide (H2S) concentration levels are high.

To keep the sulphur in a liquid state, the storage tanks are heated at a temperature between 257°F (125°C) and 293˚F (145˚C). Insufficient heating and external climatic conditions, in combination with missing insulation will cause temperature variations within the tank. Failure to maintain the desired temperature at the steel surfaces in the vapour space of the tank will lead to the solidification of the sulphur fog. The concentration of solid sulphur at the interior side walls, the roof and the vent nozzles will then cause severe corrosion that will propagate in depth and length.

After solidifying on the surface, the sulphur will act as an insulator contributing to further cooling of the surfaces. As the temperature continues to fall, traces of condensed water, formed by oxidation of hydrogen sulfide, will react with the solid sulphur and the iron from the tank walls, creating the ideal environment for the formation of iron oxide (Fe2O3) and iron suphide (FeS) that further accelerate corrosion.  

Protective linings

Field experience has shown that the corrosion mechanisms and conditions can be minimised or eliminated by employing protective internal linings.

The first step of Belzona’s high temperature lining research project was the introduction of hand-applied Belzona 1591 (Ceramic XHT) in 1998, and spray-applied Belzona 1521 (HTS1) in 1999. Over the following 16 years, the company’s R&D department analysed data from the field and researched innovative technologies and filler systems. This research has culminated in the introduction of its next generation of high-temperature vessel linings in March 2014, hand-applied Belzona 1593 and spray-applied Belzona 1523.

These two epoxy linings are designed to provide long-term corrosion and chemical resistance to equipment operating in continuous immersion at temperatures up to 140°C and 160°C, respectively. The two-part materials consist of an epoxy novolac base and a polyamine solidifier that, when mixed and cured, produce a very tightly cross-linked density.

The lining’s network is additionally supplemented by a novel secondary cross-linking mechanism initiated at temperatures above 90°C that further increases the cross-link density of the polymer matrix, making it even more difficult for the attacking reactive molecules to permeate through the film. Consequently, the materials demonstrate excellent resistance to liquid sulphur, sulphur dioxide (SO2) and hydrogen sulphide (H2S), as well as to the small amount of sulphuric acid (H2SO4) that may be present in a sulphur storage tank.

The high cross-link density required for coatings to achieve their high temperature immersion resistance can make conventional materials rigid and susceptible to cracking during thermal cycling and substrate flexing. Belzona 1523 and Belzona 1593 overcome this by the incorporation of rubbery domains that offer flexibility and inhibit crack propagation.

Excellent adhesion

The two linings demonstrate excellent adhesion. Belzona 1523 exhibits a tensile strength of 13.7 MPa and elongation rate of 0.54% when cured and tested at 100°C, while Belzona 1593 exhibits a tensile strength of 11.2 MPa and elongation rate of 0.31% when cured and tested at 160°C.

Since the materials can be deformed when under radial, circumferential and longitudinal stress, they preserve their integrity, move in sympathy with the substrate, and reduce material ruptures, breaks and fissures.

It has been proven that during the application of solvent-based coatings, issues can arise due to solvent retention within the film. In this case, solvent can be trapped within the applied linings and eventually evaporate leaving behind a void, which can then be filled by the system fluids causing bubbling and blistering. This is not the case with the two new linings since they are solvent free and thus also environmentally friendly.

Juan Lopez Galera and Anna Micahel are with Belzona Polymerics

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