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Apr-2015

Linings for sulphur storage tanks

Flexible spray coatings combat the corrosion mechanisms of sulphur stored in steel tanks.

ANNA MICHAEL and JUAN LóPEZ GALERA
Belzona Polymerics
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Article Summary
Molten sulphur is produced and used in a wide range of industries and liquid sulphur storage tanks are used worldwide in natural gas plants as well as oil refineries to hold 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 in 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 significant progress with regard to the mechanical design of sulphur storage tanks, these structures are still plagued by corrosion issues and internal corrosion is considered to be the main cause of lifetime and safety issues. Unlike external corrosion that can be easily identified, internal corrosion is out of sight and can therefore go unnoticed, with catastrophic consequences. As a result of internal corrosion, a sulphur storage tank’s service life has been reported to be as low as five years, although in general storage tanks survive for around 30 years.

Failures in sulphur storage tanks can lead not only to loss of revenue and increased costs through downtime and replacement, but can also have an impact on human health and the environment. Due to the combustible and reactive nature of sulphur, leaking storage tanks can be a significant source of pollution to soil, groundwater, streams and lakes, resulting in contamination of drinking water. In addition, fire, explosion and inhalation of dangerous vapours present further concerns. Such events may result in asset owners facing strict financial penalties from regulatory institutions.

Highly corrosive environment
The corrosion mechanisms vary according to the design and service conditions, but the most common cause of 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 a severe attack on the carbon steel, especially when the hydrogen sulphide (H2S) concentration levels are high.

In order to keep sulphur in a liquid state, 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 surfaces, 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 H2S, will react with the solid sulphur and iron in the tank walls, creating the ideal environment for the formation of iron oxide (Fe2O3) and iron sulphide (FeS) that further accelerate corrosion.

When the tank is in an anaerobic environment, FeS will not cause any hazards. However, when the tanks are opened to the atmosphere for inspection, cleaning and maintenance, FeS, being a pyrophoric material, can spontaneously combust in the presence of oxygen, igniting a sulphur fire or an explosion.

Despite the fact that there are fabrication codes for the construction of this type of tank, these codes do not include sufficient mitigation for the onset of corrosion. Therefore, corrosion problems in sulphur tanks remain unsolved and corrosion control and mitigation measures are left with the operator of the tank.

High temperature linings
Field experience has shown that corrosion mechanisms and conditions can be minimised or eliminated by employing protective internal linings. To ensure a successful outcome, the protective materials should demonstrate chemical resistance against direct exposure to acidic conditions at high temperatures.

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, following a field trial programme. Over the following 16 years, the R&D department analysed data from the field and researched technologies and filler systems that could enhance material characteristics and in-service performance that would allow for the coating to resist elevated temperatures, but at the same time remain flexible to sufficiently minimise the risk of cracking. This research has culminated in the introduction in 2014 of high temperature vessel linings, hand applied Belzona 1593 and spray applied Belzona 1523.

Increased flexibility
Belzona 1523 and Belzona 1593 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. These 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 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 attacking reactive molecules to permeate through the film. Consequently, the materials demonstrate high resistance to liquid sulphur, sulphur dioxide (SO2) and 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 high temperature immersion resistance can make conventional materials rigid and susceptible to cracking during thermal cycling and substrate flexing (see Figure 1). The new coatings overcome this by the incorporation of rubbery domains that enhance flexibility and inhibit crack propagation (see Figure 2).
 
No failure caused by solvent retention
The coatings demonstrate a high level of adhesion. Belzona 1523 exhibits a tensile strength of 13.7 MPa and elongation rate of 0.54% when cured and tested at 100°C, whilst Belzona 1593 exhibits a tensile strength of 11.2 MPa and elongation rate of 0.31% when cured and tested at 160°C (see Figure 3). Since the materials can be deformed when under radial, circumferential and longitudinal stress, they preserve their integrity, move in sympathy with the substrate, reducing material ruptures, breaks and fissures.

It has been shown 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 new coatings since they are solvent free.

Application methodology and inspection

These linings address an application issue previously experienced when applying a two coat system of high temperature linings: to by-pass the need for grit blasting between coats, provided the second coat is applied within a 24-hour overcoat window. In addition, the curing mechanism of Belzona the coatings, activated at ambient temperatures, eliminates the need for separate post curing processes as the lining will post cure in service for a faster turnaround.
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