Across all industrial sectors, tanks are used to store, separate, classify, distil or process liquid solutions that either serve as the product sold in the marketplace, a raw material used to make the product or a by-product that may be sold in a secondary market. Tanks come in all sorts of shapes and sizes and the materials of constructions varies from mild steel to fiberglass reinforced composite tanks (aka GRP tanks or FRP tanks) to concrete and also a variety of types of metal alloys and stainless steels.
In many cases, products that are stored inside the tanks may chemically react to the substrate used to construct the tank – and therefore corrode the tank wall. The reaction of the given product stored within the tank and the materials used to construct the tank is based on an ion exchange, molecule to molecule. In the case of an oxidation or corrosion reaction, the ion exchange will reduce the thickness of the tank wall or floor substrate and produce metallic salts by-product.
Sometimes, this reaction is gradual, taking many years to reduce the thickness of the tank walls. In this case, industrial asset owners may elect to combat the potential for corrosion by designing the thickness of the walls to allow for this general loss over a given period of time – this is known as corrosion allowance.
However, in certain circumstances, an industrial owner may recognize that the corrosion potential of a given service environment is high enough that it makes fiscal sense to design for the tank to be lined with a coating or membrane that will act as a protective barrier between the product stored or processed within the tank and the given substrate of which the tank is constructed.
When faced with a situation where he / or she must line a tank, the asset owner must decide on what lining to specify based on a seemingly intimidating array of factors to consider. Some of the more prominent factors to consider when determining the best tank lining are as follows:
In the case of new construction or a new tank for a new process, we find that customers value the initial cost of the lining above all other factors. In most cases, it is because the value of corrosion protection is under appreciated until a given customer experiences the pain of its effects – e.g. unplanned outages, tainted product, EHS (environmental, health, safety), increased maintenance costs, etc.
What we like to present is the life cycle costs associated with a tank lining. Life cycle cost is the cost of a lining (or lack thereof) over the course of a given time frame that represents the proposed design life of the tank. In this way, the customer can seek to determine the best lining system for a given service environment.
- Temperature Resistance
When choosing a coating, one must consider the glass transition temperature – Tg (crudely stated, “melting point”) of the resin system used in the lining or coating system. If the upper temperature limit of a coating is 180 F (82 C), one should know not to specify it for a system that normally operates at 170 F (76 C) and experiences spikes in upset conditions of 200 F (93 C). Doing so would result in a deformation of the lining, osmotic blistering and/or through film corrosions and, in the worst case, catastrophic failure.
In respect to prevention against failure, when specifying a coating or tank lining, consideration of the operating temperature and maximum operating temperature is critical. The chemical and physical resistance parameters of a coating or lining are variable on temperature – as the thermoplastic state of the lining may transition to a thermo-rubber phase thereby reducing the effect of its’ barrier properties.
- Vapor Permeation Rate
The rate at which a substance will travel through a coating is known as the permeation rate. The rate at which a vapor will permeate a coating is dependent on various factors including:
- Size of the molecule
- Physical driving force (e.g. pressure differential)
- Functionality of the resin system – i.e. how tightly the polymer chains are in the coating system
- Nature of the resin used (e.g. isophthalic polyester resin, novalac vinyl ester resin, phenolic epoxy resin, novalac epoxy resin, etc.)
- Type and quality of the resin used i.e. ceramic beads, silicon carbide, E glass flake, ECR glass flake
Generally, the slower vapor traverses through a coating film, the more effective that coating will act as a barrier lining to prevent corrosion on the substrate of a tank.
It is because of these factors, most of Corrocoat tank lining specifications utilize novalac vinyl ester resin systems and high quality ECR glass flake – ala Polyglass VEF and the Corroglass 600 series. These linings have a resistance to over 1200 chemicals, pH 1-13 at temperatures up to 230 F, immersed.
Of course, as no lining is a “miracle pill” to stop all corrosion in all service environments, we also have specialty coatings such as Corrothane XT (for high temperature service environments up to 300 F, immersed, 500 F non-immersed), Plasmet AR3 (resistant to 98% sulfuric acid), Corrocoat Zip E and Corrocoat SEL – proprietary blended epoxy lining systems for a variety of service environments. More information on how to specify coating using the aforementioned performance based specifications is available in this great video made by our founder and CEO, Charles Watkinson.
Other Factors to Consider
- Abrasion Resistance: Click for Video – The abrasion or wear associated with a given service environment relies on factors such as particle type, particle size, speed, angle of impact, wet or dry and many others. Abrasion resistant coatings must be able to be hard enough to resist the impact of abrasive material thus, preventing damage and be able to dissipate the energy caused by the impact thus, preventing damage.
- Cold Wall Effect: Click for Video
- Thermal Shock: If we are considering a rapid change from 20°C to 105°C, the difference is very large. We would experience a thermal shock of the coating. Coating failures can occur due to thermal shock. They are often characterized by an apparently sound coating film which has delaminated from the substrate, or by cracking of the coating. The problem is caused by an inability of the coating and the substrate to expand rapidly at the same rate.
- In 2017, Corrocoat USA solved a severe problem of thermal shock in a blast cabinet at a liquid nitrogen plant where the change in temperature is -320 F to 70 F in half a second. The temperature differential was causing severe spalling of the concrete below and the cause for significant maintenance work for the life of the plant. For more information about this solution, contact Corrocoat USA at email@example.com.
- Pressure: We must consider the working pressure and the maximum pressure of the environments we are considering. Pressure may have a direct bearing on the permeation into the coating. We must also consider the possibility of explosive decompression and finally, at very high pressures, the compressive strength of the coating.
- Explosive Decompression: This is more a physical attack than a chemical attack, although, it is caused by permeation. This problem is common in oil-gas separators when rapid decompression occasionally takes place. An agreement should be reached with the customer to ensure that after an extended service the pressure is released over a given period dependent upon the service pressure.
- Stresses due to movement: such as on the underside of Isotainers or at the tank floor-wall interface of storage tanks.
Tank Lining Application
After the owner and/or engineer have determined the best lining system to suit their tank, now they must pick a suitable contractor to line it. Effective tank lining contractors will have experience in surface preparation via abrasive blasting, coating application and maintain safety, quality control and environmental considerations in all that they do.
Tank lining is not for the faint of heart. In many cases, it involves working at heights, in a confined space, with dangerous and flammable chemicals under a tight schedule, while the plant is down for an outage or maintenance shutdown. Just because an industrial maintenance painter did a good job on adjacent structural steel, does not mean that they can line a process or storage tank during the next outage.
We have many mutually beneficial experiences teaming up with resident contractors to supply coating specifications, manufacture and supply the coating, and provide expertise, training and / or NACE CIP 1,2, or 3 certified inspection services for QAQC.
The right tank lining contractor will:
- Have experience and equipment suitable for abrasive blasting
- Have training to work in confined spaces and at heights – scaffolding, swing stages, rope access
- Often be local to the geographic area
- Experience applying thick film barrier coatings via airless spray pumps, by brush and by trowel
- Experience applying fiberglass laminate and fiberglass tank repairs
Still, in certain circumstances, it makes best sense to employ Corrocoat USA as a single service provider of the tank lining, inclusive of specification, manufacture, supply, surface preparation, application and inspection. Corrocoat self-employs a few crews capable of tank lining operations, which can be combined or partner with a resident contractor in order to tackle a larger tank lining project.
The advantage to the asset owner for hiring Corrocoat to take the whole project on is the benefit of granting a single source of responsibility for the integrity of the tank. In this case, we warranty the project with full responsibility for the integrity of the lining, the asset and our reputation. Over the past ten years, Corrocoat USA has supported and/or self-performed a variety of tank lining operations.
- Slurry Tanks
- Sand Slurry Grinder Tank
- Tank Lamination & Coating
- Chemical Dosing Tank
- Methanol Tank
- Sulfuric Acid Protection
- Arsenic Scrubber Tank
- Seawater Filter Vessel