Fibre-reinforced plastic tanks and vessels

Fibreglass Reinforced Plastics (FRP), often referred to as Glass Reinforced Plastics (GRP), represents a cutting-edge composite material that is increasingly utilized in the construction of tanks and vessels across various sectors, including chemical manufacturing, paper production, and food and pharmaceutical industries. These chemical equipment units can range in scale from under one meter to an impressive twenty meters, all crafted from FRP.

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The creation of FRP chemical equipment primarily employs techniques such as Hand Lay-up and filament winding, with British Standards (BS) playing a crucial role in governing these applications.

Dual Laminate Structure

The corrosion-resistant qualities of FRP enable the construction of tanks either entirely from this composite material or with an additional liner made from different materials. This configuration, referred to as dual laminate, involves an inner liner composed of distinct material properties than those of the structural composite. The term "dual" indicates the two-layer system, while "laminate" refers to the layered composite design.

When the liner is made of FRP, it is typically designed to be resin-rich, utilizing "C-Glass" for its construction, while the structural component is constructed from "E-Glass." A thermoplastic liner is usually implemented at a thickness of approximately 2.3 mm (100 mils) and does not contribute to the mechanical strength of the tank. The curing of the FRP liner generally precedes further manufacturing processes, either through a BPO/DMA system or MEKP catalyst combined with cobalt in the resin.

If the inner liner is not composed of FRP, there are several thermoplastic material options available, and the design must account for the specific chemical corrosion resistance needed. Common liners include PP, PVC, PTFE, ECTFE, ETFE, FEP, CPVC, and PVDF.

Given its vulnerability to buckling, FRP is particularly strong in resisting tensile forces and corrosion, making it particularly suitable for hydrostatic tanks. The fibers within the tank’s construction are oriented tangentially to absorb hydrostatic pressure, significantly enhancing hoop strength and allowing these tanks to be stronger than steel on a weight-for-weight basis.

The FRP layer over the liner furnishes the necessary structural integrity to handle various design conditions, including internal pressure, vacuum, hydrostatic forces, seismic loads, wind strain, and even snow loads.

Applications of FRP Tanks

FRP tanks and vessels designed according to BS standards have become indispensable in the chemical sector across numerous industries, including chlor-alkali production, fertilizer manufacturing, wood pulp and paper processing, metal extraction, refining, electroplating, brine processing, vinegar production, food preparation, and air pollution control, particularly in municipal wastewater treatment facilities.

Diverse Types of FRP Tanks

FRP tanks and vessels serve a multitude of purposes in various commercial and industrial settings, encompassing chemical processing, water and wastewater treatment, the food and beverage industry, mining and metals, energy production, and high-purity applications.

FRP Scrubbers

FRP scrubbers play a critical role in removing pollutants from scrubbing fluids. In the context of air pollution control, these scrubbers can be classified into three main categories: Dry Media, Wet Media, and Biological.

Dry Media Scrubbers

Dry media scrubbers utilize a solid media such as activated carbon, maintained in the middle of the vessel by support beams and grating. These systems operate through adsorption and absorption to control pollutant concentrations in incoming gases.

Several design factors must be taken into consideration, including:

• Loading and unloading of the media
• Corrosive effects of the fluid being treated
• Internal and external pressure management
• Environmental load tolerances
• Support structures for the grating
• Installation and loading logistics
• Media regeneration strategies
• Support structures for dual bed configurations
• Preventive maintenance redundancy
• Demisting systems to remove detrimental liquids from the media
• Condensate drainage

Wet Media Scrubbers

Wet media scrubbers operate by saturating the polluted fluid with a scrubbing solution, requiring enhanced design criteria. Key design considerations include:

• The corrosiveness of both the pollutant and scrubbing solution
• High-pressure and loading factors of the spraying system
• Aerodynamic considerations to minimize bypassing
• Internal support structures
• Reservoirs for recirculating scrubbing fluid
• Environmental load capacities
• Installation procedures and plumbing logistics
• Sump fluid drainage mechanisms

In systems like decarbonators used in reverse osmosis, the spraying liquids act as scrubbing agents, stripping gases from the water to be processed elsewhere.

Biological Scrubbers

Structurally similar to wet media scrubbers, biological scrubbers have larger designs enabling slower air movement through the system, promoting biological growth as the water, infused with nutrients, circulates through the vessel. In these setups, bacteria effectively cleanse the pollutants.

FRP Storage Tanks

A standard FRP tank typically features an inlet, outlet, vent, access points, drainage features, and overflow nozzles. Additional elements such as external ladders for easy access and slopes to facilitate drainage may also be included. The tank's design must withstand the weight of personnel accessing the top and be resistant to the corrosive nature of the stored fluids, often with an added secondary containment layer in case of structural failure.

Tank Sizing Considerations

The manufacturing capabilities for FRP vessels are largely unaffected by size constraints, with economic factors being the primary consideration. Tanks under 7,500 liters (approximately 2,000 gallons) are often constructed from more affordable materials like HDPE or PVC. Larger tanks, especially those exceeding four meters in diameter, may confront shipping limitations, prompting the use of alternative construction methods at the site utilizing concrete or steel.

For chemical storage and air quality management, forming multiple smaller tanks is advantageous. For instance, in California, the Orange County Sanitation District is set to deploy 24 vessels to treat a flow rate of 188,300 cfm (or 86,200 L/s) of odorous air, necessitating a design accommodating up to 50 ppm hydrogen sulfide concentrations. Designing a single tank to achieve equivalent treatment would necessitate an impractically large diameter of over 36 feet, leading to significant challenges in shipping, structural supports, and maintenance.

FRP Construct Limitations

Typical limitations in FRP vessel design are dictated by application requirements and the specific resins employed. Thermoplastic resins can experience creep under high-temperature conditions, potentially leading to failure. Innovations in resin chemistry have introduced materials capable of withstanding greater temperatures, significantly broadening their application range, with a typical high limit being around 200 degrees Celsius.

Moreover, fiberglass structures are sensitive to long-term UV exposure, leading to degradation from chemical reactions due to direct sunlight. This deterioration can cause surface damage that weakens the tanks, possibly resulting in reduced impact resistance and elongation characteristics. Utilizing external gel coats and sealants helps to mitigate UV damage by blocking harmful radiation from reaching the fiberglass.

The lifespan of the UV protection is influenced by factors such as resin composition, thickness, design characteristics, and pigment properties. Understanding UV performance metrics is crucial for selecting the right material for outdoor applications. For instance, nominal UV resistance ratings like UV-8 indicate durability for a duration equivalent to 4 years of standard exposure.

Protecting fiberglass substrates from UV degeneration can be likened to applying sunscreen to prevent skin damage, with gel coats functioning as protective barriers for tanks and other fiberglass constructs.

Standards and Regulations

Fiberglass tanks are regulated by several certifications, including:

• BS-87, the British standard superseded by EN standards for FRP tanks and vessels.
• ASME RTP-1, which governs reinforced thermoset plastic equipment across the United States for specific pressure and environmental conditions.

Key design standards and compliance requirements generally involve adherence to standards such as ASME RTP-1 or recognition from ASME, with ASTM specifications providing guidelines for the filament winding process rather than design mandates.

British Standards (BS)

The introduction of "unit properties" in BS standards alleviates uncertainties linked to merely specifying thickness, expressing materials in terms of performance per unit weight of reinforcement.

ASME RTP-1 Specifications

The ASME RTP-1 specifications focus on stresses linked to the physical characteristics of the composite selected, ensuring suitable designs for operational pressures and loads.

BS EN Standards

These European standards have replaced the older BS-87 specifications, ensuring current compliance markers for fiberglass tank construction.

References

1. ^Newberry, Alfred L.; Putri, Gunung (-11-18). "World's largest FRP acid storage tanks". Reinforced Plastics. 49 (10): 26-29.
2. ^[1] Page 12, Plant 2 Headworks Facility
3. ^Carollo Engineers , Orange County Sanitation District Plant No. 2 Headworks Replacement (Job No. P2-66) Specification D.1.3.A.3
4. ^Numerical calculations related to tank sizes and air handling, indicating practical dimensions and efficiencies of multiple tank installations.
5. ^Raventank.com

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