FRP tanks and vessels

FRP (Fibreglass Reinforced Plastics, also known as GRP, or Glass Reinforced Plastics) is a modern composite material of construction for chemical plant equipment like tanks and vessels. Chemical equipment that range in size from less than a metre to 20 metres [1] are fabricated using FRP as material of construction.

FRP Chemical Equipments are manufactured mainly by Hand Lay-up and filament winding processes. BS4994 still remains a key standard for this class of items.

Dual Laminate

Due to the corrosion resistant nature of FRP, the tank can be made entirely from the composite, or a second liner can be used. In either case, the inner liner is made using different material properties than the structural portion (Hence the name dual (meaning two) and laminate (a word commonly used for a layer of a composite material))

The liner, if made of FRP is usually resin rich and utilizes a different type of glass, called "C-Glass", while the structural portion uses "E-Glass". The thermoplastic liner is usually 2.3 mm thick (100 mils). This thermoplastic liner is not considered to contribute mechanical strength. The FRP liner is usually cured before winding or lay-up continues, by using either a BPO/DMA system, or using an MEKP catalyst with cobalt in the resin.

If the liner is not made of FRP, there are multiple choices for a thermoplastic liner. The engineer will need to design the tank based on the chemical corrosion requirement of the equipment. PP, PVC, PTFE, ECTFE, ETFE, FEP, CPVC, PVDF are used as common thermoplastic liners.

Due to FRP's weakness to buckling, but immense strength against tensile forces and its resistance to corrosion, a hydrostatic tank is a logical application for the composite. The tank is designed to withstand the hydrostatic forces required by orienting the fibres in the tangential direction. This increases the hoop strength, making the tanks anisotropically stronger than steel (pound per pound).

FRP which is constructed over the liner provides the structural strength requirements to withstand design conditions such as internal pressure or vacuum, hydrostatic loads, seismic loads (including fluid sloshing), wind loads, regeneration hydrostatic loads, and even snow loads.

Applications

FRP tanks and vessels designed as per BS 4994 are widely used in the chemical industry in the following sectors: chlor-alkali manufacturers, fertilizer, wood pulp and paper, metal extraction, refining, electroplating, brine, vinegar, food processing, and in air pollution control equipment, especially at municipal waste water treatment plants and water treatment plants.

Types

FRP Vessels and Tanks are used in multiple applications, requiring a strong, corrosion resistant environment.

Scrubbers

FRP Scrubbers are used for scrubbing fluids. In air pollution control technology, scrubbers come in three varieties, Dry Media, Wet Media, and Biological.

Dry Media

Dry media typically involved a dry, solid media (such as activated carbon) suspended in the middle of the vessel on a system of beam supports and grating. The media controls the concentration of a pollutant in the incoming gas via adsorption and absorption.

These vessels have several design constraints. They must be designed for

Wet media

Wet media scrubbers typically douse the polluted fluid in a scrubbing solution. These vessels must be designed to more stringent criteria. The design constraints for wet media scrubbers typically include:

In the case of a decarbonator, used in reverse osmosis systems to limit the concentration of gases in the water, the air is the scrubbing fluid and the sprayed liquid is the polluted stream. As the water is sprayed out of the scrubber, the air strips the aqueous gasses out of the water, to be treated in another vessel.

Biological

Biological scrubbers are structurally identical to the wet media scrubbers, but vary in their design. The vessel is designed to be larger, so the air moves slower through the vessel. The media is designed to encourage biological growth, and the water that sprays through the vessel is filled with nutrients to encourage bacteria to grow. In such scrubbers, the bacteria scrub the pollutant. Also, instead of a single, large support system (typically 10 feet depth of media for chemical scrubbers), there are multiple stages of media support, that can change the design requirements of the vessel. (See biofilter for similar technology that is usually performed outside of an FRP vessel.)

Tanks

A typical storage tank made of FRP has an inlet, an outlet, a vent, an access port, a drain, and an overflow nozzle. However, there are other features that can be included in the tank. Ladders on the outside allow for easy access to the roof for loading. The vessel must be designed to withstand the load of someone standing on these ladders, and even withstand a person standing on the roof. Sloped bottoms allow for easier draining. Level gauges allow someone to accurately read the liquid level in the tank. The vessel must be resistant to the corrosive nature of the fluid it contains. Typically, these vessels have a secondary containment structure, in case the vessel bursts.

Size

The size of FRP Vessels is rarely limited by manufacturing technology, but rather by economics. Tanks smaller than 7,500 liters (2,000 gallons) are easily manufactured out of cheaper materials, such as HDPE or PVC. Tanks larger than four meters are generally limited by shipping constraints, and the economics suggest a concrete or steel tank fabricated at the tank's location.

For chemical storage and air pollution control, the choice is to make multiple tanks of smaller diameters. For example, one of the largest odor control projects in California, the Orange County Sanitation District will utilize 24[2] vessels total to treat 188,300 cfm (86,200 L/s) of odorous air, with a design of up to 50 ppm of hydrogen sulfide.[3] For an equivalent single vessel to perform as well as the 13 headworks trickling filters, the single vessel would have to be over 36 feet in diameter.[4] This would be impractical due to the high shipping requirements, internal supports, spray nozzles and other internals. Plus this single vessel would not incorporate redundancy for preventive maintenance.

Limitations

Typical FRP temperature limits are almost entirely based on the resin. The thermoplastic resin will suffer from creep at elevated temperatures and ultimately fail. However, new chemistry has produced resins that claim to be able to achieve even higher temperatures, which expand this field immensely. The typical maximum is 110 degrees Celsius.

Design standards

GRP Tanks fall under regulation of several groups.

Bs4994

It is to avoid the uncertainty associated with specifying the thickness alone, that BS4994 introduced the concept of "unit properties". It is property per unit width, per unit mass of reinforcement. For example, UNIT STRENGTH is defined as load in Newton per millimeter (of laminate width) for a layer consisting of 1 kg of glass per square meter. i.e. the unit is N/mm per Kg/m2 glass

ASME RTP-1

In RTP-1 specifications, the primary concerns relate stress and strain, such as hoop stress, axial stress, and breaking stress to the physical properties of the material, such as Young's modulus (which may require an anisotropic analysis due to the filament winding process). These are related to the loads of the design, such as the internal pressure and strain.

BS EN 13121

This European standard replaces BS4994-87 which is marked now as Current, Obsolescent, Superseded.

SS245:1995

This is the Singapore Standard for sectional GRP water tank, which is current.

See also

References

  1. ↑ "World's largest FRP acid storage tanks". Reinforced Plastics 49: 26–29. 2005-11-18. doi:10.1016/s0034-3617(05)70798-0.
  2. ↑ Page 12, Plant 2 Headworks Facility
  3. ↑ Carollo Engineers, Orange County Sanitation District Plant No. 2 Headworks Replacement (Job No. P2-66) Specification 11395D.1.3.A.3
  4. ↑ Since area must be maintained to keep velocity reduced, {\sqrt{13*{{\pi}\over{4}}*10^2} \over {\pi \over 4}} = 36.055 ft

Further reading

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