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Knowledge for the Sulphuric Acid Industry

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Materials of Construction - FRP
June 19, 2003

Introduction

Fibreglass Reinforced Plastic (FRP) is used predominantly in the gas cleaning section of a metallurgical or acid regeneration plant for equipment, ducting, piping, dampers, pumps, etc.  FRP can only be used downstrream of the Quench Tower after which the gas has been adiabatically cooled, typically to less than 70°C.

Corrosion Layer

Fabrication of an FRP piece of equipment generally begins with the interior.  The interior surface will be in contact with the process and must be able to withstand the service conditions.  This layer is generally referred to as the corrosion liner.  A surfacing veil saturated with catalyzed resin forms the base for successive layers.  The corrosion layer generally consists of the surfacing veil and two layers of chopped strand mat.  As with metallic construction, the corrosion layer is not included in the total thickness when determining the wall thickness for mechanical strength.

The chemical resistance of a fibreglass composite is provided by the resin.  The surfacing veil is used to control the thickness of the resin-rich layer and gives strength to the corrosion layer.  The veil must also be resistant to the process conditions as it can be easily attacked since it is very close to the surface.  Where fluorides are present, a glass veil cannot be used since they will be readily attacked.  In this case, a veil made from polyester is used.

The corrosion layer will typically be 10 to 20 mils (0.254 to 0.508 mm) thick and contain about 90% resin to 10% veil material.

Monofilament glass or polyester fibres are commonly used as surfacing veil materials.  The most common are “C” Glass or Dacron 106 homopolymer (polyester).  “A” Glass and other polyester materials are less common but are gaining acceptance in the industry.

Typical veil materials are:

Structural Layer

Chopped Strand Mat

Chopped strand mat is a felted matrix of chopped strand “E” or “ECR” glass fibres.   Fibres are 0.5” to 2.0” long and loosely held together by a styrene soluble resin binder.

Copped strand mat is typically used with the surfacing veil to form the corrosion layer.  They are also use alone or in combination with woven roving in alternate layers to form the structural layer of the vessel.  Chopped strand mats will form a laminate of about 30% glass and 70% resin.  This combination helps to produce a laminate with good strength and excellent chemical resistance.

Typical chopped strand mats are:

Woven Roving

Woven roving is a course, heavy fabric made of long, continuous strands of glass fibres.  They are typically used in alternate layers with chopped strand mat to form the structural layer of a vessel.  The resulting laminate will be about 40-50% glass.

Typical woven rovings are:

Continuous Roving

Continuous roving comes in various densities and sizes for application in filament winding.  Gun roving is used as a substitute for chopped strand mat in the corrosion and structural layers.   The resulting laminate are generally about 30-40% glass.

Typical continuous rovings are:

Resins

For typical acid plant service the following resins are suitable:

Design

Safety factors for the design of fibreglass reinforced plastic items shall be based on the following:

Tensile or Flexural Stress - 10:1

Elasticity Instability (External Pressure, etc.) - 5:1

The ultimate tensile strength, flexural strength and flexural modulus of elasticity (tangent) requirements shall meet or exceed the values as listed in RTP-1.   Physical properties for the material should be taken at the design temperature of the process.  The corrosion barrier, internal anti-wicking barrier and exterior protective systems should not be considered as structural elements in the calaculation of the required thickness.

The design calculations should consider all loadings and forces on the equipment, piping or ducting.  These should include: internal and externl pressure/vacuum; weight of the equipment; weight of the contents; wind, snow, ice, earth quake; live and dead loads; insulation, pumps, agitators; support rings, etc.

Fire Retardant

To achieve a Class 1 flame spread rating, antomony trioxide or Nyacal APE 1545 antimony oxide shall be added up to a maximum of 5% wt to all resin in the structural layer to improve fire resistance and reduce the flame spread rating to a maximm of 15 when tested in accordance with ASTM E-84 "Standard Method of Test for Surface Burning Characteristics of Building Materials".

Ultra-Violet (UV) Inhibitor

When a piece of FRP equipment is located outdoors, UV inhibitors must be added to the exterior surface coat to prevent damage to the resin by UV light.   Typical UV inhibitors are:

• Tinuvin 326
• Cyasorb UV-24

Approximately 0.1 to 0.3% UV inhibitor is added to the surface coat.

Codes and Standards

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