G Engelsman Paper

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E X T E N D I N G H O R I Z O N S

C o m p o s i t e s A f r i c a 2 0 0 2 C o n f e r e n c e

G Engelsman

Guidelines for GRP Vessels Under Pressure.

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Composites Africa 2002 Conference and ExhibitionCaesars PalaceJohannesburgSouth Africa21 August 2002

GUIDELINES FOR GRP VESSELSBy Gavin Engelsman

ABSTRACT

The use of GRP as a long-term, cost-effective, corrosion resistant product is now beingused on a much greater scale than before. Many vessels that were previously made in steelare now being replaced with non-metallic / composite materials. In most cases thisconversion process is being carried out without re-engineering the metallic construction

concepts to suit those of GRP, unfortunately to the detriment of the end-user and the GRPIndustry. Compliance with the relevant codes of practice and specifications are alsooverlooked.

This paper shall highlight the pitfalls involved in this direct conversion process and provideguidelines for the design and construction of composite vessels.

INTRODUCTION

The following guidelines have been prepared to give the project engineer or end user a

better understanding of the statutory requirements, design codes, and minimum requireddata when specifying or redesigning a non-metallic vessel. The advantages as well asdisadvantages of non-metallic materials are stated to ensure that the engineer is au fait withthe material limitations. Guidelines for the construction of composite vessels will bedescribed, as well as a quick guide to the selection and determination of the various layersof construction and minimum wall thicknesses.

STATUTORY REQUIREMENTS

All pressure vessels shall meet the following requirements:

SABS 0227:2000 The evaluation of the technical competence of inspection authoritiesfor the certification, rectification, modification or repair of vessels under pressure :

Which states that all vessels under pressure shall have the design verified by a DesignEngineer Pr Eng, Pr Tech or Reg Cert Eng and certified by an Inspector of PressurizedEquipment. (IPE) accredited by SANAS.

OHS Act 85 of 1993

Be designed and constructed to a code of practice which is recognized by the OHS Act.

Have a nameplate that complies with the OHS act.


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The responsibility has now been placed on the manufacture to comply with the above and tosupply the following documentation with the vessel / tank:

Certificate of Manufacture.

Inspection release note.

Pressure test certificate.

Code data books.

Certified design and plans.

CODES FOR GRP VESSELS UNDER PRESSURE

It is mandatory that these vessels are designed and manufactured to recognized codes/standards and specifications. These specifications also indicate the correct non-metallicmethods of construction, as well as prescribing the various tests that are to be undertakenon the raw materials and the finished product.

It must be highlighted that unlike steel, where the manufacturer purchases a certified sheetof steel, with known properties, and forms it to construct the product. The GRP material andits properties are created at the same time as the composite product is fabricated.

Although this is essentially an advantage, stringent quality control needs to be exercised toensure that the required material properties are achieved.

The following is a summary of some of the recognized codes of practice: for tanks, vesselsand piping highlighting the key points.

Design Codes:

BS 4994-1987

Makes use of stated material properties.

Excludes the chemical barrier in the strength calculation.

Stipulates a minimum structural wall thickness of 5 mm for vessels under pressure.

Penalizes filament winding between 15 and 75

AD Merkblatt N1

Relies on previously tested stress values for use in the strength calculation


Does not penalize filament winding

RTP-1

Very detailed and complex specification

Relies on previously tested stress values for use in the strength calculation

Does not penalize filament winding

Process piping specifications

BS 6464-1984

Makes use of stated material properties.


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Penalizes filament winding between 15 and 75.

BS 7159-1989

Makes use of stated material properties .


Does not penalizes filament winding

ADVANTAGES AND DISADVANTAGES OF COMPOSITE FABRICATIONS RELATIVETO OTHER CONVENTIONAL MATERIALS

Users must understand both the advantages and disadvantages of composite constructedfabrications.

ADVANTAGES

Good corrosion resistance

Thermoplastics as well as GRP are resistant to chemical attack for a wide range of

chemicals of fluctuating concentration found in the Chlor-Alkali, Pulp and Paper andPetro-Chemical industry.

Low maintenance costs

Non-metallic equipment is resistant internally and externally to corrosion and does notrequire continual painting and preservation.

Optimization of the material properties in the required directions.

Unlike steel that is isotropic, having uniform strength in all directions. GRP materialproperties are created at the same time as the product is manufactured, thereby

allowing the fibres to be orientated in such a way as to best suit the strengthrequirements. This anisotropy property can when correctly designed result in a saving inmass and cost.

High tensile strength to weight ratio.

Density of steel: 7850 kg/m Density of GRP : 1450 to 2000 kg/m

Tensile strength of steel: 430 MPa Tensile strength of GRP

400 to 1000 MPa in direction of the fibre.

Table 1compares the properties of other metallic materials to GRP, by nature of theirweights, to yield the same mechanical properties.

Table 2 takes the properties of the materials, divides them by their relative densities andfactors them to mild steel, being 1.

Low transportation costs

Due to the lightweight construction, large non-metallic items are less expensive totransport than steel items.


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Good thermal insulation properties

GRP and plastic are good insulators thereby reducing the heat loss through the vesselwall, and reduces the degree of lagging/insulation required.

Coefficients of thermal conductivity:

Steel: 46 W/m C

GRP: 0.2 W/m C

Easy to repair and refurbish.Once a steel vessel becomes badly corroded, welding of patches no longer becomes aviable option. GRP on the other hand can have the corroded surface sandblasted and anew corrosion barrier applied throughout the entire vessel, thereby extending the lifespan of the vessel .

Dual laminate construction - best of both worlds

Vessels can be constructed using thermoplastic/ fluoroplastic liners, offering superiorchemical resistance, and reinforced with GRP, which has good mechanical properties,giving the best of both worlds.

LIMITATIONS/ DISADVANTAGES

The end user needs to be aware of the limitations of non-metallic equipment:

High temperature

The major limitation for non-metallic materials is their inability to handle hightemperatures.

Table 3 gives an indication of the maximum temperatures of the various non-metallicmaterials.

Thermoplastic and fluoroplastic lined items are difficult to replace

Plastic liners are not easy to replace and in the case of the more exotic liner materialssuch as: ECTFE, PFA, MFA and FEP, specialized welding equipment is required.

Low tensile modulus relative to steel.

GRP has a low tensile modulus when compared to steel which results in the vesselbeing very sensitive to small amounts of vacuum and if not designed accordingly canhave disastrous effects.

REDESIGNING OF METALLIC CONCEPTS, HIGHLIGHTING GOOD NON-METALLICCONSTRUCTION TECHNIQUES.

Taking an existing steel design, changing the material to a composite material is seldom themost cost-effective solution. A fresh look at what the vessel was intended for should beundertaken considering the following factors:


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Configuration / shape of the vessel

GRP has a high tensile strength as well as a high elongation, giving it a low tensile moduluswhen compared to steel. This causes GRP to have a low resistance against bending,making the construction of flat panels very thick and costly.

Where possible:

Rectangular shaped vessels should be redesigned as cylindrical vessels. Flat panels should be avoided and redesigned using semi-ellipsoidal and hemispherical

shapes.

Radiusing of all sharp corners

Sharp corners and sharp intersections should be redesigned with flowing radiuses as:

The glass mat is applied by hand and rolled into place using pig hair rollers, which cannot get into the sharp corners.

The strength of the mat is in the direction of the fibre and it is essential that the fibre can

flow from one plain to another without a break in the fibre. Sharp corners cause high stress concentrations.

Sharp corners cause resin rich zones, which are prone to exothermic reactions andcracking.

All edges should have a minimum fillet radius of 5 mm.

The design specifications stipulate minimum radii for changes in direction. The followingfigures are extracts from BS 4994 1987.

Figure 1 details the recommended radii for the intersection of a flat bottom to a cylinder.

Figure 2 details the recommended radii for the intersection of a cone to a cylinder.

Where laminates terminate they shall not end abruptly but taper into the parent material.

Figure 3 details a minimum taper of 1 in 6 for the termination of all laminates.

Nozzle attachments

Minimum flange thickness

Nozzles should be designed to accommodate not only the pressure requirement butalso induced forces caused by piping, over tightening of the bolts, man loads and otherin service operating conditions. For nozzle sizes up to 300 NB a minimum flangethickness equivalent to 600 kPa or 20 mm will accommodate any of these stray forces.

Flange design (1, , 4, Rule)

For example,

If the flange face is 30 mm thick,

The hub behind the flange should be the thickness of the flange. i.e. 15 mm,

And the length of the hub should be 4 times the thickness of the flange. i.e. 120 mm

Refer figure 4.


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Fibre direction in the flange / hub.

Ensure that there is continuity of the fibres from the hub into the face of the flange.Incorrect lay-up is the cause of many nozzle failures.

Refer figure 5.

Minimum nozzle stand offs.

A nozzle stand-off of 150 mm provides sufficient bonding length up the nozzle shank forthe attachment laminate and still allows adequate clearance for the bolts and backingring. A minimum bond length of 75 mm is required by BS 4994-1987.

Compensating pads

Load compensating pads of between 2 and 3 times the diameter of the opening shouldbe applied around all cut outs in the vessel shell.

Refer figure 6.

Gussets

Some specifications recommend the use of gussets on smaller nozzles, in these casesconical gussets should be used, as triangular plate gussets generally offer little supportas the attachment gusset laminate can not be adequately radiused in these confinedareas. A correctly constructed nozzle, with a stand off of 150 mm, should not requiregussets.

Connecting pipe work

All connecting pipe work and fittings should be independently supported so as to induceno load on the vessel.

Designing for vacuum

Do not ignore the effects of small amounts of vacuum (1 kPa(g) ) induced in the vessel.

As discussed, due to GRPs low tensile modulus, relatively low vacuum conditions need tobe considered in detail and the vessel suitably reinforced, ribbed or a vacuum breaker fitted.

MINIMUM DESIGN PARAMETERS

The following minimumdesign parameters need to be specified by the end user and takeninto account in the structural design and material of selection.

Design temperature

Design pressure.

Design vacuum.

Product.

Product concentration.

Product density.

Cyclic loading.


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RULE OF THUMB GUIDELINES.

A composite laminate is made up of various stages. The inner liner being the layer in directcontact with the product. Then followed by the backing layer and structural layer and finallythe exterior chemical barrier (C.B.). The following is a guide to the selection andconstruction of these layers.

Inner liner

Minimum of 1 surface tissue/ veil, preferably 2.

C glass tissue is normally used and is preferable for chemicals such as Chlorine.

Synthetic veils are preferred for sodium based chemicals such as Sodium Hydroxideand Soda Hypo.

ECTFE (Halar) melt blow fibre is excellent for most corrosive media, except Chlorine.

Alternatively a liner may be used.

Minimum 3 mm thick thermoplastic liner of: HDPE, PPL, uPVC, and cPVC. or

Minimum 2.3 mm thick fluoroplastic liner of: ECTFE, PTFE, ETFE, FEP and PFA.

Backing layer

1.2 kg/m Chop Strand Mat (CSM) for aggressive media 2.5 mm thick

2.4 kg/m Chop Strand Mat (CSM) very aggressive media (Chlorine) 5.0 mm thick

ECR GLAS is recommended for acid applications for long-term corrosion resistance.

Structural layer Hand lay up (HLU)

Filament wound (FW)

Exterior CB

1 layer C glass tissue, pigmented.

Resin rich waxed topcoat layer, pigmented.

Thickness comparisons

The following can be used as a quick reference table.

Laminate mass/m Thickness Glass % Hoop Design BurstStrength (HDBS)

Chopped strand mat. 1 kg 2.1 mm 25-35 6-7 MPa

Woven roving 1 kg 1.1 mm 50 10-14 MPa

Filament wound helical 1 kg 0.8 mm 50-65 27-36 MPa

Filament winding chop/ hoop 1 kg 1.0 mm 60-65 27 MPa


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Quick thickness calculation:

T = pD/ 2 in mm.

Where,

P = pressure in MPa

D = Diameter in millimeters

= Strength 27 MPa FW 9 MPa HLU

Minimum thickness.

The following are some recommended minimum total thickness for:

Piping = 4.5 mm

Fittings = 6.0 mm

Flanges = 20 mm

Tanks/vessels = 6.0 mm

CONCLUSION

There are distinct advantages in industry using non-metallic vessels, providing the vessel:

Meets the statutory requirements

Is designed to a recognized specification/ design code.

Has been correctly re-engineered using non-metallic construction principals.

Is manufactured by a reputable non-metallic manufacturer with an approved qualitysystem. i.e.: SABS ISO 9001.

Complies with the OHS Act.

Is designed, manufactured and tested under the surveillance of a government approvedAIA.

Is fitted with a OHS Act compliant nameplate.

Is supplied with a manufactures certificate of compliance.

Is supplied with pressure test certificate.

ACKNOWLEDGEMENTS

The author thanks Fibre-Wound SA and its staff for the experience gained from working withthem and to industry for their co-operation in promoting non-metallic products in the marketplace.

REFERENCES

Pilkington Reinforcements Limited Booklet

BS 4994-1987

BS 6464-1984


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BS 7159-1989

AD-Merkblatt N1

RTP-1

SABS 0227-2000

OHS Act 85 of 1993

Fibre-Wound Works Instruction Manual sections 3,4 and 9.

AUTHOR

Gavin Engelsman has been involved in the non-metallic industry for the past 12 years. Hehas designed over 300 items of composite equipment and is a member of the PolymericComposite Institute of South Africa. He is involved with industry in assisting them with therewriting and formulation of non-metallic specifications. He is also dedicated to the guidanceand training of Mechanical Engineering students in the non-metallic field.


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Attachments.

Table 1

Reference FIG 49 PILKINTON

COMPARABLE WEIGHT TO GIVEMATERIAL

SAME

RESISTANCETO

ELONGATION

SAME RESISTANCE

TO BENDING

SAME LOAD

CARRYINGCAPACITY

Steel 1.0 1.0 1.0

Aluminum alloy 0.8 0.4 0.7

Uni-directional rod(70%)

1.2 0.4 0.2

Fabric laminate (60%) 2.5 0.5 0.5

Mat laminate (30%) 5.7 0.6 0.9

Table 2

Takes the properties of the materials, divides them by their relative densities and factorsthem to mild steel, being 1.

FACTORED SPECIFIC PROPERTIES

(PROPERTIES DENSITY & FACTORED TO MS)

MATERIAL TENSILE COMPRESSIVE MODULUS

Mild Steel 1.0 1.0 1.0

Aluminum, L73 2.8 2.4 1.0

E-glass / epoxy 10.4 5.6 0.7

HM carbon / epoxy 12.3 10.2 5.0

Aramid / epoxy 19.6 3.8 2.1

HS carbon / epoxy 19.6 12.5 3.2


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Table 3

Maximum recommended temperatures of some of the more common thermoplastics andfluoroplastics.

ABBREVIATION MATERIAL IN SERVICE

TEMPERATURE FORCONTINUOUS

OPERATION (C)

GRP Glass Fibre Reinforced Products 130

ECTFE Ethylene Chlorotrifluoroethylene 150

PVDF Polyvinylidene fluoride 140

PP Polypropylene 100

CPVC Chlorinated polyvinyl chloride 100UPVC Unplasticized polyvinyl chloride 60

HDPE High density polyethylene 60

Figure 1: Flat bottom / cylinder intersection


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Figure 2: Cylinder / cone intersection


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Figure 3: Minimum taper of 1 in 6 for the termination of all laminates.

Figure 4: Flange design


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Figure 5: Fibre direction in the flange / hub.


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Figure 6: Compensation pad


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G Engelsman Paper