balmoral cylindrical tanks technical spec

BALMORAL TANKS

CYLINDRICAL GALVANIZEDSECTIONAL BOLTED TANKSFOR WATER STORAGE

www.balmoraltanks.com

CYLINDRICAL GALVANIZED SECTIONAL BOLTED TANKS FOR WATER STORAGE

Balmoral Tanks

Balmoral Tanks specialises in the design andmanufacture of GRP, steel sectional andgalvanized cylindrical steel bolted liquidstorage tanks. These tanks are primarily usedfor the storage of water in the potablewater, fire sprinkler and irrigation markets.

The galvanized cylindrical tanks are siteassembled using overlapping and boltedgalvanized steel panels that aremanufactured within the company’s facilityin the UK.

Depending upon the application or designcode, a choice of either a synthetic rubbermembrane liner or mastic is used to seal thetank together with a plastic coated troughdeck roof cover.

These galvanized tanks provide aneconomical, reliable and low maintenancesolution for water storage.

Experience gained through the supply of over5000 tanks worldwide and our ISO 9001:2008accreditation ensures consistent quality ofproduct and service for the design,manufacture and installation of liquidstorage tanks.

Scope

This document has been written to providerelevant information for cylindricalgalvanized water storage tanks designed andmanufactured by Balmoral Tanks.

These modular site bolted storage tanks aremanufactured from galvanized steel panelsand sealed using either internal syntheticrubber membrane liner or flexible mastic.

The tanks are typically used for:� Potable water storage� Fire water storage� Irrigation water storage

Contents

1 Tank overview 1

2 Tank design and key components 1

3 Standard tank accessories 4

4 Galvanizing 4

5 Epoxy and polyester coatings 5

6 Rubber tank liners 5

7 Liner repair 6

8 Liner material specification:wras approved butyl 7

9 Liner material specification: edpm 7

10 Effect of chlorinated water on butyland epdm liners 8

11 Hot dip galvanized sheet quality 8

12 Renovating damaged coatings 10

13 Galvanized products: zinc patina 11

14 Wet storage stain: prevention and cure 12

15 Liner approval 13

CYLINDRICAL GALVANIZED SECTIONAL BOLTED TANKS FOR WATER STORAGE1

1 Tank overview

The tank shell is constructed from galvanized steelpanels that are bolted together using bolts, nuts andwashers. Panels are bolted together in a definedconfiguration with the thicker panels at the bottom ofthe tank where the liquid pressure increases.

Externally fixed rolled angles are provided as a meansof securing the shell to the foundation, a means offixing the roof to the shell and to provide additionalshell stiffening as required.

The tank shell is sealed using either an internalsynthetic rubber membrane liner or mastic sealbetween the overlapping plates.

A corrugated deck roof cover, ladder, platform, pipe-work and other accessories are supplied to meetspecific requirements.

Tank panels and components can also be epoxy coatedto meet particular aesthetic site requirements.

2 Tank design and key components

2.1 General design philosophyTanks are designed using the following criteria unlessotherwise agreed:� Tank shell is designed to accommodate full

hydraulic load minus the free-board� Wind speed of 45 m/s (tank empty)� Non seismic� Live/roof load of 0.75kN/m

2.2 Tank panelsA standard tank panel is manufactured from agalvanized steel sheet having approximate overalldimensions of 2500mm, 2540mm or 2580mm long,dependent upon vertical bolt patterns, andapproximately 1250mm high dependent on the sheetand tank shell design.

Depending upon galvanizing thickness required (seeBalmoral Tanks Galvanized Information, June 2011) thepanel material will conform to the following standards:

BS EN10025 for hot dipped galvanized coating – 600g/m2

BS EN10327 pre-galvanized coating – 300g/m2.Standard panel thicknesses are 2mm, 2.5mm, 3mm,4mm, 5mm. Pre-galvanised sheets are manufacturedto a maximum thickness of 5mm. 6mm, 8mm and10mm must be manufactured in mild steel to standardEN10025.

2.3 Top and bottom rolled angleThe top and bottom of every tank shell is fittedwith a steel angle ring that is rolled to the specificdiameter of the tank. The top angle ring stiffensand maintains the concentricity of the shell as wellas creating fixing points for the roof sheeting. Thebottom angle ring provides a section that can befixed to the foundation, thus securing the tank shellto the concrete base.

The cross-section of the angles is 60x60x6mm and issupplied in lengths of 2420mm. Every angle is rolled,“toe-out”, to suit the tank shell diameter. Thevertical or rolled face of each angle has fixing holesto match the bolt pitch of the standard shell panel.

The horizontal or flat face of each bottom anglehas slots to suit the appropriate size and number ofhold-down bolts.

The angles are bolted to the tank shell through thehorizontal seams and are positioned to miss thevertical bolt seams of the shell hoop. Splice anglesstrengthen the joint between the angles and createa complete ring. The top angle ring is secured withthe horizontal face above the fixings of the tophorizontal bolt seam.

The bottom angle ring is secured with thehorizontal face below the fixing of the bottomhorizontal bolt seam.

2.4 Wind stiffening anglesAny wind stiffening requirements are fulfilled bygalvanized equal angles rolled to the tank radius.

The size of angle for any given stiffening ring isdictated by the stiffness requirement. The standardsize of the stiffening angles is 60x60x6mm, howeverin some areas of high wind loading, angle sizes canincrease up to 70 or 80mm equal angles.

The wind stiffening angles are rolled to suit eachtank radius, and are rolled either “toe-in” or “toe-out” dependent upon design requirements.

They are attached to the shell wall with 4 cleatsequally spaced across its 2420mm length. Eachcleat is fixed with a single bolt through thehorizontal seam requiring stiffening and then by asingle bolt through the angle.

Similar to the top and bottom angles, the lengths areattached to avoid the vertical seams of the tank shell.Splice angles bridge the gap and connect the individuallength to create one complete stiffening ring.


2.5 Shell fixingThe tank is secured to the foundation with fixingsthat are located externally around thecircumference of the tank shell.

The bottom angle is secured with either expandingmechanical anchors or chemically fixed anchors.Depending on the size of the tank and the windloading overturning moment, the anchors will beeither M12 and pass through the bottom angle, orM16 or larger and pass through clamping brackets.A minimum of two anchors per angle are fitted.

The anchor fixings have a Design Load = 0.6 xMinimum Specified Yield Strength. The magnitudeof lifting force due to wind loading, and thereforethe anchor fixing selection, can be determined fromthe following formula:

2.6 FreeboardA space between the top of the tank and top of theoverflow assembly is created to account for slightvariations in liquid level and sloshing that can occurfrom the filling process. This space is called the“Freeboard” and is set to a minimum of 150mmwith greater allowances being made for larger tanksthat may have an increased sloshing affect.

The freeboard may need to be further increased tocomply with certain national standards, ie, LossPrevention Certification Board’s LPS 1276 approvalstandard requires “a minimum 50mm spacebetween the maximum liquid level and the lowestsection of the roof structure”.

2.7 Dead waterWater can only be pumped out of the tank whilstthe outlet is submerged. Dead water is the volumeof water between the floor of the tank and thebottom of the outlet arrangement.

2.8 Tank capacityTank shells are designed to withstand hydraulicpressures created by the contained volume of water.The Effective Capacity is the usable volume ofliquid within the tank.

Freeboard and dead water volumes must besubtracted from the wall height volume to gain theeffective capacity.

The Effective Capacity in cubic metres is calculatedas follows:

2.9 Tank loads and stressesThe tank panels are overlapped and bolted alongvertical and horizontal seams. Bolt pitches arespaced to optimise material and bolt yield whilstmaintaining the required seal. The vertical seamsare designed to withstand all hoop stresses causedby the static head of the contents. The horizontalseams are designed to withstand all vertical loadsthat are imposed on the shell.

The hoop stress that is caused by the static load ofthe contents is greater at the bottom of the tank,ie, the greater the depth of liquid, the greater thehead of pressure. Where the hoop stresses increasebeyond allowable levels, tank plate thicknessesincrease as required. As plates thicken, thequantity of bolts within a seam pattern increases todisperse bearing stress on the shell material andshear stress on the bolts.

Vertical loads imposed on the shell are thosetransferred from the tank roof and the specified roofload. As with hoop stresses vertical loads aregreatest at the bottom of the tank. A combination ofthe roof loads and tank shell weight must bedispersed through the horizontal bolt seam at thebase of the tank. When vertical loads increasebeyond allowable levels, a close horizontal bolt pitchwill be used to reduce bearing and shear stresses.

4-nib bolts are used to fasten panels in theconstruction of the tank shell. The bolt DesignShear Stresses = 0.25 x Ultimate Tensile Strengths,which range from 510 to 1035 MPa and are used asthe shear values require.

As required, additional strength is added in theform of wind stiffening angles to prevent the shellbuckling. The angles are fixed around the outside ofthe tank on a horizontal bolt seam to create acomplete stiffening ring. A wind speed of 45m/s isused unless otherwise stated. The analysis of windstiffening requirements is on a tank by tank basis.

Under certain wind conditions there may be anoverturning moment on the tank shell. Thismoment, is calculated at the base of the tank shellwith appropriate anchor fixings being supplied tosecure the shell to the foundation.

2.10 FoundationsThe design of the concrete foundation is projectand location specific and therefore does not formpart of the tank supply for this specification.Normally the responsibility of others, the followingguideline may assist with third party design.π/4 x (Diameter(m))2 x (Wall Height(m) - Freeboard(m) - Deadwater(m))

Bolt designload

= 4 x Overturning moment

Diameter x (2xSheets in hoop)

Total shell weight

(2xPanels in hoop–

2


The ground bearing pressure must be adequateacross the whole area of the foundation to supportthe weight of the tank contents.

This load can be calculated as follows:

There is also a non-uniform vertical loadtransferred through the bottom angle. This load isthe sum of the tank shell weight (kN), the roofweight (kN) and the roof snow load (kN/m2). Thetotal circumferential force is calculated as follows:

When designing the foundation, these loads willneed to be considered. However, on small tanks,the circumferentially dispersed vertical loads willbe insignificant when compared to the weight ofthe contents.

The distance from the outside circumference of thetank to the edge of the foundation must be at least300mm. If the tank is to be constructed withexternal jack lifting equipment a larger foundationwill be needed to accommodate the jacks.

The finished foundation should be raised above thegrade by approximately 150mm

2.11 Tank sealingThe tank shell is sealed using either an internallyfitted synthetic BUTYL or EPDM rubber membraneliner. Alternatively, a polyurethane gun grademastic that is applied between the panels togetherwith an internal “seal-pour” of concrete on thebase which is generally 150mm deep.

2.11.1 MasticA grey polyurethane sealant which has been designedspecifically for sealing galvanized bolted tanks isapplied between the overlapping panel areas.

The panels are subsequently bolted together and asthe mastic cures a watertight seal is formed. Abead of mastic is also applied around the internalcircumference of the base of the tank and thistogether with layer of concrete (seal-pour) providesthe sealing mechanism.

Concrete (seal-pour) provides the sealingmechanism.

Storage conditions and expiration dates need to beobserved to ensure that the product will perform to

expectations. Typically the product should not bestored above 20º C.

2.11.2 LinerA synthetic rubber bag shaped membrane (liner)closely matching the internal dimensions of the tank isfixed to the top angle/panels.

Liner protection is provided by geo-textile or feltmatting with a density of 250g/m³ which is placed onthe base and approximately 300mm up the internaltank wall.

In addition all vertical and horizontal seams arecovered with protective 375 micron self-closingpolythene tape to protect the liner from possibledamage/puncture.

A butyl synthetic rubber WRAS liner approved foruse/contact with potable water with a nominalthickness of 1.0mm is used in potable waterapplications.

An EPDM synthetic rubber liner with a nominalthickness of 1.0mm is used in fire sprinkler andirrigation water tanks.

2.12 Trough deck roofThe tank roof is a rain, debris and light restrictingcover that is designed to withstand both wind and snowloadings. The cover is not designed for foot traffic.

Two roof loadings are currently manufactured asstandard, Light Duty (0.75kN/m²) and Heavy(1.8kN/m²). Both roofs withstand 45m/s wind speeds.Other roof loadings are available on request.

Light dutyThe light duty trough deck roof is a free-span designthat can be used on tanks up to a diameter of 18.6m.It consists of a framework of a “Z” and “C” sectionpurlins and cellular beams. Depending on the tankdiameter these items are used in various configurationsto create a debris cover that can withstand snow loadsof up to 0.75kN/m². The light duty framework iscovered with 200 micron PVC coated mild steel,corrugated decking sheets.

Heavy dutyThe heavy duty trough deck roof is also a free-spandesign that can be used on tanks up to a diameter of12.4m. The design also uses “Z” and “C” sectionpurlins and cellular beams. However, the roof canwithstand snow loads of up to 1.8kN/m². The heavyduty framework is covered with pre-insulated roofdecking sheets.

Tank wall height (m) x 9.81 x SG = kN/m2

Tank weight + Roof weight + (π/4 x Diameter2 x Snow load)

(π x Diameter)=kN/m

3

CYLINDRICAL GALVANIZED SECTIONAL BOLTED TANKS FOR WATER STORAGE 4

Column supportedFor light duty roofs above 18.6m and heavy dutyroofs above 12.4m a column supported trough deckroof is used. It is constructed from a framework ofbeams and “Z” and “C” section purlins that havecolumns to take the vertical load. The frameworkcan then be covered with either light or heavy dutydecking sheets, depending on the application.

2.13 Access ladders and platformsLadders and platforms, with an intermediate restplatform if required, are designed to meetlocal/national standards and specific tankspecifications.

Unless specified otherwise they are manufacturedfrom BS EN 10025 S275 grade steel and galvanizedin accordance with BS EN ISO 1461.

Normal practice is to include a 2.4m longremovable “hook-on” ladder that can be removedfrom the bottom of the fixed ladder to preventunauthorized access.

3 Standard tank accessories

3.1 Immersion heaterDependent upon local climatic conditions and anyrisk of freezing, an electrical immersion heater mayneed to be installed beneath the mechanical in-fillvalve to prevent the water freezing in the vicinityof the ball float valve assembly.

To meet the requirements of LPCB, immersionheater model RS102 is supplied.

3.2 Contents gaugeThe contents gauge provides an easy means ofdetermining the head of water within the tank andis fixed to a panel on the second ring of the tankwall. The gauge can be supplied complete with abrass no-loss connector which enables the gauge tobe removed from the tank without loss of water.

SpecificationDial size 150mmSensing element Bourdon tube altitude typeWetted parts Brass/bronzeMounting Direct – back connectionCase material Steel enamel black finishBezel material BrassConnection size ⅜” BSP

3.3 Vortex inhibitorA vortex inhibitor is used to prevent a vortex being

formed within the pump suction pipe and thus preventingloss of flow and potential damage to the suction pumps.This is particularly critical in fire sprinkler tankapplications.� For LPCB tanks an approved vortex inhibitor is fitted� A galvanized steel vortex plate or GRP vortex

inhibitor is supplied for all non-LPCB and FMapproved tanks

3.4 Equilibrium ball float valveA cast iron ball float valve can be supplied to control theinflow and maximum volume of water contained withinthe tank. The valve is easily connected to the back plateattached to the ball valve housing. Valves from50–300mm are available.

3.5 Ball valve chamberBall valve chamber is available to accommodate theequilibrium ball valve. The housing is supplied with apre-drilled back plate onto which the specified ball valveis fixed.

3.6 Flanges/Connections/Suctions/BracketsA variety of pipe connection components are available toensure efficient connection to third party pipework. Theyare normally galvanized but can also be epoxy coated.These include:� Drain connection� Overflow arrangement� Suction elbow connection

3.7 Tank connections and fittingsThe majority of connection openings for the tank aremade on site. Openings are cut and/or drilled to suit sitespecific requirements but connections should not crossa tank seam.

Once the position of the connection has been determinedthe tank panel is cut to suit the outside diameter of theconnecting pipe and holes drilled to match the boltingconfiguration of the connecting flanges.

For liner sealed tanks the liner is cut in the same manner.

A rubber gasket is fitted to both the inner and outersurface of the tank shell opening to provide a sealbetween mating surfaces.

Bolts are sealed using either mastic or approved sealingcompound.

4 Galvanizing

4.1 Tank panelsDependent upon the design code and/or the panel


thickness, the panels will be galvanized to one ofthe following standards:BS EN ISO 1461� Hot dip process� Average 600g/m² galvanizing weight per side� Panel thicknesses of 5mm to 10mmEN 10327 Z600� Mill galvanized� Minimum 300g/m² galvanizing weight per side� Panel thicknesses of 2mm to 5mm

4.2 Surface finishPanels that are galvanized to EN 10327 Z600 have aconsistent bright shiny finish and arepredominately used on tanks that are fitted with aliner.

Panels that require a hot dip galvanizing processwill generally have a dull/variable finish whichdiffers significantly from the mill galvanized finish.

Freshly galvanized steel progresses through anatural weathering process and the surface of theproduct will change and/or potentially be variabledependent upon the environmental and storageconditions to which the product is subjected.

4.3 Storage of galvanized panelsIt is important that galvanized panels are handledand stored properly so that the aesthetic finish ismaintained.

Adequate ventilation must be provided so that thebuild-up and retention of excessive water on thesurface of the galvanized panels is avoided in orderto limit the formation of surface staining.

5 Epoxy and polyester coatings

Galvanized tank panels and ancillary connectionscan be either epoxy or polyester coated to meetspecific requirements, ie:� Potable water specification� Colour match other site equipment� Offer extended warranty

6 Rubber tank liners

6.1 GeneralLPCB (Loss Prevention Certification Board) and FM(Factory Mutual) approved fire sprinkler tanks asmanufactured by Balmoral Tanks incorporating arubber liner that is manufactured from materialgenerally known as EPDM.

WRAS (Water Regulations Advisory Scheme) approvedpotable water tanks as manufactured by BalmoralTanks incorporating a rubber liner that is manufacturedfrom material generally known as butyl. This materialis a WRAS approved for use with potable water.

A butyl rubber liner manufactured from material thathas received WRAS approval (BS6920) and is listed bythe Water Byelaws Scheme as approved for use incontact with potable water is available for liningpotable water tanks.

6.2 MaterialEPDM and butyl are flexible synthetic rubbermembranes that are respectively ideal materials forlining fire sprinkler tanks or potable water tanks. Thesheeting provides a completely waterproof seal due itsclosely packed molecular structure making it extremelyresistant to the transmission of liquids or vapour.

Both EPDM and butyl have outstanding ageing andweather-resistant properties. Their cross-linkedmolecular structure gives excellent ageing over a longperiod of time even when exposed to the atmosphere,sunlight, ultraviolet radiation and ozone. Strength andelasticity remains virtually unchanged over manyyears, without shrinking, hardening or cracking.

Refer to material specifications for further informationrelated to the liner material.

6.3 Liner specification

6.4 HandlingDue to its high degree of flexibility, damage by roughhandling is minimised, and the material readily adaptsto surface irregularities. The material has excellentresistance to abrasive wear, tearing, flex-cracking andpuncturing. However, it may be damaged by sharptools, knives, etc, and it is recommended that cleanrubber soled footwear is used when installing orinspecting rubber liners.

Liner Material 1.0 mm EDPM or 1.0 mm butyl

Scrim reinforcement 1.5 mm thick reinforced EDPM/butyl100mm wide materialType 2 design, installed at the topof the liner

Eyelets Brass type number 4126 mm centresFitted circ 40 mm from top ofreinforcement scrim

Dimensions Generally +50mm over tank diameterand +100mm over tank height


A protective underlay, normally a geo-textile or feltmaterial is always supplied and must be fitted onthe tank base for extra protection againstpuncturing prior to installation of the liner. Strips ofPVC material or similar should be used to cover thebolts securing the tank panels or other areas toprevent damage to the liner. As an option a fullgeo-textile or felt liner can be supplied.

6.5 Storage of tank linersIt is advisable to use the liners as soon as possibleafter they have been manufactured, as over a periodof time the liner will develop creases. However,these creases are not permanent as the material haselasticity and memory which means that the mainmaterial will return to its original state.

The only exception to this is the reinforced band atthe top of the liner which, by its nature, does nothave memory due to the reinforcing scrim withinthe material. However, problems are minimised dueto the combination of the high tear strength of thescrim, and large number of fixing eyelets assist inpulling the reinforcing scrim back into shape.

The following guidelines should be followed whenstoring the liner:� Liner is best stored indoors at ambient conditions� The liner is normally supplied in a layer of

geotextile matting, strapped on a pallet andshrink-wrapped for extra protection

� Additional protective crating can be supplied onrequest

� Do not stack items on top of the liner orimmediately adjacent to it, even when it is in itspackaging, as there is always the chance that asharp item could puncture the liner. If in doubtprovide additional protection

� Liner should not be moved around excessively asthis creates a chance of the liner being damaged

6.6 RepairAlthough EPDM and butyl rubber liners areextremely tough and resistant to puncture, damagecan occur. A repair can be carried out simply,quickly and economically on site. In most cases arepair using a heat/pressure process, similar to thatused by the manufacturer, is utilised.

Refer to the “Liner tank assembly guide” for furtherinformation. Repair kits are available from BalmoralTanks upon request.

6.7 InstallationRefer to the “Liner tank assembly guide” forinstallation instructions.

7 Liner repair

For punctures that are not immediately visible theycan be located using a bright light placed on one sideof the liner and viewed from the other - two peoplewill be required for this.

Once the puncture has been located it should bemarked so that a repair can be affected using aStickseal patch.

Stickseal patches should only be used on flatsupported areas where tensile stresses to the linerare minimal. It should not be used to repair adamaged liner that is likely to be folded or creased. Ifthis is unavoidable the liner must be repositioned toensure the repaired area is laid flat and free fromfolds.

7.1 Instructions for useNormally the damage is a small puncture but if thedamage is more extensive then ensure the damagedarea is free from any jagged edges that are likely tocontinue tearing if any stress is applied. This isachieved by rounding off likely edges/tears with asharp pair of scissors.

Clean the liner around the damaged area and at least200mm beyond with a scouring pad and clean water.If the liner is older than five years then use a wirebrush to reveal fresh membrane.

Ensure the cleaned area is completely dry by carefullyheating the damaged area with a hot air gun.

Prepare a Stickseal patch making sure that the patchoverlaps the damaged area by a minimum of 100mmon all sides. Also ensure that the corners of the patchare rounded to prevent uplift when applied.

Position the prepared patch centrally over thedamaged area then, working from the centreoutwards, apply the patch to the membrane using asuitable 40/50mm roller. Roll flat with an outwardmotion until the entire patch has been applied.

To prevent uplift of the Stickseal edge apply a 1cmbead of 5590 Lapseal sealant to the entire edge ofthe patch ensuring the bead overlaps the membraneand patch equally and to a depth of 4mm. This willprotect the tacky exposed Stickseal edge fromcontamination and uplift and provide a secondary sealbetween the membrane and Stickseal patch.

Leave for a minimum of one hour for the Stickseal toharden before submersing in water.


8 Liner material specification: wras approved butyl

9 Liner material specification: epdm

Physical properties Unit Requirements Test Methods

Hardness IRH 65±5 BS 903 A26

Modulus at 300% elongation Mpa min 4,5 BS 903 A2

Tensile strength Mpa min 9,0 BS 903 A2

Elongation at break % min 350 BS 903 A2

Tear strength kN/m min 23 BS 903 A3 C

Properties after aging - 168h/121 C BS 903 A19

Tensile strength Mpa min 7,5 BS 903 A2

Elongation at break % min 300 BS 903 A2

Ozone resistance 96h/30 C - No Cracks BS903 A43

50 pphm and 80% elongation - max –30 BS 903 A25

Additional requirements

UK Approvals Water byelaws scheme, approval to BS 6920 Test report M010093, List No 5044.

Thickness Nominal ± 10%

Physical properties Unit Req/typical Value Test Methods

Hardness IRH 65 ± 5 65 BS903 A26

Modulus at 300% elongation Mpa 5,0 6,9 BS 903 A2

Tensile strength Mpa min 9,0 10,1 BS 903 A2

Elongation at break % min 300 405 BS 903 A2

Tear strength kN/m min 30 37 BS 903 A3C

Properties after ageing ºC 168/121 BS 903 A19

Tensile strength Mpa min 7,5 9,7 BS 903 A2

Elongation at break % min 300 345 BS 903 A2

Brittle point ºC max. –40 -53 BS 903 A25

Additional Requirements

Approvals DIN 7864 part. 1 1984, Swedish Type Approval No 2224/82.

Thickness Nominal ±10%


10 Effect of chlorinated water onbutyl and epdm liners

10.1 BackgroundButyl and EPDM liners have been used in waterstorage tanks since the mid 60’s, and have a provenrecord of long service life.

However, to ensure maximum service life specialattention must be given to the use of chlorines forsterilisation of the water.

10.2 Chlorine: basic informationChlorine is very poisonous and will cause deatheven at low concentrations.

Chlorine is a very reactive oxidizing agent, whichwill react instantly with almost anything organicand most inorganic materials. Of the elementswithin the periodic system only Fluorine is morereactive. It is because of this powerful oxidizingproperty that chlorine is such an effectivesterilizing agent. It is important to understand,however, that chlorine will react not only with thechemicals in water, but also with any organicmaterial in the tank. Butyl and EPDM liners are suchorganic materials.

Chemically, chlorine is a halogen, and always wantsto add an electron to its outer electron shell. Thiselectron must be taken from another material, withwhich the chlorine reacts.

The chlorine dosed in water will always, whencorrectly dosed, initially react with materials in thewater then with the materials in contact with thewater, such as a liner, and the surplus volume ofchlorine will evaporate from the water into the airas a poisonous gas.At an ideal chlorine dosing level, the chlorine iscompletely consumed or neutralised by sterilisingthe bacteria and micro-organisms in the water. Ifthe chlorine is not fully consumed by the water, theremaining chlorine will continue to react with theliners, reducing their life, until it is fully consumedor evaporated into the air.

10.3 Factors which influence liner degradationAs stated above, chlorine is extremely reactive andwill react with all materials. However, four factorssignificantly influence the degree of attack on theliner material:

10.3.1 Concentration of chlorineLinear relation.

10.3.2 Time exposure to chlorineLinear relation.

10.3.3 TemperatureAs a rule of thumb, a 10°C increase in temperaturewill double the effect of chlorine.

10.3.4 Liner installation qualityFolds and tensions in the installed liner will reduce lifetime, as folds will be the point of chlorine attack.

10.3.5 Liner resistance to chlorineAll flexible liners will be attacked by chlorine butthose produced from fluorocarbon rubbers, fluorsilicones and PTFE will a have “fair” resistance.However, these types of products are notcommercially available and are significantly moreexpensive than butyl or EPDM.

Butyl and EPDM are considered “non-resistant” andbutyl is rated as better than EPDM as butyl remainsmore flexible under chlorine attack and has extremelylow gas permeability (chlorine is a gas). Themaximum allowable chlorine dosage on Butyl andEPDM is 0.2ppm.

11 Hot dip galvanized sheet quality

11.1 Quality assuranceBalmoral Tanks insists upon the highest qualitystandards from its suppliers, who are required toprocess work to BS EN ISO 1461 standards. Therequirements of these standards ensure that the zinccoating is continuous and of the required thicknesstogether with an acceptable aesthetic appearance.

Quality assurance for the industry has been enhancedby the introduction of the BS EN ISO 9000 series ofstandards – ‘Quality Systems’ and where possibleBalmoral’s suppliers are accredited to this standard.

11.2 Coating weight or thickness measurementThe nature of the galvanizing process ensures that, inmost cases, if the coating has formed, it willautomatically be of sufficient weight to meet therequirements of BS EN ISO 1461. There are a numberof inspection techniques which can be used whennecessary.

11.3 Coating finishThe table below summarises variations in finish whichmay be observed and whether or not they areacceptable. The acceptability of the coating is,however, judged primarily on its long-termperformance and corrosion resistance.


11.3.2 Dull grey or dark grey coatingSilicon is sometimes added to steel as a de-oxidantduring production and this speeds up the reactionbetween the steel and the molten zinc. When thegalvanized article is removed from the bath, butstill remains hot, the reaction may continue andconvert all or part of the surface zinc layers tozinc-iron alloys. These are dark grey compared withthe light grey of pure zinc although after a periodof exposure the difference in grey colour becomesless pronounced.

The dark grey coating surface may develop stainingafter a relatively short period of exposure, even inmild, damp conditions. This is only a surface effectand does not indicate serious deterioration of thecoating: the galvanized coating remains andcontinues to protect the steel.

11.3.3 Staining and discoloration by rustSound galvanized steel with many years ofcorrosion-free life remaining can sometimes be ruststained or discolored. This may give an incorrectimpression that the coating has failed and mayoccasionally be visually unacceptable. It may be theresult of one or more of the following factors:

� Direct contact of galvanized articles withunprotected or inadequately protected steel, eg,galvanized steel sections fastened withunprotected, electroplated or painted steel bolts

11.3.1 Galvanized finish

Appearance Acceptability of protection (Not necessarily of appearance)

Dull grey coating (All alloy, no free zinc) Acceptable

Cracking in pooled zinc Acceptable

Rust stains Not acceptable

General roughness Acceptable within Balmoral standard

Lumpiness and runs Acceptable within Balmoral standard

Pimples Acceptable within Balmoral standard

Bulky white deposit (wet storage stain) Acceptable (provided coating weight remains in compliance withBS EN ISO 1461)

Flux staining Not acceptable

Bare spots Not acceptable

� Deposits of iron and steel, dust and swarf fromother operations or sources on the galvanized surface

� Water draining from unprotected or poorlyprotected steelwork, eg, from dam-aged areas onpainted steelwork

� From cleaning residues in welds. During cleaning,acid may penetrate into the weld area via pinholesor other gaps in the welding

� Rusting of areas welded after galvanizing andsubsequently left unprotected or inadequatelyprotected

� Water running off other materials, notably metalssuch as copper and certain hardwoods, eg, oak.This effect may occur whenever water can dissolvematerials from one surface and deposit them onthe galvanized steel

Discoloration and staining from most external sourceshave no effect on the life of the coating. However,affected areas may be cleaned to improve theappearance of the component. Generally, wirebrushing or the use of a scouring powder will removethe stain and leave a sound galvanized coating.

11.3.4 General roughnessBS EN ISO 1461 demands that a galvanized coatingshall be ‘smooth’ but points out that smoothness is arelative term and that coatings on fabricated articlesshould not be judged by the same standards as thoseapplied to mechanically wiped products such as mill-galvanized sheet, tube and wire.


An uneven coating is usually due to excessive oruneven growth of the alloy layers because of thecomposition or surface condition of the steel. Anuneven coating is often thicker than a conventionalcoating and, therefore, has a longer life.

11.3.5 Lumpiness and runsLumps and runs caused by uneven drainage of zincfrom an article when it is removed from the bath mayoccur due to shape or thinness of the component andare not harmful to the life of the coating. Sharppoints of excess solidified zinc are not acceptable asthey may present a hazard during handling.

11.3.6 PimplesPimples are caused by inclusions of dross (a pastyzinc/iron alloy residual that forms in the galvanizingbath) in the coating. These may arise from iron saltscarried over on the work from the cleaning tank andunable to escape from the surface of the coating.Contamination may also arise from agitation of thedross layer at the bottom of the bath. Dross has asimilar corrosion rate to that of zinc and its presencein the coating as finely dispersed particles isacceptable but major dross inclusions tend to causebrittleness of the coating and will be rejected basedon Balmoral Tanks’ standards.

11.3.7 Wet storage stainWet storage stain is the white corrosion products anddark stains which may be seen on the surfaces ofnewly galvanized articles when they have beenclosely stacked and stored or transported under dampor wet conditions. Where wet storage stain hasformed, the coating beneath may be stained darkgrey or black.

To minimise wet storage stain, zinc coated tanksheets are transported and stored under dry and wellventilated conditions prior to onward shipment to thetank construction site. If stored outdoors at site, thesurfaces should not be in close contact: freecirculation of air is necessary to prevent condensationand retention of moisture. Capillary action canattract water into closely contacting surfaces such asnested sheets. Components should not be stored indirect contact with the ground.

If heavy wet storage stain deposits do exist theyshould be removed. This can usually be achieved bybrushing with a stiff bristle brush or light abrasives.

11.3.8 Flux and dirt stainingWhere flux is used during the dipping process, fluxresidues may adhere to the surface after immersionand pick up moisture to form white corrosion

products. Although this is a surface effect, fluxstains may be detrimental to the life of thecoating and are not acceptable to Balmoral Tanks’quality standards.

Dirt may be picked up on the surface of thecoating from the site, truck beds or from contactwith other articles. These are readily washed offto reveal a sound coating underneath and are not,therefore, harmful.

11.3.9 Bare spotsBalmoral Tanks’ quality standard does not acceptany bare spots. However due to the sacrificialaction of zinc, small localised flaws up to 5mmmaximum width are usually self-healing and havelittle effect on the life of coating.

12 Renovating damaged coatings

Small areas of damage may occasionally occur intransport and erection. Due to the sacrificial actionof zinc, small localised flaws do not reduceprotection. Nevertheless, it is often aestheticallydesirable to renew the coating even over such smallareas.

Adequate corrosion resistance will be achieved atany damaged area if a weight of zinc is depositedequivalent to the weight of the undamagedcoating. The following techniques are acceptableaccording to BS EN ISO 1461:

� Thoroughly wire brush the affected area andapply sufficient coats of zinc rich paint (bybrush or aerosol spray) to give a coatingthickness at least equivalent to the originalgalvanizing

� Grit blast the affected area and thermal zincspray. A 100µm thermally sprayed zinc coatingconfers corrosion protection equivalent to an85µm galvanized coating

Zinc rich paint is much the simplest to apply,especially on site. Thermal zinc spraying is usuallyonly economic when applied in the workshop.

BS EN ISO 1461 requires that the coatingthickness on renovated areas shall normally be30µm more than the local coating thicknessrequirement for the galvanized coating. Anexception to this is when the coating is to beover-coated and the renovated area requires anequivalent thickness.


2.1 Examples of hot dip galvanized finish

13 Galvanized products:zinc patina

Freshly galvanized steel progresses through anatural weathering process and the surface of theproduct will change and/or potentially be variabledependent upon the environmental and storageconditions to which the product is subjected.

During the first few weeks after an article has beengalvanized it develops a natural protective patina.If allowed to develop properly, the patina itselfprovides a corrosion protection layer for the activezinc metal.

The formation of the zinc patina begins with thedevelopment of a thin layer of zinc oxideparticulates on the freshly coated surface.

These particulates react with water, from rainfall ordew, to form a porous, gelatinous zinc hydroxide.

During drying, this product reacts with carbondioxide present in the atmosphere and converts intoa thin, compact and tightly adherent layer ofcorrosion products consisting mainly of basic zinccarbonate.

The rate of patina formation varies according to the

Dull grey coating Rust stains General roughness

Pimples Wet storage stain Flux staining

Bare spots

environmental conditions. Typically, it takesapproximately 6-12 months to fully develop.

13.1 StorageHandling and storage conditions can inhibit theformation of the patina.

Storage areas that are high in humidity and lack aircirculation tend to promote excessive growth ofzinc oxide and zinc hydroxide. Adequate ventilationmust be provided so that the build-up and retentionof excessive water on the surface of the galvanizedsteel are avoided leading to the formation of wetstorage stain - also referred to as white rust.

The zinc patina begins its development with exposure tooxygen in the atmosphere. Moisture from rain or humid airreacts with this layer to form zinc hydroxide. This layerthen reacts with carbon dioxide present in the atmosphereto form the tightly adherent, insoluble zinc patina.


14 Wet storage stain:prevention and cure

14.1 IntroductionOne of the commonly encountered problems withgalvanized coatings of all kinds is white storagestain - also referred to as white rust.

It is manifested as a bulky, white, powdery depositthat forms rapidly on the surface of the galvanizedcoating under certain specific conditions. Whiterust is usually detrimental to the galvanizedcoating's appearance.

The surface of galvanized coatings is almost 100%zinc. It is the durability of the zinc that providesthe outstanding anti-corrosion performance forsteel, yet zinc is a relatively ‘reactive’ metal. It isthe stable oxides that form on the zinc's surfacethat determine its durability, and these oxides areformed progressively as the zinc is exposed to theatmosphere. Carbon dioxide in particular is acontributor to the formation of these stable oxides.

With newly galvanized steel, the zinc's surface hasbeen subjected to little oxidation and is at its mostvulnerable. For this reason, galvanizers use achromate passivation in conjunction with itsgalvanizing operations to provide protection to thegalvanized coating during the ‘youth’ period of thecoating. This passivation coating provides shortterm protection to the zinc to give the stableoxides time to form on the surface.

14.2 White rust formationPure water (H2O) contains no dissolved salts orminerals and zinc will react quickly with pure waterto form zinc hydroxide, a bulky white and relativelyunstable oxide of zinc. Where freshly galvanizedsteel is exposed to pure water (rain, dew orcondensation), in an oxygen deficient environment,the water will continue to react with the zinc andprogressively consume the coating.White rust can occur when galvanized products arenested together and tightly packed, or when watercan penetrate between the items and remain therefor extended periods.

14.3 Avoiding white rust formation on tank sheetsBalmoral Tanks takes the following steps to reducethe potential for the formation of white rust.

Foam strips are placed between the individual tanksheets that allow air to circulate between adjacentsheets and/or the edges of each pack of plates aresealed with adhesive tape.

Palletised sheets are covered with waterproofplastic to prevent the ingress of rainwater.

14.4 Treating galvanized surfaces affected bywhite rustOnce the galvanized surface has been attacked andthe zinc hydroxide compounds have formed it isdesirable to remove the oxide products from thesurface because:

Their presence inhibits the formation of stablecarbonate based oxides

They are unsightly.

The effect on the galvanized coating can rangefrom very minor to severe and different levels ofremedial treatment are available to deal with whiterust problems at the various levels at which theyare likely to occur.

The following treatments are recommended to dealwith white rust on galvanized products and shouldbe carried out prior to installation.

14.5 Light white rustingThis is characterised by the formation of a lightfilm of white powdery residue.

Provided the items are well ventilated and welldrained, white rust rarely progresses past thissuperficial stage.

It can be brushed off if required but will generallywash off in service under normal circumstances.

No remedial treatment is generally required for thislevel.

14.6 Moderate white rustingThis is characterised by a noticeable darkening andapparent etching of the galvanized coating underthe affected area, with the white rust formationappearing bulky.

The galvanized coating thickness should be checkedto determine the extent of attack on the coating.

In the majority of cases, less than 5% of thegalvanized coating will have been removed and thusno remedial work should be required as long as theappearance of the affected area is not detrimentalto the use of the product and the zinc hydroxideresidues are removed by wire brushing. If theappearance is unacceptable, the white rustaffected area can be treated as follows:


14.8 SummaryWhite rust is a post-galvanizing phenomenon thatgenerally manifests itself as an aestheticproblem, particularly related to tank panels.

Its prevention lies in the manner in which it ispacked, handled and stored prior to thegalvanized product’s installation and use at thefinal destination.

Balmoral Tanks take various precautions duringthe packing of galvanized tank sheets to minimiseor eliminate the formation of white rust.

If white rust does form on delivered products, itis normally a reflection of the length of time andthe environment in which the product has beenstored since being dispatched from the BalmoralTanks factory. It is not a reflection on thegalvanized coating's performance and is more anaesthetic issue that can normally be easilyrectified on site.

15 Liner approval

15.1 Potable Water Butyl liners are manufacturedusing WRAS approved materials.

� Wire brush the affected area to remove allwhite corrosion products

� Using a cloth pad wet with aluminium paint, rubthe surface with the pad to apply a thin film ofaluminium paint to the affected area to blend itwith the adjacent unaffected galvanizedsurfaces

14.7 Severe white rustingThis is characterised by very heavy oxide depositsand items may be stuck together.

Areas under the oxidized patches may be almostblack or show signs of red rust.

A coating thickness check will determine the extentto which the galvanized coating has been damaged.

Remedial treatment to reinstate the coating shouldbe undertaken as follows:

� Wire brush or buff the affected area to removeall oxidation products and rust if any

� Apply one or two coats of approved epoxy zinc-rich paint to achieve required dry film thicknessof 100 microns minimum

BALMORAL TANKS

Balmoral Park, Loirston, Aberdeen AB12 3GY

Tel +44 (0)1224 859000 Fax +44 (0)1224 859123

Email [email protected] www.balmoralltanks.com

TNK-CYL-REV0-0811

Balmoral Tanks believe that the information printed in this document is accurate and published for information only. No warrants, express or implied, are contained therein, nor does any legal liability attach toBalmoral Tanks for any reason whatsoever. Property rights of the subject belong to Balmoral Tanks and transfer of these rights is not granted by possession of this document. The company’s policy is one ofcontinuous product improvement and we reserve the right to make alterations to our range and specification without prior notice.

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