Floating Roof Crude Tank-Workshop

WORKSHOPFloating Roof Seals

Tank-Technik

IMHOF

WORKSHOP Floating Roof Sealspage 1

RS/22.02.05Workshop komplett.doc

Tank-Technik

IMHOF

Contents

1. General problems of hydrocarbon liquid storage

1.1 Critical data of hydrocarbon vapours

1.2 General advantages of floating roof tanks versus fixed roof tanks for liquids with high vapourpressure

2. Tank types and floating roof types

3. Codes of tank construction

• API 650• BS 2654, substituted by EN 14015• DIN 4119, substituted by EN 14015

4. National and public codes of hydrocarbon emission control

• EC• Germany• Switzerland• USA

5. Historical development of tank construction and emission control

5.1 Tanks with passive emission control

5.2 Tanks with active emission control

6. Determination of hydrocarbon emission and definition of floating roof efficiency

6.1 Calculation methods of API-Manual of Petroleum Measurement Standards, Chapter 19,Section 1 and 2

6.2 Definition of floating roof efficiency

6.3 Floating roof efficiency

6.3.1 Efficiencies attainable of external floating roof tanks, depending on turnover rate and tankdiameter

6.3.2 Efficiencies attainable of internal floating roof tanks, depending on turnover rate and tankdiameter

6.4. Emission rates per year and meter of rim space

6.5. Evaluation of emission by practical tests at TAMAG/Switzerland

7. Limits of floating roof technology

8. Possible future improvements of passive emission control

9. Risks of tank geometry and rim space tolerances

10. Centring of floating roof

11. Standard requirements for floating roof seals

12. Design criteria of floating roof seals (EN 14015)

13. Highlights of IMHOF seals

14. Reduction of emission and costs involved

15. Sealing membrane materials

Status: 02/2005



Tank-Technik

IMHOF

1. General Problems of Hydrocarbon Liquid Storage

1.1 Critical Data of Hydrocarbon Vapours

Approximate VapourPressure at 20°C

[mbar]

Limits of ExplosiveVapour Concen-

tration [% Volume]

Flash Point[°C]

Temperature ofVapour Ignition

[°C]

Pentane 560 ~ 1,3 – 8,0 285

Gasoline 400 ~ 1,0 – 7,5 < - 20 240 - 280

Crude Oil 400 ~ 0,7 – 5,0

Hexane 170 ~ 1,2 – 7,4 < - 20 240

Methanol 125 ~ 5,5 –26,5 11 455

Benzene 100 ~ 1,2 – 8,0 < - 11 555

Ethanol 60 ~ 3,5 –15,5 12 425

Heptane 50 ~ 1,0 – 6,7 215

Jet Fuel 3 ~ 0,7 – 5,0 42 - 72 257

Heating Oil 0,5 ~ 0,6 – 6,5 55 220

1.2 General Advantages of Floating Roof Tanks versus Fixed RoofTanks for Liquids with high Vapour Pressure

• No filling losses• No breathing losses (daily)• No risks of tank explosion or tank implosion• Effective protection against lightning strikes and tank fires• Effective cooling of storage product• No problems of fixed roof corrosion• No need for costly vapour balancing and vapour treatment systems



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IMHOF

2. Tank Types and Floating Roof Types

3. Codes of Tank Construction

In the past large aboveground hydrocarbon storage tanks in Europe and in the world were builtaccording to national codes, more or less identical to the American tank code API 650 (BS 2654,DIN 4119, etc.).

Since several years a new European tank standard is worked out, named EN 14015: „Specificationfor the design and manufacture of site built, vertical, cylindrical, flat-bottomed, above ground,welded, metallic tanks for the storage of liquids at ambient temperatures and above“.The new European tank standard is more detailed and precise as API 650 and takes better note toEuropean steel qualities, quality control and environmental and safety regulations in Europe.

Floating roof tanks with ring pontoon for diameters> 12 m and < 70 m (EFRT)

Floating roof tanks with double deck fordiameters < 12 m and > 70 m (EFRT)

Fixed roof tanks without roof column (IFRT)

Fixed roof tanks with roof column (IFRT)



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IMHOF

4. National and Public Codes of Hydrocarbon Emission Control

EC

94/63 EC:

Since 1995 all EC countries are to follow the rules of directive 94/63/EC for storage and distribution ofmotor gasoline.The following efficiency factors for EFRT and IFRT are to be met, in comparison to a fixed roof tankwith the same yearly turnover, having a vacuum/pressure valve but no internal floating cover.

a) Tank existing before 31.12.1995- EFRT: 95% efficiency EFRT: External Floating Roof Tank- IFRT: 90% efficiency IFRT: Internal Floating Roof Tank

b) Tanks built after 31.12.1995- EFRT: 95% efficiency by using primary and secondary seals- IFRT: 95% efficiency by using primary and secondary seals

The efficiency factors can be calculated by using Chapter 19, Section 1 and Section 2 of API´sEvaporation Loss Measurement Standards.

IPPC:

The European Directive 96/61/EC asks for Integrated Pollution Prevention and Control (IPPC).Technical work groups of the EC member states worked out a BREF document on BAT (BestAvailable Technique) and describes Emission Control Measures (ECM).

GERMANY

Besides the above EC codes for storage and distribution of gasoline, for all other storage tanks in tankfarms and refineries, Germany uses its rules of TA-Luft 2002.Here efficiencies of floating roof systems (external or internal) shall be better than 97 %, when storingproducts with vapor pressures above 13 mbar at 20° C.

SWITZERLAND

The Swiss health agency (Lufthygieneamt) has specified the most stringent rules of emission controlin Europe. For any size of tank farm in Switzerland the emission of hydrocarbons shall stay below amaximum of 3 kg/hr, or alternatively the maximum hydrocarbon concentration emitting to theatmosphere shall stay below 150 mg/m³.

USA

„At present there is no uniform federal program which regulates aboveground storage tanks. Instead,there is a complex, confusing, and overlapping network of miscellaneous federal statutes andregulations that directly or indirectly govern tanks as well as local requirements imposed by state andlocal authorities.“ (Philip E. Myers, Aboveground Storage Tanks, 1997)According to the Clean Air Act (CAA) all floating roof tanks are to be equipped with primary seal andwith secondary seal.For fixed roof tanks with internal floating cover the regulations ask for liquid mounted single seals, likea shoe type seal, or a vapour mounted primary seal with secondary seal.The US regulations also specify the sealing requirements for guide poles, sample wells andother fittings of the floating roof.Seal gaps at floating roof tanks are to be measured once per year. For internal floating covers a yearlyvisual inspection of seals is required.



Tank-Technik

IMHOF

5. Historical Development of Tank Construction and Emission Controlfor Hydrocarbon Storage Tanks

The emission rates and efficiencies stated below are given for the following example:

Tank ∅: 30 m gasoline tm = 10 °CTank height: 13 m 600 mbar RVP vm = 3,0 m/sTank volume: 8730 m3 M = 64 g/mol pm = 1013 mbar12 Turnovers/year Tank colour: white

5.1 Tanks with Passive Emission Control

Fixed Roof Tank with free vents,Internal Floating Cover as Steel Panwith Single Foam Filled Seal

Floating Roof Tank withDouble Seal (rim mounted)Guide Pole SealRoof Leg Seals

Floating Roof Tank withShoe Type Primary Seal only

Fixed Roof Tank with P/V – Valve

Base Case ofEC – Directive 94/63

~ 1925

e = 9.665 kg/yearη = 88,9 % tank

~1960

e = 1.625 kg/yearη = 98,1 % tank

~ 1990

e = 914 kg/yearη = 99,0 % tank

~ 1920

e = 86.815 kg/yearη = 0 % tank



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IMHOF

5.2 Tanks with Active Emission Control

Résumé:

Tanks or tank systems subject to active emission control measures often apply vapour balancingsystems, gasholders and vapour recovery units. Residual emission is given by all flange connections,the function of safety valves, gasholder systems with huge rubber membranes, and the efficiency ofthe vapour recovery unit. Depending on the details of equipment, the total efficiency of such systemscan vary between approximately 99,4 and 99,9 %, based on the application of a multistage vapourrecovery system with efficiencies of 99,85 %.

Alternatively floating roof tanks with multifold sealing systems and vapour draw-off lines can be runwith small and constant vapour quantities, requiring small and single stage recovery units only.

The total investment and the power consumption required leads to much higher costs for thealternative with vapour balancing systems and without floating roof technology.

Furthermore the installation of additional equipment and the extra power consumption createsquantities of substitute emissions of CO2, CO, SO2, NOX, dust, cooling water, in power stations, steelmills and other plants of raw material supply.

Hence the total balance of emissions for systems using vapour balancing and vapour recovery units inrelation to systems with floating roofs is negative. The big breathing volumes of fixed roof tanks (andgasholders) with high vapour concentrations do not lead to economical and ecological soundsolutions.

Fixed Roof Tank withVapour Balancing System,Gasholder,Multistage Vapour RecoveryUnit(η = 99,985%)

Floating Roof Tank withMultifold Seal,Vapour Draw-Off System,Single Stage VapourRecovery Unit(η = 97 %)

~1990

e ≈ 100-500 kg/yearη ≈ 99,4-99,9 % total

2004

e < 100 kg/yearη > 99,9 % total



Tank-Technik

IMHOF

6. Determination of Hydrocarbon Emission and Definition of FloatingRoof Efficiency

6.1 Calculation Method of API-Manual of Petroleum MeasurementStandards, Chapter 19, Section 1 and Section 2

6.2 Definition of Floating Roof Efficiency

The efficiency of a floating roof tank (external or internal) is defined as relation of emissions betweenthe floating roof tank in question and a fixed roof tank, equipped with pressure / vacuum valve only,having the same tank dimensions (diameter, height, volume) and the same yearly throughput with thesame storage product as in the floating roof tank.

Now calculated with Chapter 19 – Section 1

EmissionFormerly calculatedwith API 2518



Now calculated with Chapter 19 – Section 2, Edition April 1997



Tank-Technik

IMHOF

6.3 Floating Roof Efficiency

6.3.1 Efficiencies attainable of External Floating Roof Tanks, dependingon Turnover Rate and Tank Diameter

84

86

88

90

92

94

96

98

100

0 10 20 30

number of filling actions per year

effic

ienc

y of

EFR

T [%

]

D= 12,0 m;V= 1075 m³;H= 10,0 m

D= 15,0 m;V= 1847 m³;H= 11,0 m

D= 20,0 m;V= 3581 m³;H= 12,0 m

D= 30,0 m;V= 8730 m³;H= 13,0 m

D= 40,0 m;V= 16713 m³;H= 14,0 m

D= 50,0 m;V= 27980 m³;H= 15,0 m

D= 60,0 m;V= 42977 m³;H= 16,0 m

Calculation of efficiency according to API-Manual of Petroleum Measurement Standards Chapter 19, Sec. 1 and Sec. 2 (April 1997)

storage conditions:

product: gasolineReid vapor pressure: 600 mbaraverage wind speed: 3,0 m/sdaily average ambient temperature: 10,0 °C

All EFRT are equipped with shoe type primary seal plus rim-mounted secondary seal plus guidepole seal and roof leg seals.

1

1

2

2

3

3

456

4

5

7

6

7



Tank-Technik

IMHOF

6.3.2 Efficiencies attainable of Internal Floating Roof Tanks, dependingon Turnover Rate and Tank Diameter

88

90

92

94

96

98

100

0 10 20 30

number of filling actions per year

effi

cien

cy o

f IF

RT

[%

]

D= 12,0 m;V= 1075 m³;H= 10,0 m

D= 15,0 m;V= 1847 m³;H= 11,0 m

D= 20,0 m;V= 3581 m³;H= 12,0 m

D= 30,0 m;V= 8730 m³;H= 13,0 m

D= 40,0 m;V= 16713 m³;H= 14,0 m

Calculation of efficiency according to API-Manual of Petroleum Measurement Standards Chapter 19, Sec. 1 and Sec. 2 (April 1997)

storage conditions:

product: gasolineReid vapor pressure: 600 mbardaily average ambient temperature: 10,0 °C

All IFRT are equipped with shoe type primary seal plus guidepole seal and roof leg seals.

1

12

2

3

3

4

5

4

5



Tank-Technik

IMHOF

6.4 Emission rates per year and meter of rim space, given for standardseal types as tested by API

0,1

1,0

10,0

100,0

1000,0

0,00 2,00 4,00 6,00 8,00

wind velocity [m/s]

Em

issi

on o

f ri

m s

pace

[kg

/m*a

]

Shoe seal primary only (1)

Shoe seal + shoe mounted secondary (2)

Shoe seal + rim mounted secondary (3)

Liquid mounted primary seal only (4)

Liquid mounted primary + weather shield (5)

Liquid mounted primary + rim mountedsecondary (6)Vapor mounted primary seal only (7)

Vapor mounted primary + weather shield (8)

Vapor mounted primary + rim mountedsecondary (9)

Storage Product: GasolineRVP = 600 mbar

average stock temperature: TS= 14°C

7

1

8

9

2

4

5

3

6

Yearly Emission of Floating Roof Rim Spaces per Meter of Tank Circumference, depending on Type of Seal and Wind Velocity, according API - Manual of

Petroleum Measurement Standards Chapter 19, Section 2, April 1997 (Data calculated by IMHOF Tank-Technik)

The diagram shows the data for tight-fitting seals.

The data for average-fitting sealsor damaged seals are higher.

1 2 3

7 8 9

4 5 6



Tank-Technik

IMHOF

6.5 Evaluation of Emission by Practical Tests, carried out at TAMAG /Switzerland

Total storage capacity of tank farm: 750.000 m3 21 Fixed roof tanks; ∅ 44,0 m4 Fixed roof tanks; ∅ 19,6 m

Tank Service: 13 Tanks; ∅ 44,0 m gasoline 95 and 984 Tanks; ∅ 19,6 m gasoline 95 and 985 Tanks; ∅ 44,0 m heating oil3 Tanks; ∅ 44,0 m diesel oil

Hydrocarbon emission limit requested by the Swiss Health Agency (Lufthygieneamt) in 1990:

max. 3 kg / h for the complete tank farm at any time of the year, or alternativelymax. 150 mg / m3 outlet concentration at any tank.

Alternative Solutions taken in consideration:

Alternative 1:Closing of all ventilation openings of tanks, installation of vapour balancing pipework and installation ofvapour recovery unit (VRU)→ costs expected: ~ 20 Mio SFr

Alternative 2:Installation of improved sealing systems, and application of additional passive emission controlmeasures like shadowing, insulation, cooling of tanks, etc.→ costs expected: unknown

In view of the high installation and operation costs of alternative 1, TAMAG decided for alternative 2.The necessary emission tests were specified and carried out by EMPA (EidgenössischeMaterialprüfungsanstalt).

All tanks originally hadbeen equipped with pan-type, welded internalfloating roofs with conven-tional lip seals only. Theroofs are equipped withventilation openings.



Tank-Technik

IMHOF

Continuation: 6.5 TAMAG / Switzerland

Step by Step Improvement of Tank System and Test Results:

1991

In tank no. 12 (∅ 44,0 m), used for storage of gasoline 95, the lip-type seal was substituted by a liquid-mounted foam seal, type "Slimline" (see Fig. 1).

The emission measurement carried out in November showed values close to the allowable maximumbut was not acceptable to the authorities due to the low temperature level in November.

1992

Emission measurement at tank no. 12 was carried out in May.

During discharge of gasoline hydrocarbon concentration above the floating cover was found between300 and 1500 mg/m3 being 2 to 10 times higher than acceptable.

1993

Tank no. 8 was equipped with a "Slimeline" foam seal with double foam elements.Tank no. 9 was equipped with a double compression plate seal, type "Double-Flex A".

Emission tests were carried out in June and August; storage product was gasoline 95. No significantreduction of emissions was found in relation to the single foam filled primary seal of tank no. 12, testedin 1992.

1994

Tank no. 5 was equipped with a double seal IM-IGT + IM-WSII (see Fig. 1), supplied by IMHOF.

First emission tests showed a reduction of emission of approx. 50 % in relation to the tests in 1993.But it was uncertain if this reduction was sufficient at all times of the year.

1995

Tank no. 11 was equipped with a multifold sealing system IM-IGS + IM-WSII (see Fig. 1), supplied byIMHOF.

Emission tests were carried out between end of July and mid of August. Short after tank filling thegasoline temperatures reached 25 °C and the emission values were above limit. As from beginning ofAugust the gasoline temperatures dropped and the emission stayed well below its limit. In general theemission was approx. 50 % of the values given by the test in 1994.

1996:

Emission tests in tanks no. 5, 11 and 12 were carried out in parallel, during May to October. Thestorage product was gasoline 95, with slight differences in composition:

Tank no. 5 (∅ 44,0 m): Seal IM-IGT + IM-WSII (supply in 1994)Tank painted grey

Tank no. 11 (∅ 44,0 m): Seal IM-IGS + IM-WSII (supply in 1995)Tank roof painted white; Tank shell painted grey

Tank no. 12 (∅ 44,0 m): Seal type "Slimline" (supply in 1991)Tank roof painted white; Tank shell painted grey



Tank-Technik

IMHOF


In May the gas concentrations above the floating cover had been (see Fig. 2):Tank no. 5: 35 mg HC/m3

Tank no. 11: 20 mg HC/m3

Tank no. 12: 60 mg HC/m3

During July and August the gasoline temperature was well above 20 °C and the limits of emissioncould not be met. The customer decided to repeat this test with the same tanks in 1997 using a waterirrigation at the tank roofs.

1997:

Emission tests in tanks no. 5, 11 and 12 were carried out in parallel, during May to July. Although thetank roofs were irrigated with pond water 20 °C, the gasoline temperatures reached values of 23 °C.The emission values were below the values of 1996 but the limit of gas concentration of 150 mgHC/m3 could not be met at any time during this period.

During these tests IMHOF had carried out numerous temperature measurements at tank wallsinside and outside the tanks. On the base of these temperature measurements the customercould be convinced that it is necessary to paint the tanks completely white.

1998:

Emission tests in tanks no. 5 and 11, were carried out in parallel, during May to September.

Due to the fact that all tanks had been painted completely white the maximum temperatures of thestorage product stayed below 20 °C (see Fig. 3) and the requested maximum emission level of 3 kg/hfor the whole tank farm was met.

Furthermore in future years the tank farm would store gasolines according to EC-Directive 98/70 withapproximately 30 % lower vapour pressures.

Résumé:

The stringent limit of a maximum emission of 3 kg/h for the complete tank farm with 25 tanks, as wellas the limit for the vapour concentration above the floating cover (150 mg HC/m3), can be met:

♦ with direct contact floating cover (steel pan),

♦ with multifold periphery seals,

♦ with tanks painted white.



Tank-Technik

IMHOF


Fig. 1: Seals tested in parallel, 1996 and 1997, at TAMAG / Mellingen

Con

cent

ratio

n of

gas

[C/m

³]

Tank 12

Tank 5

Tank 11

200

180

160

140

120

100

80

60

40

20

0

Limit (equivalent to 150 mgHC/m³)

05.0

5.96

07.0

5.96

09.0

5.96

11.0

5.96

13.0

5.96

15.0

5.96

17.0

5.96

19.0

5.96

21.0

5.96

23.0

5.96

25.0

5.96

27.0

5.96

29.0

5.96

31.0

5.96

Fig. 2: Concentrations of gas above the floating cover, may 1996,TAMAG / Mellingen

Fig. 3: Influence of the tank colour (comparison of the emissions measured at tank no. 11 in 1997and 1998)

T a n k 1 1

10

12

14

16

18

20

22

24

23.

Jun

30.

Jun

07

. Ju

l

14

. Ju

l

21

. Ju

l

28

. Ju

l

04.

Aug

11.

Aug

18.

Aug

25.

Aug

01.

Sep

Date

Tem

per

atu

re [°

C]

0

4 0 0

8 0 0

1 2 0 0

1 6 0 0

2 0 0 0

2 4 0 0

Gas

con

cent

ratio

n

[mg

C/m

³]

Ave rage t empe ra tu reo f gaso l i ne 1997(on ly tank roo f wh i te )

Ave rage tempera tu reo f g a s o l i n e 1 9 9 8( tank comple te ly wh i te )

gas concent ra t ion 1997above f loat ing roof(on ly tank roof whi te)

gas concent ra t ion 1998above f loat ing roof( tank complete ly whi te)

→

→←

←



Tank-Technik

IMHOF

7. Limits of Floating Roof Technology

When storing hydrocarbon liquids in vertical tanks with floating roof (EFRT) or floating cover (IFRT)the efficiency of the system floating roof/ floating cover plus seals can always be upgraded with theapplication of better seals and/or multifold sealing systems.

The expected efficiencies are e.g.:

External Floating Roof Tank Internal Floating Roof Tank

Primary seal η = 88 % η = 95 %

Double seal + guide pole seal η = 97,5 % η = 99 %

Triple seal + guide pole seal η = 99,5 % η = 99,8 %

etc. etc.

For vertical storage tanks with diameters above approximately 8 m the application of improved sealingtechnologies, economically as well as ecologically always is the winning decision against systems withvapour balancing and vapour recovery, provided the temperature and the vapour pressure of the liquidstored stays below a certain limit.

For gasoline in winter quality having a Reid Vapour Pressure of 90 kPa as used in the past, this limit oftemperature was found in practical tests at 19.6 °C, with a vapour pressure of 48 kPa. Gasolineaccording to Euronorm 2000 will reach this vapour pressure at approximately 31 °C, which means thistemperature will not be reached under normal operating conditions.

The above limit was found for a fixed roof tank, 44 m diameter, with pan-type internal floating cover,equipped with a multifold sealing system.

We expect this vapour pressure of 48 kPa at storage temperature to be about the limit pressure forother storage products also, when using passive emission control measures only.

For hydrocarbon liquids having a vapour pressure higher than approximately 48 kPa at the expectedstorage temperature of the bulk liquid, active emission control with vapour recovery/ vapourdestruction is justified economically and ecologically.

But the use of vapour balancing, gasholders and vapour recovery units are not the only answer ofactive emission control for refinery tanks.

In many cases systems with floating roofs/ floating covers in the form of full-contact or steel-weldeddesign, combined with a continuous and small vapour draw-off from the interspace of multifold sealingsystems, followed by vapour recovery/ vapour destruction, is the more economical solution and leedsto much higher overall emission control.



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IMHOF

8. Possible Future Improvements of Passive Emission Control

Tests on fixed roof tanks with internal floating covers (TAMAG/ Switzerland) have shown a biginfluence of temporally and locally high tank wall temperatures at areas of high sun radiation. As longas the bulk liquid temperature stays below a certain limit temperature (see TAMAG report) the amountof emission is closely linked to respective tank wall temperatures. This can be explained by some kindof film evaporation at the inner tank wall. All other relevant temperatures (liquid bulk temperature,temperature of floating roof, temperature of vapour space above the floating roof) do not show a biginfluence on emission quantities. Therefore measures for passive emission control for fixed roof tanksare:

For floating roof tanks the characteristic of the seals (rim seal loss factor), the wind velocity and thevapour pressure (temperature) of the product stored have the highest impact on emission. Hence themeasures of passive emission control for floating roof tanks are:

♦ Solid design of internal floating cover usingfull contact covers, preferably steel-welded,pontoon-type or pan-type

♦ Multifold seal with submerged primary sealelement

♦ Tank paint with high solar heat reflection

and alternatively:♦ Application of sun shields or insulation of

tank walls and tank roof

♦ Multifold seals with submerged primaryseal element,

♦ Tank paint with high solar heatreflection

and alternatively:♦ Application of sun shields or insulation

of tank walls, respectively tank in tankdesign

♦ Natural cooling of floating roof byevaporation of rain water

sunshield

rainwaterevaporation

sunshield

insulation



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IMHOF

9. Risks of Tank Geometry and Rim Space Tolerances

Measurement of Tank Deformation

No tank code in the world, neither API 650, BS 2654; DIN 4119; EN 14015, nor any other tank codedealing with floating roof tanks can guarantee that a certain floating roof tank built in accordance withthose standards, and a standard floating roof seal installed will fit together to seal the rim space at allfloating conditions.The reason for this unsatisfying situation is that all existing tank codes bypass the realisticdefinitions of the floating roof system.The point were all tank codes are wrong is the fact that they all talk about “the rim space”, whichmeans a rim space at any point of the tank circumference. And they talk of deformations of the tankshell and tolerances of the tank radius or tank diameter only.These definitions do not give a complete base for the tolerances a floating roof seal has to deal with.Of course not only the tank shell but also the floating roof has its deformations and deviations againstthe design geometry. In addition to this the roof can float into any direction. This means all tolerancesof two diametrical points of the tank shell and the pontoon can add up at one side of the floating roofonly.The technically sound and successful evaluation of existing rim space dimensions to be used for thedesign of floating roof seals - according to our design criteria - is worked out as follows:

a) Add up the rim space measurements of two diametrical rim spaces (A1+A2, B1+B2, C1+C2,etc.) to find the max. total clearance between tank shell and floating roof at the tankcircumference. The distance between two measuring points at the tank circumference should notbe bigger than approx. 3 m.

b) Repeat the above measurement procedure for different heights of flotation.

A1

A2

B1

B2C1

C2

< 3

m



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IMHOF

Typical Tank Deformation

Data taken from an existing tankDesign Tank Diameter: 62,000 mDesign Rim Space: 275 mm

- Tank Diameter at Bottom- Tank Diameter at Top- Pontoon Diameter- Design Diameter

12 3

45

67

8

9

10

11

12

13

14

15

16

17

18

19

20

21

2223

2425

262728

293031

3233

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

4950

5152

53 54

RS I

RS II

Rim Space Measurement with Floating Roof in Top Position

RSI RSII RSI + RS II RSI RSII RSI + RSII____________________________________________________________________________

1 220 28 409 629 15 323 42 268 5912 235 29 357 592 16 344 43 305 6493 211 30 398 609 17 363 44 236 5994 223 31 421 644 18 382 45 262 6445 276 32 451 727 19 364 46 274 6386 235 33 382 617 20 362 47 290 6527 228 34 272 500 21 369 48 245 6148 250 35 240 490 22 330 49 224 5549 230 36 240 470 23 351 50 213 564

10 251 37 263 514 24 366 51 205 57111 246 38 284 530 25 335 52 196 53112 284 39 345 629 26 294 53 146 44013 344 40 360 704 27 341 54 144 48514 312 41 328 640



Tank-Technik

IMHOF

Definition of Rim Space Tolerances

By working out the above data for a certain tank you get the successful design base for the floatingroof seal.Hence the correct definition of tolerances of a floating roof system would read as follows:

RSn = nominal Rim Space or design Rim Space

A1 = measured rim space at a certain point at tank circumference

A2 = measured rim space at a point diametrical to A1

tD = total tolerance or diametrical tolerance of a floating roof system

It would be very much advisable for the owner of a tank farm to ask a tank builder to specifyand guarantee his value of tD for the total clearance between tank shell and floating roof beforean order for a new tank construction is placed.

And of curse it is necessary to inform the seal supplier about this value of tD the tank builder is able toguarantee.A workable value for tD will be:

Especially for individual and spring forced primary or secondary seals or compression plate seals, theknowledge of the possible max. rim space is an absolute must.Typical shoe type seals with pantograph system or similar do not encounter the same risks of damageas individual spring forced seals. But the risk of the pantograph type system is that the seal may notspan over extreme rim spaces, with limited sealing efficiency, when the effective value of tD isunknown.

DTank – DRoof = A1 + A2 = 2 RSn + tD

DTanktD = ____________

1000



Tank-Technik

IMHOF

10. Centring of Floating Roof

Proper sealing of a floating roof rim space as well as the general life expectance of a sealing systemare based on adequate and sufficient centring of the floating roof.Foam filled primary seals and spring loaded seals can offer sufficient centring forces. Liquid filledseals provide little centring forces and require perfect tank roundness.The centring characteristic and the specific centring forces of a certain sealing system can bedescribed by comparing the compression forces existing at two diametrical points of the floating roof,as shown in the following diagram. Sealing systems with good centring characteristic showprogressive increase of sealing forces when the rim space is reduced.

Diagram of Centring Forces

RS 1 RS 2

Centring Forces

Compression Forces at RS 2


Com

pres

sion

For

ces

RS1 minRS2 max

RS norm RS1 maxRS2 min

Centring Forces



Com

pres

sion

For

ces

RS1 minRS2 max

RS norm RS1 maxRS2 min

System with poor centring System with good centring



Tank-Technik

IMHOF

11. Standard Requirements for Floating Roof Seals

The standard requirements for floating roof seals can be described as follows:

♦ Ample working capacity and safe function of tank seal under extreme rim space variations.♦ Outstanding reduction of emission.♦ Water free storage in case of "white" storage products.♦ Wax free tank walls and floating roof deck in case of crude oil tanks.♦ Perfect centring of floating roof.♦ Minimum loss of storage capacity (by low height secondary seals).♦ Ease of fire fighting.♦ Application of all relevant safety rules.♦ Guarantee for gasfreeness when tank is emptied.♦ Ease of repair and long service life expectance.

12. Design Criteria of Floating Roof Seals as given by EN 14015

The clearance between the floating roof and tank wall shall allow the floating roof to slideup and down the tank wall. The seal design shall prevent the vapours of the productcontained therein from escaping. The rim seal shall also prevent the penetration of rainwater into the tank.

The seals shall take account of the nature , characteristics and the temperature of the liquidto be stored.

Seals shall be designed to:

♦ withstand friction against the tank shell;

♦ be resistant to the products contained in the tank;

♦ tolerate the construction tolerances in the tank shell and the floating roof;

♦ tolerate within limits, the lateral movements of the floating roof;

♦ tolerate deformations caused by changes in the climatic conditions.

In order to achieve the best possible sealing, a lower element of the seal shall project intothe stored liquid, close to the tank shell.

NOTE: Metallic seal elements completely covering the rim space between floating roofand tank shell should be equipped with foam ports to allow the entry of fire-fighting foam under fire conditions.

All metal parts of the seals shall be earthed and all non-metals parts shall be of anti-staticquality. Low sparking and low corrosion metals shall be preferred.

Magnesium alloys, copper and copper alloys shall not be used.



Tank-Technik

IMHOF

13. Highlights of IMHOF seals

We have the concepts of success for your storage tanks with floating roofs (external or internal). Highstandards of process safety and environmental protection combined with low costs are not in contrast.The successful sealing of a floating roof is no matter of art. However, it requires the solution of a set ofstandard problems of mechanics and materials as well as the solution of specific problems of tankoperation.Here you can see some examples of successful innovations of IMHOF:

1. Water seals for all "white" storage productsSo far, tank owners assumed that water-sensitive storage liquids like finished products, gasoline,alcohol, MTBE, etc. should not be stored in open top floating roof tanks. The reason is the unavoidablypenetration of quantities of rain water along the vertical inner tank walls into the storage product.Besides the damage of storage product by the rain water entered, the tank bottom will suffer fromincreased corrosion.In addition the necessary rain water drainage and waste water treatment will lead to further costs ofoperation.

All these problems of floating roof tanks are solved successfully by IMHOF since more than 20 yearsby the use of secondary seals with a water-absorbent felt element contacting the tank wall. With thiscontact element all rain water will be removed from the inner tank wall and is directed to the top of thefloating roof.Hence, the suitable secondary seal of IMHOF can avoid further investments for an additional tank roofmade of steel or aluminium. In most cases an additional roof made of steel is not possible because ofproblems of statics of the tank structure. An aluminium dome roof can avoid these problems of staticsand will lead to lower investment costs. But in general aluminium roofs in petro chemical plants involveextreme risks of fire.

Note: Burning aluminium sheets fly like paper, with temperatures above 2000 °C, setting in fire vastareas of plant.

Fig. 4 and 5 show suitable secondary seals with water-absorbent felt elements.

Fig. 4: Secondary seal IM-WSW Fig. 5: Secondary seal IM-WW



Tank-Technik

IMHOF

2. Seals with wax scrapers for crude oil tanksIn crude oil storage tanks you often find heavy soiling of tank walls and floating roof, when storingcrude with high paraffin wax. In extreme cases the floating roof carries big quantities of paraffin waxand the rain water drainage system is inoperative.By the application of suitable shoe type seals with spring-loaded wax scrapers tank walls and floatingroofs keep clean from wax (see Fig. 6, 7, 8).

Fig. 6: IM-GLXS2 (with primary curtain instandard position)

Fig. 7: IM-GHXS2 (with primary curtain in lowposition)

Fig. 8: Efficiency of wax scraper equipment by a practical example

Crude oil tank with conventional shoe type seal. The same tank after installation of a shoe typeseal with wax scrapers.



Tank-Technik

IMHOF

3. Seals with progressive centring forcesIn case of tanks with extreme rim space deviations (e.g. due to tank deformation by fire effect orfoundation setting) standard rim seals are unsuitable. In these cases only shoe type seals of specialdesign incorporating high centring forces are satisfying. IMHOF developed seals for extreme changesof rim space (e.g. 100 - 600 mm) and with progressively increasing centring forces (see item 11;Diagram of centring forces).

4. Secondary seals with small working heightThe interest of the tank owner for maximum use of tank volume and the request of the fire-brigade forfast and effective fire fighting ask for small sealing heights over the top end of the pontoon.

IMHOF developed suitable systems for this problem

♦ by the application of U-shaped compression springs for separate secondary seals (see Fig.4above), or

♦ by the application of integrated primary - secondary seals, based on a primary shoe type seal (seeFig. 6 and 7 above).

5. Guide pole sealsAs calculations based on the API show, the emission losses from vertical roof openings which are notspecifically sealed are much higher than the losses from a floating roof rim gap with double seal. Theemission losses from gauge and guide poles as well as from the roof legs are particularly excessive.In order to avoid those emission losses, IMHOF can offer corresponding seals as seen in Fig. 9 (guidepole seal) and Fig. 10 (roof leg seal).

Fig. 9: Gauge and guide pole sealtype IM-PRS

Fig. 10: Roof leg sealtype IM-RDS

Floatingroof deck

membrane

Hoseclamp

Fabricreinforced

cap

Roofleg

Sealing ring

Submergedsleeve

Sealing ring

Gauge andguide pole

Guide profile

Sliding plate

Pontoon



Tank-Technik

IMHOF

6. High efficient sealsThe continuous request for substantial reduction of hydrocarbon emission in many cases resulted indrastic cost increases for process plants and utilities. In most cases the fight for extreme reductions ofone substance of emission resulted in much higher emissions of other air pollutants like CO2, SO2,NOX, dust, etc. at some other places (e.g. power plant, steel mill etc.).This problem does not exist for a floating roof tank with appropriate sealing system. As IMHOF provedseveral times, each request for emission reduction can be met by good engineering and high efficienttank seals, without substantial increase of costs.(see results at TAMAG / Switzerland, item 6.5).

7. Potential further improvementsFor even stronger requirements of emission delimitation from tanks with floating roof IMHOFalready marked the way for further useful improvements under cost aspects.

♦ Reduction of heating up of storage product by the use of the suitable natural cooling, as well as byshadowing or isolation of certain tank walls.

♦ Continuous evacuation of small quantities of product vapours from an interspace of the sealingsystem and economical destruction/recuperation with well-known process methods.



Tank-Technik

IMHOF

14. Reduction of Emission and Costs involved

14.1 Floating Roof Tank with Ø 20,0 m, Storage of GasolineStanding Storage Losses calculated acc. to API-Manual of PetroleumMeasurement

6.680

4.282

49

121533

112

0

2.000

4.000

6.000

8.000

10.000

12.000

Situation before erection of improvedsealing systems at guide pole and roof

legs - Rim seal consisting of primaryshoe type seal only

Situation after erection of improvedsealing systems at guide pole and roof

legs - Double rim seal consisting ofshoe type primary seal plus rim

mounted secondary seal

Hyd

roca

rbon

em

issi

on [

kg/a

]

Emission of roof legs

Emission of rim space

Emission of guide andmeasurement pole,slotted version

Storage data used:RVP = 600 mbarv = 3,73 m/s (mean wind speed)Ts = 10,0 °C (mean temperature)

* Required investment forsupply and erection ofimproved system



Tank-Technik

IMHOF

14.2 Floating Roof Tank with Ø 50,0 m, Storage of GasolineStanding Storage Losses calculated acc. to API-Manual of PetroleumMeasurement

6.680

10.705

152

1211.332

345

0

2.000

4.000

6.000

8.000

10.000

12.000

14.000

16.000

18.000

20.000

Situation before erection of improvedsealing systems at guide pole and rooflegs - Rim seal consisting of primary

shoe type seal only




Hyd

roca

rbo

n e

mis

sio

n [

kg/a

]


Emission of rimspace

Emission of guideand measurementpole, slotted version

Storage data usedRVP = 600 mbarv = 3,73 m/s (mean wind speed)Ts = 10,0 °C (mean temperature)




Tank-Technik

IMHOF

14.3 Floating Roof Tank with Ø 50,0 m, Storage of Crude OilStanding Storage Losses calculated acc. to API-Manual of PetroleumMeasurement

2.458

3.939

56

44490

127

0

1.000

2.000

3.000

4.000

5.000

6.000

7.000

Situation before erection of improvedsealing systems at guide pole androof legs - Rim seal consisting of

primary shoe type seal only

Situation after erection of improvedsealing systems at guide pole and

roof legs - Double rim seal consistingof shoe type primary seal plus rim


Hyd

roca

rbo

n e

mis

sio

n [

kg/a

]


Emission of rim space

Emission of guide andmeasurement pole,slotted version





Tank-Technik

IMHOF

14.4 Floating Roof Tank with Ø 70,0 m, Storage of Crude OilStanding Storage Losses calculated acc. to API-Manual of PetroleumMeasurement

2.458

5.515

90

44686

201

0

1.000

2.000

3.000

4.000

5.000

6.000

7.000

8.000

9.000

Situation before erection of improvedsealing systems at guide pole and rooflegs - Rim seal consisting of primary

shoe type seal only




Hyd

roca

rbon

em

issi

on [

kg/a

]


Emission of rimspace

Emission of guideand measurementpole, slotted version





Tank-Technik

IMHOF

15. Sealing Membrane Materials

Recommended membrane materials for Primary Seals

Storage liquid (under ambient temperature) With abrasion *(e.g. IM-PNS)

Without abrasion **(e.g. IM-PGT)

♦ Crude oil, Heating oil, Jet fuel, Diesel,Gasolines with up to 40 % aromatics

PU ~ 1,0 mmNBR 2-3 mm

PU ~ 0,6 mmNBR ~ 1,5 mm

♦ Gasolines/hydrocarbons with 40 – 80 %aromatics

PU ~ 1,0 mm PU ~ 0,6 mm

♦ Hydrocarbons with > 80 % aromatics andpure Benzene, Toluene, Xylene

---PTFE ~ 0,3 mmPU ~ 1,0 mm

♦ Gasoline with alcohol, pure alcohol NBR 2-3 mmNBR ~ 1,5 mmCSM ~ 0,7 mm

♦ Gasoline with MTBE PU ~ 1,0 mm PU ~ 0,6 mm

♦ Pure MTBE ---PTFE ~ 0,3 mmPU ~ 1,0 mm

♦ Acidic liquids and sea water --- CSM ~ 0,7 mm

♦ Ketones, esters, nitro- and chlorinatedhydrocarbons

--- PTFE ~ 0,3 mm

Recommended membrane materials for Secondary Seals

Storage liquid (under ambient temperature) With abrasion *(e.g. IM-WS2)

Without abrasion **(e.g. IM-WSW)

♦ Crude oil, Heating oil, Jet fuel, Diesel,Gasolines with up to 40 % aromatics

PU ~ 1,0 mmNBR 2-3 mmCSM + PU

CSM ~ 0,7 mm

♦ Gasolines/hydrocarbon with 40 – 80 %aromatics

PU ~ 1,0 mmNBR 2-3 mmCSM + PU

CSM ~ 0,7 mm

♦ Hydrocarbons with > 80 % aromatics andpure Benzene, Toluene, Xylene

PU ~ 1,0 mm PU ~ 1,0 mm

♦ Gasoline with alcohol, pure alcoholNBR 2-3 mmCSM + PU CSM ~ 0,7 mm

♦ Gasoline with MTBE PU ~ 1,0 mm CSM ~ 0,7 mm

♦ Pure MTBE PU ~ 1,0 mm CSM ~ 0,7 mm

♦ Acidic liquids and sea water NBR 2-3 mm CSM ~ 0,7 mm

♦ Ketones, esters, nitro- and chlorinatedhydrocarbons

NBR 2-3 mm CSM ~ 0,7 mm

* with contact of sealing membrane to the tank wall** without contact of sealing membrane to the tank wall

Documents

Floating Roof Crude Tank-Workshop