mp14p01

Version 0

Aboveground Welded Steel Storage Tanks - Design and Construction

MP 14-P-01

July 1998

Scope

This ExxonMobil Engineering Practice (EMEP) covers basic requirements for the design, fabrication, erection and testing of welded, cylindrical above-ground vertical atmospheric [up to 17 kPa (2.5 psig)] storage tanks fabricated of carbon steel and austenitic stainless steel. This document applies only to tanks whose entire bottom is uniformly supported. Tanks for refrigerated products below a temperature of 5C (41F) are not included.

MP 14-P-01 Aboveground Welded Steel Storage Tanks - Design and Construction July 1998

ExxonMobil Oil, 1997 2 of 44

Table of Contents

Scope.................................................................................................................................................. 1

Table of Contents............................................................................................................................. 2

Table of Figures ............................................................................................................................... 5

Table of Tables ................................................................................................................................. 6

1. References ................................................................................................................................. 7

1.1. Master PracticesExxonMobil Engineering Practices .............................................. 7

1.2. ExxonMobil Data Sheets ............................................................................................... 7

1.3. ExxonMobil Practice Tutorials ...................................................................................... 8

1.4. NFPANational Fire Protection Association............................................................... 8

1.5. APIAmerican Petroleum Institute ............................................................................... 8

1.6. ASCEAmerican Society for Testing Material ........................................................... 8

1.7. ASMEAmerican Society of Mechanical Engineers ................................................. 8

1.8. ASTMAmerican Society for Testing and Material ................................................... 9

2. General........................................................................................................................................ 9

2.1. Tanks for Production Liquids ........................................................................................ 9

3. Design........................................................................................................................................10

3.1. Design Loading .............................................................................................................10

3.2. Tank Bottoms ................................................................................................................10

3.3. Shells ..............................................................................................................................10

3.4. Roof ................................................................................................................................10

3.5. Nozzles...........................................................................................................................11

4. Materials ....................................................................................................................................11

4.1. Bolting .............................................................................................................................11



5. Corrosion Protection.............................................................................................................11

6. Fabrication, Erection, Welding and Tolerances.............................................................12

6.1. Fabrication and Erection .............................................................................................12

6.2. Welding ..........................................................................................................................12

6.3. Tolerances .....................................................................................................................13

7. Inspection and Testing .........................................................................................................13

7.1. Radiographic Inspection..............................................................................................13

7.2. Materials Identification.................................................................................................14

7.3. Testing ............................................................................................................................14

8. Tank Appurtenances .............................................................................................................15

8.1. Connections and Manholes ........................................................................................16

8.2. Venting ...........................................................................................................................16

8.3. Loss Prevention Systems............................................................................................17

8.4. Stairways, Ladders and Platforms .............................................................................17

Appendix A: Floating Roof Requirements .............................................................................19

1. External Floating Roofs (for Open-Top Tanks)..............................................................19

1.1. Design ............................................................................................................................19

1.2. Seals ...............................................................................................................................19

1.3. Vertical Guide Pipe.......................................................................................................20

1.4. Drains .............................................................................................................................21

1.5. Fire Protection Systems ..............................................................................................21

2. Internal Floating Roof Tanks ...............................................................................................22

Appendix B: High Strength Steel Requirements..................................................................25

1. Design........................................................................................................................................25

2. Materials ....................................................................................................................................25



3. Welding and Weld Inspection .............................................................................................26

3.1. Welding ..........................................................................................................................26

3.2. Weld Inspection ............................................................................................................27

4. Erection.....................................................................................................................................27

5. Coatings....................................................................................................................................27

Appendix C: Figures ....................................................................................................................28

Appendix D: ExxonMobil Hydrostatic Tank Leak Test .......................................................43

1. Background..............................................................................................................................43

2. Applicability .............................................................................................................................43

3. MHTLT Procedure:.................................................................................................................43



Table of Figures

Figure 1: ExxonMobil Hydrostatic Leak Test Connections............................................... 16

Figure 2: Shell Insulation Flashing ..........................................................................................18

Figure A1: Catenary Foam System - Ladder Piping Details ........................................... 23

Figure A2: Foam Standpipe with Baffle Figure ..................................................................24

Figure C1: Location of Shell and Bottom Connections and Appurtenances Typical Layout and Dimensions............................................................................. 28

Figure C2: Location of Roof Connections and AppurtenancesTypical Layout and Dimensions.................................................................................................................. 30

Figure C3: Water Drawoff ArrangementsFor Nonfreezing Areas .............................. 32

Figure C4: Water Drawoff ArrangementsFor Freezing AreasType 1.................... 33

Figure C5: Separate Suction and Inlet Piping..................................................................... 34

Figure C6: Separate Inlet Piping for Floating Roof Tanks............................................... 35

Figure C7: Combination Suction and Inlet Piping ............................................................. 36

Figure C8: Stripping Arrangements ...................................................................................... 37

Figure C9: Gauging Datum Plate ............................................................................................ 37

Figure C10: Aluminum Dome Roofs...................................................................................... 38

Figure C11: Storage Tanks Double Steel Bottom with Liner and Leak Detection .... 40



Table of Tables

Table 1 .14

Table A 1: Minimum Guide Pipe Size According to Tank Size.......................................20

Table A 2: Foam and Access Appurtenance .......................................................................21

Table C 1 ........................................................................................................................................33

Table C 2 ........................................................................................................................................34

Table C 3 ........................................................................................................................................36

Table C 4 ........................................................................................................................................39

Table C5 .41



1. References

The following publications form a part of this Practice. Unle ss otherwise specified herein, use the latest edition.

1.1. Master PracticesExxonMobil Engineering Practices

MP 00-P-10 Meteorological Data for Plant Design

MP 01-P-08 Foundation Design

MP 01-P-14 Containment Systems for Aboveground Storage Tanks

MP 03-P-03 Positive Materials Identification

MP 32-P-22 Storage Tank Inventory Instrumentation

MP 70-P-02 Fire Protection - Storage Tanks

1.2. ExxonMobil Data Sheets

ExxonMobil Data Sheets

ExxonMobil Data Sheet Home Page

T1401C01 Aboveground Welded Steel Storage Tanks - Design & Construction - Customary Units

T1401M01 Aboveground Welded Steel Storage Tanks - Design & Construction - Metric Units

T1401C02 Aboveground Welded Steel Storage Tanks - Design & Construction - Customary Units

T1401M02 Aboveground Welded Steel Storage Tanks - Design & Construction - Metric Units

D1401C01 Aboveground Welded Steel Storage Tanks - Documentation Requirements Sheet

D1401C02 Aboveground Welded Steel Storage Tanks - Documentation Requirements Sheet

I1401C01 Aboveground Welded Steel Storage Tanks - Inspection & Testing Requirements Sheet

I1401C02 Aboveground Welded Steel Storage Tanks - Inspection & Testing Requirements Sheet



1.3. ExxonMobil Practice Tutorials

EPT 01-T-04 Thermal Insulation

EPT 16-T-01 Tank Heating and Heat Loss

1.4. NFPANational Fire Protection Association

NFPA 11 Standard for Low-Expansion Foam (National Fire Codes, vol. 1)

NFPA 30 Flammable and Combustible Liquids Code (Includes Tentative Interim Amendment - 1996) (National Fire Codes, vol. 1)

1.5. APIAmerican Petroleum Institute

API SPEC 12D Specification for Field Welded Tanks for Storage of Production Liquids Tenth Edition

API SPEC 12F Specification for Shop Welded Tanks for Storage of Production Liquids Eleventh Edition

API STD 650 Welded Steel Tanks for Oil Storage Ninth Edition; Addendum 1 - 1994; Addendum 2 - 1995; Addendum 3 - 1996

API STD 653 Tank Inspection, Repair, Alteration, and Reconstruction Second Edition; Errata February 1996; Errata May 1996; Follow Up to Errata May 1996; Addendum 1-December 1996

API STD 2000 Venting Atmospheric and Low-Pressure Storage Tanks Nonrefrigerated and Refrigerated Fourth Edition

1.6. ASCEAmerican Society for Testing Material

ASCE 7 Minimum Design Loads for Buildings and Other Structures

1.7. ASMEAmerican Society of Mechanical Engineers

ASME SEC IX BPVC SECTION IX Qualification Standard for Welding and Brazing Procedures, Welders, Brazers, and Welding and Brazing Operators Addenda - 1995; Addenda - 1996; Interfiled (Boiler and Pressure Vessel Codes)



1.8. ASTMAmerican Society for Testing and Material

ASTM A123 REV A

Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products (AASHTO M111)

ASTM A153/A153M

Standard Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware (AASHTO No. M232)

ASTM A307 Standard Specification for Carbon Steel Bolts and Studs, 60 000 psi Tensile Strength

2. General

Tanks shall be designed, fabricated, inspected and tested in accordance with API STD 650 and as modified herein.

The design, fabrication, erection and testing of welded, cylindrical above-ground vertical atmospheric storage tanks fabricated of carbon steel and austenitic stainless steel shall be in accordance with requirements of this EMEP, unless superceded by more stringent local regulations.

2.1. Tanks for Production Liquids

For tanks for production liquids ordered to API SPEC 12D or API SPEC 12F, the following apply:

Maximum capacity 477 m3 (3,000 bbl).

Consult MP 70-P-02 for spacing, layout and fire protection.

Painting and coating specifications are required.

Frangible roofs are required for 12D tanks.

No center roof supports for diameters less than 6.7m (22 ft).

Qualification of welders and welding procedures shall be in accordance with the ASME SEC IX.

Consider high level alarms.

Tanks with steels having minimum specified yield strengths of 262 MPa (38,000 psi) and greater shall be in accordance with Appendix B.



3. Design

3.1. Design Loading

When design for earthquakes is required, the design criteria of API STD 650, Appendix E, shall be used.

Wind and earthquake loading shall not be applied simultaneously.

Either overturning stability shall be verified on the empty tanks in the corroded condition or anchorage shall be provided.

3.2. Tank Bottoms

When leak detection and/or secondary containment are required, one of the following alternatives shall be specified:

1. An elevated tank bottom (generally for small tanks on concrete pads).

2. Impervious concrete or an impervious liner in the foundation (refer to MP 01-P-14).

3. A double steel bottom (refer to Figure C-11).

3.3. Shells

The tank nameplate shall show the maximum permissible product specific gravity based on actual shell thickness less corrosion allowance.

Calculations for intermediate wind girders for tanks where the shell corrosion allowance exceeds 1.5 mm (1/16 in) shall be based on the nominal shell thickness minus the corrosion allowance.

3.4. Roof

Freely vented cone roofs shall be frangible in accordance with API STD 650.

External and internal floating roofs shall be in accordance with Appendix A.

Aluminum dome roofs shall be in accordance with Figure C-10.

Where an internal coating for a fixed carbon steel roof is required, one of the following shall be used:



a) A steel roof with any support members being external, usually a dome or umbrella roof. Roof plates shall be fillet welded on both sides or butt welded. Any support members shall be continuously welded to the roof and have drain slots.

b) An aluminum dome roof. 44

3.5. Nozzles

When subsurface foam injection is specified, connections shall be provided in accordance with NFPA 11.

The tank vendor shall provide the following API STD 650, Appendix P information for bottom shell course nozzles NPS 6 and larger on tanks with design temperatures greater than 66C (150F) or of high strength steels (refer to Appendix B):

Nozzle rotations, deflections and stiffness coefficients

Nomograms of the allowable loads (refer to API STD 650, Figures P-5A and P-5B)

4. Materials

A carbon steel material other than those listed in API STD 650 may be used with prior ExxonMobil approval.

For Steels having minimum specified yield strength of 262 MPa (38,000 psi) and greater, refer to Appendix B.

For austenitic stainless steel tanks, the type of steel will be specified on the ExxonMobil Data Sheets for this Master Practice. Substitution requires ExxonMobil approval.

4.1. Bolting

All carbon steel bolting for flanged connections shall conform to ASTM A307, Grade B as a minimum. Bolts shall consist of regular unfinished machine bolts with one heavy semifinished hex nut.

5. Corrosion Protection

A minimum corrosion allowance of 1.5 mm (1/16 in) shall be provided on carbon steel shells, bottoms and welded components contacted by the liquid (except floating roofs) except where internally coated. Corrosion allowances specified on the ExxonMobil Data Sheet (ExxonMobil Data Sheets for this Master Practice) shall govern if larger.



When underbottom cathodic protection is required, an impressed current system is recommended.

6. Fabrication, Erection, Welding and Tolerances

6.1. Fabrication and Erection

No holes shall be made in shell plates for erection purposes.

After windgirder completion, the windgirders shall be flooded with water and areas that accumulate water shall be marked. A drain hole of 16mm (5/8 in) minimum diameter shall be located in the low spot of each area, prior to painting, to completely drain the girders.

6.2. Welding

Flux cored welding and electrogas welding require The Companys approva l.

Downhill welding is not permitted on vertical shell seams without The Companys approval.

Fillet welds for external attachments shall be continuous. Wind girders shall be seal welded to the shell on their underside.

Weld interpass temperature for stainless steel is limited to 177C (350F).

Internal and external welds to be coated or painted shall have neither sharp edges nor excessive weld metal buildup.

Supplemental welding requirements applicable to shell joints are as follows:

1. Square-groove butt joints shall be permitted to a maximum plate thickness of 6 mm (1/4 in). This may be increased to 9 mm (

3/8 in) plate thickness if automatic welding machines are used, or if one side of the weld is back-gouged to form a groove.

2. Automatic welding of vertical joints in tanks built to API STD 650, Appendix A or J, does not require The Companys approval. In all other cases, automatic welding of vertical joints is limited to tank minimum design metal temperatures warmer than 10C (50F) and is not permitted unless approved by The Company. Submittals for approval shall include the following information:

a) Minimum expected Charpy V-notch impact energy of base material, weld material and heat affected zone at the minimum design temperature.

b) Welding procedures proposed, including heat treatment if any, and the resultant mechanical properties of welded joints.



c) Methods of quality control during tank erection, such as production weld test plates, ultrasonic inspection and magnetic particle inspection.

d) Lists of other tanks with similar materials and procedures.

When impact testing of the base material is required, production weld test plates shall be made and tested, and results approved by The Company prior to tank erection as follows:

1. The test plates made shall represent the bottom-course, mid-course and top-course vertical and horizontal weld seams.

2. Each test plate shall be from one of the heats that produced the plates for the tank. It shall be welded, in the field, using the same qualified welding procedure, joint detail, number of weld layers, position and consumables required for the tank joint.

6.3. Tolerances

Dimensional tolerances shall be in accordance with API STD 650, with the following modifications:

1. Tolerance for banding shall be 9 mm (3/8 in) maximum.

2. Tolerance for out-of-plumbness of the top of the shell relative to the bottom of the shell shall not exceed 50 mm (2 in).

3. No single shell course shall be out-of-plumb by more than 1/200 of the course height. Plumbness shall be measured and reported to The Company immediately after erection of each shell course at intervals not to exceed one half of the shell plate width.

Cone roofs shall have no depressions in which water can accumulate.

7. Inspection and Testing

7.1. Radiographic Inspection

Radiographic inspection of butt joints shall be in accordance with API STD 650 and the following supplemental requirements:

1. The film for spot radiographs at junctions between vertical and horizontal joints shall be positioned with the greater length on the vertical seam.

2. For automatically welded horizontal joints, one spot radiograph shall be taken in the first 3 m (10 ft) of weld after each setup of the automatic welder.

3. For automatically welded vertical joints in API STD 650, Appendix A, the requirements of API STD 650, Paragraph 6.1.2.2.b shall apply.



4. For manually welded joints, radiographic inspection requirements shall apply to each welder.

7.2. Materials Identif ication

For stainless steels, this shall be in accordance with MP 03-P-03.

7.3. Testing

Tank bottom welds shall be tested using vacuum test equipment with a vacuum of 35 kPa to 48 kPa (5 psi to 7 psi). The bottom-to-shell weld(s) shall be tested using penetrating oil after completion of the inner fillet weld for 12 hours. The external fillet weld shall be either magnetic or dye penetrant tested.

Before any internal coating is applied, the complete tank shall be hydrotested, using fresh water, as follows:

1. Complete all tank fabrication.

2. Perform nondestructive examinations, including a vacuum box test of new floor welds. Repair defects and retest.

3. Conduct a full hydrostatic test.

4. Conduct a ExxonMobil Hydrostatic Tank Leak Test (MHTLT), if available, as the last part of the hydrostatic test, per Appendix D of this EMEP. If the MHTLT is not available, observe the level change of the full tank, as follows:

Table 1

Tank Diameter Holding Time

21 m to 30.5 m (70 to 100 ft) 5 days

>30.5 m (100 ft) 7 days

5. Repeat the vacuum box test of new floor welds. Repair and retest with vacuum box.

6. Abrasive-blast and apply a lining (if required).

7. Conduct a holiday pinhole test of the lining.

8. Fill tank with water.

9. Notes:



a) Where a floating roof is to be installed and the bottom is to be lined, the lining may be applied prior to roof installation with proper protection of the lining during roof installation. Alternatively, the lining may be applied after roof installation. The lining may be applied prior to roof perimeter seal installation to increase ventilation.

b) All welding or cutting in the tank shall be completed prior to conducting the hydrostatic test or MHTLT. Any welding or cutting performed after the above -noted operations shall be done with The Companys supervision. Alternatively, the MHTLT may be performed as the last step before returning the tank to service.

For austenitic stainless steel tanks, the chloride content shall be less than 50 ppm.

Tanks shall be tested for bottom leaks using The Companys Hydrostatic Tank Leak Test in accordance with Appendix D or a ExxonMobil approved alternative test.

Hydrotest settlement readings shall be taken for every 2.4 m (8 ft) fill increment and just before draining is begun at the following number of equal spaces (N) around the tank:

N = D/3 (D = diameter in meters)

N = D/10 (D = diameter in feet)

Round N to the nearest even integer but no less than four. If the allowable differential settlements (refer to MP 01-P-08) are exceeded, filling shall be stopped and ExxonMobil notified. Hydrotest water shall be held for one day unless settlement is occurring.

All of the above tests shall be performed prior to any painting or coating of the welds being tested, except as noted in 7.3, Item 9(b) or if otherwise specified.

8. Tank Appurtenances

Appurtenances shall be furnished as specified in the ExxonMobil Data Sheets for this Master Practice.

Shell and bottom connections and appurtenances shall be in accordance with Figure C-1 unless otherwise specified.

Roof connections and appurtenances shall be in accordance with Figure C-2 unless otherwise specified.



8.1. Connections and Manholes

On the shell, no screwed connections larger than NPS 2 shall be permitted, except for the non-freezing type of water draw-off valves. Nozzles shall be of the API regular type.

Water drawoff valve connections shall be in accordance with Figure C-3.

Connections for The Company Hydrostatic Tank Leak Test shall be provided in accordance with Figure 1, unless otherwise specified.

Flange bolt holes shall straddle center lines.

Figure 1: ExxonMobil Hydrostatic Leak Test Connections

8.2. Venting

Vents for fixed roof tanks shall be sized in accordance with API STD 2000.

- Flame arresters shall not be supplied unless specified and approved by The Company.



- Coarse mesh stainless screens shall be supplied on all vents not susceptible to plugging.

- Tank vents shall not be located closer to stairs or platforms than shown in Figure C-2.

Fixed roof tanks subject to pressure surges (e.g. hot oil, slop-oil and relief tanks) shall have added venting capacity. A minimum of one venting-type roof manway shall be provided.

8.3. Loss Prevention Systems

Fire protection systems and tank grounding systems shall be in accordance with MP 70-P-02.

8.4. Stairways, Ladders and Platforms

Open-grating stair treads on stairways and rolling ladders and grating-type floor plates on platforms and walkways shall be galvanized, except as follows:

a) Galvanized parts shall not be in contact with stainless steel parts. Bolted attachments with insulators may be used.

b) In locations with sour (H2S) fluids, structures shall be painted and not galvanized.

Galvanizing shall be in accordance with ASTM A123 REV A for grating and structural parts, and with ASTM A153/A153M for hardware.

Railings conforming to local requirements shall be provided on the open sides of all exposed stairways, stair platforms and gauging/sampling areas on cone roofs (refer to Figure C-2). An intermediate rail is not required unless the spacing of the posts exceeds 225 mm (9 in) on center.

When the roof slope of a cone roof exceeds 1:12 or snow or icing conditions are expected, access to roof gauges, gauge hatches and the manway or combination manway/vent nearest the roof stair shall be provided by platforms with grating.

A fixed stair rise shall not exceed 45 degrees and the treads shall be of the non-skid type.

8.4.1. Insulated Tanks Stair treads shall not be welded to the shell. They shall be mounted on a stringer supported from the shell at no less than 3 m (10 ft) intervals, with 75 mm (3 in) clearance from the insulation.

The distance between flange bolting and the insulation shall be sufficient to allow insertion of a box wrench.



Shell insulation flashing at the top of the shell shall be provided as shown in Figure 2.

Figure 2: Shell Insulation Flashing

Level gauges and alarms shall be in accordance with MP 32-P-22.



Appendix A: Floating Roof Requirements

1. External Floating Roofs (for Open-Top Tanks)

1.1. Design

Floating roofs shall be of the multiple compartment, single -deck pontoon or double deck type (refer to Table A-1) and shall be provided with the specified type of seals.

Pontoon bulkheads shall be completely welded for liquid tightness.

Roofs shall be designed to avoid fatigue failure caused by wind effects.

- Single-deck roofs in which the single deck exceeds 52 m (170 ft) in diameter shall be stiffened, and the design shall be submitted for The Companys approval.

Roof support legs shall be adjustable to carry the roof in an upper cleaning position with 2 m (6.5 ft) minimum clearance and a lower operating position with 75 mm (3 in) clearance from tank internals and the tops of nozzle openings.

Rolling ladders shall have self-leveling non-skid treads.

Overflow slots are prohibited.

Where a floating roof with a foam-filled seal may descend to the manhole level, a filler plate conforming to the tank inside diameter shall be attached to the manhole cover.

1.2. Seals

Stainless steel shoe primary seals shall be used unless otherwise specified.

Non-metallic primary seals shall be foam filled.

a) Other types may be submitted for The Companys approval.

b) These seals may be vapor mounted if secondary seals are provided, unless regulations require liquid mounting (as in the United States, for example).

c) A metallic weather shield shall be provided if secondary seals are not furnished.

Non-metallic seal material shall be satisfactory for use with petroleum products having an aromatic content up to 75 percent, unless otherwise specified.



Liquid-filled seals are prohibited.

When a secondary seal is not required, provision shall be made for future installation of a secondary seal with the tank in service.

Each steel shoe section shall be electrically bonded to the roof with stainless steel shunts, at maximum intervals of 3 m (10 ft), on the tank circumference. Insulated joints or jumper bonds are not required. Shunts shall extend from the shoe to the roof, above the seal area.

Shunts shall be provided for secondary seals that have no electrical bond between the shell and roof, with shunt spacing, as noted in the above paragraph.

Minimum deviations from nominal rim spaces to be accommodated by the seal shall be:

a) + 200 mm (8 in) for tanks over 82.3 m (270 ft) diameter.

b) + 150 mm (6 in) for tanks from 30.5 m (100 ft) to less than 82.3 m (270 ft) diameter and greater thank 14.6 m (48 ft) high.

1.3. Vertical Guide Pipe

Roof rotation shall be prevented by means of a vertical guide pipe. The minimum pipe size shall be in accordance with Table A-1.

This pipe shall extend to the top of the stairway platform to permit gauging. If used for sampling, the pipe shall be slotted or perforated. (Refer to Figure C-9.)

Any environmental regulations concerning sealing the guide -pipe shall be met.

Table A1: Minimum Guide Pipe Size According to Tank Size

Tank Height, m (ft)

12.2 (40) 14.6 (48) 17 (56) 19.5 (64) >19.5 (64)

Minimum Pipe Size (NPS)*

Maximum Tank Diameter, m (ft)

4 28.3 (93) 26.2 (86) 25.3 (83) 24.4 (80) 6 44.8

(147) 36.6 (120)

32.0 (105)

29.3 (96) 27.4 (90)

8 79.2 (260)

57.9 (190)

46.0 (151)

39.0 (128)

34.4 (113)

10 100.6 (330)

74.4 (244)

58.8 (193)

48.8 (160)

12 112.2 (368)

85.0 (279)

68.0 (223)



14 106.1 (348)

83.2 (273)

16 113.7 (373)

* NPS = nominal pipe size, inches.

1.4. Drains

Roof drains shall be limited to the following types:

1. Flexible steel/elastomer pipes equal to those manufactured by the Mesa Rubber Co., Coflexip & Services, Inc. or Dunlop Ltd. Non-metallic material shall be satisfactory for use with petroleum products with aromatic content up to 75 percent, unless otherwise specified.

2. Pipes with wire-reinforced hose pivot joints equal to that manufactured by Pivot Master, Inc., with The Companys approval. Expansion devices shall be provided for freezing climates.

A steel gate valve shall be placed on the tank shell at the lower end of the roof drain piping. For a single -deck roof, a gate valve may be specified for the roof end of the drain pipe and located to allow temporary removal of the check valve specified by API STD 650, Appendix C.

Emergency drains shall be provided for double -deck roofs.

1.5. Fire Protection Systems

External floating roof tanks shall be equipped in accordance with Table A-2.

Table A2: Foam and Access Appurtenance

Diameter m (ft) Shell Height m (ft)

Appurtenances/Requirements

>45.7 (150) Any A fixed or semifixed foam system, in accordance with Figure A-1, with a standpipe to grade (for connection to piping terminating outside the dike).

36.6 (120) to and including 45.7 (150) Any

15.2 (50)

An NPS 3 standpipe terminating at the stairway top landing, with a baffle to direct the foam down the tank shell (as shown in Figure A-2).

30.5 (100) to



Diameter m (ft) Shell Height m (ft)

Appurtenances/Requirements

1.1 m (3.5 ft) below the top of the shell or shell extension and have a handrail for the entire circumference. Access shall be provided from the circular stairway to the wind girder.

In addition, a circular foam dam shall be provided at the peripheral seal, except as follows:

1. For steel shoe primary seals, when fixed foam devices discharge below the continuous fabric seal.

2. For other primary seals, when fixed foam devices discharge below a non-combustible weathershield or non-combustible secondary seal, and the primary seal is more than 150 mm (6 in) below the top of the roof rim plate.

3. Where there are no fixed foam devices or standpipes and any weathershields or secondary seals are combustible.

Foam dams shall be continuously welded or otherwise tightly fastened to the roof. The dam shall extend at least 50 mm (2 in) above the highest seal and shall be at least 300 mm (12 in) high.

The foam dam shall be located one to two feet from the edge of the roof. The dam shall be slotted to allow drainage of rain water.

The height of the drain slots shall not exceed 9.5 mm (3/8 in), and the total of the drain slot areas shall be 280 mm2/m2 (0.04 in2/ft2) of area between the foam dam and the tank shell.

2. Internal Floating Roof Tanks

Internal Floating Roof Tanks shall conform to API STD 650, Appendix H, and the following requirements.

The floating roof design shall be one of the following types, defined in Appendix H of API STD 650, with supplemental requirements as noted:

1. Steel pontoon or double -deck roofs. Pontoon bulkheads shall be completely welded for liquid tightness.

2. Metallic roof on floats. The deck skin shall be deformed at its seams by the clamping members to effect a tight seal (refer to Figure A2).

The floating roof perimeter seal may be either metallic or non-metallic type, and a double or secondary seal shall be provided where required.

Floating roof support legs shall conform to Section 1.1, fourth item (Appendix A).



Combined circulation vents/inspection hatches in accordance with API STD 650, Appendix H shall be provided on freely vented fixed roofs (excluding aluminum dome roofs) and shall have hinged hoods with clamps.

Gas blanketed tanks shall have pressure tight, gasketed, clear plastic plates with hinged metal covers on the roof manways, so that the internal floating roof and its seals may be viewed.

Overflow slots are prohibited.

Figure A1: Catenary Foam System - Ladder Piping Details



Figure A2: Foam Standpipe with Baffle



Appendix B: High Strength Steel Requirements

This appendix covers additional requirements for steels having minimum specification yield strength of 262 MPa (38,000 psi) and greater.

1. Design

The annular bottom plate to which the shell course is welded shall be a minimum of 9 mm (3/8 in) thick, including corrosion allowance.

The welds attaching the floor plate to the annular bottom plate shall be made with a minimum of two passes.

All shell opening connections shall be attached by full-penetration welds, including attachments to insert-type reinforcement.

Attachments shall be designed so that the primary bending stress plus the primary membrane stress is not greater than 11/2 times the shell allowable stress.

2. Materials

Material for annular bottom plates shall have physical and chemical properties similar to those of the shell material.

High-strength steel materials, other than those listed in API STD 650, may be used with prior ExxonMobil approval. Approval shall be based on the following information:

1. Complete chemical and mechanical specification limits, including steel process and heat treatment.

2. Description of measures employed to ensure uniformity of mechanical properties and microstructure.

3. Minimum expected Charpy V-notch impact energy of base material, weld material and heat-affected zone at the minimum design temperature.

4. Welding procedures proposed, including heat treatment if any, and the resultant mechanical properties of welded joints.

5. Methods of quality control during tank erection, such as production weld test plates, ultrasonic inspection and magnetic particle inspection.



6. List of other tanks fabricated of the proposed material.

Micro-alloy elements in fine -grain carbon manganese steels are restricted as follows:

Vanadium shall not exceed 0.05 percent.

Niobium shall not exceed 0.05 percent.

Their combined content shall not exceed 0.08 percent.

When these elements are used, the soluble aluminum-to-nitrogen ration shall not be less than 2:1.

The ratio of actual yield strength to actual ultimate tensile strength shall not exceed 0.85.

3. Welding and Weld Inspection

3.1. Welding

Welding procedure requalification will be required for:

1. Individual weld layer thickness shall not be greater than that used in the procedure qualification tests.

2. Electrodes not of the same size and AWS classification as that in the procedure qualification test.

3. Nominal preheat and interpass temperatures shall be the same as those in the procedure qualification tests.

4. Automatic welding if flux composition is changed or if speed and heat input are changed beyond the range specified.

Fillet welds connecting the lowest shell course and bottom plates to the annular bottom plate shall be made with low-hydrogen electrodes equivalent to AWS classification EXX16 or EXX18.

Steels with minimum specified yield strengths greater than 262 MPa (38,000 psi) and mechanical properties enhanced by heat treatment shall have a minimum of two layers of weld metal on all scars, pickups, structural attachments or other welds made on the plate surface.

Hardness surveys across the procedure qualification welds and heat-affected zones shall be as follows:

1. Made with an instrument having an indentor of 1.6 mm (1/16 in) maximum diameter.

2. Hardness shall not exceed 225 HB (238 HV10) except that peak hardness of up to 250 HB (260 HV10) is acceptable in the heat affected zone (HAZ).



3. For steels with minimum yield strengths equal to or greater than 345 MPa (50,000 psi), peak HAZ hardness shall not exceed 310 HB (325 HV10).

A hardness test shall be made on each 30.5 m (100 ft) of the shell weld seams and on main nozzle welds, and it shall not exceed 225 HB (238 HV10).

3.2. Weld Inspection

Weld inspection shall be in accordance with API STD 650 and the following requirements:

Horizontal Joints

- One radiograph in the first 3 m (10 ft) of each seam plus one for each 15.2 m (50 ft) of seam length.

Vertical Joints

- Complete radiographing for all welds in plates thicker than 19 mm (3/4 in). For plates 19 mm (3/4 in) and less, radiographing of all intersections plus one additional radiograph for each 7.6 m (25 ft) of vertical seam length in each course.

Structural attachment welds and welds attaching the lowest shell course to the annular bottom plates shall be examined by the magnetic particle or dye penetrant method to detect cracks and other linear discontinuities.

Longitudinal welds in shell and floor nozzles and manways shall be examined by radiography in accordance with the requirements for vertical shell joints. There shall be at least one spot radiograph for each weld.

4. Erection

Lugs, clips and similar items attached by welding for purposes of erection only shall be removed. The plate in these weld areas and at other scars shall be ground smooth and examined by the magnetic particle or dye penetrant method.

5. Coatings

The bottom course and annular bottom plate shall be epoxy coated on the inside when the minimum specified tensile strength of the plate material is greater than 586 MPa (85,000 psi).



Appendix C: Figures

Figure C1: Location of Shell and Bottom Connections and Appurtenances Typical Layout and Dimensions



Legend for Type 1 and Type 2 Bottoms

1. Shell Manholes, tank diameters:

- Up to 25 ftone NPS 24

- > 25 ft to 100 fttwo NPS 24 (180 apart)

- > 100 ftthree NPS 24 (180 apart)

2. Inlets and suction nozzles

3. Water drawoff

4. Foam chambers (note 1)

5. Sump (for Type 1 bottoms, number required same as number of shell manholes)

6. Datum (striker) plate (note 1)

7. Roof drain nozzle (note 2)

8. Thermowell connection NPS 1/CL 3000 coupling with round head plug 3 ft above tank bottom

9. Stripping nozzle (Note 1)

10. H-level & HH-level alarms (located near dike entrance or main path through tank farm.)

Notes :

1. Provide only when shown on the ExxonMobil Data Sheets for this Master Practice.

2. For open-top floating roof tanks only.

3. Connections may be parallel. Recommended for large-diameter piping.

4. Slope shall be increased to compensate for predicted center-to-edge settlement, so that slope after predicted settlement is 1 inch in 10 ft.



Figure C2: Location of Roof Connections and AppurtenancesTypical Layout and Dimensions

Legend for Location of Roof Connections and Appurtenances

12. Circular stairway unless otherwise specified

13. Roof manholes, tank diameters

- Up to 50 ftone NPS 24

- > 50 ft to 100 fttwo NPS 24 (180 apart)

- > 100 ftthree NPS 24 (180 apart)

Hatch-type combination manways/vents (shown by dashed line) located on the roof perimeter shall be used for freely vented cone roof tanks with internal floating roofs.

14. Vent connections for PV valve or vapor recovery in pressurized tanks

15. Scaffold cable support or center vent for freely vented tanks

16. Railing

17. Pontoon manholes, NPS 20 minimum, one per compartment

18. Roof drain

19. Emergency drain for double deck floating roofs

20. Gauging/anti-rotation pipe (Notes 3 and 4)

21. Rim vent for shoe type seals (Note 2)

22. Roof supports (number as required)



23. Automatic bleeder vent, leg operated

24. Rolling ladder with articulated steps

25. Water gauging connection over sump (Note 4)

26. Temperature, gauging and sampling hatch (Note 3)

27. Automatic gauge, ground or remote reading (Notes 1 & 3)

28. Averaging thermometer (Note 1)

Notes:

1. Provide only when shown on ExxonMobil Data Sheets for this Master Practice.

2. Manufacturer's design.

3. Location to be accessible from platform.

4. For single deck floating roofs, provide extension to avoid product flowing onto roof.



Figure C3: Water Drawoff ArrangementsFor Nonfreezing Areas

Notes:

1. For tanks with Type 1 bottom (peripheral sumps): provide one NPS 3 water drawoff to each sump.

2. For tanks with Type 2 bottom:

- Provide one NPS 2 water drawoff for tanks up to 50 ft in diameter.

- Provide one NPS 3 water drawoff for tanks 50 ft to 90 ft in diameter.

- Provide one NPS 2 and one NPS 4 water drawoff connection for tanks over 90 ft in diameter; see plan for arrangement.

3. Use gate valve; water drawoff pipe and valve to be same size.

4. Elbows may be welded ells, socket welded, or screwed malleable iron fittings



Figure C4: Water Drawoff ArrangementsFor Freezing AreasType 1

Table C1

Sizing Table NPS

Size of Value Pipe Y Pipe Z

2 3 11/2

3 4 3 *

4 6 3 *

* Pipe Z shall be NPS 4 for sumps in tanks with Type 1 bottom

Notes:

1. For tanks with Type 1 bottom (peripheral sumps): provide one NPS 3 water drawoff to each sump.

2. For tanks with Type 2 bottom:

- Provide one NPS 2 water drawoff for tanks up to 50 ft in diameter.

- Provide one NPS 3 water drawoff for tanks 50 ft to 90 ft in diameter.

- Provide one NPS 2 and one NPS 4 water drawoff connection for tanks over 90 ft in diameter; see plan for arrangement.

3. Use gate valve; water drawoff pipe and valve to be same size.

4. Elbows may be welded ells, socket welded, or screwed malleable iron fittings.



Figure C5: Separate Suction and Inlet Piping

Table C2

Nozzle Diameter NPS Tee Size NPS Tee Outlet Hole B Diameter

4 6 x 4 2 in

6 8 x 6 3 in

8 10 x 8 4 in

10 10 6 in

12 12 8 in

14 14 8 in

16 16 10 in

18 18 10 in

20 20 12 in

24 24 16 in

30 30 18 in



Nozzle Diameter NPS Tee Size NPS Tee Outlet Hole B Diameter

36 36 22 in

Figure C6: Separate Inlet Piping for Floating Roof Tanks

Note:

1. Detail C may be used for inlet velocities 0.9 m/s (3 ft/s) where no possibility of vapor slugs exists.



Figure C7: Combination Suction and Inlet Piping

Table C3

Slot Dimensions

Nozzle & Pipe Diameter (NPS)

A B X

8 22 in 3 in H

10 26 in 4 in H

12 30 in 5 in H

14 30 in 6 in H

16 35 in 7 in H

18 39 in 8 in H

20 43 in 9 in H

24 52 in 11 in H -3 in

30 64 in 14 in H -6 in

36 76 in 17 in H -9 in



Figure C8: Stripping Arrangements

Figure C9: Gauging Datum Plate



Figure C10: Aluminum Dome Roofs

Notes:

1. Aluminum dome roofs in accordance with API STD 650, Appendix G are permitted with the following provisions:

- Where snow loading applies, they shall be in accordance with the unbalanced snow load of ASCE 7-88, unless local regulations are more severe.

- Pedestal-type roof supports resting on the wind girder shall be provided when there is no top rim angle on the shell or where support loads would exceed the following:



Table C4

Shell Thickness Load at Each Support Point

mm (in) kg (lbs)

4.8 3/16 5400 12,000

6.4 1/4 13,600 30,000

8.0 5/16 26,300 58,000

9.5 3/8 45,400 100,000

* Top Shell Course in corroded condition

2. Where an internal floating roof is required, provide the following:

- Inspection hatches, 0.6 m (2 ft) square, at a maximum spacing of 17 m (56 ft) around the dome perimeter. Only one hatch is required for tanks with diameters of 9 m (30 ft) and smaller.

- Access to these hatches via the shell wind girder, if of sufficient size and with a handrail, or via a walkway with an outer handrail on the dome roof.

- Skylights located between inspection hatches with additional two near the center.



Figure C11: Storage Tanks Double Steel Bottom with Liner and Leak Detection

NOTES:

This design shall be considered for smaller diameter tanks [ 9 m (30 ft)].

For larger diameter tankers, refer to MP 01-P-14.

For design, fabrication and installation, see next page.

NOTES:

1. Melt zinc and weld steel wire core of anode to support ring as close as possible to upper bottom.

2. Cone up both floors to center so after predicted center to edge settlement occurs, slope is 1:120.

3. Liner:

- Petrogard 10 - 1 mm (0.04 in) thick, with factory seams for field joining.

- Other, specify.



4. Shell may not overhang support ring by more than 1.5 mm (1/16 in) in either direction.

5. Sand Fill:

- Use dry, clean, commercial silica sand, pH neutral, non-corrosive, non-conductive and salt free with the following sieve analysis:

Table C5

Square Opening Sieve Size Percent Passing by Weight

#4 90 - 100

#200 10 maximum

The sand shall have electrical resistance greater than 50,000 Ohms/cm. Certified reports shall be furnished for each truckload of sand. A minimum of five random samples from the as-received sand shall be tested for chlorides and resistivity. One supplier of high quality sand in the U.S. is Uniman Sand Inc. Use the "soil box" technique (as manufactured by the MCM Miller company or other acceptable equivalent) to determine sand resistivity. An alternate test to the soil box is ASTM G-57, Standard Method for Field Measurement of Soil Resistivity using the Wenner Four-Electrode Method. Maximum acceptable chloride concentration is 2.0 ppm.

6. Installation of Liner Material

- No welding inside the tank or on the roof where sparks or molten metal might damage the liner is permitted during membrane installation.

- Seam sealing may be performed by the contractor if the contractor is approved by The Company and the liner manufacturer. If not, the liner manufacturer shall supply seam sealing personnel.

- Vacuum test liner seams at 7 to 14 kPa (1 to 2 psi) in the presence of The Company. The liner shall also be tested for pin holes using a holiday detector with a minimum of 10,000 volts. Defective areas may be patched and retested.

- The contractor's personnel shall protect the liner from damage while preparing the tank bottom for installation of the sand pad.

7. Installation of Sand

- Sand shall be covered and kept dry during storage and installation. It is desirable to have the sand delivered in closed trucks and installed at the time of delivery. Blowing the sand into the tank via a hose is the fastest and easiest method.

- Sand installation shall be done in two equal layers so that the cathodic protection can be installed equidistant from the two bottoms.

- Grade the sand from the doorway into the tank so that equipment is not operated on the liner material. Spreading equipment, if used, shall have low pressure pneumatic tires. Sand spreading tools shall not have sharp edges or points.

8. Installation of Bottom



- Picks or pry bars shall not be used to position the plate due to potential for liner damage.



Appendix D: ExxonMobil Hydrostatic Tank Leak Test

1. Background

The Company developed a test to detect tank bottom leaks.. The test uses a small column of water (or liquid contained by the tank) that is connected to the tank and is instrumented to detect and record a drop in tank water level of as low as 0.025 mm (0.001 in) over a four to eight hour period. Since leak sensitivity decreases with increasing tank diameter, the test is recommended, but not required, for tanks over 46 m (150 ft) diameter.

The test may also be used on existing tanks to check for bottom leaks in conjunction with or in between internal inspections.

Bottom leaks in new tank floors have occurred even after a full vacuum box inspection of floor welds. In some cases, a leak has occurred from a weld arc strike or cutting operation remote from a weld. Leaks have occurred in the field in shop fabricated and shop tested tank bottoms as a result of transportation or lifting stresses or field work such as installation of a sump. The cost of cleaning one tank exceeds the cost of a large number of tests.

2. Applicability

All tanks shall be tested except the following:

1. Clean water tanks

2. Tanks with products that do not flow at ambient temperatures

3. Elevated tanks, provided that they are filled with at least 1.2 m (4 ft) of water, in their erected position, prior to filling with product

4. Tanks where testing is waived by ExxonMobil

3. MHTLT Procedure:

1. Set up the testing date and ensure that test apparatus is available.

2. Blank all valves/lines that are below the intended liquid level.



1. Prior to the test day, fill the tank with 1.2 m (4 ft) of water. For tanks with floating roofs, the roof must be landed on its legs and the liquid level must be below the upper test tap.

2. Hold the liquid level for the time recommended by the testing company.

3. After the test, obtain a copy of the test results. Retain copies in the tank files, as well as in the project files.

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