API 650: Welded Tanks for Oil Storage

Question 1

To what types of tanks does this standard apply? (API 650-13, Section 1.1.1)

Answer 1

This standard establishes minimum requirements for material, design, fabrication, erection, and inspection for vertical, cylindrical, aboveground, closed- and open-top, welded storage tanks in various sizes and capacities for internal pressures approximating atmospheric pressure (internal pressures not exceeding the weight of the roof plates), but a higher internal pressure is permitted when additional requirements are met (see 1.1.13). This standard applies only to tanks whose entire bottom is uniformly supported and to tanks in non-refrigerated service that have a maximum design temperature of 93 °C (200 °F) or less (see 1.1.20).

Question 2

What is a butt weld? (API 650-13, Section 5.1.1.1)

Answer 2

A weld placed in a groove between two abutting members. Grooves may be square, V-shaped (single or double), or U-shaped (single or double), or they may be either single or double beveled.

Question 3

What is a fillet weld? (API 650-13, Section 5.1.1.4)

Answer 3

A weld of approximately triangular cross-section that joins two surfaces at approximately right angles, as in a lap joint, tee joint, or corner joint.

Question 4

What is a tack weld? (API 650-13, Section 5.1.1.8)

Answer 4

A weld made to hold the parts of a weldment in proper alignment until the final welds are made.

Question 5

What is the size of the groove weld based on? (API 650-13, Section 5.1.2.1)

Answer 5

The size of a groove weld shall be based on the joint penetration (that is, the depth of chamfering plus the root penetration when specified).

Question 6

What is the minimum size of fillet welds on plates 5mm (3/16”) thick? (API 650-13, Section 5.1.3.3)

Answer 6

On plates 5 mm (3/16 in.) thick, the weld shall be a full-fillet weld, and on plates more than 5 mm (3/16 in.) thick, the weld thickness shall not be less than one-third the thickness of the thinner plate at the joint and shall be at least 5 mm (3/16 in.).

Question 7

What type of weld is used for joining ring sections? (API 650-13, Section 5.1.5.8)

Answer 7

Full-penetration butt-welds shall be used for joining ring sections.

Question 8

What are the requirements for horizontal shell joints? (API 650-13, Section 5.1.5.3)

Answer 8

Horizontal shell joints shall have complete penetration and complete fusion; however, as an alternative, top angles may be attached to the shell by a double-welded lap joint.

Question 9

What is the minimum nominal thickness required for bottom plates? (API 650-13, Section 5.4.1)

Answer 9

All bottom plates shall have a corroded thickness of not less than 6 mm (0.236 in.).

Question 10

Bottom plates of sufficient size shall be ordered so that when trimmed, there is a projection of beyond the outside edge of the weld attaching the bottom to the shell plate. (API 650-13, Section 5.4.2)

Answer 10

Bottom plates or annular plates of sufficient size shall be ordered so that, when trimmed, at least a 50 mm (2 in.) width will project beyond the outside surface of the shell plate or meet the requirements given in 5.1.5.7 e, whichever is greater.

Question 11

How shall isolated radial loads on the tank shell be distributed? (API 650-13, Section 5.6.1.5)

Answer 11

Isolated radial loads on the tank shell, such as those caused by heavy loads on platforms and elevated walkways between tanks, shall be distributed by rolled structural sections, plate ribs, or built-up members.

Question 12

What is the largest flanged or threaded nozzle that can be installed in a tank shell without reinforcement? (API 650-13, Section 5.7.2.1)

Answer 12

Openings in tank shells larger than required to accommodate a NPS 2 flanged or threaded nozzle shall be reinforced.

Question 13

If work is expected to be carried on through the roof manhole opening while the tank is in use, what must be done to the roof structure around the manhole? (API 650-13, Section 5.8.4)

Answer 13

The effects of loads (other than normal personnel access) applied at the roof manhole and supporting roof structure shall be considered. Examples of such loads may include fall protection anchorage, hoisting, or personnel retrieval. The roof structure and plate around the manhole shall be reinforced as necessary.

Question 14

What is the purpose of stiffening rings (wind girders)? (API 650-13, Section 5.9.1.1)

Answer 14

To protect the tank from blow in due to high winds. Additionally, the effects due to vacuum pressure might require stiffening rings to oppose the tank shell sucking in on itself.

Question 15

If a stiffening ring is to be formed or built up, what is the minimum nominal thickness of the steel plate that should be used? (API 650-13, Section 5.9.3.1)

Answer 15

The minimum nominal thickness of plate for use in formed or built-up stiffening rings shall be 6 mm (0.236 in.).

Question 16

If material needs straightening, how should this be accomplished? (API 650-13, Section 6.1.1.2)

Answer 16

When material requires straightening, the work shall be done by pressing or another noninjurious method prior to any layout or shaping. Heating or hammering is not permissible unless the material is maintained at forging temperature during straightening.

Question 17

After tank bottom plates are laid out and tacked together, in what sequence should they be welded together? (API 650-13, Section 7.2.2.1)

Answer 17

After the bottom plates are laid out and tacked, they shall be joined by welding the joints in a sequence that the Manufacturer has found to result in the least distortion from shrinkage and thus to provide as nearly as possible a plane surface.

Question 18

What is the maximum misalignment allowed for vertical shell joints when the plate thickness is greater than 5/8 inch? (API 650-13, Section 7.2.3.1)

Answer 18

Misalignment in completed vertical joints for plates greater than 16 mm (5/8 in.) thick shall not exceed 10 % of the plate thickness or 3 mm (1/8 in.), whichever is less.

Question 19

What is the maximum misalignment allowed for vertical shell joints when the plate thickness is less than or equal to 5/8 inch? (API 650-13, Section 7.2.3.1)

Answer 19

Misalignment in completed vertical joints for plates less than or equal to 16 mm (5/8 in.) thick shall not exceed 1.5 mm (1/16 in.).

Question 20

For horizontal butt joints on the tank shell, what percentage of the upper plate thickness may project beyond the face of the lower plate? What is the maximum projection allowed for this type of joint? What is the maximum projection allowed if the upper plate is less than 5/16 inch thick? (API 650-13, Section 7.2.3.2)

Answer 20

In completed horizontal butt joints, the upper plate shall not project beyond the face of the lower plate at any point by more than 20 % of the thickness of the upper plate, with a maximum projection of 3 mm (1/8 in.); however, for upper plates less than 8 mm (5/16 in.) thick, the maximum projection shall be limited to 1.5 mm (1/16 in.).

Question 21

What type of weld procedure must be used when the shell plate thickness is greater than 1.5 inches thick? What is the maximum thickness allowed for a single weld pass? (API 650-13, Section 7.2.3.5)

Answer 21

For circumferential and vertical joints in tank shell courses constructed of material more than 40 mm (11/2 in.) thick (based on the thickness of the thicker plate at the joint), multipass weld procedures are required, with no pass over 19 mm (3/4 in.) thick permitted.

Question 22

What are five ways that the initial weld pass of the shell-to-bottom weld inside the shell may be tested? (API 650-13, Section 7.2.4.1)

Answer 22

1) magnetic particle;
2) applying a solvent liquid penetrant to the weld and then applying a developer to the gap between the shell and the bottom and examining for leaks after a minimum dwell time of one hour;
3) applying a water-soluble liquid penetrant to either side of the joint and then applying a developer to the other side of the joint and examining for leaks after a minimum dwell time of one hour;
4) applying a high flash-point penetrating oil such as light diesel to the gap between the shell and the bottom, letting stand for at least four hours, and examining the weld for evidence of wicking.
5) applying a bubble-forming solution to the weld, using a right angle vacuum box, and examining for bubbles.

Question 23

When conducting vacuum testing of weld seams, what is the indicator that porosity is present in the seam? (API 650-13, Section 8.6.9)

Answer 23

The presence of a through-thickness leak indicated by continuous formation or growth of a bubble(s) or foam, produced by air passing through the thickness, is unacceptable.

Question 24

What vacuum should be maintained within the vacuum box when testing weld seams using vacuum testing? (API 650-13, Section 8.6.3)

Answer 24

A partial vacuum of 21 kPa (3 lbf/in.2, 6 in. Hg) to 35 kPa (5 lbf/in.2, 10 in Hg) gauge shall be used for the test.

Question 25

What are three methods that may be used to test newly welded bottom joints? (API 650-13, Section 7.3.3)

Answer 25

1) A vacuum-box test in accordance with 8.6.
2) A tracer gas test in accordance with 8.6.11.
3) After at least the lowest shell course has been attached to the bottom, water (to be supplied by the Purchaser) shall be pumped underneath the bottom. A head of 150 mm (6 in.) of liquid shall be maintained using a temporary dam to hold that depth around the edge of the bottom. The line containing water for testing may be installed temporarily by running it through a manhole to one or more temporary flange connections in the bottom of the tank, or the line may be installed permanently in the subgrade beneath the tank. The method of installation should be governed by the nature of the subgrade. Reasonable care shall be taken to preserve the prepared subgrade under the tank.

Question 26

When testing reinforcement plate welds, how much pressure should be applied between the tank shell and the reinforcement plates? (API 650-13, Section 7.3.4)

Answer 26

After fabrication is completed but before the tank is filled with test water, the reinforcing plates shall be tested by the Manufacturer by applying up to 100 kPa (15 lbf/in.2) gauge pneumatic pressure between the tank shell and the reinforcement plate on each opening using the telltale hole specified in 5.7.5.1.

Question 27

If water is not available to test the shell of a completed tank, what are two procedures that may be used to test the tank shell? (API 650-13, Section 7.3.6)

Answer 27

1) applying highly penetrating oil, such as automobile spring oil, to all of the joints on the inside and examining the outside of the joints for leakage; or
2) applying vacuum to either side of the joints or, if above the liquid level, applying internal air pressure as specified for the roof test in 7.3.8 and visually examining the joints for leakage

Question 28

How may pinhole leaks in tank bottom joint be repaired? (API 650-13, Section 7.4.2)

Answer 28

Pinhole leaks or porosity in a tank bottom joint may be repaired by applying an additional weld bead over the defective area.

Question 29

If a defect is discovered after a tank is filled with water, where must the water level be relative to the defect being repaired? (API 650-13, Section 7.4.4)

Answer 29

Repairs of defects discovered after the tank has been filled with water for testing shall be made with the water level at least 0.3 m (1 ft) below any point being repaired or, if repairs have to be made on or near the tank bottom, with the tank empty.

Question 30

What is the maximum out-of-plumbness allowed for the top of the tank shell relative to the bottom of the shell? (API 650-13, Section 7.5.2)

Answer 30

The maximum out-of-plumbness of the top of the shell relative to the bottom of the shell shall not exceed 1/200 of the total tank height.

Question 31

What is the maximum deviation (peaking) allowed on vertical weld joints? What tool shall be used to measure peaking? (API 650-13, Section 7.5.4)

Answer 31

Deviations (peaking) at vertical weld joints shall not exceed 13 mm (1/2 in.). Peaking at vertical weld joints shall be determined using a horizontal sweep board 900 mm (36 in.) long. The sweep board shall be made to the nominal radius of the tank.

Question 32

What is the maximum deviation (banding) allowed on horizontal weld joints? What tool shall be used to measure banding? (API 650-13, Section 7.5.4)

Answer 32

Deviations (banding) at horizontal weld joints shall not exceed 13 mm (1/2 in.). Banding at horizontal weld joints shall be determined using a straight edge vertical sweep board 900 mm (36 in.) long.

Question 33

When should measurements to check a newly constructed tank against specifications be conducted? (API 650-13, Section 7.5.1)

Answer 33

Measurements shall be taken prior to the hydrostatic water test.

Question 34

For what types of weld joints is radiographic inspection required? (API 650-13, Section 8.1.1)

Answer 34

Radiographic examination is required for shell butt-welds (see 8.1.2.2, 8.1.2.3, and 8.1.2.4), annular-plate butt-welds (see 8.1.2.9), and flush-type connections with butt-welds (see 5.7.8.11).

Question 35

For what types of weld joints is radiographic inspection NOT required? (API 650-13, Section 8.1.1)

Answer 35

Radiographic examination is not required for the following: roof-plate welds, bottom-plate welds, welds joining the top angle to either the roof or shell, welds joining the shell plate to the bottom plate, welds in nozzle and manway necks made from plate, or appurtenance welds to the tank.

Question 36

What is the minimum weld length that each radiograph must clearly show? (API 650-13, Section 8.1.2.8)

Answer 36

Each radiograph shall clearly show a minimum of 150 mm (6 in.) of weld length.

Question 37

What are eight general methods that can be used to improve subgrade support conditions beneath an aboveground tank? (API 650-13, Section B.2.4))

Answer 37

1) Removing the objectionable material and replacing it with suitable, compacted material.
2) Compacting the soft material with short piles.
3) Compacting the soft material by preloading the area with an overburden of soil. Strip or sand drains may be used in conjunction with this method.
4) Stabilizing the soft material by chemical methods or injection of cement grout.
5) Transferring the load to a more stable material underneath the subgrade by driving piles or constructing foundation piers. This involves constructing a reinforced concrete slab on the piles to distribute the load of the tank bottom.
6) Constructing a slab foundation that will distribute the load over a sufficiently large area of the soft material so that the load intensity will be within allowable limits and excessive settlement will not occur.
7) Improving soil properties by vibro-compaction, vibro-replacement, or deep dynamic-compaction.
8) Slow and controlled filling of the tank during hydrostatic testing. When this method is used, the integrity of the tank may be compromised by excessive settlements of the shell or bottom. For this reason, the settlements of the tank shall be closely monitored. In the event of settlements beyond established ranges, the test may have to be stopped and the tank releveled.

Question 38

How much higher than the surrounding ground should the surface on which the tank bottom rests be located? (API 650-13, Section B.3.1)

Answer 38

The grade or surface on which a tank bottom will rest should be constructed at least 0.3 m (1 ft) above the surrounding ground surface.

Question 39

What are three reasons for having the surface on which the tank bottom rests higher than the surrounding ground? (API 650-13, Section B.3.1)

Answer 39

1) provide suitable drainage,
2) help keep the tank bottom dry, and
3) compensate for some small settlement that is likely to occur.

Question 40

What type of material is recommended for the surface on which the tank bottom plates will rest? What types of materials should be avoided? (API 650-13, Section B.3.2)

Answer 40

To minimize future corrosion problems and maximize the effect of corrosion prevention systems such as cathodic protection, the material in contact with the tank bottom should be fine and uniform (clean washed sand). Gravel or large particles shall be avoided.

Question 41

What should be the minimum thickness of a concrete ringwall? (API 650-13, Section B.4.2.2)

Answer 41

The ringwall shall not be less than 300 mm (12 in.) thick.

Question 42

When designing a concrete ringwall, what forces should the ringwall be reinforced against? (API 650-13, Section B.4.2.3)

Answer 42

A ringwall should be reinforced against temperature changes and shrinkage and reinforced to resist the lateral pressure of the confined fill with its surcharge from product loads.

Question 43

What is the maximum spacing allowed between tank anchors? (API 650-13, Section 5.12.3)

Answer 43

The anchor center-to-center spacing measured along the tank circumference at the shell outer diameter shall not exceed 3 m (10 ft).

Question 44

What is the minimum diameter allowed for anchor bolts? (API 650-13, Section 5.12.6)

Answer 44

When anchor bolts are used, they shall have a corroded shank diameter of no less than 25 mm (1 in.).

Question 45

What are seven general requirements for undertank leak detection systems? (API 650-13, Section I.2)

Answer 45

1) Leaks through the tank bottom shall be detectable by observation at the tank perimeter. If a leak is detected, it shall be collected.
2) The use of electronic sensors for the detection of vapors and liquids is acceptable; however, the requirements of Item a above shall be satisfied. Any such sensor shall be fail-safe or have provision for calibration.
3) The materials of construction shall be chemically resistant to the range of products to be stored at the temperature range expected in service. Other physical properties shall be specified by the tank owner.
4) The permeability of the leak detection barrier shall not exceed 1 × 10–7 cm (4 × 10–5 mils) per second.
5) The material in contact with the subgrade shall be suitable for below-grade service or be protected against degradation.
6) The leak barrier shall be of one-piece construction, or the joints shall satisfy the leak tightness, permeability, and chemical resistance requirements for the base leak-barrier material. The Manufacturer and a complete description of the leak barrier material shall be identified to the tank owner.
7) The installation of sumps and pipes below the tank bottom is acceptable; however, the required leak detection and leak tightness shall be maintained. See Figure I.8 and Figure I.9 for typical details.

Question 46

Appendix M specifies additional requirements for tanks operating in what temperature range? (API 650-13, Section M.1.1)

Answer 46

This Annex specifies additional requirements for API Standard 650 tanks with a maximum design temperature exceeding 93 °C (200 °F) but not exceeding 260 °C (500 °F).

Question 47

The finished tank grade should be crowned from its outer periphery to its center. What are two reasons for doing this? (API 650-13, Section B.3.3)

Answer 47

The crown will partly compensate for slight settlement, which is likely to be greater at the center.
It will also facilitate cleaning and the removal of water and sludge through openings in the shell or from sumps situated near the shell.

Question 48

Under what conditions may satisfactory foundations for aboveground tanks be constructed from earth materials? (API 650-13, Section B.4.1.1)

Answer 48

When an engineering evaluation of subsurface conditions that is based on experience and/or exploratory work has shown that the subgrade has adequate bearing capacity and that settlements will be acceptable, satisfactory foundations may be constructed from earth materials.

Question 49

What are four things that an acceptable earth foundation should accomplish? (API 650-13, Section B.4.1.1)

Answer 49

1) provide a stable plane for the support of the tank;
2) limit overall settlement of the tank grade to values compatible with the allowances used in the design of the connecting piping;
3) provide adequate drainage;
4) not settle excessively at the perimeter due to the weight of the shell wall.

Question 50

What are five advantages of a concrete ringwall? (API 650-13, Section B.4.2.1) Which of these advantages also apply to a crushed stone or gravel ringwall? (API 650-13, Section B.4.3.1)

Answer 50

1) It provides better distribution of the concentrated load of the shell to produce a more nearly uniform soil loading under the tank. (also for gravel)
2) It provides a level, solid starting plane for construction of the shell.
3) It provides a better means of leveling the tank grade, and it is capable of preserving its contour during construction. (also for gravel)
4) It retains the fill under the tank bottom and prevents loss of material as a result of erosion. (also for gravel)
5) It minimizes moisture under the tank.

Question 51

Where a concrete ringwall is provided under the shell, how level must the top of the ringwall be within any 30 feet of the circumference? How level must the top of the ringwall be over the total circumference? (API 650-13, Section 7.5.5.2)

Answer 51

Where a concrete ringwall is provided under the shell, the top of the ringwall shall be level within ±3 mm (1/8 in.) in any 9 m (30 ft) of the circumference and within ±6 mm (1/4 in.) in the total circumference measured from the average elevation.

Question 52

Where a concrete ringwall is not provided under the tank shell, how level must the foundation under the shell be within any 10 feet of the circumference? How level must the foundation under the shell be over the total circumference? (API 650-13, Section 7.5.5.2)

Answer 52

Where a concrete ringwall is not provided, the foundation under the shell shall be level within ±3 mm (1/8 in.) in any 3 m (10 ft) of the circumference and within ±13 mm (1/2 in.) in the total circumference measured from the average elevation.

Question 53

What are the specific characteristics listed in this document for fill material used to replace objectionable material or to raise the grade to a suitable height? (API 650-13, Section B.2.5)

Answer 53

The fill material used to replace muck or other objectionable material or to build up the grade to a suitable height shall be adequate for the support of the tank and product after the material has been compacted. The fill material shall be free of vegetation, organic matter, cinders, and any material that will cause corrosion of the tank bottom. The grade and type of fill material shall be capable of being compacted with standard industry compaction techniques to a density sufficient to provide appropriate bearing capacity and acceptable settlements. The placement of the fill material shall be in accordance with the project specifications prepared by a qualified geotechnical engineer.

Question 54

What should be considered when the ringwall width exceeds 18 inches? (API 650-13, Section B.4.2.3)

Answer 54

When the ringwall width exceeds 460 mm (18 in.), using a footing beneath the wall should be considered.

Question 55

What type of aboveground tank foundation should be used when the soil bearing loads must be distributed over an area larger than the tank area? What may be required for proper tank support? (API 650-13, Section B.4.4.1)

Answer 55

When the soil bearing loads must be distributed over an area larger than the tank area or when it is specified by the owner, a reinforced concrete slab shall be used. Piles beneath the slab may be required for proper tank support.

Question 56

If an anchored tank is not properly designed, its shell can be susceptible to tearing. What should be the relationship between the strength of the anchorage attachments and the yield strength of the anchors? (API 650-13, Section 5.12)

Answer 56

The anchors themselves should yield before the attachments to the shell do.

Question 57

What should be the minimum slope of the top deck of a double deck floating roof?

Answer 57

1/64

Question 58

What’s the minimum diameter of roof drains on a floating roof tank?

Answer 58

• NPS 3 for diameter less than or equal to 120’

- NPS 4 for diameter greater than 120’

Question 59

In pin-connected tension members, the net section across the pinhole, transverse to the axis of the member, shall not be less than ________ of the net section of the body of the member.

Answer 59

135%

Question 60

In pin-connected tension members, the net section beyond the pinhole, parallel to the axis of the member shall not be less than _______ of the net section of the body of the member.

Answer 60

90%