API 620: Design and Construction of Large, Welded, Low-pressure Storage Tanks

Question 1

The minimum distance allowed between the outer edge of an opening reinforcement and any significant discontinuity in the curvature of the tank walls is _______ inches or _______ the nominal thickness of the wall plate, whichever is greater. (API 620-13 Section 5.14.5)

Answer 1

6 in; 8 times

Question 2

What is the minimum number of manhole openings that must be provided in order to afford access to the interior of the tank for inspection and repair? (API 620-13, Section 5.15)

Answer 2

Each tank shall be provided with at least two manhole openings to afford access to its interior for inspection and repair.

Question 3

What is the minimum size allowed for an inspection manhole? (API 620-13, Section 5.15)

Answer 3

Manholes shall in no event be smaller than 20 in. along any inside dimension.

Question 4

When certain conditions are met, single openings in tanks do not require additional reinforcement. What are three such conditions described in this document? (API 620-13, Section 5.16.2.1)

Answer 4

1) 3 in., or less, pipe size welded connections in tank walls 3/8 in. or less;
2) 2 in., or less, pipe size welded connections in tank walls over 3/8 in.;
3) threaded connections in which the hole in the tank wall is not greater than 2 in. pipe size.

Question 5

Under certain conditions, two or more adjacent tank openings may require combined reinforcement. What are two such conditions mentioned in this document? (API 620-13, Section 5.17.1)

Answer 5

1) The distance between the centers of any two adjacent openings is less than two times their average diameter so that their required areas of reinforcement overlap.
2) Any two adjacent openings are spaced so that if they are reinforced separately, the distance between the outer edges, or toes, of their reinforcing plate fillet welds or insert welds is (1) less than 6 in. at any point, or if this be larger, (2) eight times the nominal thickness of the fillet weld around the thicker reinforcing plate or eight times the nominal thickness of the insert butt-weld for an insert-type connection. Where the periphery weld has been stressrelieved before the welding of the adjacent shell joint, the spacing may be reduced to 6 in. from the longitudinal or meridional joints or 3 in. from circumferential or latitudinal joint, provided that in either case, the spacing is not less than 2½ times the shell thickness. In no case shall any portion of a cross section be considered to apply to more than one opening, that is, to be evaluated more than once in a combined area. Curved sections that form the outer boundary of a combined reinforcement shall be connected by straight lines, large-radius reverse curves tangent to the curved sections, or a combination of these two elements; in no case shall there be any re-entrant angles therein.

Question 6

When adjacent tank openings are provided with combined reinforcement, what is the minimum distance required between any two of these openings? (API 620-13, Section 5.17.2)

Answer 6

When two or more adjacent openings will be provided with a combined reinforcement, the minimum distance between the centers of any two of these openings shall preferably be at least 1.5 times their average diameter, and the area of reinforcement between them shall be at least equal to 50 % of the total required for these two openings on the cross section being considered.

Question 7

What must a reinforced flush-type shell connection be preassembled into in order to perform thermal stress relief? (API 620-13, Section 5.27.3)

Answer 7

The reinforced connection shall be completely preassembled into a sidewall plate. The completed assembly, including the sidewall plate that contains the connections, shall be thermally stress relieved at a temperature of 1100 °F to 1200 °F for a period of 1 hour per in. thickness of sidewall-plate thickness, td.

Question 8

What is the maximum “out-of-plumbness” allowed between the top and bottom of the shell for tanks with cylindrical sidewalls? (API 620-13, Section 6.5.2.1)

Answer 8

For cylindrical sidewalls, 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 9

If a tank foundation includes a concrete ringwall, how level must the top of the foundation be? (API 620-13, Section 6.5.6.2)

Answer 9

The top of the foundation with a concrete ringwall shall be level within ±1/8 in. in any 30 ft of circumference and within ±1/4 in. in the total circumference.

Question 10

Immediately before any welding operation, how should the surface to be welded be prepared? What type of film is not considered detrimental to the welding operation? (API 620-13, Section 6.10.1)

Answer 10

Immediately before any welding operation, the surface to be welded or to which weld metal is to be applied shall be cleaned thoroughly of all scale, slag, grease, and any oxide that would lower the quality of the deposited weld metal. A light oxide film resulting from flame cutting is not considered detrimental.

Question 11

How should the reverse side of double-welded butt joints be prepared prior to welding? Why must this be done? (API 620-13, Section 6.10.3)

Answer 11

The reverse side of double welded butt joints shall be prepared by chipping, grinding, or melting out to ensure sound metal at the base of the weld metal first deposited before weld metal is applied from the reverse side. This operation shall be done to ensure complete penetration and proper fusion in the final weld.

Question 12

What are three weather related conditions under which welding must not be done? (API 620-13, Section 6.11)

Answer 12

1) when the surfaces to be welded are wet from rain, snow, or ice,
2) when rain or snow is falling on such surfaces, or
3) during periods of high winds, unless the welder and work are properly shielded.

Question 13

When merging weld with plate surfaces, the maximum permissible undercutting of longitudinal or meridional butt joints is _________. (API 620-13, Section 6.13)

Answer 13

1/64 in

Question 14

What are the criteria for a weld to be acceptable by visual inspection? (API 620-13, Section 7.15.5.3)

Answer 14

a) The weld has no crater cracks or other surface cracks.
b) Undercut does not exceed the applicable limit in 6.13.
c) The frequency of surface porosity in welds does not exceed one cluster (one or more pores) in each 4 in. of length, and the maximum diameter of each cluster does not exceed 3/32 in.
d) Complete fusion and required penetration exists at the joint between the weld metal and the base metal.

Question 15

Before hydrostatic testing of a tank, what method must be used to inspect the attachment welding around all openings and their reinforcements in the walls of the tank? (API 620-13, Section 7.18.2.2)

Answer 15

The attachment welding around all openings and their reinforcements in the walls of the tank shall be examined by the magnetic-particle method both inside and outside the tank.

Question 16

To test reinforcement plate welds, ______ pounds per square inch of pneumatic pressure should be applied. (API 620-13, Section 7.18.2.3)

Answer 16

15

Question 17

Before hydrostatic testing of a tank, what method shall be used to test all bottom plate joints in cases where the tank bottom rests directly on the tank grade? What alternate method can be used? (API 620-13, Section 7.18.2.4)

Answer 17

In cases in which the bottom of the tank rests directly on the tank grade (preventing access to the underside of the bottom of the tank), all joints between the bottom plates shall be tested on the inside of the tank by applying a solution film to the joints and pulling a partial vacuum of at least 3 lbf/in.2 gauge by means of a vacuum box with a transparent top.
As an alternate to vacuum box testing, a suitable tracer gas and compatible detector can be used to test the integrity of welded bottom joints for their entire length if an appropriate tracer gas testing procedure has been reviewed and approved by the Purchaser.

Question 18

Safety and relief valves 1/2 inch and larger must be marked with certain information. This information should include: (API 620-13, Section 9.6.1.1)

Answer 18

a) name or identifying trademark of the manufacturer;
b) Manufacturer’s design or type number;
c) size of valve (pipe size of the valve inlet);
d) set pressure, in lbf/in.2 gauge;
e) full open pressure, in lbf/in.2 gauge;
f) capacity of valve, in ft3 or air 20 per minute (60°F and 14.7 lbf/in.2 absolute), see 9.6.1.2.

Question 19

The lowest footing course for a concrete tank foundation should be bedded directly against the sides of the excavation when the sides are self-supporting. What steps should be taken just before the concrete is poured to prepare the excavation? (API 620-13, Section C.8.2)

Answer 19

Before the concrete is poured, adjacent dry soils shall be thoroughly moistened by sprinkling with water. Likewise, all loose material from cave-in, plus any soft rain-soaked soil, shall be removed from the bottom of the excavation.

Question 20

How far beyond steel baseplates should the tops of concrete foundations project? (API 620-13, Section C.8.4)

Answer 20

The tops of foundations shall be large enough to project at least 3 in. outside of any steel baseplates of the superstructure.

Question 21

Except for the tops of concrete pedestal and wall foundations, how far below final grade should exposed concrete surfaces be smooth finished? (API 620-13, Section C.8.5)

Answer 21

The exposed surfaces, other than the tops of concrete pedestal and wall foundations, shall be smooth finished down to 6 in. below the proposed final grade.

Question 22

What should be done as a final check on the adequacy of the tank foundation during the hydrostatic test? (API 620-13, Section C.11)

Answer 22

As a final check on the adequacy of the foundations and subgrade, the Purchaser shall take level readings with surveyor’s instruments around the entire periphery of the tank before water is introduced into the tank for the hydrostatic test. The readings shall be continued at reasonable intervals during the entire filling operation and shall be plotted promptly in suitable form to indicate whether any undue or uneven settlement is occurring.