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API 653 practice questions

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Question 1Tank Design and Construction

Under API 650, the one-foot method for calculating shell plate thickness may be used only for tanks whose diameter does not exceed:

  1. 250 ft (76 m)
  2. 150 ft (46 m)
  3. 200 ft (61 m) Correct answer.
  4. 100 ft (30 m)

Explanation

API 650 permits the one-foot method for tanks up to 200 ft (61 m) in diameter. Above that diameter the variable-design-point method must be used, because the point of maximum circumferential stress in a course can lie well above the one-foot design point.

Question 2Tank Design and Construction

Under API 650, the minimum nominal thickness of shell plates for a tank 50 ft to 120 ft (15 m to 36 m) in diameter is:

  1. 1/4 in (6 mm) Correct answer.
  2. 3/8 in (10 mm)
  3. 5/16 in (8 mm)
  4. 3/16 in (5 mm)

Explanation

API 650 sets minimum nominal shell thickness by tank diameter: 3/16 in below 50 ft, 1/4 in from 50 ft to 120 ft, and 5/16 in above 120 ft. This floor applies even when the hoop-stress calculation gives a smaller value.

Question 3Tank Design and Construction

In the API 650 one-foot method shell equation t = 2.6D(H - 1)G/S + CA, the term G represents:

  1. Specific gravity of the stored liquid Correct answer.
  2. The design internal gauge pressure of the tank
  3. The shell joint efficiency factor used in design
  4. The acceleration due to gravity acting on the liquid

Explanation

G is the design specific gravity of the product, which scales the hydrostatic pressure term of the equation. Where a specific gravity is not specified, a value of 1.0 is normally used.

Question 4Tank Design and Construction

A tank 100 ft in diameter stores a liquid of specific gravity 1.0 at a design liquid height of 40 ft. Using the one-foot method with S = 20,000 psi, E = 1.0, and a corrosion allowance of 0.0625 in, the required thickness of the bottom shell course is approximately:

  1. 0.570 in Correct answer.
  2. 0.440 in
  3. 0.582 in
  4. 0.507 in

Explanation

The one-foot method uses H - 1 = 39 ft: 2.6 x 100 x 39 x 1.0 / 20,000 = 0.507 in. Adding the 0.0625 in corrosion allowance gives 0.570 in, which is then compared with the minimum nominal thickness for the tank diameter.

Question 5Tank Design and Construction

API 650 requires that roof plates of a supported cone roof have a minimum nominal thickness of:

  1. 3/16 in (5 mm) Correct answer.
  2. 3/8 in (10 mm)
  3. 1/4 in (6 mm)
  4. 5/16 in (8 mm)

Explanation

Roof plates are not pressure-retaining, so API 650 sets a modest minimum nominal thickness of 3/16 in (5 mm). The plates must also be thick enough to carry the roof live load and any attached equipment.

Question 6Tank Design and Construction

Supported tank roofs are designed for a minimum uniform live load of:

  1. 10 lb/ft² (0.5 kPa)
  2. 50 lb/ft² (2.4 kPa)
  3. 25 lb/ft² (1.2 kPa) Correct answer.
  4. 20 lb/ft² (1.0 kPa)

Explanation

API 650 requires tank roofs to be designed for a minimum live load of 25 lb/ft² (1.2 kPa) applied over the horizontal projected area, in addition to dead load and any specified external loads.

Question 7Tank Design and Construction

API 650 specifies a minimum slope for a supported cone roof of:

  1. 1-1/2 in per ft (1:8)
  2. 3/4 in per ft (1:16) Correct answer.
  3. 1/2 in per ft (1:24)
  4. 1 in per ft (1:12)

Explanation

Supported cone roofs are built with a slope of at least 3/4 in per foot of radius (about 1:16). The slope ensures drainage and lets the roof plate act largely as a membrane rather than being loaded in bending.

Question 8Tank Design and Construction

Where annular plates are required, their radial width measured from the outside of the shell shall be at least:

  1. 6 in (150 mm)
  2. 24 in (600 mm) Correct answer.
  3. 12 in (300 mm)
  4. 18 in (450 mm)

Explanation

API 650 requires the annular ring to project at least 24 in (600 mm) outward from the outside of the shell. This places adequate thicker plate under the high-stress shell-to-bottom junction and keeps the critical zone away from the tank grade.

Question 9Tank Design and Construction

When a horizontal sweep board is used to check a vertical shell butt weld, the maximum permitted peaking is:

  1. 3/8 in (10 mm)
  2. 1/4 in (6 mm)
  3. 1/8 in (3 mm) Correct answer.
  4. 1/2 in (13 mm)

Explanation

Peaking at a vertical seam is limited to 1/8 in (3 mm) when measured with a 36 in (900 mm) sweep board. Excessive peaking concentrates stress at the weld and increases the risk of brittle fracture.

Question 10Tank Design and Construction

A horizontal sweep board used to check a shell course for flat spots (banding) may reveal a maximum gap of:

  1. 1 in (25 mm) Correct answer.
  2. 1-1/2 in (38 mm)
  3. 1/2 in (13 mm)
  4. 2 in (51 mm)

Explanation

Banding is limited to 1 in (25 mm) measured with a 36 in (900 mm) horizontal sweep board. Flat spots reduce the shell's resistance both to internal pressure and to wind-induced buckling.

Question 11Tank Design and Construction

The maximum permitted out-of-plumbness of a completed tank shell is:

  1. 1/50 of the total tank height
  2. 1/200 of the total tank height Correct answer.
  3. 1/100 of the total tank height
  4. 1/500 of the total tank height

Explanation

API 650 limits shell plumbness to 1/200 of the total tank height measured along the shell. This keeps the shell from leaning, which would otherwise interfere with floating roofs and internal structures and induce secondary stresses.

Question 12Tank Design and Construction

A new tank is filled with water to the design liquid level for the hydrostatic test and held at that level for at least:

  1. 8 hours
  2. 24 hours Correct answer.
  3. 1 hour
  4. 4 hours

Explanation

API 650 requires the water to be held at the design liquid level for at least 24 hours before the shell and bottom welds are examined. The holding period lets the foundation settle and allows any seepage or leakage to become visible.

Question 13Tank Design and Construction

Welding procedures used for API 650 tank construction must be qualified in accordance with:

  1. ASME Section IX Correct answer.
  2. AWS D1.1 structural welding code
  3. ASME Section VIII, Division 2
  4. API Recommended Practice 577

Explanation

API 650 requires welding procedure specifications and welder performance qualifications to be qualified in accordance with ASME Boiler and Pressure Vessel Code Section IX, the recognized code for procedure and performance qualification.

Question 14Tank Design and Construction

The minimum nominal thickness of tank bottom plates, excluding any corrosion allowance, is:

  1. 3/16 in (5 mm)
  2. 1/4 in (6 mm) Correct answer.
  3. 7/32 in (5.5 mm)
  4. 5/16 in (8 mm)

Explanation

API 650 requires bottom plates to be at least 1/4 in (6 mm) nominal thick, exclusive of corrosion allowance. Any specified corrosion allowance is added on top of this minimum.

Question 15Tank Design and Construction

The minimum diameter of a shell manhole in an API 650 tank is:

  1. 30 in (760 mm)
  2. 12 in (300 mm)
  3. 20 in (500 mm) Correct answer.
  4. 24 in (600 mm)

Explanation

Shell manholes must be at least 20 in (500 mm) in diameter so that a person wearing protective equipment can pass through for inspection, cleaning, and repair.

Question 16Tank Design and Construction

The primary function of a top wind girder on an open-top tank is to:

  1. Anchor the floating roof rim seal
  2. Carry the weight of the roof structure
  3. Keep the shell round under wind load Correct answer.
  4. Resist overturning moments from wind

Explanation

The top wind girder stiffens the open top of the shell so that wind pressure and suction do not buckle the shell. It is not a roof support, an anchorage, or part of the floating roof seal system.

Question 17Tank Design and Construction

API 650 applies to welded storage tanks with a design internal pressure that does not exceed approximately:

  1. 15 psi (103 kPa)
  2. 2.5 psi (17 kPa) Correct answer.
  3. 50 psi (345 kPa)
  4. 5 psi (35 kPa)

Explanation

API 650 covers flat-bottomed, cylindrical, welded tanks with internal pressure approximately equal to atmospheric, conventionally stated as not exceeding 2.5 psig. Tanks with higher internal pressure fall under API 620 or the ASME pressure vessel code.

Question 18Tank Design and Construction

In the one-foot method, the thickness of a shell course is calculated using the liquid level measured to a point:

  1. At the mid-height of the course
  2. 1 ft above the bottom edge of that course Correct answer.
  3. At the top edge of the course being designed
  4. Just below the top edge of the course

Explanation

The one-foot method evaluates hoop stress one foot above the bottom edge of each course, the elevation where restraint from the course below has effectively disappeared. This is why H is measured to the design liquid level and then reduced by 1 ft in the equation.

Question 19Tank Design and Construction

The emergency venting capacity required for a storage tank is determined in accordance with:

  1. API Standard 2000 Correct answer.
  2. NFPA 30
  3. API Recommended Practice 575
  4. ASME Section VIII, Division 1

Explanation

API Standard 2000 covers the venting of atmospheric and low-pressure storage tanks, including normal and emergency venting capacity. API 650 relies on it for sizing the vents that protect the shell and roof from overpressure or vacuum.

Question 20Tank Design and Construction

The minimum design metal temperature specified for a new tank is used principally for:

  1. Setting the lowest product storage temperature
  2. Establishing the hydrostatic test water temperature
  3. Selecting the thickness of shell insulation
  4. Determining whether impact testing is required Correct answer.

Explanation

The minimum design metal temperature, together with the material specification and plate thickness, determines whether Charpy impact testing is required and what energy level must be met. This prevents brittle fracture when the tank is cold, whether from climate or from refrigerated service.

Question 21Tank Design and Construction

The concrete ringwall under the shell of a large storage tank is provided to:

  1. Support the shell and spread the load Correct answer.
  2. Resist wind uplift on the shell
  3. Keep groundwater out of the foundation
  4. Provide a level surface for the floating roof

Explanation

A ringwall carries the concentrated shell and annular plate loads and distributes them over a wider area of the subgrade. The interior of the tank is normally supported by a compacted pad of sand or select fill rather than by the ringwall.

Question 22Tank Design and Construction

Anchor bolts are required on a welded storage tank when the tank:

  1. Is subject to net uplift from wind or seismic loads Correct answer.
  2. Stores liquid above 60 °C (140 °F)
  3. Exceeds 100 ft (30 m) in diameter
  4. Is fitted with a self-supporting cone roof and top angle

Explanation

Anchor bolts are provided only where the calculated overturning or seismic uplift exceeds the resisting dead weight of the shell, roof, and contents. Otherwise the tank is stable on its foundation by its own weight.

Question 23Tank Design and Construction

When a nozzle opening is cut into a shell course, reinforcement must be provided because:

  1. Metal carrying hoop stress is removed and must be replaced Correct answer.
  2. The nozzle alters the specific gravity of the product
  3. The nozzle adds concentrated weight to the shell
  4. The opening interrupts the protective corrosion barrier of the shell

Explanation

An opening removes shell material that resists the circumferential (hoop) stress, so the lost cross-sectional area must be replaced by excess thickness in the nozzle neck, a pad, or the shell itself within defined limits around the opening.

Question 24Tank Design and Construction

Two tanks are built from the same steel; Tank A has all shell butt welds fully radiographed (E = 1.0) and Tank B has them spot radiographed (E = 0.85). Compared with Tank A, the required shell thickness of Tank B will be:

  1. Greater, because the lower joint efficiency governs Correct answer.
  2. The same, because radiography does not affect design
  3. The same, provided the minimum nominal thickness is met
  4. Less, because less examination allows thinner plate

Explanation

Joint efficiency appears in the denominator of the shell thickness equation, so a lower efficiency produces a greater computed thickness. Full radiography raises the efficiency and therefore permits a thinner shell than spot radiography for the same design conditions.

Question 25Tank Design and Construction

A tank is designed to store a liquid with a specific gravity of 1.2. Which condition governs the required thickness of the bottom shell course?

  1. The product condition, because water is lighter Correct answer.
  2. Both conditions equally, since the thicknesses are averaged
  3. Neither, since the minimum nominal thickness always governs
  4. The hydrostatic test, because water is heavier

Explanation

The shell must satisfy both the product condition at G = 1.2 and the water test condition at G = 1.0. Because the product is heavier than water, the product condition produces the higher stress at the design liquid level and controls the thickness.

Question 26Tank Design and Construction

Excluding any corrosion allowance, what is the minimum nominal thickness permitted for roof plates on an API 650 storage tank?

  1. 5 mm (3/16 in) Correct answer.
  2. 8 mm (5/16 in)
  3. 3 mm (1/8 in)
  4. 6 mm (1/4 in)

Explanation

API 650 requires roof plates to be at least 5 mm (3/16 in) nominal thickness, with any corrosion allowance added by the purchaser on top of that value. Thinner plate lacks the stiffness needed to span between rafters and to be welded reliably.

Question 27Tank Design and Construction

In API 650 design, how is the corrosion allowance for a new tank established?

  1. It is set by the fabricator from the design pressure
  2. It is included in the minimum thickness tables
  3. It is fixed at 3 mm by the standard for all services
  4. It is specified by the purchaser Correct answer.

Explanation

The standard does not impose a fixed corrosion allowance. The purchaser specifies the allowance for the intended service, and it is added to the thicknesses calculated from the design equations as well as to the tabulated minimum nominal thicknesses, which do not include allowance.

Question 28Tank Design and Construction

The primary stress produced in the cylindrical shell of a tank by the stored liquid is:

  1. Longitudinal, parallel to the axis
  2. Torsional, about the vertical axis
  3. Radial, through the plate thickness
  4. Circumferential (hoop) tension Correct answer.

Explanation

Internal liquid pressure acts radially outward and generates hoop, or circumferential, tension in the shell, which is the stress that normally governs shell plate thickness. The longitudinal stress caused by the same pressure is roughly half the hoop stress and is not governing for pressure.

Question 29Tank Design and Construction

In a tank built from the same material throughout, the nominal thickness of the shell courses generally:

  1. Decreases from the bottom course to the top Correct answer.
  2. Is governed only by wind girder requirements
  3. Increases toward the top of the shell
  4. Is the same for all courses to simplify fabrication

Explanation

Hoop stress is proportional to the depth of liquid above the point considered, so the bottom course is the thickest. Thickness is reduced in each higher course until the minimum nominal thickness for the tank diameter is reached.

Question 30Tank Design and Construction

Using the 1-foot method with Sd = 160 MPa and G = 1.0, what is the minimum calculated shell thickness at the bottom course of a tank 30 m in diameter with a design liquid level 15 m above the bottom of that course?

  1. 13.5 mm Correct answer.
  2. 6.8 mm
  3. 13.8 mm
  4. 12.2 mm

Explanation

The 1-foot method uses t = 4.9D(H − 0.3)G/Sd, with the design point taken 0.3 m above the bottom of the course. Substituting D = 30 m, H = 14.7 m, G = 1.0 and Sd = 160 MPa gives t = 4.9 x 30 x 14.7 / 160, or about 13.5 mm.

Question 31Tank Design and Construction

The 1-foot method gives a required shell thickness of 9.6 mm for a course, and the purchaser specifies a corrosion allowance of 3.0 mm. What is the minimum nominal plate thickness for that course?

  1. 12.6 mm Correct answer.
  2. 15.6 mm
  3. 9.6 mm
  4. 6.6 mm

Explanation

The corrosion allowance is added to the thickness required by the design equation, so the minimum nominal thickness is 9.6 + 3.0 = 12.6 mm. The result is also checked against the tabulated minimum nominal thickness for the tank diameter.

Question 32Tank Design and Construction

What circumstance requires the addition of an intermediate wind girder to the shell of an open-top tank?

  1. When the shell is too tall to remain stable without stiffening Correct answer.
  2. Whenever the shell is built from more than six courses
  3. Only when the tank is anchored to its foundation
  4. Whenever the tank diameter exceeds 30 m, whatever the thickness

Explanation

API 650 gives a maximum unstiffened shell height that depends on shell thickness, diameter and design wind speed. When the actual shell height exceeds that value, one or more intermediate wind girders must be provided so the shell does not buckle under wind.

Question 33Tank Design and Construction

What structural function does the roof-to-shell junction serve in a fixed roof tank?

  1. It seals the shell-to-roof joint against vapour leakage
  2. It supports the shell against wind overturning moments
  3. It resists circumferential compression from roof loads Correct answer.
  4. It transfers the shell loads into the bottom plate

Explanation

Roof dead and live loads produce a radial thrust at the top of the shell, which the junction ring (usually a top angle or a fabricated compression ring) resists as circumferential compression. The ring also helps keep the top of the shell circular.

Question 34Tank Design and Construction

How is the required area of reinforcement for a nozzle opening in a tank shell determined?

  1. Opening diameter times the required shell thickness Correct answer.
  2. Twice the opening diameter times the nominal thickness
  3. Opening area times the design stress
  4. Opening circumference times the shell thickness

Explanation

Reinforcement follows the area-replacement principle: the metal removed by the opening must be replaced, and the required area equals the finished opening diameter multiplied by the required shell thickness. The replacement may be taken from excess shell thickness, a thicker nozzle neck or a reinforcing pad.

Question 35Tank Design and Construction

API 650 Appendix E sets out requirements for:

  1. Seismic design of storage tanks Correct answer.
  2. Qualification of welding procedures
  3. Design of tanks with stainless steel shells
  4. Design of tanks under external pressure

Explanation

Appendix E covers seismic design, including the hydrodynamic forces exerted by the contents and the resulting shell, roof and anchorage requirements. Other parts of the standard address materials, welding and different tank configurations.

Question 36Tank Design and Construction

A tank stores a liquid with a specific gravity of 0.8 and will be hydrostatically tested with water. Which condition governs the required shell thickness?

  1. The water test, because water has the higher specific gravity Correct answer.
  2. The product condition, because it is the condition the tank sees in normal operation
  3. Neither condition governs, because minimum plate thickness always controls the design
  4. Both conditions govern equally, because specific gravity does not affect thickness

Explanation

The shell must satisfy both the product condition and the water test condition, and the greater required thickness governs. Water at a specific gravity of 1.0 is heavier than the 0.8 product, so the test condition controls the shell design.

Question 37Tank Design and Construction

Excluding corrosion allowance, what is the minimum nominal thickness of shell plates for a tank 50 m in diameter?

  1. 10 mm (3/8 in)
  2. 6 mm (1/4 in)
  3. 8 mm (5/16 in) Correct answer.
  4. 5 mm (3/16 in)

Explanation

The tabulated minimum nominal shell thickness increases with tank diameter: 5 mm below 15 m, 6 mm from 15 m to 36 m, 8 mm from 36 m to 60 m, and 10 mm at 60 m and above. A 50 m tank therefore requires 8 mm plate before any corrosion allowance is added.

Question 38Tank Design and Construction

How must shell thickness be determined for a tank with a diameter of 80 m?

  1. By the 1-foot method applied to the bottom course only
  2. By the variable-design-point method Correct answer.
  3. By the 1-foot method applied to each course
  4. By treating the shell as a vertical beam in bending

Explanation

The 1-foot method is permitted only for tanks not exceeding 60 m in diameter. Above that size the variable-design-point method is used, because it accounts for the maximum hoop stress in a course occurring above the bottom of that course.

Question 39Tank Design and Construction

In a frangible-roof tank, what is intended to happen if the internal pressure rises excessively?

  1. The shell-to-bottom junction yields first, letting the shell lift
  2. The roof-to-shell junction fails and relieves the pressure Correct answer.
  3. The anchor bolts stretch and the tank lifts off intact
  4. The roof plates stretch elastically and then recover

Explanation

A frangible roof is deliberately designed so that the roof-to-shell junction is the weakest link in the pressure envelope. On excessive internal pressure, that junction yields and vents the pressure before the shell or the shell-to-bottom junction is ruptured.

Question 40Tank Design and Construction

Under what condition are annular bottom plates required beneath the shell?

  1. Whenever the tank diameter exceeds 30 m
  2. When the bottom course exceeds 13 mm Correct answer.
  3. Only when the tank is anchored to its foundation
  4. Whenever the shell has more than three courses

Explanation

Annular plates are required when the bottom shell course thickness, excluding corrosion allowance, exceeds 13 mm (1/2 in), and also when the product of the design liquid specific gravity and the tank diameter exceeds 15 m (50 ft). They spread the shell loads onto the foundation and provide heavier plate for the shell-to-bottom corner weld.

Question 41Inspection, Examination, and Testing

Which examination method is used to check lap-welded bottom seams of a storage tank for through-thickness leaks?

  1. Radiographic examination
  2. Ultrasonic thickness measurement
  3. Magnetic particle examination
  4. Vacuum box test Correct answer.

Explanation

A vacuum box draws a vacuum over a soap-solution-coated lap weld; bubbles that form at the seam show a leak path through the plate or weld. It is the standard leak test for lap seams in tank bottoms because it directly demonstrates tightness rather than internal soundness.

Question 42Inspection, Examination, and Testing

Before any other nondestructive examination is performed on a completed weld in a tank, the weld shall be:

  1. Machined flush
  2. Hydrostatically tested
  3. Visually examined Correct answer.
  4. Stress relieved

Explanation

Visual examination is the first required examination step for every weld. Welds that do not meet visual acceptance criteria must be repaired before other nondestructive methods are applied, and no other method substitutes for visual examination.

Question 43Inspection, Examination, and Testing

Which examination method is appropriate for detecting surface-breaking discontinuities in austenitic stainless steel tank welds?

  1. Magnetic particle examination
  2. Liquid penetrant examination Correct answer.
  3. Ultrasonic thickness measurement
  4. Radiographic examination

Explanation

Austenitic stainless steels are non-ferromagnetic, so magnetic particle examination cannot be applied to them. Liquid penetrant examination detects surface-breaking discontinuities in any nonporous material regardless of magnetic properties, provided the surface is clean.

Question 44Inspection, Examination, and Testing

Which method is used to determine the remaining thickness of in-service tank shell plates with access to only the outside surface?

  1. Liquid penetrant examination
  2. Magnetic particle examination
  3. Ultrasonic gauging Correct answer.
  4. Radiographic examination

Explanation

Ultrasonic thickness gauging measures remaining wall thickness using access to one surface only, which suits in-service shell surveys. The instrument is calibrated on a reference block of known thickness before and during the survey.

Question 45Inspection, Examination, and Testing

Which technique is commonly used for scanning large areas of a tank bottom plate for corrosion?

  1. Vacuum box testing
  2. Radiographic examination
  3. Liquid penetrant examination
  4. Magnetic flux leakage Correct answer.

Explanation

Magnetic flux leakage scanning, including automated floor scanners, covers large bottom areas quickly and identifies regions of metal loss in ferromagnetic plate. Point methods such as penetrant or radiography are then used to follow up on suspect areas.

Question 46Inspection, Examination, and Testing

The primary purpose of the hydrostatic test performed after major repairs is to:

  1. Qualify the welding procedure
  2. Verify foundation bearing pressure
  3. Measure shell thickness
  4. Confirm leak tightness Correct answer.

Explanation

A hydrostatic test fills the tank with water so the repaired shell, bottom, and joints can be observed under load for leakage and distortion. It demonstrates tightness and structural adequacy of the completed work, not material thickness or welding procedure capability.

Question 47Inspection, Examination, and Testing

Before using an ultrasonic thickness gauge for a shell survey, the instrument shall be calibrated:

  1. Using a radiographic film reference
  2. Against a reference block Correct answer.
  3. Only once per year by the manufacturer
  4. Only when readings fall below the minimum

Explanation

Ultrasonic thickness gauges are calibrated on a reference block of known thickness and similar material, and the calibration is rechecked at intervals during the survey and whenever the transducer or material changes. Accurate calibration is the basis for every thickness reading taken.

Question 48Inspection, Examination, and Testing

Routine in-service visual inspections of a tank between internal inspections are typically performed by:

  1. The welding inspector only
  2. The tank manufacturer
  3. The designated inspector Correct answer.
  4. The API 653 authorized inspector only

Explanation

Routine in-service visual checks may be carried out by owner/operator personnel or a designated inspector working to a written procedure. The authorized inspector is reserved for the formal inspection and suitability-for-service evaluations.

Question 49Inspection, Examination, and Testing

Which discontinuity is most reliably detected by radiography of a shell butt weld?

  1. A tight, planar toe crack
  2. Internal porosity Correct answer.
  3. A lamination parallel to the plate surface
  4. A surface scratch on the weld face

Explanation

Radiography responds to differences in density and thickness along the beam path, so volumetric features such as porosity and slag are readily imaged. Tight planar cracks may be missed unless oriented favorably, and features parallel to the plate surface produce little contrast.

Question 50Inspection, Examination, and Testing

Magnetic particle examination of a carbon steel weld is most effective for finding:

  1. Weld metal chemistry
  2. Laminations deep in the plate
  3. Surface and near-surface defects Correct answer.
  4. Measuring remaining wall thickness

Explanation

Magnetic particle examination reveals discontinuities that distort the magnetic field at or just below the surface. Its useful depth of detection is limited, so deeper internal features require ultrasonic or radiographic examination.