Storage tanks are the vessels used to store petroleum products and constitute one of the core facilities in the storage and transportation system of oil depots. For a long time, many in-service storage tanks in China have exhibited defects such as cracks and corrosion to varying degrees. Coupled with inadequate management during operation, these issues have led to frequent safety incidents, resulting in equipment damage and even casualties. Therefore, conducting nondestructive testing on in-service storage tanks is of great significance.

Currently, abroad, acoustic emission technology is primarily used for online detection of active defects on the shell plates of atmospheric storage tanks and of corrosion and leakage signals on the tank bottom plates; magnetic flux leakage methods are employed for periodic inspection of corrosion and leakage on the tank bottom plates; and ultrasonic testing is utilized to inspect the shell and roof plates.

In recent years, routine inspection of atmospheric storage tanks has only just begun in China, with ultrasonic testing, magnetic particle testing, radiographic testing, penetrant testing, and ultrasonic thickness measurement being the primary methods employed. In contrast, acoustic emission and magnetic flux leakage scanning—techniques that are now widely used abroad—currently lack corresponding inspection standards and codes in China and are still in the experimental and pilot-promotion stage.

The selection of inspection methods during tank construction is determined by the type, size, orientation, and location of defects, as well as the geometry, dimensions, weld locations, and material properties of the tank components being inspected.

1. Principles for the Application of Nondestructive Testing Methods for Storage Tanks

Atmospheric storage tanks are primarily fabricated by prefabricating the roof, shell plates, and bottom plates on-site, followed by assembly and welding. The roof and shell plates are typically joined by butt welds, while the bottom plates generally use lap joints. For the steel plates used in the bottom ring and the first ring of the tank wall, when the plate thickness is ≥23 mm, ultrasonic testing shall be conducted in accordance with ZBJ 74003-1988 “Ultrasonic Testing of Steel Plates for Pressure Vessels,” and only those meeting Grade III requirements shall be deemed acceptable. For steel plates with a yield strength ≤390 MPa, a random inspection shall be carried out on 20% of the total number of plates; if any nonconforming plates are found, each plate must be individually inspected. For steel plates with a yield strength >390 MPa, each plate must be inspected individually.

1. Nondestructive testing of the tank bottom weld:

(1) All bottom-plate welds shall be subjected to a tightness test using the vacuum box method, with a test negative pressure of not less than 53 kPa; the welds are considered合格 if no leakage is detected.
(2) For edge-plate butt welds with a specified yield strength greater than 390 MPa, penetrant testing shall be performed after completion of the root pass, and a second round of penetrant testing or magnetic particle testing shall be conducted upon completion of the final layer.
(3) For tank bottom edge plates with a thickness of ≥10 mm, radiographic testing shall be performed on the outer 300 mm of each butt weld; for tank bottom edge plates with a thickness of <10 mm, at least one weld per welder shall be subject to random inspection in accordance with the aforementioned method.
(4) After the root passes of lap joints in the overlapping sections of the three-layer steel plates of the bottom plate and of tee joints connecting to the tank bottom plate have been completed, penetrant testing shall be performed within a 200 mm range in each of the three directions. Upon completion of all welding, penetrant testing or magnetic particle testing shall be conducted.
(5) Magnetic particle or penetrant testing shall comply with SY/T 0444-1998 “Technical Standard for Magnetic Particle Testing of Atmospheric Steel Welded Storage Tanks and Pipelines” and SY/T 0443-1998 “Technical Standard for Penetrant Testing of Atmospheric Steel Welded Storage Tanks and Pipelines.”

2. Nondestructive testing of tank wall welds:

(1) For longitudinal welds, for each welder and each plate thickness (plates with a thickness difference of no more than 1 mm may be considered equivalent), a 300-mm section shall be selected at an arbitrary location along the initial 3-meter length of weld for radiographic examination. Thereafter, regardless of the number of welders, a 300-mm section shall be selected at an arbitrary location within every 30 meters of weld, as well as at the end of each 30-meter segment, for radiographic examination. Of the sections subjected to radiographic examination, 25% shall be located at tee joints, with a minimum of two such locations per tank.
(2) For circumferential butt welds, for each plate thickness (based on the thinner plate), a 300-mm section shall be selected at any location along the initial 3-meter weld segment for radiographic examination. Thereafter, for each plate thickness, a 300-mm section shall be selected at any location within every 60 meters of weld and any remaining fractional length thereof for radiographic examination. The above inspection requirements do not take into account the number of welders.
(3) When the thickness of the bottom circumferential shell plate is ≤10 mm, 300 mm shall be randomly selected from each longitudinal weld for radiographic examination; when the plate thickness is 10 mm ≤ t ≤ 25 mm, two 300-mm sections shall be selected from each longitudinal weld for radiographic examination, one of which shall be located close to the bottom plate.
(4) For circumferential shell plates with a thickness of 25 mm ≤ t ≤ 38 mm, radiographic examination shall be performed on every longitudinal weld; for shell plates with a thickness greater than 10 mm, radiographic examination shall be performed on all tee joints.
(5) Except for T-joints, ultrasonic testing may be used in place of radiographic testing; however, 20% of such joints shall be subject to radiographic re-inspection.
(6) When radiographic or ultrasonic testing results are unsatisfactory, additional inspection shall be carried out by extending 300 mm at each end of the inspected length; however, if the defect is located more than 75 mm from the edge of the radiograph or from the ultrasonic examination area, no further extension is required. If the inspection results for the extended areas remain unsatisfactory, the inspection shall be continued by further extending the examination.
(7) Radiographic testing shall be conducted in accordance with GB/T 3323-2005 “Radiographic Examination of Fusion-Welded Joints in Metallic Materials.” The acceptance criterion shall be Level III; however, for welds in steels with a yield strength greater than 390 MPa, or in ordinary carbon steel with a thickness of 25 mm or more, or in low-alloy steel with a thickness of 16 mm or more, the acceptance criterion shall be Level II. Ultrasonic testing shall be performed in accordance with JB 1152-1981 “Ultrasonic Testing of Butt Welds in Boilers and Steel Pressure Vessels,” with an acceptance criterion of Level II.

3. Nondestructive testing of internal fillet welds in T-joint tanks:

Nondestructive testing of the internal fillet welds at the T-joints between the bottom ring tank wall and the tank bottom:

(1) When the thickness of the tank bottom edge plate is ≥8 mm and the thickness of the bottom ring wall plate is ≥16 mm, or when steel plates of any thickness have a yield strength >390 MPa, penetrant or magnetic particle testing shall be performed on the internal fillet welds after completion of both the internal and external fillet welds. Following the water-filling test of the oil tank, a re-inspection shall be conducted using the same methods, with flaw detection requirements and standards identical to those for the bottom plate inspection.
(2) For steel plates with a yield strength greater than 390 MPa, penetrant testing shall also be performed after the completion of the first pass of the internal fillet weld.

II. Online Nondestructive Testing Technology for Storage Tanks

Tank inspection can be categorized into offline (open-tank) inspection and online inspection. Offline inspection requires shutdown and tank emptying, with inspectors safely entering the tank to carry out the examination, typically using conventional nondestructive testing methods.

Tank inspection and maintenance entail lengthy schedules and high costs, which can disrupt production; therefore, it is crucial to establish an appropriate maintenance interval for storage tanks. Both excessively long and excessively short intervals are suboptimal: overly long intervals leave tanks exposed to heightened risks due to delayed maintenance, while overly short intervals result in unnecessary maintenance-related losses and increased production costs. Moreover, production constraints often make it difficult to adhere to the prescribed maintenance schedule, leading to delayed identification of potential tank hazards and compromising safe tank operation.

Corrosion is the primary cause of damage to steel storage tanks and their associated components; therefore, leak detection and assessment of corrosion severity are critical components of tank inspection, with corrosion monitoring playing a particularly important role. Statistical data indicate that the bottom plate of in-service storage tanks is the most severely corroded area. Among all known tank accidents, those resulting from bottom-plate corrosion and subsequent leaks account for more than 20%, making this the leading cause of tank failures. Consequently, strengthening corrosion monitoring of tank bottoms is an essential measure for enhancing tank safety, and inspection and maintenance efforts should be focused on the tank bottom.

Online tank inspection enables safety assessment of tank bottom plates without opening the tank or halting production, serving as a method for comprehensive surveys and initial screening of bottom-plate corrosion conditions and offering a degree of predictive capability. By conducting qualitative assessments through online inspection to classify the structural integrity of the tank bottom plate and developing corresponding maintenance plans, enterprises can allocate limited maintenance resources more efficiently, extend the inspection intervals for “good” tanks, promptly identify and repair “defective” tanks, reduce and prevent environmental pollution caused by leaks, lower operation and maintenance costs, and mitigate losses resulting from potential accidents.

Currently, the main online corrosion monitoring technologies for tank bottom plates internationally include acoustic emission, guided wave testing, and robotic inspection.

III. Acoustic Emission Testing Technology

The phenomenon in which a localized source within a material rapidly releases energy, generating transient elastic waves, is known as acoustic emission, sometimes also referred to as stress-wave emission.
When corrosion defects are present in the tank bottom plate, its structural strength is reduced, leading to localized micro-deformations under the pressure of the liquid level. These deformations can cause the detachment and spalling of corrosion products, generating acoustic emission signals. In the event of a leak, the flow of the process medium also produces continuous acoustic emission signals. An online acoustic emission monitoring system uses a sensor array mounted on the lower portion of the tank’s outer wall to detect the acoustic emission signals generated by corrosion and leakage in the bottom plate, and then analyzes these signals to assess the corrosion condition of the tank bottom structure.

IV. Ultrasonic Guided Wave Testing Technology

Ultrasonic guided waves are elastic waves that propagate along the length of a structure and are confined and guided by the structure’s geometric boundaries; they are commonly used for defect detection in plates and for structural health assessment. Waves that propagate in plates whose thickness is comparable to their wavelength are referred to as plate waves (also known as plate-guided waves). These waves are primarily detected using Lamb waves and exhibit characteristics such as long propagation distances, rapid and convenient inspection, and insensitivity of detection accuracy to the medium inside the tank, making them well suited for online inspection of tank bottom and shell plates.

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