Non Destructive Testing On Tank

NDT (non destructive testing) is most widely used in inspection by Quality Control Inspector and Third Party. Main aim of NDT is to check the defects without any damage to our product  ,equipment , labor and environment. This article is all about NDT used on oil storage tank during construction which  I had understand and visualize in my Internship at PSO Zulfiqarabad Oil Terminal.

 In this article I shall be elaborating about these Non Destructive Testing ,

1. Radiography Testing                 (RT)

2. Vacuum Box Testing                 (VBT)

3. Die Penetrate Testing               (DPT)

4. Magnetic Particle Inspection    (MPI)

5. Ultrasonic Testing                     (UT)

6. Oil Penetrate Testing                 (OPT)

1. Radiography Testing (RT) is a sort of NDT used for internal defects at welding joints. One of the major advantage of RT is that we have proper documentation of this test in the form of films at particular joint. Although it is not possible to conduct RT of whole tank usually 2.5% of tank is to be tests by RT normally. I visually saw the process of RT at PSO it was quite hazardous for human as the source which emits gamma rays was of 14Q . Major instrument used for RT is Survey meter , collie meter , Pocket  Dossi meter , File and source of Radium 192.

Above is the source of radium 192 which emits radiations and a testing film is pasted at the other side of the  tank to catch the rays leaving the tank walls.

This Schematic shall elaborate about the RT , at one side of the tank wall is our source and at the other side is our film at the center is our weld joint. Source emits radiation which passes from tank walls and make a photo of the weld portion and these films are than inspected to check the internal defects .

 One of its disadvantage is that we can perform it only for 2.5% of our joints , it cannot be used for Annular plate to Shall plate joint, limited to 72 mm thickness ,it is hazardous for human RT workers used Survey meter to check the intensity of radiation and Pocket Dossi meter to determine the rays injected in one person.   

2. Vacuum Box Testing (VBT) is one of the efficient NDT for butt and fillet joints , pin holes and point holes can easily be determined by VBT. It consist of a glass box , soapy solution , and a vacuum for suction purpose.

 Usually a suction or vacuum pressure of -21 to -35 KPa is provides by the vacuum. The principle is simple that bubble shall form at the point or pin hole which is a welding defect. First of all soapy solution is applied to the surface than vacuum box is place on the surface and vacuum is applied gradually to visualize the defects. One of its disadvantage is that it cannot be used for Annular plate to Shall plate joint.

 3. Die Penetrate Testing (DPT) is quite efficient NDT because it can be used in any joint in the tank either its Annular to Shall , Shall to Shall , Full plate to Sketch Plate , Sketch plate to annular plate , Butt weld , filet weld and can also be used for roof welding of tank too. In DPT usually three chemicals are used.

a ) Cleaner                                      b ) Penetrant                                                c) Developer

 These all are applied in order a in the form of continuous layer on welded part to inspect any defects . Its results can easily be understand as penetrant liquid rushes out from the developer layer in 5 minutes.

Like the figure above after DPT shows the welding defect on left and a complete crake on the right figure on flange.

4. Magnetic Particle Inspection (MPI)    is a (NDT) process for detecting surface and shallow subsurface discontinuous in tank . The process puts a magnetic field into the part. The piece can be magnetized by direct or indirect magnetization. Direct magnetization occurs when the electric current is passed through the test object and a magnetic field is formed in the material. Indirect magnetization occurs when no electric current is passed through the test object, but a magnetic field is applied from an outside source. The magnetic lines of force are perpendicular to the direction of the electric current. I was unable to saw MPI at PSO site because of some personal issues .Basic Schematic is shown in figure.

  5. Ultrasonic Testing (UT) is also a sort of rays like testing of tank or pressure vessel for this test it is mandatory to empty your whole tank or pressure vessel before conducting the UT. This test uses high frequency sound waves beyond our hearing limit. High frequency sound waves are very directional, and they will travel through a medium  of tank or vessel until they encounter a boundary with another medium usually air or could be water at which point they reflect back to their source. By analyzing these reflections it is possible to measure the thickness of a test piece, or find evidence of cracks or other hidden internal flaws.  

Usually during UT a sort of graph is formed which is being inspected  by Quality Control Inspector and Third Party at the site to visualize the internal defects and depth of the plate . UT could be used for a wide range too usually 100m from both side of the probe.

Some of disadvantages or we can say that limitation in UT is that , Our tank shall be empty , shall not be performed on concrete tank , flowing gases must be removed , tank shall not be underground.

6. Oil Penetrate Testing (OPT) is also a sort of NDT which is quite efficient for annular to shall joint. OPT is usually done after the first pass of Annular plate to shall plate joint. First of all the inner side of the shall to annular joint is covered with limestone powder and oil (usually diesel is used at our site) is sprayed from outside of the tank to the annular to shall joint . After the holding time of 4 hour inspection is done if any crack is present a sort of leakage shall be visible by naked eye.

Special Thanks To Our Site Mentor;

Engineer Imran Sahab