This article provides information about the inspection of storage tanks and storage tank testing in a construction shop and site and storage tank in-service inspection in operating units. In addition, I will be publishing articles on more subject matters, such as storage tank inspection procedures and storage tank ITP. 

What is the Application of a Storage Tank?

There are two types of storage tanks, the Atmospheric Storage Tank, and the Low-Pressure Storage tank.

The maximum pressure for vapor or gas space of an atmospheric storage tank is 2.5 psi, and for a low-pressure storage tank is 15 psi.

If the pressure is more than 15 psi, liquid in the storage tank is not allowed, and it must be stored in a pressure vessel.

Atmospheric storage tanks store crude oil, heavy oil, gas oils, furnace oils, naphtha, gasoline, non-volatile chemicals, etc.

A low-pressure storage tank stores light crude oil, gasoline blending stock, light naphtha, pentane, volatile chemicals, liquid oxygen, liquid nitrogen, etc.

The physical shape of the above-stated storage tanks might be similar, but the construction code, design, and inspection requirements are different.

What are the Above Ground Storage Tank Inspection Requirements in the Manufacturer Shop and the field?

The construction code for the atmospheric storage tank is API STD 650, and for a low-pressure storage tank is API STD 620. These cover the minimum design, materials, fabrication, inspection, and testing requirements.

With the above explanation, your inspection and test plan (ITP) for storage tank construction needs to meet API STD 650 or API 620.

The storage tank inspection and test plan summarize all inspection requirements in several pages and tabular format.

The storage tank inspection and test plan are prepared in two sections, one for the supply, cutting, rolling, and beveling of storage tank plates in the manufacturer’s shop and the other section for construction activities in the field.

The prepared plates in the shop are transferred to the field for assembly. Depending on the client’s contract, sometimes there is only one contractor for shop and field activities, and sometimes there are two contractors, one for the shop and one for field assembly.

This table determines each party’s responsibility (i.e., construction contractor, third party inspector, and client).

For more detailed information on above-ground storage tank inspection, both in-shop and on-site, review the following articles:

What are the Above Ground Storage Tank Inspection Requirements in Operating Plants?

The In-Service Inspection code for your atmospheric storage tank is API STD 653. The content in this standard covers tank inspection, repair, alteration, and reconstruction.

You must use this standard with the API Recommended Practice 575, titled “Guidelines and Methods for Inspection of Existing Atmospheric and Low-pressure Storage Tanks.”

As you see above, the API STD 653 addresses only the atmospheric storage tank, but API RP 575 covers both atmospheric and low-pressure storage tanks.

The API STD 653 is designated for atmospheric above-ground storage tank inspection for a tank built based on API 650. Still, you may use this code for any steel tank with specific consideration and operating details.

Other recommendations for your above-ground storage tank inspection are API RP 651 for tank bottom plates cathodic protection system, API RP 652 for tank internal lining system, API RP 571 for damage mechanism, and API 577 for welding inspection and metallurgy. In addition, many of the design, welding, examination, and material requirements of API 650 can be applied in the maintenance inspection, rating, repair, and alteration of in-service tanks.

But suppose you found a conflict between the construction code API 650 and API 653. In that case, the requirement of API 653 must govern this standard, as it -specifically addresses above-ground storage tank inspections for operating plants.

API 653 above-ground storage tank inspectors are qualified for the inspection of storage tanks, as they are certified by API for such activities. 

What are Important Points in the Above Ground Storage Tank Inspection in a shutdown?

For detailed information, you need to refer to API STD 653 and API RP 575.

Internal Inspection-Shell Plate

  • Shell plate inspection for corrosion and deformation
  • Shell plate circumferential and longitudinal welding joints inspection for corrosion and cracking
  • Internal coating inspection (if any)

Internal Inspection- Bottom Plate

  • Thickness measurement and remaining life calculation on bottom plates and annular plates
  • Bottom plates and annular plates inspection for corrosion and deformation
  • Bottom plates and annular plates welding joints inspection for corrosion and cracking, specifically the annular plate joints with first shell course plate
  • Sump inspection
  • Internal coating inspection (if any)
  • Inspection of sample plate, which must be cut to find any external corrosion
  • Vacuum testing inspection and supervision

Internal Inspection-Fix Roof

  • Roof plate inspection for corrosion and deformation
  • Roof structure members inspection for corrosion and deformation
  • Roof plates and structure members welding joints inspection for corrosion and cracking
  • Internal coating inspection (if any)

Internal Inspection-Floating Roof

  • Roof plate inspection for corrosion and deformation
  • Roof structure members inspection for corrosion and deformation
  • Roof plates and structure members welding joints inspection for corrosion and cracking
  • Roof sealing system inspection, i.e., rubber seal, seal plate, etc.
  • The roof leg supports inspection
  • Pontoon Gasoil leak testing
  • Internal coating inspection (if any)
  • Roof drain system inspection

Internal Inspection-Internal Equipment

  • Gage pole inspection for corrosion and deformation
  • Support and structure members inspection for corrosion and deformation
  • Inspection of columns for corrosion and deformation
  • Fixed coil inspection and test for corrosion

External Inspection-Shell and Roof

  • Thickness measurement and remaining life calculation
  • Shell and roof plates inspection for corrosion and deformation
  • Shell and roof plate circumferential and longitudinal welding joints inspection for corrosion and cracking
  • Nozzles, manways, flange welding joints inspection for corrosion and cracking
  • The external part of annular plates inspection, specifically welding joint with shell plates
  • Side vents inspection
  • Gasket seating surface inspection of nozzles and manways flange joints
  • Top angle, wind girder, and stairway inspection for corrosion and deformation
  • Roof sump inspection (floating roof only)
  • Pontoon and pontoon structure welding joints inspection for corrosion and cracking (floating roof only)
  • Weather shield rubber and pontoon doors rubber inspection (floating roof only)
  • Pontoon air leak testing (internal floating roof only)
  • Painting or insulation inspection

External Inspection-Foundation 

  • Concrete foundation inspection for corrosion and cracking
  • Earthling system inspection
  • Asphalt inspection (around shell)
  • Cathodic protection system inspection