This article provides information about the inspection of storage tanks and storage tank testing. We will cover construction shops, sites, and storage tank in-service inspection in operating units.
As tank owners, you must inspect your above-ground storage tanks. Hazardous waste leaks, spill prevention, costly repair, and environmental damage can be easy to avoid. The key is having a safely confined and proactive inspection and maintenance program.
What is an Above-Ground Storage Tank?
An above-ground storage tank is a large, heavy-duty container used to store oil and other material, and chemicals. The weight of an AST and the liquid it stores produce immense pressure on the ground beneath it. Over time, this pressure can wear away at the tank’s surface, causing pitting and crevice corrosion.
What is the Application of Above-Ground Storage Tanks?
There are two types of storage tanks, the Atmospheric Storage Tank, and the Low-Pressure Storage tank.
The greatest pressure for the 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 storage must be in a pressure vessel.
Atmospheric storage tanks store crude oil. As well as heavy oil, gas oils, furnace oils, naphtha, gasoline, non-volatile chemicals, etc.
A low-pressure storage tank stores light crude oil. As well as volatile chemicals, liquid oxygen, liquid nitrogen, etc.
The physical shape of the above-stated storage tanks might be similar. Yet, the construction code, design, and inspection requirements are different.
Inspection Requirements in the Manufacturing Equipment 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 are get prepared in two sections. One for the supply, cutting, rolling, and beveling of storage tank plates in the manufacturer’s shop. The other section is for construction activities in the field.
The prepared plates in the shop get transferred to the field for assembly. Based on the contract, in some cases, only one contractor does work for both 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 (construction contractor, third-party inspector, and client).
For more detailed information on above-ground storage tank inspection, 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. However, API RP 575 covers both atmospheric and low-pressure storage tanks.
The API STD 653 is gets 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.
Recommendations for your above-ground storage tank inspection are API RP 651 for tank bottom plates cathodic protection system. API RP 652 for the tank internal lining system, API RP 571 for the damage mechanism, and API 577 for welding inspection and metallurgy.
But suppose you found a conflict between the construction code API 650 and API 653. In that case, the need for API 653 must govern this standard. It addresses above-ground storage tank inspections for operating plants.
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
- Sump inspection
- Internal coating inspection (if any)
- Inspection of the 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 inspected 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 inspected 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 the inspection
- Pontoon Gasoil leak testing (Oil filled electrical)
- Internal coating inspection (if any)
- Roof drain system inspection
Internal Inspection-Internal Equipment
- Gage pole inspection for corrosion and deformation
- Support and structure member’s 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, and 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 the shell)
- Cathodic protection system inspection
With above-ground storage tanks, there are potential dangers. If you want help mitigating those risks, PMET is here to help.