Types of Pressure Vessel Inspections According to API 510

The primary purpose of performing inspections on equipment at an industrial plant is to ensure that the equipment is currently operating safely and to determine whether it will continue to be safe until the next inspection interval. This process is crucial to prevent catastrophic failures, ensure mechanical integrity, and maintain plant operability.

Inspection plans must be thorough and specific, developed to cover all possible damage mechanisms and meet both applicable country-specific and code requirements. These plans include different types of inspections, Non-Destructive Testing (NDT) methods, and inspection intervals that must be selected based on the specific characteristics and circumstances of each piece of equipment.

Internal PV inspection:

The main objective of internal inspection is to identify damage that cannot be detected by regular monitoring of external Corrosion Measurement Locations (CML). This inspection requires removing the equipment from service and is usually carried out through a manhole. However, in cases where the vessel is too small to allow safe entry, alternative methods such as internal inspections using borescopes, drones, and robotic crawlers can be used.

API RP 572 provides more information on the internal inspection of pressure vessels and should be used when performing this type of inspection.

Some considerations to keep in mind:

• It is not necessary to completely remove internal elements if safety conditions are guaranteed.
• Spot NDTs may be required in selected areas, with the pressure vessel completely empty, to determine the condition of the vessel surface.
• Internal linings (e.g., refractory, cladding) should be closely examined for indications.

In-service PV inspection:

The objective of in-service inspection is to anticipate, as accurately as possible and using specific techniques, the damage mechanisms associated with service from the outside before performing an internal inspection. For certain pieces of equipment, in-service inspection may be acceptable in lieu of internal inspection, particularly when corrosion rates are well known. In-service inspection can be carried out while the vessel is depressurized or under pressure.

External PV Inspection:

External inspection is a crucial evaluation to ensure operational integrity and safety without the need to remove equipment from service. This inspection allows for the identification of potential problems visible from the outside.

The following points should be considered during this inspection:

• Vent holes in connection gussets should remain open to provide visual evidence of possible leaks.
• Special attention should be paid to insulated areas that may allow moisture ingress, as vessels are susceptible to corrosion under insulation (CUI).
• Conduct a visual examination for indications of bulging, out of roundness, buckling, and distortion.

Thickness examination

Thickness measurement is an essential technique for verifying the thickness of pressure vessel components. The data obtained from these measurements are used to determine corrosion rates (primary method) and the remaining useful life of the equipment.

Although thickness measurements are not always required during in-service inspections, they are the primary method for accurately monitoring corrosion rates.

The owner/operator is responsible for ensuring that examiners are trained and qualified as UT examiners in accordance with applicable procedures.

Inspection for CUI/CUF:

Inspection for Corrosion Under Insulation (CUI) and Corrosion Under Fireproofing (CUF) should be considered for externally insulated vessels and those in intermittent service or operating at temperatures between:

1. a) -12º C and 177º C for carbon and low alloy steels
2. b) 60º C and 177º C for austenitic stainless steels
3. c) 138º C and 177º C for duplex stainless steel

The most susceptible areas include above reinforcing rings, nozzles and manholes, other penetrations such as clips or brackets, damaged insulation (water ingress), areas with failed insulation, top and bottom headers, or other areas prone to accumulating moisture.

CUI inspection may require removal of some/all of the insulation.

For more information: API 510 | Pressure Vessels Inspection and Repair