Compliance

API 510 Pressure Vessel Inspections: What Gets Inspected and When

Pressure vessels hold gases and liquids under pressure in refineries, gas plants, and chemical facilities. Separators, scrubbers, and receivers are all vessels. When one fails, the result can be deadly. API 510 exists to catch that failure before it happens. It sets how often vessels get inspected, who may inspect them, and what the inspector has to check.

What API 510 covers

API 510, the Pressure Vessel Inspection Code, applies to pressure vessels in petroleum and chemical service. That covers the fired and unfired vessels that hold hydrocarbons, process fluids, steam, and chemicals. The American Petroleum Institute publishes the standard. It sets the rules for in-service inspection, repair, alteration, and rerating of pressure vessels.

The vessels in question include the three phase separators that split oil, gas, and water, plus scrubbers, knock-out drums, and other process vessels. Compression plants are full of them, as covered in our gas compression equipment guide. Fired heaters and piping fall under other codes. Vessels in petroleum refining, gas processing, chemical plants, and similar service are the ones API 510 governs.

Who can inspect

Not everyone can inspect a pressure vessel. API 510 requires inspections to be carried out by a qualified API 510 inspector. These are individuals who have passed an API certification exam and met the experience requirements. The certification is administered by API.

The inspector determines the condition of the vessel, judges whether it is fit for continued service, and sets the interval for the next inspection. The owner or operator stays responsible for the integrity of the vessel. But the certified inspector is the one who signs off on the work.

The inspection cycle

API 510 lays out a cycle of inspections. An external visual inspection checks the outside of the vessel for visible damage, leaks, and deterioration. An internal inspection looks at the inside, where corrosion and cracking usually live. There is also on-stream inspection, which can be done while the vessel stays in service using methods like ultrasonic thickness measurement.

The intervals have set maximums. An external visual inspection is due at least every five years. An internal inspection is due at least every ten years. In between, the vessel may need thickness measurement at intervals set by the rate of corrosion.

The corrosion rate drives the schedule. A vessel corroding quickly gets inspected more often. A vessel with a low corrosion rate can run longer. The inspector calculates the corrosion rate from past thickness readings and uses it to set the next internal inspection date, within the limits the code allows.

Thickness measurement points and corrosion rates

Thickness measurement is the core of vessel integrity work. The inspector establishes thickness measurement locations, often called TMLs, at points on the shell and heads where corrosion is most likely. These locations are recorded so the same spots get measured every time.

Readings are compared against the minimum required thickness for the vessel. From the readings, the inspector works out the corrosion rate, which is how fast the wall is thinning. The corrosion rate feeds directly into the remaining life calculation and the next inspection interval. This is why consistent, comparable readings matter. If the wrong point gets measured, the data is worthless.

What the inspector looks for

The inspector hunts for the failure modes that take vessels down. Corrosion, both general wall loss and local pitting. Cracking, which can be fatigue cracks, stress corrosion cracking, or cracks at welds. Distortion, which suggests the vessel has been overstressed or overheated. Leaks at nozzles, flanges, and welds. Bulges, dents, and mechanical damage.

The inspection also checks supports, foundations, insulation, and coatings, because damage can hide underneath. Every finding is compared against the acceptance criteria in the code. The result decides whether the vessel stays in service, needs repairs, or must be taken out of service.

Repairs and alterations

When a vessel fails inspection, it cannot simply be patched. API 510 sets requirements for repairs and alterations. Repairs restore the vessel to a safe condition. Alterations change the design, such as adding a nozzle or changing the design conditions.

Both require proper procedures, qualified welders, and materials that match the original design. After the work is done, the vessel is verified before it returns to service. Entry into a vessel for internal work is hazardous, which is why permit to work systems matter on site. Records of the repair become part of the vessel file.

Records

The inspection is not finished when the inspector leaves the vessel. API 510 requires records that document the condition of the vessel and the basis for the next interval.

Inspection reports record what was found and what was done. Thickness data is kept so corrosion rates can be tracked over the life of the vessel. Remaining life calculations justify the interval that was set. The vessel file should hold the original design data, the inspection history, repair records, and the current condition assessment. These are the records the next inspector, and any regulator, will read.

API 510 and OSHA mechanical integrity

In the United States, vessel inspection is not just good practice. It is a legal requirement. OSHA's process safety management standard, 29 CFR 1910.119, covers facilities that handle highly hazardous chemicals. Its mechanical integrity requirements mandate written procedures, training, and the inspection and testing of pressure vessels and other critical equipment.

Inspections must follow recognized and generally accepted good engineering practices. That is exactly where API 510 comes in. Following API 510 gives a facility a defensible basis for its inspection program. A vessel that is not inspected on schedule is a compliance gap, and one an inspector can cite.

Scheduling the next inspection

The interval is only as good as the follow-through. When the inspector sets the next due date, someone has to make sure the vessel actually gets inspected by then. In a plant with hundreds of vessels, dates slip.

Facilities schedule the next inspection against the vessel list so the interval never slips. Software like OpsFlo, which tracks field tickets, dispatch, approvals, timesheets and documents for oilfield service companies, keeps vessel work visible and accounted for. The inspection date, the findings, and the repair records belong in the same system.

Sources and further reading

• API 510: Pressure Vessel Inspection Code
• OSHA 29 CFR 1910.119: Process safety management of highly hazardous chemicals