API 510 · 570 · 650/653 · 520/521 · RP 2003 · RP 552 · RP 14C

API inspection codes explained

What each of the common API codes covers (vessels, piping, tanks, relief systems, static and lightning, signal transmission, offshore safety systems), who uses it, and which inspector certifications go with it.

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Which API document covers what

Document	Covers	Who uses it	Certification that goes with it
API 510	In-service inspection, rating, repair, and alteration of pressure vessels	Refinery / chemical plant inspection departments, repair contractors	API 510 Authorized Pressure Vessel Inspector
API 570	In-service inspection, rating, repair, and alteration of process piping	Plant inspection groups, piping repair contractors	API 570 Authorized Piping Inspector
API 650	Design and construction of new welded atmospheric storage tanks	Tank fabricators and erectors, engineers	Construction inspection — often AWS CWI for welding
API 653	Inspection, repair, alteration, and reconstruction of existing tanks	Tank owners, repair contractors	API 653 Aboveground Storage Tank Inspector
API 520 (Parts I and II)	Sizing and selection (Part I) and installation (Part II) of pressure-relief devices	Process and relief-system engineers; installers for Part II	Engineering — no field inspector cert
API 521	Pressure-relieving and depressuring systems: relief scenarios, flares, and disposal	Process safety and flare-system engineers	Engineering
API RP 2003	Preventing ignition from static electricity, lightning, and stray currents	Tank farms, loading racks, fueling operations, electrical contractors	None specific
API RP 552	Instrument signal transmission systems — wiring, shielding, grounding, and routing	I&E designers and instrumentation contractors	None specific (ISA certifications common)
API RP 14C	Safety analysis and shutdown systems for offshore production facilities	Offshore operators, controls integrators, I&E techs	None specific

Our own summary, not the text of any API document. Use the edition your owner, jurisdiction, or insurer requires — consult the standard from the publisher. Not engineering advice.

Where a contractor runs into each one

You are…	The document you will be asked about	What you typically provide
Doing a weld repair on a vessel in a refinery	API 510	Repair procedure, WPS/PQR, NDT, and sign-off by the authorized inspector
Replacing a section of process piping	API 570	Material certs, weld records, pressure-test records, inspector acceptance
Erecting a new field-welded tank	API 650	Fabrication and welding records, NDT, hydrotest, and settlement survey
Replacing a tank floor or shell course	API 653	Repair plan approved by the inspector, weld and NDT records, hydrotest as required
Installing a relief valve and its piping	API 520 Part II (installation)	Install to the engineer’s drawings — inlet and outlet piping arrangement matters
Bonding and grounding a tank farm or loading rack	API RP 2003 (with NFPA 77 and NFPA 780)	Bonding/grounding drawings, resistance test records
Running instrument cable and shields in a plant	API RP 552	Cable routing, segregation, and shield-termination per the I&E spec
Commissioning offshore shutdown logic	API RP 14C	Function test records against the SAFE chart

Our own summary, not the text of any API document. Use the edition your owner, jurisdiction, or insurer requires — consult the standard from the publisher. Not engineering advice.

Worked example — corrosion rate and remaining life (the arithmetic API 510 and 570 use)

Input / step	Value	How
Previous thickness reading	0.500 in.	Six years ago, same location
Current thickness reading	0.440 in.	UT today
Required (minimum) thickness	0.300 in.	From the engineer’s pressure-design calculation
Corrosion rate	0.010 in./yr	(0.500 − 0.440) ÷ 6
Remaining life	14 years	(0.440 − 0.300) ÷ 0.010
Next internal inspection (vessel, API 510)	7 years	Lesser of half the remaining life or 10 years

Illustration of the method only. Piping (API 570) intervals depend on the piping class set by the owner; required thickness and inspection plans belong to the inspector and engineer.

How to read an RP 14C SAFE chart (concept)

Part of the chart	What it tells you	What you check in the field
Rows — safety devices	Each sensor or device, such as high-pressure and low-level switches or fire detection, with its tag	The installed device matches the tag, range, and set point on the list
Columns — shutdown actions	Each valve, pump, or burner shutdown the system can take	The final element actually moves when tripped
Marks at the intersections	Which devices cause which actions	Trip each device and confirm every marked action occurs and no others do
Reference to the safety analysis	Why each device exists (the safety analysis table and checklist)	Any bypassed or deleted device has a documented justification

SAFE = Safety Analysis Function Evaluation. Our own summary, not the text of any API document. Use the edition your owner, jurisdiction, or insurer requires — consult the standard from the publisher. Not engineering advice.

Common field mistakes

Mistake	What to do instead
Treating API 650 as the repair code for an existing tank	New construction is API 650; repairs and alterations to an existing tank fall under API 653
Starting a vessel or piping repair without the inspector’s authorization	Under API 510/570 the authorized inspector approves the repair plan before work starts
Changing relief-valve inlet or outlet piping in the field	Relief piping arrangement is engineered — send an RFI before changing it
Landing instrument shields at both ends because it seemed safer	Follow the I&E spec (RP 552 practice is usually one end) to avoid ground loops
Assuming a bond wire is fine because it is connected	Test and record bonding resistance; static protection depends on a continuous path

Our own summary, not the text of any API document. Use the edition your owner, jurisdiction, or insurer requires — consult the standard from the publisher. Not engineering advice.

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Which API code is the inspector talking about?

This page maps the documents; it does not reproduce them. All of them are published by the American Petroleum Institute, and inspector certifications are run through API Individual Certification Programs. Use the edition required by the owner or jurisdiction.

The in-service trio: 510, 570, 653. Equipment gets built to a construction code (ASME Section VIII for vessels, ASME B31.3 for process piping, API 650 for tanks). Once it is in service, the inspection code takes over: API 510, 570, or 653. These codes define who can authorize repairs, how thickness readings become corrosion rates and remaining life, and how inspection intervals are set. At PSM-covered plants, they are how the owner meets the mechanical-integrity requirement in 29 CFR 1910.119.

Relief systems: 520 and 521. API 520 Part I covers sizing and selection, Part II covers installation, and API 521 covers the overpressure scenarios and the disposal side (flares and blowdown). This page deliberately gives no sizing method. Relief sizing belongs to the engineer of record. The contractor’s job is to install exactly what was engineered, including inlet and outlet piping, and to flag any conflict.

The electrical and instrument practices. RP 2003 explains how static, lightning, and stray currents ignite hydrocarbon vapors and how bonding, grounding, and operating practices prevent it. RP 552 covers how instrument signals are carried: cable types, shielding, segregation from power, and grounding. RP 14C is the offshore safety-system practice behind SAFE charts and shutdown function testing.

Related guides: hazardous area classification (API RP 500/505), API 1104 weld acceptance, ISA instrument tag letters, and insulation resistance testing. Our free equipment inspection checklist and weld log help build the turnover package, and Field PM keeps inspection records, photos, and NDT reports on the job.

FAQ

What is the difference between API 510, 570, and 653?+

They are the three in-service inspection codes. API 510 covers pressure vessels, API 570 covers process piping, and API 653 covers aboveground storage tanks. Each addresses inspection, rating, repair, and alteration of equipment already in service, and each has its own API inspector certification.

What is the difference between API 650 and API 653?+

API 650 is the standard for designing and building new welded storage tanks. API 653 governs inspecting, repairing, altering, and reconstructing tanks that are already in service. A new tank is built to 650 and maintained under 653 for the rest of its life.

Do API 520 and 521 tell me how to size a relief valve?+

API 520 Part I covers sizing and selection of pressure-relief devices, Part II covers installation, and API 521 covers the relief scenarios and disposal systems (flares, blowdown). Relief sizing is engineering work done by the engineer of record. Contractors install to the engineered design and should not size or substitute relief devices in the field.

Why do OSHA PSM plants care about API codes?+

OSHA’s Process Safety Management standard (29 CFR 1910.119) requires mechanical integrity inspection and testing to follow recognized and generally accepted good engineering practices. For vessels, piping, and tanks, API 510, 570, and 653 are the practices most plants use to meet that requirement.

What is a SAFE chart?+

A Safety Analysis Function Evaluation chart from API RP 14C. It is a matrix that lists every offshore safety device against the shutdown actions it must trigger. Technicians use it to function-test the safety system: trip each device and confirm every marked action happens.

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