Facility managers call inspection providers every day asking for an “API inspection.” The trouble is that API publishes more than a hundred standards, and three of them, API 510, API 653, and API 570, govern the in-service inspection of three completely different categories of equipment.
Picking the wrong code, or assuming a single inspector can cover all three with one certification, leads to compliance gaps, bid mismatches, and inspections that fail an audit even when the work itself was performed correctly. This article walks through which code applies to which equipment, what the inspections actually cover, and how the three fit together at sites that operate all of them.
Quick reference: the three codes at a glance
Before getting into specifics, here is the practical comparison most readers want first.
| Code | Equipment Type | Key Threshold | Primary Inspection Methods | Typical Interval | Inspector Certification |
|---|---|---|---|---|---|
| API 653 | Above-ground storage tanks | Atmospheric or low-pressure (under 2.5 psi) | Visual, UT thickness, MFL floor scan, settlement survey | 5-year external; 10 to 20-year internal | API 653 certified inspector |
| API 510 | Pressure vessels | Internal pressure 15 psi or above | Visual, UT, RT, dye penetrant, magnetic particle | Up to 5-year external; 10-year internal (RBI may extend) | API 510 certified inspector |
| API 570 | In-service process piping | Piping carrying hydrocarbons or other flammable / toxic fluids | Visual, UT, RT for welds, profile RT | Class-dependent: 5 to 10 years | API 570 certified inspector |
API 653: above-ground storage tank inspection
API 653, formally titled “Tank Inspection, Repair, Alteration, and Reconstruction,” governs the in-service inspection of welded steel above-ground storage tanks (ASTs) operating at near-atmospheric pressure. The standard covers tanks built to API 650 (the new-construction standard for welded steel tanks) and tanks built to predecessor or other standards that have been brought into API 653 service through evaluation.
What qualifies as an AST under API 653
The defining characteristics are construction type and operating pressure. API 653 applies to:
- Welded steel tanks of carbon steel or stainless steel construction
- Tanks operating at atmospheric pressure or low pressure (typically below 2.5 psi)
- Tanks with diameters generally greater than 30 feet (smaller shop-built tanks usually fall under STI SP001)
- Vertical, cylindrical, above-ground tanks storing petroleum, chemicals, water, or other liquids
API 653 does not cover underground storage tanks, pressure vessels, or shop-built tanks under STI SP001 jurisdiction. It also excludes new tank construction, which is governed by API 650.
What an API 653 inspection includes
API 653 defines three categories of inspection. Routine in-service inspections are owner-performed visual walkdowns conducted at least monthly. External inspections are formal inspections by a certified inspector while the tank remains in service, typically every five years, covering shell condition, foundation, anchorage, secondary containment, and ultrasonic thickness measurements. Internal inspections require the tank to be taken out of service, cleaned, and gas-freed, and they happen every 10 to 20 years depending on calculated corrosion rates and risk-based analysis.
Magnetic flux leakage (MFL) scanning is the standard method for evaluating tank floor integrity during internal inspections. UT thickness gauging covers shells, roofs, and nozzles.
API 510: pressure vessel inspection
API 510, “Pressure Vessel Inspection Code: In-Service Inspection, Rating, Repair, and Alteration,” governs vessels designed and built to ASME Section VIII or earlier pressure vessel codes. This is the standard that applies to most refining, petrochemical, and gas-processing pressure equipment outside of pipelines and storage.
What qualifies as a pressure vessel under API 510
The threshold is internal pressure. API 510 applies to:
- Vessels with internal design pressure of 15 psi or above
- Vessels designed and built to ASME Section VIII Division 1 or 2, or to predecessor codes that have been deemed equivalent
- Process vessels including separators, accumulators, scrubbers, reactors, columns, and heat exchanger shells
API 510 explicitly does not apply to atmospheric storage tanks (those go to API 653), to pressure piping (which is API 570), or to vessels exempt under ASME Section VIII rules such as some small-diameter or low-pressure vessels.
What an API 510 inspection includes
Pressure vessels undergo periodic external visual inspections, typically every five years, and internal inspections at intervals of up to ten years (or longer with a documented risk-based inspection program). Internal inspections evaluate shell and head thickness, weld condition, internal attachments, and corrosion under insulation (CUI) when applicable. Inspectors use UT thickness gauging, radiographic testing of welds, dye penetrant or magnetic particle testing for surface defects, and visual examination.
API 510 inspections often coordinate with planned plant turnarounds because internal access requires the vessel to be depressurized, isolated, and cleaned. Coordinating multiple vessel inspections in a single turnaround keeps downtime to a minimum.
API 570: process piping inspection
API 570, “Piping Inspection Code: In-Service Inspection, Rating, Repair, and Alteration of Piping Systems,” covers in-service metallic piping that carries hydrocarbons, flammable fluids, toxic chemicals, or other process media. It is the in-service counterpart to ASME B31.3, which is the design and construction code for process piping.
What qualifies as process piping under API 570
API 570 applies to in-service piping systems that fall into one of three classes based on consequence of failure:
- Class 1: services with the highest consequence of release, including flammable fluids, toxic services, and certain high-pressure services
- Class 2: services with intermediate consequence, including most flammable fluids and acids
- Class 3: services with low consequence, such as combustible fluids that do not flash on release
The standard does not cover utility piping (compressed air, plant water, plumbing), pipelines under DOT jurisdiction, or piping internal to fired equipment. Utility and plumbing systems are governed by other codes entirely; mistaking process piping for plumbing is a common confusion that leads to misapplied inspections.
What an API 570 inspection includes
Inspection intervals depend on piping class and corrosion rate. Class 1 piping is typically inspected on visual external intervals up to five years and thickness measurements at the same or shorter intervals. Inspectors map condition monitoring locations (CMLs), perform UT thickness gauging at established points, conduct visual exams of supports and insulation, and review valves, flanges, and small-bore connections that are common failure points.
Radiographic testing is used selectively for weld evaluation, particularly on welds that show indications during visual or UT inspection.
Decision logic: which code applies to your equipment
When new equipment shows up at a site, or when an owner inherits equipment with incomplete records, the right code is usually determinable from three questions:
1. Is it a tank holding liquid product at near-atmospheric pressure?
If yes, API 653 applies. If the tank is shop-built and under 30 feet in diameter, also consider STI SP001 as an alternative inspection standard.
2. Is it a pressure-containing vessel rated at 15 psi or above?
If yes, API 510 applies. ASME Section VIII rules determine whether the vessel was originally in scope at construction; API 510 applies to in-service inspection of vessels in that scope.
3. Is it interconnecting piping carrying process fluids between vessels, tanks, or units?
If yes, API 570 applies. The piping class (1, 2, or 3) determines inspection frequency and rigor.
Equipment that does not fit any of these categories is likely covered by another code. Underground storage tanks fall under EPA and state environmental regulations. Pipelines crossing public right-of-way fall under DOT 49 CFR rules. Boilers fall under ASME Section I and the National Board Inspection Code.
When a site has all three
Most refineries, midstream gas plants, and chemical facilities have all three categories of equipment in service simultaneously. A typical refinery might have hundreds of pressure vessels under API 510, dozens of storage tanks under API 653, and thousands of feet of process piping under API 570. The inspection programs are usually managed under a single mechanical integrity program with separate inspection plans for each code.
The practical advantage of working with one inspection provider that holds all three certifications is consistency: a single point of accountability for scheduling, a unified reporting format, and an inspector team that understands how the equipment categories interact. When a corrosion finding on one piping circuit indicates conditions that may affect a downstream vessel, having the same provider see both leads to faster root-cause analysis and better-targeted follow-up inspections.
It also simplifies audit response. Mechanical integrity audits, whether internal or from regulatory bodies like OSHA under the Process Safety Management standard, frequently span all three asset classes at once. A unified provider produces unified documentation.
Other related codes worth knowing
Three additional standards come up frequently in conversations about API 510, 653, and 570 and are worth knowing about even when they are not the primary code in scope:
STI SP001 is the Steel Tank Institute standard for shop-built aboveground storage tanks. It serves as the primary inspection standard for smaller tanks (typically under 30 feet in diameter) and is widely accepted by AHJs as an alternative to API 653 for those tanks. For a deeper comparison of how STI SP001 stacks up against API inspection requirements, see our breakdown of STI vs API inspections.
API 580 and API 581 establish the framework for risk-based inspection (RBI). RBI is referenced by API 510, 653, and 570 as a method for extending base inspection intervals when supported by documented engineering analysis. Sites that operate under an RBI program can defer internal inspections beyond the default code intervals.
API 650 is the new-construction standard for welded steel storage tanks. It is the design code that API 653 references when evaluating existing tanks. API 650 itself does not govern in-service inspection.
Working with a provider that covers all three codes
NDT Tanknicians holds API 510, API 653, and API 570 certifications and serves clients with mixed equipment portfolios across all 50 states. Our inspectors coordinate inspection schedules across asset classes to minimize downtime and produce consistent reporting that simplifies audit response.
If you are evaluating which code applies to your equipment, scheduling a multi-code inspection at a site with mixed assets, or trying to standardize your mechanical integrity program across vessels, tanks, and piping, contact us to discuss your inspection plan.
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