API 650 vs 620 vs 653: which tank standard you need

Someone asks you for a tank and the first thing that shows up is alphabet soup: API 650, API 620, API 653. They sound alike and they do different jobs. Pick the wrong one and you get a design that doesn’t comply, an inspection that doesn’t pass, or a repair that doesn’t count.

The short version: 650 and 620 are for new tanks and split by pressure; 653 is for the tank you already have standing. We’ll show it with diagrams and a decision table.

The three standards at a glance

All three come from the American Petroleum Institute (API) and live together on any storage project. Each one covers a different stage of the tank’s life: low-pressure design, atmospheric design and in-service upkeep.

API 650: atmospheric tanks

API 650 is the reference for welded steel tanks that store at atmospheric pressure, or close to it. It’s most of the vertical flat-bottom tanks you see in a plant: crude, firewater, diesel, chemicals.

It sets the design, materials, fabrication, erection and testing of a new tank, from shell plate thickness to the bottom’s tightness test. That’s where tank fabrication comes in: cut, roll, weld the shell and bottom, and test before handover. The welding of those joints falls under ASME Section IX, with qualified welders.

API 620: low pressure

When the product needs a bit more than atmospheric, API 620 takes over. It covers tanks designed up to 15 psi, a range meant for products that demand it: liquefied gases, ammonia, some cryogenics. Not the water tank on the corner; the one holding something that pushes back on the walls.

The 15 psi line isn’t arbitrary. Above that pressure the equipment stops counting as a tank and becomes a pressure vessel, designed under ASME Section VIII, not the tank standards. The code changes, the math changes, the inspection changes.

API 653: inspection and repair

API 650 builds the tank; API 653 keeps it alive. It’s the standard for tanks already in service: inspection, repair, alteration and reconstruction of atmospheric tanks built to API 650 or earlier.

653 work stands on three legs: the external in-service inspection (visual, tank full), the internal inspection (tank out of service and cleaned), and ultrasonic thickness readings, which tell you how much steel is left and how much life the bottom and shell have.

This is where the Caribbean hits hard. The marine environment attacks steel, and a 20-year-old tank loses thickness year after year. When a reading drops below the minimum, it’s time to repair plate or reinforcement, and that’s done by API 653 qualified personnel and certified welders.

Which one applies to your tank

The decision fits in two questions: is the tank new or existing, and what pressure does it run at.

• Existing tank, whatever it is: API 653 for inspection and repair.
• New tank, atmospheric: API 650.
• New tank, up to 15 psi: API 620.
• Above 15 psi: it’s no longer a tank, it’s an ASME pressure vessel.

Why it matters in the Caribbean

On an island, a tank doesn’t only hold the product: it holds salt air, humidity and sun all year. Choosing the standard right when you build (650 or 620) and respecting the inspection intervals when you operate (653) is what separates a tank that lasts decades from one that starts pitting in a few years.

At TAS we fabricate, erect and inspect tanks under these standards, with certified welding and qualified personnel. If you have a tank project, new or existing, tell us and we’ll say which standard applies and what scope it needs, backed by our industrial welding team.

Frequently asked questions