How API 650 and API 620 differ in scope, design pressure, materials and inspection — and how the standard drives fabrication, welding qualification and testing on a real tank package.
Storage tanks are one of the most standardised pieces of process equipment on any industrial site, and yet the wrong choice of code at the enquiry stage still causes redesign, requalification and schedule loss further down the line. For welded steel atmospheric and low-pressure tanks, the two codes that decide almost everything about the fabrication route are API 650 and API 620. Both are published by the American Petroleum Institute, both cover shop-built and field-erected tanks, and both are recognised by every serious owner and EPC contractor in the region — but they are written for different service envelopes and they drive different design, welding and inspection decisions.
This guide compares the two standards from a fabricator's point of view: where each one applies, how the design pressure limit changes the shell and roof, what the material and welding rules look like on the shop floor, and how the inspection package is built up. If you are specifying a tank for a new terminal, refinery expansion or water treatment plant in Egypt or the Gulf, use it to decide which code belongs in your enquiry — and what to expect from the fabricator once it is awarded.
Scope: what each standard covers
API 650 covers vertical, cylindrical, above-ground, welded steel storage tanks operating at atmospheric pressure — the internal gas or vapour pressure is limited to the weight of the roof plates, effectively a fraction of a psi above atmosphere. It is the default code for crude oil, refined product, water, firewater and most bulk liquid storage on tank farms and process plants.
API 620 covers welded low-pressure storage tanks operating up to 15 psi (about 1 barg) of internal gas pressure, and at design temperatures down to −325 °F for refrigerated service. It is the code used for LNG and LPG storage, ammonia, ethylene, and any process tank where a positive gas blanket is part of the design intent rather than a nuisance to be vented.
The scope decision is not aesthetic. A tank specified to API 620 is a pressure-retaining vessel with a stress-based design, tighter material toughness rules and a heavier inspection package than an atmospheric tank of the same volume built to API 650.
Design pressure and shell thickness
API 650 shell thickness is driven almost entirely by hydrostatic head — the height of the liquid column and its density — with a small allowance for roof and wind loads. The one-foot method and the variable-design-point method in the code give the fabricator a direct formula for each shell course, and the shell thickens toward the bottom in a staircase profile that is familiar to anyone who has walked a tank farm.
API 620 designs the shell on a membrane-stress basis that combines liquid head with the specified internal gas pressure. Even a modest 5–10 psi blanket changes the numbers meaningfully — shell courses run thicker, the top-course-to-roof junction becomes a designed compression ring, and the roof itself is often a self-supporting dome rather than a supported cone. For refrigerated service, low-temperature toughness requirements push material selection toward fine-grained carbon steels or nickel alloys, and the fabrication route inherits every one of those requirements.
Materials and welding qualification
API 650 material selection is governed by Figure 4.1a of the code, which pairs plate thickness with minimum design metal temperature to pick a permitted plate grade. In practice, most Middle East atmospheric tanks are built from A36, A283 Grade C or A516 Grade 65/70 plate, with impact testing required only for cold-service tanks or thicker shell courses. Welding is qualified under Section IX of the ASME code, and standard SMAW and SAW procedures cover the shell, bottom and roof.
API 620 tightens both ends of that picture. Materials are selected against the design temperature with mandatory impact testing across the whole shell, roof and nozzle envelope; welding procedures are requalified with impact tests on the weld metal and heat-affected zone; and welder qualifications carry through to every joint on the tank. On refrigerated service, the fabricator typically runs a dedicated WPS matrix and preserves samples of each production heat for retrospective testing.
Inspection, NDT and testing
The inspection package for an API 650 tank centres on the bottom weld vacuum-box test, radiography of vertical shell seams to the extent required by service class, ultrasonic testing of the annular-to-shell weld, and a hydrostatic test with a settlement survey once the tank is filled. The code's Section 8 lays out the acceptance criteria and hold points, and the owner's inspector typically countersigns the fabricator's records at each stage.
API 620 adds full radiography of shell and roof pressure-retaining welds, tighter dimensional tolerances on the top-course compression ring, and a pneumatic pressure test in addition to the hydrostatic proof. On cold-service tanks, a leak test with helium or a tracer gas is common, and the whole package is often witnessed by a third-party authorised inspector before the tank is released to insulation and commissioning.
Roof choice and appurtenances
API 650 supports supported cone roofs, self-supporting cone and dome roofs, and internal or external floating roofs — the choice is driven by product vapour pressure, emissions requirements and site preference. Floating-roof tanks are a specialised sub-discipline within the code, with their own seal, drain and rolling-ladder standards.
API 620 tanks are almost always fixed-roof designs — a floating roof is not compatible with a pressurised gas blanket. The roof is engineered as a pressure boundary, nozzles and manways are reinforced accordingly, and pressure- and vacuum-relief devices are sized to protect the tank across every credible upset scenario.
What this means for the fabricator's proposal
When a fabricator prices an API 650 tank, the bulk of the cost sits in plate weight, welding hours, shop or field erection sequencing, and coating. The inspection line is a defined, well-understood package, and a competent shop can commit to first-time-right rates with confidence.
When the same fabricator prices an API 620 tank, the picture shifts: material specifications tighten and lead times extend, welding qualification and NDT hours climb, and the project schedule has to accommodate a heavier third-party inspection cycle. A serious proposal for a 620 tank names the material specifications, the WPS references, the NDT plan and the pressure-test methodology up front — not as annexes to be resolved later.
Talk to TARRADCO about your next storage tank
TARRADCO fabricates storage tanks to API 650 and low-pressure vessels to related standards for clients across Egypt and the Gulf, with in-house plate rolling, welding qualification and sandblasting-and-coating capacity. If you are scoping a tank package — bulk product, firewater, process or specialty service — send the datasheet and site conditions and our engineering team will confirm the code, materials and inspection route in a written technical proposal.