API 650 storage tank: reading the design basis before anyone quotes an inspection

An API 650 tank is a welded, field-erected, atmospheric steel tank, and the phrase covers everything from a small bulk tank behind a generator building to a terminal shell tall enough to need an elevator. What they share is a design basis, and reading it is the part of preparation most owners skip. The construction standard itself is described in the API 650 article.

Diameter and height, and what they do to a bid

Shell area drives how many thickness readings are taken, how much scaffolding or rope access is required and how long the work takes. Height drives the hydrostatic load on the lower courses, which is where the required minimum thickness is largest. Two tanks of equal volume with different proportions do not cost the same to inspect, and a bid that does not ask for both numbers is a template. Access is the multiplier on top. A tank in the open with clear standoff on every side is a different proposition from one boxed between two neighbours inside a dike, where the scaffold has to be built in sections and part of the shell can only be reached by rope. None of that changes the inspection itself; all of it changes the week.

Specific gravity, the number that is usually wrong on file

The design specific gravity sets the shell calculation. The stored product may have changed several times since construction, which is common on older terminals and almost universal on sites that have been sold. Establishing the actual current product, and whether it is denser than the design assumed, belongs at the start of a scope rather than in the findings.

The bottom, where the money is

Most of the cost and most of the risk sit in the floor. It cannot be examined from outside, it corrodes from the soil side as well as the product side, and the decision to open a tank is expensive. The design detail that matters here is whether there is an annular ring, what the plate thickness was, and whether a release prevention barrier or liner exists underneath. All three change the examination method. A liner in particular changes it twice over: it has to be assessed in its own right, and it stands between the scanning equipment and the plate it is supposed to be reading. Knowing a liner exists before the tank is opened is the difference between a planned method and an improvised one.

Roof type changes the whole job

A fixed roof, an external floating roof and an internal floating roof are three different inspections with three different safety plans. Floating roofs add seals, drains, legs and the roof structure itself to the scope, and they add confined space and gas hazards to the plan. Naming the roof type in a request for quotation removes a common source of change orders.

Foundation and containment

A ringwall, an earthen pad and a slab settle differently, and differential settlement is one of the findings that most often turns an inspection into a project. The dike or containment around the tank is not an API 650 feature, but it is nearly always part of the site requirement, and it belongs in the same walk. The consequences of what gets found are set out in the article on what triggers each level of examination.

Putting it into a scope document

Diameter, height, roof type, product and design specific gravity, foundation type, year built and standard edition, plus the record of previous examinations. With those, three bidders are quoting the same job. Without them, the cheapest bid is usually the one that assumed the least, and the difference appears as extras once the tank is open. The scope specification exists to force those fields to be answered before anyone puts a number on the work.