API 653 cheat sheet

API 653 applies to aboveground steel storage tanks built to API 650 or API 12C whose internal design pressure does not exceed 2.5 psig (17 kPa gauge).

One-foot method shell thickness, US customary: t = 2.6D(H - 1)G/(SE); SI: t = 4.9D(H - 0.3)G/(SE), with H measured from the bottom of the course under consideration to the design liquid level.

Joint efficiency E for shell butt joints: 1.00 with full radiography, 0.85 with spot radiography, 0.70 with no radiography.

Minimum nominal plate thickness for new construction: shell 3/16 in. (5 mm) and bottom 1/4 in. (6 mm), exclusive of corrosion allowance.

Normal and emergency venting must be sized per API 2000; emergency vent capacity must cover fire exposure and abnormal vapor generation.

API 650 Appendix E governs seismic design and requires evaluation of impulsive and convective liquid components, shell compressive stress, sloshing wave height, and anchorage.

Frangible roof construction makes the roof-to-shell attachment the weak link so that overpressure lifts the roof and vents the tank before the shell-to-bottom joint fails.

Shell openings such as nozzles and manholes require reinforcement equal to the area removed, placed within the specified limits around the opening.

Changing shell height, diameter, or foundation changes the structural configuration and requires re-evaluation of shell thickness, stability, and anchorage.

External inspection intervals shall not exceed 5 years; intervals are shortened when corrosion rates or service conditions warrant.

Internal inspection interval shall not exceed 20 years and shall not exceed one-half the estimated remaining corrosion life of the shell or bottom.

Corrosion rate = (t previous - t current) / years between measurements; remaining life = (t current - minimum required thickness) / corrosion rate.

Minimum required thickness for each shell course is computed with the one-foot method using the liquid level expected at the next inspection, the product specific gravity, and the allowable stress.

Hydrostatic test: fill with water, hold typically 24 hours, then visually examine all welded joints and measure shell settlement; test water is normally at least 50 deg F (10 deg C).

Examination methods: radiography of butt welds, magnetic flux leakage or ultrasonic scanning of bottom plates, magnetic particle or liquid penetrant for surface flaws, and vacuum box testing of bottom lap welds.

Flaws found during inspection may be evaluated for continued service using the fitness-for-service procedures of API 579-1/ASME FFS-1.

Inspectors must have near vision corrected to Jaeger 1 at 12 in. and color vision adequate for the examination method used.

Pressure-vacuum vents and other relief devices shall be inspected and function-tested to confirm they open at the design settings.

Repair restores a tank or part of a tank to a condition suitable for continued service; alteration changes structural design or configuration; reconstruction is reassembly of a tank that has been dismantled and relocated.

All welding for repairs, alterations, and reconstruction shall use welding procedures and welders qualified under ASME Boiler and Pressure Vessel Code Section IX.

Shell insert plates shall be at least 12 in. (300 mm) wide, have rounded corners, and be at least as thick as the minimum required at that location.

Peaking at vertical shell joints, measured with a 36 in. (900 mm) sweep board, shall not exceed 1/2 in. (13 mm).

Shell butt-weld repairs shall be examined by radiography to at least the extent required for the original joint.

A hydrostatic test is required after major repairs or alterations such as shell plate replacement, new or enlarged openings, or foundation work; the authorized inspector may waive testing for minor repairs where it is impractical.

Adding or enlarging a shell opening larger than 12 in. nominal pipe size, changing shell height, or changing the foundation are alterations requiring design re-evaluation.

Completed repairs and alterations are recorded on the nameplate or a supplemental plate and in the permanent tank records.

Confined-space entry requires a permit, oxygen between 19.5 percent and 23.5 percent, flammable vapor below 10 percent of the lower explosive limit, and toxic gas within exposure limits per API 2015.

Hot work requires a permit, a combustible gas reading below 10 percent of the lower explosive limit, isolation of connected lines, and continuous atmospheric monitoring.

Initial filling is limited to about 3 ft/s (1 m/s) until the inlet is submerged, to control static charge generation.

Steel tank bottoms in soil are adequately protected when polarized to -0.85 V or more negative measured against a copper-copper sulfate reference electrode.

API 2350 requires overfill prevention including level alarms, an independent high-high sensor, and procedures to stop filling before overflow.

Routine water draw-off and sludge removal limit the water bottoms that drive pitting corrosion and underside corrosion.

Normal and emergency vents, flame arresters, and roof drains require routine inspection and cleaning to preserve design venting capacity.

The 5-year external inspection is supplemented by owner walk-arounds that check for leaks, settlement, coating breakdown, and secondary containment condition.

Out-of-service tanks shall be cleaned, gas-freed, and positively isolated from connected piping with blinds or a double block and bleed arrangement.

Before an out-of-service tank returns to service, the authorized inspector determines the inspection and testing required based on the reason and duration of the outage.

A dismantled and relocated tank is a reconstruction and must meet new-construction requirements for foundation, shell roundness, welding, and hydrostatic testing.

Reused shell plates must satisfy the minimum required thickness for the new service and be verified by thickness surveys before erection.

The original nameplate remains with the tank; reconstruction or relocation data are recorded on a new or supplemental nameplate attached near the original.

Construction, inspection, repair, and alteration records shall be maintained for the life of the tank and made available to the authorized inspector.

Inspection records include thickness measurements with locations and dates, calculated corrosion rates and remaining life, examination reports, and test results.

Permanently decommissioned tanks must be freed of product and sludge and then dismantled or left in a condition that prevents structural collapse and release of contained residues.