Changes in API 650, Welded Steel Tanks for Oil Storage
By Vincent A. Carucci and Stephen J. Gliebe
The following highlights several changes to API 650 that are contained in
the Eleventh Edition dated June 2007. It should be noted that the addition
of Reference Section 2 and Definition Section 3 resulted in changes to
paragraph numbers for all the subsequent sections.
Section 1: Scope
- Appendix L is now a mandatory appendix (Para. 1.1.18). It provides
the Data Sheet and the Data Sheet instructions for listing required
information to be used by the Purchaser and the Manufacturer.
- A new Appendix W provides commercial and documentation
recommendations that may be applied when specified by the purchaser on
the tank data sheet.
- A new section (Para. 1.3) covers the responsibilities of both the
tank manufacturer and purchaser. It clearly states that the
manufacturer is responsible for complying with all the provisions of
API 650, and the purchaser’s inspection, review, and even acceptance
of the tank does not relieve the manufacturer of his responsibility.
Section 2: References
References have been moved to Section 2, and several new reference
standards have been added. Among them are three Process Industry Practices
(i.e., PIP) and several from the US EPA and OSHA.
Section 3: Definitions
A new Section 3 with definitions has been added.
Section 4: Materials
- Materials limitations have been added. Rimmed or capped steels are
not permitted (Para. 220.127.116.11). Use of cast iron for any pressure part
or any part attached to the tank by welding is not permitted (Para.
18.104.22.168). Cadmium-plated components shall not be used without the
expressed consent of the Purchaser (Para. 22.214.171.124).
- Flange bolting and gasket requirements are now specified (Paras.
4.7 and 4.9).
Section 5: Design
- Weld pass restrictions have been specified (Para. 126.96.36.199).
- The top surfaces of bottom welds must be ground flush where they
contact the bottoms of the shell, insert plates, or reinforcing plates
- A new Figure 5-4 has been added to illustrate the basis for
determining the net working capacity of the tank.
- Additional data has been added to clarify corrosion allowance
requirements (Para. 5.3.2).
- A new Figure 5-5 has been added to illustrate a suggested detail
for a foundation drip ring to prevent ingress of water between the
tank bottom and foundation.
- The use of Table 5.1 for determining the thickness of the annular
bottom-plate thickness is now restricted to tanks with an effective
product height of H x G ≤ 23 m (75 ft).
- The minimum thickness of the first shell course of tanks with
diameters less than 15 m (50 ft) but greater than 3.2 m (10.5 ft) has
been increased from 5 mm (3/16 in.) to 6 mm (1/4 in.) (Para. 188.8.131.52).
- New requirements for shell nozzle flanges have been provided
(Para. 184.108.40.206.a). Depending on the size, shell nozzle flanges shall
meet the requirements of ASME B16.5, ASME B16.47 Series A or B, or
ASME Section VIII, Division 1.
- Added requirement that shell manhole covers less than 34 kg (75
lb) have two handles (Para. 220.127.116.11). Covers weighing more than 34 kg
(75 lb) shall be equipped with either a hinge or a davit.
- A requirement for protecting all free vents with
corrosion-resistant coarse-mesh bird screens has been added (Para.
- New requirements for flanged and threaded roof nozzles have been
provided (Paras. 18.104.22.168 and 22.214.171.124).
- OSHA 29 CFR 1910, Subpart D, has been added to the list of
required references for platforms, walkways, and stairways. Examples
of acceptable details have been provided in three Process Industry
Practices (Para. 5.8.10).
- Design requirements for floating suction lines have been added
- A new Figure 5-23 has been added to show the details of a
grounding lug (Para. 126.96.36.199).
- Additional design details have been provided for the drain holes
in stiffening rings (Para. 188.8.131.52).
- The minimum width of a tank walkway must be adjusted to consider
all projections (Table 5-17.2).
- The minimum width of stairs has been increased from 610 mm (24
in.) to 710 mm (28 in.) (Table 5-18.2).
- A requirement that circumferential stairways extend from the
bottom of the tank up to a roof edge landing or gauger’s platform has
been added (Table 5-18.10).
- The requirements for openings for stairways in an intermediate
stiffener have changed (Para. 184.108.40.206).
- A new Figure 5-26 has been added to show some acceptable column
base details for a supported cone roof. The details are intended to
provide for load distribution, corrosion and abrasion protection, free
vertical movement, and prevention of lateral movement (Para.
- When tank anchorage is required, a minimum of 4 anchors shall be
installed (Para. 5.12.2).
Section 6: Fabrication
- A new paragraph covers the use of materials used to aid
fabrication. It states that materials (e.g., lubricants, crayons)
shall not have a detrimental effect on the structural integrity of the
tank. Attachments that contain zinc or cadmium may not be welded to
the tank within 12 mm (1/2 in.) of a weld (Para. 220.127.116.11).
- Prior to installing shell and roof plates, and before inserting a
nozzle into a plate, the manufacturer shall visually inspect all edges
of the plate to determine if laminations are present. Guidance on
evaluating and repairing laminations is provided (Para. 6.2.4).
Section 7: Erection
- Requirements for foundation and grade work, and for removal of
internal and external temporary attachments have been expanded and
clarified (Para. 7.1).
- A new paragraph (Para. 7.3.6) covering hydrostatic testing
requirements has been added. It provides details on items such as
manufacturer responsibilities, purchaser responsibilities, tank metal
temperature, fill and discharge rates, and settlement measurements.
- Installation tolerances have been provided for nozzles and
manholes (Paras. 7.5.6 and 7.5.7).
Section 8: Inspection
The criteria for determining the number of spot radiographs required on a
tank have been changed. The new criteria are based on a per tank basis,
rather than the prior practice which allowed selection based on a group of
tanks (Para. 18.104.22.168).
Appendix A: Optional Design Basis for Small Tanks
Conformance to toughness requirements has been expanded to include bottom
reinforcing plates in flush-type connections and flush-type shell
connections, and flush-type fitting necks attached to the shell (Para.
Appendix C: External Floating Roofs
There are changes to most of the paragraphs in Appendix C for external
floating roofs. A complete review is recommended. Some of the changes to
the design section are highlighted below.
- General – Para. C.3.1
- Requirements detailing overflow protection and alarms have
been added. Overflow slots shall not be used as the primary means
of detecting an overflow incident (Para. C.3.1.1).
- Minimum wall thickness requirements for sleeves and fittings
that penetrate the roof deck have been specified (Para. C.3.1.2).
- Detailed electrical bonding requirements are provided (Para.
- Decks – Para. C.3.3.6 – All covers for roof openings, except roof
drains and vents, shall have gaskets or other sealing surfaces and
shall be provided with an air tight cover.
- Ladders – Para. C.3.7 – New requirements covering step assemblies,
ladder slope, wheels, and ladder grounding have been added.
- Roof Drains – Para. C.3.8 – New requirements covering drain hoses,
sump design, roof drain trash stops, and freeze prevention have been
- Supporting legs – Para. C.3.9 – Minimum leg thickness and
reinforcement requirements for leg sleeves have been specified.
- Roof Manholes – Para. C.3.11 – The minimum number of manholes has
been specified based on the tank diameter.
- Anti-rotation devices – Para. C.3.12 – Guide poles must be used as
an anti rotation device.
- Peripheral seals (rim seals) – Para. C.3.13 – Details specific to
the types of primary, secondary, and mechanical shoe seals have been
- Other Roof Accessories – Para. C.3.15 – A new section covering wax
scrapers, foam dams, sample hatches, and automatic level gauges has