Title: /
Definition of “Nominal Thickness”
Date: / 01/06/2009Contact: / Name: / Larry Hiner
Company: / Chicago Bridge & Iron Company
Phone: / 815-439-6125
E-mail: /
Purpose: / This agenda standardizes and sets the definitions for thickness used in the standard to avoid misunderstandings.
Source: / Larry Hiner comment on 650-624, Date 4/08
Revision: / 4 Dated May 20, 2009
Impact: / Minimal
Background: / Originally this agenda item attempted to define “as-built thickness”. Based on comments and additional research, the focus changed to a unified approach that centers around the more common terminology of “nominal thickness”.
Thickness terminology and number of times used in the Standard
As-built thickness - 14 times
Nominal thickness - 59 times
As constructed thickness - 11 times
As- ordered thickness - 5 times
Review of the standard shows the use of these terms interchangeably and many times incorrectly. The intent of all thickness requirements in API 650 can be defined by two terms “nominal” and “corroded”. Where nominal refers to the as ordered thickness and corroded thickness is the nominal thickness excluding corrosion allowance.
A term indicating the supply basis thickness plus any corrosion allowance is necessary to define the thickness used in determining toughness, PWHT requirements, weld spacing, minimum and maximum thickness limitations, etc. These requirements must be known at the time of tank design and material order and can't wait until the material is delivered to the shop or construction site. Nominal thickness is the preferred terminology.
Proposal: / Proposed changes
Under Section 3.0 Definitions - add the definition for nominal thickness and corroded thickness
corroded thickness: the nominal thickness excluding any specified corrosion allowance.
nominal thickness: the ordered thickness of the new item. This thickness is inclusive of any specified corrosion allowance. This thickness is used for determination of PWHT requirements, weld spacing, minimum and maximum thickness limitations, etc.
Note: The thickness used in the final structure is the nominal thickness plus or minus any tolerance allowed by the applicable material specification or otherwise accepted by design review and/or analysis at the point of consideration. The requirements of 4.2.1.2.3 shall also be met.
minimum nominal thickness: the smallest nominal thickness that shall be used for the component being considered.
· Change "as-built thickness" to "nominal thickness" in the following locations: 5.7.6.4, Table 5-21 a, Table 5-21b, F.7.5, and V.5
· Change as “constructed” to "nominal thickness" in the following locations: Table 5-1 a/b, 5.6.1.1, A.4.2, AL.5.2.2
· Change as-ordered thickness to nominal thickness in the following locations: 5.7.5.1, 5.9.7.2, AL 5.5.2
· Modify all uses of "minimum thickness" to "minimum nominal thickness".
Rationale: / This agenda item clarifies requirements and eliminates conflicts.
Discussion: / The following list provides the typical situations or reasons why the final thicknesses may be different than the nominal thickness value. In the following list items 3 through 5 require Engineering review and possible subsequent Purchaser approval.
1. Each material specification (ASTM A6 for reference) has tolerances that set the bounds of the supplied thickness. API 650 paragraph 4.2.1.2.3 specifically allows this variation as long as the under tolerance does not exceed 0.01”
2. Machining tolerances – typically extra material is added to the design thickness (usually at flanges) to account for machining to get a proper surface for seating gaskets. For better integrity and reduced machining cost, any excess thickness is typically left on the component.
3. Changes from the ordered thickness made to suit in-stock material for availability. Typically this results in a slightly greater thickness used than necessary in the design.
4. Last minute material switches in field to solve problems or accommodate Purchaser changes. Material ordered to decimal thickness is not typically available on short notice and thus requiring use of gage material from stock sources.
5. Last minute requests from field to accept discovered under tolerance plate. Acceptance granted only after design review and any subsequent necessary analysis.
Specific Changes
Table 5-1a—(SI) Annular Bottom-Plate Thicknesses (tb)
aPlate thickness refers to nominal shell plate thickness exclusive of corrosion allowance for product design and nominal thickness as constructed
for hydrostatic test design.
bThe stress to be used is the maximum stress in the first shell course (greater of product or hydrostatic test stress). The stress
may be determined using the required thickness divided by the thickness from “a” then multiplied by the applicable allowable
stress:
Product Stress = (td/as-constructed nominal t exclusive of CA) (Sd)
Hydrostatic Test Stress = (tt/as-constructed nominal t) (St)
Note: The thicknesses specified in the table, as well as the width specified in 5.5.2, are based on the foundation providing
uniform support under the full width of the annular plate. Unless the foundation is properly compacted, particularly at the
inside of a concrete ringwall, settlement will produce additional stresses in the annular plate.
Table 5-1b—(USC) Annular Bottom-Plate Thicknesses (tb)
aPlate thickness refers to nominal shell plate thickness exclusive of corrosion allowance for product design and nominal thickness as constructed
for hydrostatic test design.
bThe stress to be used is the maximum stress in the first shell course (greater of product or hydrostatic test stress). The stress
may be determined using the required thickness divided by the thickness from “a” then multiplied by the applicable allowable
stress:
Product Stress = (td/as-constructed nominal t exclusive of CA) (Sd)
Hydrostatic Test Stress = (tt/as-constructed nominal t) (St)
Note: The thicknesses specified in the table, as well as the width specified in 5.5.2, are based on the foundation providing
uniform support under the full width of the annular plate. Unless the foundation is properly compacted, particularly at the
inside of a concrete ringwall, settlement will produce additional stresses in the annular plate.
5.6 SHELL DESIGN
5.6.1 General
5.6.1.1 The required shell thickness shall be the greater of the design shell thickness, including any corrosion allowance, or the
hydrostatic test shell thickness, but the shell thickness shall not be less than the following:
Nominal Tank Diameter Nominal Plate Thickness
(m) (ft) (mm) (in.)
< 15 < 50 5 3/16
15 to < 36 50 to < 120 6 1/4
36 to 60 120 to 200 8 5/16
> 60 > 200 10 3/8
Notes:
1. Unless otherwise specified by the Purchaser, the nominal tank diameter shall be the centerline diameter of the bottom shell-course plates.
2. Nominal plate thickness refers to the tank shell as constructed. The thicknesses specified are based on erection requirements.
3. When specified by the Purchaser, plate with a minimum nominal thickness of 6 mm may be substituted for 1/4-in. plate.
4. For diameters less than 15 m (50 ft) but greater than 3.2 m (10.5 ft), the minimum thickness of the lowest shell course only is increased to
6 mm (1/4 in.).
5.10.3.2 Minimum Thicknesses
The minimum nominal thickness of any structural member, including any corrosion allowance on the exposed side or sides, shall not beless than 6 mm (0.236 in.) for columns, knee braces and beams or stiffeners which by design normally resist axial compressive forces or 4 mm (0.17 in.) for any other structural member.
5.10.4.7 Roof support columns shall be provided at their bases with details that provide for the following:
a. Load Distribution: Column loads shall be distributed over a bearing area based on the specified soil bearing capacity or foundation
design. Where an unstiffened horizontal plate is designed to distribute the load, it shall be have a minimum nominal thickness of 12 mm (1/2 in.) thick. Alternatively, the column load may be distributed by an assembly of structural beams. The plate or members shall be designed to distribute the load without exceeding allowable stresses prescribed in 5.10.3.1.
b. Corrosion and Abrasion Protection: At each column a wear plate with a minimum nominal thickness of 6 mm (1/4 in.) thickness shall be welded to the tank bottom with a 6 mm (1/4 in.) minimum fillet weld. A single adequate thickness plate may be designed for the dual functions
of load distribution and corrosion/abrasion protection.
5.10.5 Self-Supporting Cone Roofs
Note: Self-supporting roofs whose roof plates are stiffened by sections welded to the plates need not conform to the minimum thickness requirements, but the nominal thickness of the roof plates shall not be less than 4.8 mm (3/16 in.) when so designed by the Manufacturer, subject to the approval of the Purchaser.
5.10.5.1 Self-supporting cone roofs shall conform to the following requirements:
θ ≤ 37 degrees (slope = 9:12)
θ ≥ 9.5 degrees (slope = 2:12)
In SI units:
Minimum nominal thickness =
Maximum nominal thickness = 13 mm, exclusive of corrosion allowance
where
D = nominal diameter of the tank shell (m),
T = greater of load combinations (e)(1) and (e)(2) of Appendix R (kPa),
θ = angle of cone elements to the horizontal (deg).
In US Customary units:
Minimum nominal thickness =
Maximum nominal thickness = 1/2 in., exclusive of corrosion allowance
5.10.6.1 Self-supporting dome and umbrella roofs shall conform to the following requirements:
Minimum radius = 0.8D (unless otherwise specified by the Purchaser)
Maximum radius = 1.2D
In SI units:
Minimum nominal thickness =
Maximum nominal thickness = 13 mm, exclusive of corrosion allowance
where
D = nominal diameter of the tank shell (m),
T = greater of load combinations (e)(1) and (e)(2) of Appendix R (kPa),
rr = roof radius (m).
In US Customary units:
Minimum nominal thickness =
Maximum nominal thickness = 1/2 in., exclusive of corrosion allowance
8.1.8 Record of Radiographic Examination
8.1.8.1 The Manufacturer shall prepare an as-built radiograph map showing the final location of all radiographs taken along with the film identification marks.
A.4.1 The minimum nominal thicknesses of shell plates shall be computed from the stress on the vertical joints, using the following
formula:
In SI units:
where
t = minimum nominal thickness, in mm (see 5.6.1.1),
In US Customary units:
where
t = minimum nominal thickness (in.) (see 5.6.1.1),
A.4.2 The nominal thickness of shell plates (including shell extensions for floating roofs) shall not be less than that listed in
3.6.1.1. The nominal thickness of shell plates refers to the tank shell as constructed. The nominal thicknesses given in 5.6.1.1 are
based on erection requirements.
AL5.2.2 Annular Bottom Plate Thickness
The nominal thickness of annular bottom plates shall equal or exceed the requirements given in Table AL-4a and Table AL-4b.
Diameter (m) Size (mm)
Table AL-4a—(SI) Annular Bottom Plate Thickness
Nominal Thickness of
First Shell Course (mm)
(as constructed)
Table AL-4b—(USC) Annular Bottom Plate Thickness
Nominal Thickness of
First Shell Course (in.)
(as constructed)
E.6.2.1.2 Mechanically-Anchored
When mechanical anchorage is required, the anchor embedment or attachment to the foundation, the anchor attachment assembly and the attachment to the shell shall be designed for PA. The anchor attachment design load, PA, shall be the lesser of the load equal to the minimum specified yield strength multiplied by the as-built nominal cross-sectional area of the anchor or three times PAB.
E.7.1.2 Mechanically-Anchored
When mechanical-anchorage is required, at least six anchors shall be provided. The spacing between anchors shall not exceed 3 m(10 ft).
When anchor bolts are used, they shall have a minimum diameter of 25 mm (1 in.), excluding any corrosion allowance. Carbon steel anchor straps shall be 6 mm (1/4 in.) minimum nominal thickness and have a minimum corrosion allowance of 1.5 mm (1/16 in.) on each surface for a distance at least 75 mm (3 in.) but not more than 300 mm (12 in.) above the surface of the concrete.
F.7.5 The counterbalancing weight, in addition to the requirements in 5.12, shall be designed so that the resistance to uplift at the bottom of the shell will be the greatest of the following:
a. The uplift produced by 1.5 times the design pressure of the corroded empty tank (minus any specified corrosion allowance) plus the uplift from the design wind velocity on the tank.
b. The uplift produced by 1.25 times the test pressure applied to the empty tank (with the as-built nominal thicknesses).
I.7.3.2 The required minimum nominal thickness of the bottom plate supported on grillage shall be determined by the following
equation:
J.3.6.1 Manholes, nozzles, and other connections in the shell shall be constructed and attached as specified in 5.7, but it is unlikely that reinforcing plates will be required for manholes and nozzles in the tank shell. The need for reinforcement shall be checked according to the procedure given in 5.7.2. Since the minimum nominal shell-plate thicknesses given in J.3.3 will normally exceed the calculated thickness, the excess material in the shell should satisfy the reinforcement requirements in nearly all cases.
Appendix L
31. Seals
– Minimum nominal Shoe Thickness*: Include units. See C.3.13 and H.4.4.4.
32. Data for All Floating Roofs:
Minimum Deck Thickness*: Specify a minimum nominal deck thickness greater than that stated in C.3.3.2. If not specified, the Manufacturer shall insert the thickness stated in the above reference.
M.5.1 The requirements of 5.10.5 and 5.10.6, which are applicable to self-supporting roofs, shall be modified. For a maximum design temperature above 93°C (200°F), the calculated minimum nominal thickness of roof plates, as defined in 5.10.5 and 5.10.6,shall be increased by the ratio of 199,000 MPa (28,800,000 lbf/in.2) to the material’s modulus of elasticity at the maximum design temperature.
S.1.4 The minimum nominal thicknesses in this appendix do not contain any allowance for corrosion.
S.3.2.1.1 The required minimum nominal thickness of shell plates shall be the greater of the design shell thickness plus corrosion allowance, test shell thickness, or the nominal plate thickness listed in 5.6.1.1
S.3.3.3 Shell manholes shall be in conformance with 5.7.5 except that the minimum nominal thickness requirements shall be multiplied by the greater of (a) the ratio of the material yield strength at 40°C (100°F) to the material yield strength at the maximum design temperature, or (b) the ratio of 205 MPa (30,000 psi) to the material yield strength at the maximum design temperature.
S.3.4.2 All stainless steel components of the roof manhole shall have a minimum nominal thickness of 5 mm (3/16 in.).