13.1 Fabrication Tolerances
13.1.1 Fabrication tolerances shall be in accordance with the more stringent of the applicable Code and PIP document VEFV1100, with the following exceptions to tolerances on page 1 of drawing VEFV1102:
Tolerance 1: Height from base line to face of top nozzle shall be the smaller of 4 mm per 3000 mm of height or 19 mm.
Tolerance 2: Flange face of nozzle with agitator shall be aligned within ±¼ degree of indicated plan in any direction.
Tolerance 7: Unless more stringent tolerances are specified by the process licensor, alignment of flange face of nozzle
without an agitator shall be within ±½ degree of specified plan, but not to exceed 5 mm across the diameter.
Tolerance 12: Bottom of vessel support to base line 0 mm, -6 mm.
Tolerance 13: For supports located above baseline, tolerances shall be 0 mm, + 6 mm.
Tolerance 14: Maximum difference in peak deviations from straight, applied to the shell at any location along the
circumference clear of openings, shall be: 3 mm in any 3000 mm of length, 12 mm in any 15000 mm of length, and shall not exceed 19 mm over the overall length of shell. In addition, distortion caused by welding of longitudinal or circumferential joints shall not exceed 6 mm maximum depth in a 900 mm length of shell centered on the weld.
Tolerance 20: Deviation from average I.D. (as determined by strapping) from nominal I.D. shall be as follows:
a) ±3 mm for I.D. ≤ 1200 mm b) ±6 mm for I.D. > 1200 mm
Out-of- roundness tolerances shall be according to applicable ASME SEC VIII.
Tolerance 26: Supports out of level shall be within ±3 mm.
Deviation from flatness of support base plate is not acceptable, i.e., support base plates shall be in full direct contact with the foundation.
Tolerance 28: Delete.
Tolerance 31: Distance between centerlines of support bolt holes shall be within ±6 mm and maximum diagonal measurements shall be within ±6 mm.
13.1.2 Use of fitness-for-service assessment methodology to qualify the design of components that do not satisfy the fabrication tolerances according to this specification is prohibited.
13.1.3 Dished heads shall achieve at least the minimum required thickness in all areas after forming.
13.2 Forming and Assembly
13.2.1 Tapered transitions shall be made only on the external surface of the vessel, according to the rules of the applicable Code, in the following conditions:
a) There will be an interference with the removal of a vessel's internals.
b) Vessels that have strict requirements regarding smooth internal profiles for flow or cyclic loading conditions or internal volume constraints.
13.2.2 The beveled edges of weld preparations for carbon steel plates with thickness 25 mm and thicker and all ferrous alloy plates shall be
magnetic particle examined for linear discontinuities. Defects shall not exceed limits as per ASME SA-20.
13.2.3 Plate edge laminations revealed by magnetic particle examination shall be completely removed and repaired.
13.2.4 Each shell section shall be completely welded longitudinally and corrected for out of roundness and peaking of the weld seam prior to welding to adjoining shell or head.
13.2.5 All re-rolling or forming of the shell sections is to be completed prior to radiography.
13.2.6 Welds Encroachment
13.2.6.1 It is the responsibility of the manufacturer to ensure that the outer edge of welds attaching manways, nozzles (with and without reinforced pads) and other structural attachments (with and without reinforced pads), except those in paragraph 10.2.11, to pressure-retaining components shall not be closer than 1 inch from the adjacent edge of any other weld. It is the responsibility of the manufacturer to ensure that requirements of paragraph 7.11.4 of this specification are met in the vicinity of the welds.
Commentary Note:
Weld spacing requirements for skirt-to-vessel junctures shall be according to paragraph 10.2.11 of this specification.
13.2.6.2 Where the optimized fabrication layout and/ or process design requirements do not absolutely allow meeting the spacing requirement in paragraph 13.2.6.1 of this specification, NDE per paragraph 14.6 shall be performed.
13.2.6.3 It is prohibited to cover butt welds in wall of vessels that will undergo PWHT by structural attachments (with or without reinforcing pads).
13.2.7 Telltale Holes in Reinforcing Pads
13.2.7.1 ¼ - inch telltale vent holes drilled and tapped for ⅛ -inch NPT shall be provided in reinforcing pads for welded attachments, including nozzles and manways, per the following:
1) One hole in single piece reinforcing pad.
2) Where a pad is split, each segment shall have at least one hole.
13.2.7.2 Telltale holes shall be located at the lowest position accessible for inspection with center of the hole 25 mm from edge of the pad. This is applicable to each segment of a split reinforcing pad.
Commentary Note:
In case of reinforcing pads for attachments, other than nozzles and manways, center of telltale hole shall be 25 mm from the closest edge of the pad.
13.2.7.3 Telltale holes in reinforcing pads for external welded attachments shall be plugged with grease or other materials adequate for the operating temperature but not capable of retaining pressure, to prevent moisture ingress between the pad and the vessel pressure-retaining component. Telltale holes in internal attachment pads shall be seal welded.
13.2.8 Segments of split reinforcing pad shall be welded together without using a backing strip.
13.2.9 All internal and external attachments, including clips, welded directly to pressure-retaining parts, shall be fully seal welded, except for blank square nuts used for external insulation where tack welding is allowed.
13.2.10 No tack welding is permitted between heads and skirts on the inside of skirts.
13.2.11 Vessels with large diameter and/ or overall length which fabrication cannot be completely done in shop shall be designed to minimize the amount of field welding, radiography and heat treatment. Where adjacent sections are of such a size that shop fabrication and field assembly is required, the sections shall be match marked to ensure proper field fit up.
13.2.12 Forming
13.2.12.1 General
a) Cold forming is performed at temperatures within the range of above 20°C (68°F) and below 120°C (248°F).
b) Hot forming is any forming performed above the austenite phase start temperature of 740°C (1364°F).
c) All tempering heat treatments must be at least 25°C (45°F) above the nominal PWHT temperature as given in the applicable ASME code for the respective material.
13.2.12.2 Hot Forming
1) All quenched and tempered materials must be completely heat-treated after hot forming to achieve the original material properties.
2) All hot forming procedures require approval of the Saudi Aramco Engineer as defined in this
specification prior to commencement of any of forming activities. Hot forming procedure shall describe all heat treatment operations and tests to be performed. The tests shall include, but not limited to, all of the mechanical tests required by the original material specification.
3) Normalized materials that are hot formed need to be heat treated unless the below rules are followed:
a) Normalized materials that are hot formed in the range of 750°C (1382°F) to 950°C (1742°F) and still air-cooled.
b) Normalized materials that are hot formed in a multi-step sequence must be cooled to below 200°C (392°F) prior to the last step. The material will then
be re-heated within the range of 750°C (1382°F) to 950°C (1742°F) for forming in the last step.
c) Normalized and tempered materials that are formed in accordance with either 13.2.14.2(3)(a) or
13.2.14.2(3)(b) of this specification need only receive a tempering heat treatment. The tempering temperature must not exceed the temperature stated in the steel manufacturers Material Test Certificate.
13.2.12.3 Cold Forming
a) Heat treatment requirements for Carbon Steels (P-1) and Low Alloy Steels (P-3, 4, 5, 9A and 9B) that undergo cold forming (by pressing or cold spinning) shall be as follows:
Material Fiber Elongation Strain εf (%) Heat Treatment Requirement
Carbon Steels P-1
Less than or equal to 5
None
(Exception: PWHT per the applicable Code shall be performed for cold spun heads) Greater than 5 and
equal to or less than 10 PWHT per the applicable Code
Greater than 10
Normalizing, Normalizing and tempering or quenching and tempering, as required to maintain original material properties.
Low Alloy Steels P-3, P-4, P-5, P-9A & P9B
Less than or equal to 3
None
(Exception: PWHT per the applicable Code shall be performed for cold spun heads) Greater than 3 and
equal to or less than 10 PWHT per the applicable Code
Greater than 10
Normalizing, normalizing and tempering or quenching and tempering, as required to maintain original material properties.
High Alloy Materials Table 6.2 of ASME Section VIII,
Division 2 Table 6.2 of ASME Section VIII, Division 2 Non-ferrous Materials Table 6.3 of ASME Section VIII,
Division 2 Table 6.3 of ASME Section VIII, Division 2
b) Calculation of forming fiber elongation strain εf (%) shall be according to the following:
Type of Part Being Formed Fiber Elongation Strain εf (%) For double curvature heads that are formed from
one-piece or welded multi-piece blanks by any process that includes dishing or cold spinning (e.g., dished heads or cold spun heads)
εf = 100 ln[Db/(Df -2ta)]
For heads that are assembled from formed segments (e.g., spherical dished shell plates or dished segments of ellipsoidal or torispherical heads)
εf = 100 tb / Rfd
Cylinders and cones formed from plate εf = (50 tb / Rfc) [1-(Rfc / Ro)]
Where:
ln is the natural logarithm
Db is the diameter of unformed blank plate or diameter of intermediate product Df is original outside diameter
Rfd is the smallest mean radius of curvature of formed segment (mean radius of spherical segment, mean knuckle radius of knuckle segment of multi sectional semi-ellipsoidal or torispherical heads)
Rfc is the mean radius of curvature of finished product (mean radius of cylinder, mean radius of the smaller diameter of cone)
Ro is the mean radius of initial product (flat plate) or the intermediate product (in case of unformed initial product equals to infinity)
ta is the nominal thickness of the plate before forming or intermediate product tb is the nominal thickness of the plate before forming
Commentary Notes:
i) Cold spun heads with nominal thickness exceeding 50 mm shall be heat treated by normalizing, normalizing and tempering or quenching and tempering, as required to maintain original material properties), irrespective of the calculated fiber elongation strain.
ii) For semi-ellipsoidal and torispherical heads formed from one-piece or welded multi-piece blanks, maximum calculated extreme fiber elongation strain among all head’s zones shall be used to determine the need of heat treatment. Separate calculation for each zone (spherical crown, knuckle area, etc.) shall be made, using the greatest measured thickness and smallest radius of curvature of the zone after forming.
iii) Separate calculation of extreme fiber elongation shall be made for each formed segment (e.g., spherical dished shell plates or dished segments of ellipsoidal or torispherical heads).
Need for heat treatment shall be determined for each segment individually using the greatest measured thickness and smallest radius of curvature after forming.
iv) In case of different forming steps without intermediate heat treatment are employed, extreme fiber elongation is the total amount of elongation of the individual forming steps. In case of intermediate heat treatment, the deformation is that elongation achieved after the last previous heat treatment. This is applicable for all types of formed part.
v) Filler metal used in items subjected to hot forming temperatures, or normalized, shall satisfy the weld joint design requirements after such heat treatment. This is considering that such welds will generally suffer significant strength reduction.
13.2.13 Bolt tensioning device shall be used for bolting up flanged connections with stud bolts of diameter 1-½ inch and above. Bolt up of flanges, irrespective of bolt diameter shall be according to ASME PCC-1 requirements.
13.2.14 Correction of fit-up offsets of the closing longitudinal butt joint in a rolled shell ring shall be achieved by only employing rolling machine operation until the deviations are within the specified Code tolerances.
13.2.15 Alignment of pre-formed sections of multi-piece vessel head at butt joints, with fit-up deviations exceeding the Code tolerances, shall be achieved by only reforming (employing pressing machine) the head segments until the deviations are within the specified limits.
13.2.16 Alignment of completely fabricated sections at girth joints (shell ring-to-shell ring and head-to-ring-to-shell ring), with fit-up deviations exceeding the Code tolerances, shall be achieved by only reforming the shell (using rolling machine) or head (employing pressing machine), whichever is out-of-true, until the deviations are within the specified limits.
13.3 Welding
13.3.1 All welding shall be in accordance with the requirements of SAES-W-010.
13.3.2 Dissimilar metal welds (DMW) are not permitted in sulfide stress cracking environment as defined in this specification. Welds in clad systems are acceptable if the DMW interface with the ferritic steel is not in contact with the sour fluid.
13.3.3 Following maximum allowable carbon equivalent, based on thickness (t) shall be met for pressure vessels intended for sulfide stress cracking environment.
Thickness (mm) Carbon Equivalent (%)
6 < t < 60 0.43
60 < t < 100 0.45
t > 100 0.48
13.3.4 The method of weld overlay shall be such as to produce a minimum of 3.2 mm thickness meeting the specified chemical composition of the specified weld overlay material.
14 Nondestructive Examination