Quiz- Annex B In-service Welding- CWI Part C- 9 Questions
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Annex B: In-service Welding
This annex provides welding practices for making repairs to and installing appurtenances on piping systems that are in service. Of particular importance are the welds that melt into the carrier pipe since they cool at an accelerated rate due to the quenching effect of the fluid flowing through the carrier pipe.
The welds that melt into the carrier pipe are either fillet welds or branch connection welds. Fillet welds are used to join the ends of encirclement sleeves to the carrier pipe. These fillet welds are also referred to as “sleeve welds” and “circumferential welds.” Groove welds are used to join split sleeves using longitudinal welds, but these, unlike sleeve welds and branch connection welds, do not melt into the carrier pipe.
Welding attachments onto in-service pipelines pose two major risks: (1 ) burning through the pipe wall and (2) hydrogen cracking. Burning through the pipe wall is unlikely when the wall thickness is 0.250 inches or greater, although many companies successfully weld onto thinner wall pipe with regularity. Although Annex B explains how to prevent both of these problems, the major focus of Annex B is on the prevention of hydrogen cracking.
For hydrogen cracking to occur, three conditions must be satisfied simultaneously:
(1 ) Hydrogen in the weld,
(2) A crack susceptible microstructure, and
(3) Tensile stress acting on the weld. Conversely, to prevent hydrogen cracking, one of these must be reduced or eliminated. Since these welds are made on pipelines that have been in service, hydrogen is always present and cannot be reduced to sufficient levels. In addition, residual tensile stresses are present in every weld due to the inherent shrinkage resulting from solidification and subsequent cooling from high temperatures. As a result, the primary technique used to prevent hydrogen cracking in in-service welds is the prevention of a crack-sensitive microstructure.
To make matters worse, in-service welds are made onto pipe carrying flowing fluids which accelerate the cooling rate and, therefore, promote the formation of crack-sensitive phases in steels, particularly martensite. Because of this combination of factors, the most effective approach for preventing hydrogen cracking in in-service welds is the use of a welding procedure that has a high enough heat input to overcome the quenching effects of the fluids flowing through the carrier pipe. As an alternative, temper bead welding sequences are also effective at producing multi-pass welds in which the heat-affected zone in the carrier pipe has a tempered microstructure, known to be more resistant to
B.2 Qualification of In-service Welding Procedures
This subsection lists the essential variables and tests required to qualify in-service welding procedures.
Paragraph B.2.1 refers the reader to Section 5 for the basic requirements for qualifying welding procedures for making fillet welds and states that the requirements and exceptions in this annex will then be added to those requirements.
Paragraph B.2.2 lists the additional information required to be addressed on the procedure specification. One of these variables is the carbon equivalent of the carrier pipe to be welded. For determining the carbon equivalent, the following equation is provided:
CEIIW = %C + %Mn/6 + (%Cu + %Ni)/1 5 + (%Cr + %Mo + %V)/5
Also included here is the provision that carbon equivalents may be grouped for the purposes of procedure qualification, as will be discussed shortly.
The pipeline operating conditions must also be addressed on the procedure specification. This can be addressed by specifying the fluid type and its flow rate. Again, conditions may be grouped.
For procedures designed to overcome the quenching effect of the flowing contents by using a sufficiently high heat input, the heat input range should be specified. The minimum value of this range should be that used to weld the procedure qualification coupon. Heat input is given in the Note as Heat Input = Amps x Volts x 60 / Travel Speed.
For procedures designed to overcome the quenching effect of the flowing contents by using a temper bead sequence, the weld deposition sequence, including bead size and overlap, should be specified. The required heat input range for each temper bead layer should also be specified on a temper bead procedure specification.
Paragraph B.2.3 lists additions and changes to the essential variable rules specified in Section 5. For in-service welds, specified minimum yield strength is no longer an essential variable, meaning that in-service welding procedures are valid for all pipe grades. However, since hydrogen cracking is more likely on pipe materials having greater hardenability (as measured by the carbon equivalent), an increase in the carbon equivalent over that used to qualify the procedure is now an essential variable. So, for in-service welds, the MTR of the carrier pipe to be welded should be available so that the carbon equivalent can be calculated from the listed composition. When the in-service welding procedure is qualified, the carrier pipe used in the test coupon should have a carbon equivalent no less than that of the production carrier pipe. Ideally, the carrier pipe used for the qualification test should have a very high carbon equivalent. In that case, the in-service welding procedure would be qualified for a large range of carbon equivalents, up to and including that used in the qualification test weld.
For in-service fillet and branch welds, pipe wall thickness is no longer an essential variable, so these procedures are qualified for all wall thicknesses; however, the note in paragraph B.2.3.1 .3 provides an exception, which states that, for weld deposit repairs, the welding procedure is qualified only for wall thicknesses equal to or greater than that used in the qualification test.
Finally, for in-service welds, pipeline operating conditions are an important essential variable, with an increase in the severity of the conditions being a cause for requalification. So, the procedure should be qualified under the most severe quenching conditions. This can be accomplished by making the test weld on the water canister set up at a 45 ° angle as shown in Figure B.2 on page 1 05. By implication, this variable rule also suggests that the use of a heat input value less than that used to qualify a “heat input the procedure” is also an essential variable and would require the requalification of the procedure. By the same logic, paragraph B.2.3.1 .4 states that, for a temper bead procedure, a change in the deposition sequence or the bead spacing (or overlap) from that qualified is also an essential variable and would require requalification of the procedure.
Paragraph B.2.4 addresses the welding of the test joints and refers to Figure B.2 on page 1 05 showing the carrier pipe coupon filled with flowing water with the sleeve being welded to it using fillet welds or branch connection welds as appropriate. This condition has been shown to produce thermal conditions more severe than typical in-service welding applications. Therefore, a procedure qualified under this condition qualifies for all in-service conditions.
Paragraph B.2.5 addresses the testing of the welded joints. In general, in-service welds that melt into the carrier pipe, including repairs to weld depositions, will be tested using the nick break tests described in subsection 5.8, except that the test locations are shown in Figures B.3 and B.4 on pages 107 and 108, respectively, and the number of specimens required is specified in Table B.1 on page 109. The additional tests required in this table are macro section tests, detailed in paragraph B.2.5.4, and face bend tests, detailed in paragraph B.2.5.5, and both are described below.
Longitudinal seam welds of full-encirclement sleeves should be tested with the tension tests, bend tests, and nick break specimens required in subsection 5.6, as a function of sleeve wall thickness and diameter. Branch and sleeve welds should be tested with the nick break tests in Section 5.8 plus the additional required tests in B.2.5.1.
Paragraph B.2.5.4 gives details about the macro section tests to be removed from the in-service branch and sleeve welds. These are shown schematically in Figure B.5 on page 1 09. They may be machine-cut or oxygen cut oversized.
When they are oxygen cut, the oxygen-cut surface must be machined by a non-thermal process to remove at least ¼ inch from the side to be examined. These weld cross-sections shall then be ground, polished, etched with a suitable etchant, and examined visually without dye penetrants and with little or no magnification (typically at 1 0X or less).
Two of the four micro etch specimens for branch and sleeve welds and both of the specimens for weld deposition repair shall be examined by hardness testing in accordance with ASTM E384. At least five indentations shall be made in the coarse-grained HAZ at the toe of each weld cross-section using a Vickers indenter and a 1 0 kg load to determine the maximum hardness.
Paragraph B.188.8.131.52 gives the requirements for visual examination of each micro etch specimen:
(a) The weld should show complete fusion at the root of the joint.
(b) The weld must be free from cracks.
(c) The legs of the fillet welds must meet the requirements of the procedure specification.
(d) The fillet weld surface (convexity and concavity) must be fat within +/- 1 /1 6 inches.
(e) The undercut depth should not exceed the lesser of 1 /32 inch or 1 2.5% of the pipe wall thickness.
(f) Heat-affected zone (HAZ) hardness values over 350 HV should be evaluated to determine the risk of hydrogen cracking
Paragraph B.2.5.5 describes the face bend specimens that are required. Specific specimens may be cut for these tests or the remaining portion of the nick break specimens can be used. Both options are shown in Figure B.6 on page 110. They may be machine-cut or oxygen cut oversized. When they are oxygen cut, they must be machined by a nonthermal process to remove at least 1 /8 inch from each side. When remnants of the sleeve or branch welds are used, the sleeve and branch welds must be removed to, but not below, the surface of the sleeve. Undercut should not be removed.
Paragraph B.184.108.40.206 addresses the method of bending the specimens. This is identical to the requirements in paragraph 220.127.116.11 except that the specimens shall not be tested sooner than 24 hours after welding.
The acceptance criteria for these bend specimens is exactly the same as that found in paragraph 5.6.4.
B.3 In-service Welder Qualification
Paragraph B.3.1 requires welders performing in-service welding to qualify by making a single qualification test weld in accordance with subsection 6.2 using the specific procedure (heat input or temper bead) used to overcome the quenching effects of the fluid flowing through the pipeline. A welder with only that qualification is qualified for in-service welding in accordance with the essential variable limits of subsection 6.2. However, this annex modifies those qualification ranges as explained in the next few paragraphs.
If that qualification test is welded on pipe less than 1 2.750 inches in OD, the welder is qualified for diameters up to and including the diameter on which he tested. If that qualification test is welded on pipe 1 2.750 inches in OD or larger, the welder is qualified to weld all pipe diameters.
A welder who meets both the multiple qualification requirements of subsection 6.3 and the additional qualification test of this annex is qualified as an in-service branch or sleeve welder in accordance with the essential variable rules of subsection 6.3. Welders who perform weld deposition repairs are limited based on the positions in which they perform the test welds.
Paragraph B.3.2 states that the qualification test required by this annex should be welded on a coupon that simulates the ability of the flowing contents to remove heat from the pipe during welding. Similar to the weld coupon required for the procedure qualification test, falling the carrier pipe with flowing water during welding should produce conditions equal to or more severe than typical in-service conditions.
Welders qualified on such a coupon are therefore qualified for all typical in-service applications. The coupon should also be welded following either the heat input, temper bead, or weld deposition repair welding procedure, as applicable.
Paragraph B.3.3 requires the test coupon to meet the visual inspection requirements of subsection 6.4 and the mechanical testing requirements of subsection 6.5. For longitudinal seam welds in full encirclement sleeves, the type and number of test specimens required for welder qualification are listed in Table B.2 on page 1 1 1.
Paragraph B.3.4 requires that records of these qualifications be maintained.
B.4 Suggested In-service Welding Practices
When making actual in-service welds, the requirements of Section 7 apply, except for the alternative or additional requirements of subsection B.4. Paragraph B.4.1 reminds the reader that safety is the primary concern when welding onto these in-service lines. Factors such as operating pressure, flow conditions, and minimum wall thickness in the area to be welded should be considered. Ultrasonic testing of the pipe wall in the area to be welded is commonly used to determine minimum wall thickness and/or to verify that there are no laminations in the pipe wall that could compromise in-service weld quality or pressure integrity. Additional safety precautions can be found in API’s Recommended Practice 2201.
Paragraph B.4.2 addresses important alignment and ft.-up issues. For saddle and sleeve welds, the gap between the sleeve and the carrier pipe should be minimized to permit easy fusion of the carrier pipe. Weld metal build-up on the carrier pipe is one way to minimize any gap that might be present. Clamping devices are recommended. For longitudinal butt welds of full encirclement sleeves, the root opening should be sufficient to permit full penetration. Use of a mild steel backup strip or suitable tape may be necessary to prevent penetration into the carrier pipe.
Recommended welding sequences, sleeve designs, and geometries are provided in paragraph B.4.3 and shown graphically in Figures B.7 through B.12 on pages 112 through 115. For full encirclement sleeves requiring circumferential fillet welds, the longitudinal seams should be completed before beginning the sleeve welds to minimize the residual stresses on the sleeve welds. When making the circumferential fillet welds, one sleeve weld should be completely welded before beginning the other.
Regardless of the type of fitting used, the welding sequence should always be chosen to minimize residual stresses. Heat input limits and/or temper bead sizes and locations as specified in the welding procedure must be followed.
Paragraph B.4.4 recommends that in-service beads should be deposited in the circumferential direction when possible.
B.5 Inspection and Testing of In-service Welds
In-service welds must meet the acceptance criteria of Section 8 except for the additional or alternative requirements in subsection B.5. Since hydrogen cracking is the primary weld quality concern and most hydrogen cracks are located under the weld bead and do not break the surface, the inspection method must be able to detect under bead and toe cracks. A combination of magnetic particle and ultrasonic testing is recommended for inspecting sleeve-to-saddle and branch-to-carrier pipe welds. Radiographic testing is not a good candidate for detecting these types of cracks.
Since it takes time for hydrogen trapped in the weld to diffuse to the coarse-grained region of the HAZ to cause the cracking, it is important to establish a suitable delay time after welding prior to the inspection to ensure that inspection is conducted after the cracking has had adequate time to develop.
B.6 Standards of Acceptability: NDT (Including Visual)
The standards of acceptability in Section 9 apply to imperfections located in in-service welds. For weld deposition repair, the weld length is defined as the maximum weld length in the direction in which the flaw is oriented.
B.7 Repair and Removal of Defects
The requirements in Section 1 0 apply to the repair and removal of defects found in in-service welds. In addition, care should be taken to ensure that excavation of the defect does not reduce the wall thickness of the pipe to less than that required to contain the pipe’s operating pressure.