Codificación de los discursos mediáticos

Welding procedures are composed of two parts: the procedure specification and the procedure qualification. The procedure specification form is shown in Exhibit A of API STD 1104 and the information to be filled in ranges from process to speed of travel. These will be discussed individually later on. The procedure qualification form shown in Exhibit B of API STD 1104 documents the mechanical properties (such as strength, ductility, and hardness) of the welding procedure established in the welding procedure specification. Mechanical properties are determined by destructive testing of a test weld. After the welding procedure is qualified, changes to the procedure specification may be made providing they are not changes to the essential variables. Any changes to the essential variables require requalification of the welding procedures and revision of the welding procedure specification. The essential variables that have to be considered for the SMAW process are:

• Yield strength range of the pipe group

• Major change in joint design

• Welding position

• Wall thickness group

• Filler metal group

• Time lapse between root and hot pass

• Direction of welding

• Travel speed

There are additional essential variables for automatic welding, and API STD 1104 Section 9.0 should be consulted for these.

49 CFR 192 Transportation of Natural and Other Gas by Pipeline 49 CFR 195 Transportation of Hazardous Liquids by Pipeline

ANSI/ASME B31.4 Liquid Transportation Systems for Hydrocarbons, Liquid Petroleum Gas, Anhydrous Ammonia, and Alcohols ANSI/ASME B31.8 Gas Transmission and Distribution Piping Systems API Standard 1104 Standard for Welding Pipelines and Related Facilities API RP 1107 Recommended Pipe Line Maintenance Welding Practices ASME Section IX Welding and Brazing Qualifications

Welding Procedure Specification

The following is a discussion of the individual entries on the API STD 1104 proce-dure specification form.

Process (Essential Variable). Each process is identified by name and as manual, semiautomatic, or automatic. The most common process is shielded metal arc welding (SMAW), which is a manual process. Other processes are also recognized by API STD 1104. These are:

• Gas metal arc welding (GMAW)

• Gas tungsten arc welding (GTAW)

• Flux cored arc welding (FCAW)

• Submerged arc welding (SAW)

SAW is often used for double jointing of pipe where productivity gains can be achieved through automation. The other welding processes (GMAW, GTAW, and FCAW) can be used either semiautomatically or automatically depending upon the application.

Material (Essential Variable). Pipe and fitting materials are identified as to specifi-cation (e.g., API SPEC 5L and grade, or ASTM number). The materials are

grouped into three strength ranges based on specified minimum yield strength (SMYS). These are:

• SMYS of 42¹ksi or less

• SMYS of more than 42 ksi but less than 65¹ksi

• SMYS over 65 ksi (each grade requires separate qualification)

Generally, the Company does not have experience with line pipe grades above API SPEC 5L X65, although X70 has been used by others.

Diameter and Wall Thickness (Essential Variables). The separate groups are:

Joint Design (Essential Variable). The most frequently used joint design is a V-groove having the configuration shown in Figure 600-3. Offset (high-low) during fitup should be restricted to 1/16 inch maximum. Offset greater than 1/16 inch should be reduced by equally distributing it around the circumference of the pipe.

Filler Metal (Essential Variable) and Number of Beads. The American Welding Society specification and electrode classification is listed. For SMAW, AWS Specification A5.1 or A5.5 is used, depending upon the minimum tensile strength of the electrodes. Note that minimum tensile strength (ksi) is indicated by the first

1. Qualification at the maximum strength qualifies all of the lower strength materials within the group.

Diameter Groups Thickness Groups Under 2-3/8 in. Less than 3/16 in.

2-3/8 to 12-3/4 in. 3/16 to 3/4 in.

Over 12-3/4 in. Over 3/4 in.

two digits of the electrode classification and is different from the pipe groupings that are based on SMYS. Electrode sizes and minimum number of beads commonly used are shown in the table and with the sketch on Page 2 of the procedure specifi-cation. Company practice requires a minimum of three weld passes and limits maximum electrode sizes as follows:

Electrodes furnished by Lincoln Electric Company are most commonly used. These are listed in Figure 600-4 by trade name, AWS class and specification, group, and typical API material grade application.

(1) For use on root and hot pass only on all grades.

Electrical Characteristics. The normal electrical characteristics for cellulosic elec-trodes are direct current-reverse polarity (i.e. the electrode is positive). An excep-tion to this is that straight polarity has been sometimes used for the root pass for better penetration control. This practice is acceptable provided that it is included in the procedure qualification. Voltage and amperage ranges for each electrode size should be shown in the table of the procedure specification.

Position (Essential Variable). Roll welding and position welding are terms used to describe whether the pipe is being rotated or is fixed during welding. Roll welding Fig. 600-3 V-groove weld joint

Stringer Bead Hot Pass, Filler and Cap Passes 5/32 in. max. 3/16 in. max.

Fig. 600-4 Lincoln Electric Company Electrode Specifications

Trade Name AWS Class AWS Specification API Group API Material Grades

Fleetweld 5P E6010 A5.1 1 B, X42, X46

Fleetweld 5P + E6010 A5.1 1 ⁽¹⁾

Shield-Arc Hyp E7010-G A5.5 1 X42–X66

Shield-Arc 85 E7010-A1 A5.5 1 X42–X56

Shield-Arc 70 + E8010-G A5.5 2 X60, X65

is done with the pipe rotated about a horizontal axis and the welding performed near or at the top center for a flat position weld. Position welding can be done with the pipe axis horizontal, vertical, or sloping. When the pipe axis is horizontal, the position of the weld is vertical. If the pipe axis is to be other than horizontal, it should be clearly described.

Direction of Welding (Essential Variable). The direction of welding for position welding using cellulosic electrodes is normally downhill for pipeline welders.

Downhill welding is much quicker than uphill welding. The direction of welding does not apply if the position of the weld is flat (roll welding) or horizontal (where the pipe axis is vertical).

Number of Welders. For position welding, the number of welders varies with pipe size. Generally, two welders can be used for pipe sizes over NPS 8 and three for pipe sizes over about NPS 24. The use of more than one welder helps to balance shrinkage stresses and increase productivity.

Time Lapse Between Passes (Essential Variable). For welding with cellulosic electrodes, the time lapse between completing the stringer bead and starting the hot pass is important to avoid cracking. Good practice is to start the hot pass within five minutes of completing the stringer bead. Where the hot pass cannot be started within five minutes, the stringer bead should be reheated to 100°F minimum and checked for cracking prior to welding. Weld joints that have not had the stringer bead completed at the end of the day should be rejected.

Type of Lineup Clamp. This refers to the method of aligning the pipe and whether internal or external lineup clamps are used. In rare cases, lineup clamps will not be used and "none required" should be stated.

Removal of Lineup Clamp. The percent completion of the stringer bead required before removal of the lineup clamp should be specified. For internal lineup clamps, generally 100% of the stringer bead is completed before removal is permitted. For external lineup clamps, API STD 1104 requires not less than 50% of the stringer bead to be completed in equal segments around the circumference before removal is permitted.

Cleaning. Standard pipeline procedure is to grind the root pass and power wire brush all remaining passes.

Preheat and Stress Relief (Postweld Heat Treatment). Preheat requirements will vary with pipe grade, carbon equivalent, and wall thickness. Preheat is generally not required except for low initial pipe temperature, repair welds, and heavier wall thicknesses. Both Codes B31.4 and B31.8 require preheat for carbon steel when the carbon content exceeds 0.32% or the carbon equivalent (C +Mn/4) exceeds 0.65%

(this is a simplified carbon equivalent used only for determining the need to preheat). These are extreme cases for most pipe materials and rarely will be cause for preheat. For pipe temperatures below 40°F, preheat of 100°F minimum should be used. A preheat of 200°F minimum should be used for wall thicknesses of one inch or greater and all weld repairs.

Stress relief is normally not required for wall thicknesses of 1-1/4 inches and less.

(Code B31.4 permits up to 1-1/2 inches with 200°F preheat over 1-1/4 inches.) When stress relief is required, the temperature range and holding time should be specified.

Shielding Gas and Flow Rate (Essential Variable). Applies only to the gas shielded processes (i.e., GTAW, GMAW, and FCAW).

Shielding Flux (Essential Variable). Applies only to the granular flux used for submerged arc welding.

Speed of Travel (Essential Variable). Travel speed should be specified as a range.

The following are typical ranges for vertical down welding with cellulosic elec-trodes on larger pipe (e.g., over NPS 6 ).

Procedure Qualification Test Results

Procedure qualification test requirements are described in Section 2.6 of API STD 1104 for butt welds and Section 2.8 for fillet welds. Results are recorded on the coupon test report form in Exhibit B of API STD 1104. This is a record of the actual results for the tensile, bend, and nick break tests. Additional tests, such as Charpy V-notch and hardness, are recorded on a separate sheet.

The testing laboratory performing the tests normally prepares this form or one similar to it. For work performed by contractors, the Company can either require them to qualify their welding procedures or permit them to use a Company-quali-fied welding procedure.