Circumferential welds

Beneath each tubercle was a dish-shaped depression. Some depressions were as deep as half the intact wall thickness. Many shallow striation-streaked depressions were present on internal surfaces (Fig. 3.24). Circumferential welds were preferentially attacked (Fig. 3.27). 

Figure 6.9 Irregular deposit and corrosion-product mounds containing concentrations of sulfate-reducing bacteria on the internal surface of a 316 stainless steel transfer line carrying a starch-clay mixture used to coat paper material. Attack only occurred along incompletely closed weld seams, with many perforations. Note the heat tint, partially obscured by the deposit mounds, along the circumferential weld.

A weeping leak developed in a carbon steel emergency service water pipe at a circumferential weld emplo3nng a weld-backing ring. A rubberized saddle clamp was used to plug the leak temporarily. After several weeks the section was cut out of the system and the failure was examined. 

Figure 6.21 A longitudinally split carbon steel emergency service water pipe, as in Fig. 6.10. Note the circumferential weld-backing ring, which is consumed on one side.

Figure 15.20 Internal surface of 14-in. diameter stainless steel pipe. The circumferential weld is in the center of the specimen.

Figure 16.1 shows part of a steel tank which came from a road tank vehicle. The tank consisted of a cylindrical shell about 6 m long. A hemispherical cap was welded to each end of the shell with a circumferential weld. The tank was used to transport liquid ammonia. In order to contain the liquid ammonia the pressure had to be equal to the saturation pressure (the pressure at which a mixture of liquid and vapour is in equilibrium). The saturation pressure increases rapidly with temperature at 20°C the absolute pressure is 8.57 bar at 50°C it is 20.33 bar. The gauge pressure at 50°C is 19.33 bar, or 1.9MN m . Because of this the tank had to function as a pressure vessel. The maximum operating pressure was 2.07 MN m" gauge. This allowed the tank to be used safely to 50°C, above the maximum temperature expected in even a hot climate. 

While liquid was being unloaded from the tank a fast fracture occurred in one of the circumferential welds and the cap was blown off the end of the shell. In order to decant... 

This section covers piping system components other than assemblies consisting of pipe and fittings joined by circumferential welds. 

All dents that affect the curvature of the pipe at the longitudinal weld or any circumferential weld shall be removed. All dents that exceed a maximum depth of in. in pipe NPS 12 and smaller or 2% of the nominal pipe diameter in all pipe greater than NPS 12... 

Pipeline systems shall be tested after construction to the requirements of this Code except for pretested fabricated assemblies, pretested tie-in sections, and tie-in connections. The circumferential welds of welded tie-in connections not pressure tested after construction shall be examined by radiographic, ultrasonic, or other nondestructive methods in accordance with para. PL-3.19.2. 

Seamless Shell with a Circumferential Weld Shell with a Longitudinal Weld ... 

Figure 3-5 Allowable out-of-roundness of cylindrical shells to be lined. A = greatest diameter, B = smallest diameter, A - B < 0.75%A. Template length = (OS m) or 5% of circumference, whichever is greater. Maximum deviation from true arc = /is" (1.6 mm), except / " (3.2 mm) permitted at longitudinal (not circumferential) welds.

A Longitudinal or spiral welds in the main shell, necks or nozzles, or circumferential welds connecting hemispherical heads to the main shell, necks, or nozzles ... 

In fact, circumferential weld shrinkage alone, particularly in heavy-wall pipe, may create complex bending stresses at the joint. Although post-weld heat treatments should relieve many of these stresses, the subsequent cooling can reintroduce harmful stresses if there is much restraint. The fact that the much higher thermal expansion and contraction of the austenitic stainless steels may introduce unexpected restraint stresses, as well as being troublesome in piping layouts, should not be overlooked. 

Circumferential paths (e.g., tank ends) require a moderate level of flexibility. A single-axis machine can be used for circumferential welding, with the aid of an external rotary positioner. 

Retractable pin tool In cases where there is no other solution, a retractable pin tool can be considered. The retractable pin tool is described in section 11.5, Secial Peripheral Equipment. This is a special peripheral solution that allows the pin of the tool to retract up into the shoulder over time. This can be used in circumferential welding or FSP apphca-tions. The use of this peripheral equipment increases the stiffness and intelhgence requirements of the machine. To be effective, the retractable pin technology requires the retraction of the pin to occur in a prewelded area or in an area of parent material away from the joint line. This may require the FSW machine to have additional axes. 

Stiffeners should never be located over circumferential weld seams. If properly spaced they may also double as insulation support rings. Vacuum stiffeners, if combined with other stiffening rings, such as cone reinforctunent rings or saddle stiffeners on horizontal vessels, must be designed for the combined condition, not each independently. If at all... 

On three occasions in 1987, 1988 and 1990, small leaks occurred through cracks in the circumferential welds in the PFR reheater vessels, which were manufactured in type 321 stainless steel. 

This repair consisted of replacing the circumferential welds on the hot sub-headers and the hot bends. In addition the longitudinal welds on the hot subheaders and the cold bends were also examined. This repair, currently underway, involves the following operations ... 

Calibration checks should be carried out on the whole system at the start and end of the examination plus intervals of no more than 4 hrs during long examinations such as circumferential welds. If there is any change in examination personnel, for instance during a shift change, these checks should also be carried out. 

A particularly dangerous breach in a pressure vessel happened in a steam generator where a crack (about 70 per cent of the circumference with a depth equal to a significant fraction of the thickness) was discovered on a circumferential weld. At some points the crack was through wall . It was discovered during a visual inspection thanks to the humidity patch on the external insulation layer of the component. 

After the circumferential weld seam was completed, the tie strap on the inside of the RPV was removed, the surface back-grooved to remove any potential slag inclusions, an MT inspection performed, and the weld was completed manually to fill in the back-groove on the inside surface of the RPV. A UT examination of the weld was performed, followed by PWHT, radiographic inspection and back-cladding. 

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