Force-Fitting a Steam Header Flange
A 250 mm NPS carbon steel steam header, 12.7 mm wall, operates at 2.0 MPa and 260 degrees C. A new 18 m run is being tied into an existing flange. The pipe is already supported on spring hangers set to cold load. When the spool is offered up, the flange faces show a 6 mm parallel offset and a 3 degree angular misalignment; bolt holes are 12 mm out. The flange is a raised-face weld-neck with a spiral-wound gasket. The foreman proposes to pull the flanges together with two chain come-alongs and then tighten the bolts. The line has a 90 degree elbow 2 m from the flange, and the next anchor is 12 m away. The design drawings show an expansion loop but it has not been installed. Ambient temperature is 18 degrees C. The system will be hydrotested at 1.5 times design pressure before insulation. The piping specification requires cold spring of 50 percent of thermal movement where loops are not installed.
1. Estimate the thermal expansion of the 18 m run and explain why the 6 mm offset and 3 degree angular misalignment are not ordinary fit-up tolerance.
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Use delta L = alpha x L x delta T, with alpha about 12 x 10^-6 per degree C for carbon steel. Delta T = 260 - 18 = 242 degrees C, and L = 18,000 mm. Delta L = 0.000012 x 18,000 x 242 = 52.3 mm. The line will grow about 52 mm from cold to operating temperature. A 6 mm offset is small compared with that movement, but pulling it out imposes bending and shear at the flange and nearby elbow. The 3 degree angular misalignment is more serious: across a 250 mm flange, it produces roughly 250 x tan 3 degrees, or about 13 mm of gap variation across the face, which is far beyond gasket seating tolerance. The spring hangers and anchors will be preloaded by the pull, so the problem is not just the flange joint.
2. Evaluate the chain come-along proposal and describe the acceptable correction.
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The come-along proposal should be rejected. Pulling a raised-face weld-neck flange with a spiral-wound gasket into alignment will distort the flange, load the gasket unevenly, and leave locked-in stress in the pipe and supports. It can also crack the nozzle or elbow weld after the load is released. The correct correction is to cut the spool at a convenient location, usually at the elbow or a straight section, and re-fit it with the flange faces parallel and the bolt holes aligned. Align with dogs or clamps only for temporary positioning, never to close a gap under load. Install the expansion loop shown on the drawings, set the cold spring to 50 percent of 52.3 mm, about 26 mm, and adjust the spring hangers to their cold-load settings before bolting up.
3. What support, cold-spring, and hydrotest checks are needed before the line is returned to service?
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Confirm the expansion loop is installed and that anchors, guides, and spring hangers match the design. Set the cold spring to about 26 mm and remove any shipping stops or hanger pins as specified. Check flange parallelism and bolt-hole alignment without force, then torque the bolts in the correct sequence with the specified gasket. For hydrotest, use 1.5 x 2.0 = 3.0 MPa. Fill with water, vent air, use calibrated gauges, and provide temporary supports for the test weight. Do not let temporary supports or come-alongs restrain normal expansion during the test. After the test, inspect flanges and supports for leakage or movement, dry the line, and verify spring hanger travel before insulation.