Pump Suction Flange Will Not Close
You are fitting a new 6-inch, 150-lb carbon steel suction line to a centrifugal pump. The pump is set and grouted, and the pipe anchor is 4 ft upstream. With the pipe on temporary cribbing, the flange gap measures 3/16 in at the bottom and 1/16 in at the top; the flange faces are parallel within about 1/32 in. A fitter proposes hooking a chain fall to the pump base to pull the pipe into alignment before bolting. The gasket is 1/8-in compressed fiber sheet, and the bolting is new. The line is at ambient temperature and has not been hydrotested. The pump manufacturer requires flange loading to be checked and prohibits using the pump as an anchor. The pipe is 8 ft long between the last support and the flange. You are the pipefitter responsible for the joint.
1. What is the first thing you should check before deciding how to close the flange gap, and why?
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First, verify that the pipe is free to move and that the gap is not caused by a support or anchor problem. Check the temporary cribbing, the anchor, and the last hanger; a bound support or a pipe already pulled by another joint can show up as an uneven flange gap. Use a feeler gauge at four points and check flange face parallelism, not just the gap. Also confirm the pump is not soft-footed and that the pump flange is true. The reason is that pulling a pipe with a chain fall converts the mismatch into pump casing load and shaft misalignment. The gap may be a fit-up error, but you must first prove the pipe is not restrained. If the pipe is free and the faces are parallel, a small gap can be closed by normal bolting; if the gap is tapered or the faces are not parallel, the spool must be corrected.
2. The fitter says the chain fall will only need a little pull. What is your response, and what acceptable methods might you use if the pipe is genuinely misaligned?
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Even a small pull can put hundreds of pounds of load on the pump and later show up as vibration, seal leakage, or coupling wear. The pump manufacturer prohibits using the pump as an anchor, so the chain fall is not acceptable. Stop and check the pipe layout. Acceptable corrections include adjusting or adding a support, re-cutting and re-fitting the last spool piece, rotating the flange to correct bolt-hole alignment, or using a flange alignment tool that does not load the pump. If the gap is within the engineering tolerance and the faces are parallel, bring the joint together with lubricated bolts in a star pattern and watch the gap; do not use the pump to pull. If the correction requires moving the pump, that is a millwright/alignment task, not a pipefitting force fit.
3. After bolting, what steps verify the joint is not imposing strain on the pump?
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Before final torque, check the gap at four points as the bolts are snugged; the gap should close evenly. Torque in stages, usually 30%, 60%, and 100% of the specified value, in a star or crisscross pattern. After torque, re-check flange parallelism and gap. Then check pump shaft alignment with a dial indicator or laser alignment; if the pump was aligned before the pipe was connected, a change after bolting indicates pipe strain. Also check that the pipe support is carrying the pipe and that the flange bolts are not being used to support the pipe weight. During startup, check vibration and seal condition. If alignment changes, loosen the flange and correct the pipe, not the pump.