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Thread: Standard face to face tolerances on welded components.

  1. #1
    Associate Engineer
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    Standard face to face tolerances on welded components.

    I'm looking for the end to end acceptable tolerance for welded components. We manufacture valves here and we are getting the flanges (2" 600 sch80 in this specific case) welded to the end connections for the valve. The best answer I can find is in ASME B16.10 section 5.1 (straightway valves) where they list a tolerance of "+/- 0.06in shall be allowed on face-to-face and end-to-end dimensions of valves NPS10 and smaller". My issue the valves it seems to show in this spec look like fully casted valves so i was wondering if there is a different spec for welded end connection valves or components.

    We are trying a new welding shop and are getting push back on the tolerance i've listed on the weld drawing of +/- 0.03" for the face to face of the end connection (2 ends at 0.03 each gives 0.06). They are telling me that they weld pipe all the time and are used to 1/8" tolerances. Any help would be greatly appreciated.

    I've been pointed to pipe world which seems to answer every other question except where they reference the "Tolerance on Linear Dimensions".

  2. #2
    Associate Engineer
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    Unless someone has a hard answer, the best i've come up with is i will continue to ask the welders (maybe unreasonably so) to try for +/- 0.03" per weld and accept up to 0.06" per weld. My findings are below.

    All valve specs either copy or directly reference ASME B16.10 with sites a +/- 0.06" (2mm) for valve face to face but all their valves look casted and machined (easier to hold tolerances). If welded this would be 2 welds giving +/- 0.03" per weld.

    The welding spec PFI SE-3 shows at minimum 0.125" for a spool which would at minimum contain 2 welds giving +/- 0.06" per weld.

  3. #3
    Administrator Kelly Bramble's Avatar
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    Weldments are challenging to hold tight tolerances without fixtures, post machining etc. Distortions are common due to the extreme temperature variations at the weld affected zone (WAZ) during the welding operations and mitigation is challenging.

    The general design rule for length is +/- a tape measure increment +/-1/8" or greater. If less variability is required then be prepared to spend on fixturing or design with flanges having extra material (machine allowance) that will be removed at the next manufacturing step.

    Good luck
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  4. #4
    Administrator Kelly Bramble's Avatar
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    Also, consider that in some installation applications a flex pipe (bellows or other) can allow for more generous tolerance specifications in the primary piping installation.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  5. #5
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    I would consider post machining, however none of our competitors do this and the extra costs would price us right out of the market. It hasn't been an issue over the many many valves we've sold so, if it ain't broke... We were reaching out to a new machine shop because the ones we deal with were booked up and we go through this with every new shop. I was really trying to find some hard spec info but it looks like "get it as close as you can" is good enough because i'm sure that all the other aren't holding to +/-0.03 as they aren't using a fixture. Like i said earlier we haven't had one complaint and its the same process our competitors do so i guess it works. Thank you for your input.

  6. #6
    Administrator Kelly Bramble's Avatar
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    There is almost always a new supplier issue with existing tough-to-build parts.

    Chances are that existing suppliers have built fixtures or have worked a special process to build your end item. We designers and engineers do tend to over specify (tight) tolerances.

    In other words - just because it was built elsewhere does not mean it is easy without special processing.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

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