allowable offset to piping at chemicals plants

It really depends on the design and size of your piping system and how much flexibility is already present in it - of even more concern is what your piping is connected to - pump, machinery, tank, vessel - because you do not want to induce excessive stress on these.

The following is taken from para. 335.1 (c) of ASME B31.3

(c) Flanged Joints. Before bolting up, flange
faces shall be aligned to the design plane
within 1 mm in 200 mm (1⁄16 in./ft) measured
across any diameter; flange bolt holes shall
be aligned within 3 mm (1⁄8 in.) maximum
offset.

As stated, the alignment tolerance applies to each individual flange, measured to the design plane. Some practitioners consider this to be excessive and use half of this tolerance, thereby making the alignment tolerance 1⁄2 mm in 200 mm (1⁄32 in./ft). Conversely, it is recognized that misalignment greater than that permitted by para. 335.1 (c) can generally be accepted for small diameter piping, particularly if it is not connected to load-sensitive equipment. It is understood that more stringent alignment
tolerances may be required for large piping connected to load-sensitive equipment such as machinery. For machinery, refer to API 686, Chapter 6, Sections 4.6 through 4.9 and Fig. B-4; and you may even want to refer to WRC 449 or WRC 513.

There is even interpretation 15-07 (with some relevance to your situation):

Question: In accordance with ASME B31.3c-1995 Addenda, para. 335.1.1(c), prior to bolting up a flanged joint, may the flange faces be out of alignment from the design plane by more than 1/16 in./ft (0.5%), provided the misalignment is considered in the design of the flanged assembly and attached piping in accordance with para. 300(c) (3)?

Reply: Yes.

With your torquing procedure you may want to look at something like this (of course it depends on the size of your flange and number of bolts):

1. install 4 studs, snug to align the flanges
2. install the remaining 4 studs torqued to 1/2 final value (leaving the initial 4 out of the sequence)
3. remove the original 4 studs and replace with new and torque to 1/2 final value
4. proceed with your torque sequencing on all studs.

An appropriate test of whether the alignment is acceptable is to check any machinery alignment (if present) with and without the piping bolted to it.

The last point (that I probably shouldn't even mention but I will for arguments sake) and this will probably ruffle a few feathers , but there is even a slight chance that by torquing this misalignment into place it will lower the stress values on the piping when at temperature - similar to cold springing. However, cold springing needs to be explicitly designed and is quite the art. (but forget I even mentioned this )

If you are stuck with the misalignment and can't replace the flange, nor do an analysis on your piping system - you are on your own ........... it may be a problem, and it may not .....................

oh yeah, another thought - IF you can remove and replace the flange and it is a buttwelded flange:

ASME B31.3 - 328.4.2:

6) Buttweld fittings manufactured in accordance
with ASME B16.9 may be trimmed to produce an angular
joint offset in their connections to pipe or to other buttweld
fittings without being subject to design qualifications
in accordance with para. 304.7.2 provided the total

angular offset produced between the two jointed parts does not exceed three degrees.

And just curious - what size/style of flange are you dealing with?