Thinning of elbows on the outer radius (extrados) is common. Some manufacturers will manufacture them from pipe of the next schedule up. For example, they might use shedule 40 start pipe to make a schedule 10 elbow, schedule 80 for a schedule 40 and so on. I should think large manufacturers will order pipe to their own tolerance. I've also seen some manufacturers who measure the wall thickness of the start pipe and ensure the thickest part on the circumference is turned to be the back wall of the pipe. As you say the start pipe can be 12.5% down on thickness to start with so there would be no chance of forming an elbow or bend without the extrados falling below minimum wall thickness. To answer your question I would return them as rejects to the supplier.
All well and good provided minimum wall thickness was specified in the original PO or engineering specification or code of construction that was referenced in the above. If not, you have no basis for rejection.
gafoorkti;
Please see the excerpt from B16.9 below regarding design of the fitting.
2.2 Design of Fittings
The design of fittings shall be established by mathematical
analyses (e.g., ACME B16.49 for bends) contained
in nationally recognized pressure vessel or piping codes,
or at the manufacturer’s option by proof testing in accordance
with para. 9 of this Standard. In order to meet
design or manufacturing requirements, it is expected
that some portion of formed fittings may have to be
thicker than the pipe wall with which the fitting is
intended to be used. The mathematical analyses, if used,
may take into account such thicker sections. Records of
mathematical analysis and/or successful proof test data
shall be available at the manufacturer’s facility for
inspection by the purchaser.
Unfortunately, ASTM A 234 does not provide any guidance for minimum wall thickness for a elbow, only material specification information.
As I mentioned previously, you need to know the code of construction for installation of this fitting. If the fitting is certified to B16.9 from the supplier, discuss with the supplier the minimum wall thickness.
If the process Code is B31.3, here is what is required and please review it:
303 GENERAL
Components manufactured in accordance with standards
listed in Table 326.1 shall be considered suitable
for use at pressure-temperature ratings in accordance
with para. 302.2.1 or para. 302.2.2, as applicable. The
rules in para. 304 are intended for pressure design of
components not covered in Table 326.1, but may be used
for a special or more rigorous design of such components,
or to satisfy requirements of para. 302.2.2. Designs
shall be checked for adequacy of mechanical strength
under applicable loadings enumerated in para. 301.
When you calculate the max allowable pressure of that 87%-wall thinned ell, I'll bet it comes out over 10,000 psi. Most of the thickness in c/s pipe is to resist bending stress so that the pipe supports can be farther apart than 1-2 feet. Most of the remaining thickness of that c/s pipe for Corrosion Allowance.
When you pull out API-570 and calculate the [pressure-only] Tmin, you will find out that very little wall thickness is needed to contain the pressure of the fluid in your pipe. This is why the industry standard 87.5% has been around for over 50 years, without causing any problems.
