Presumably the eccentric reducer is a B16.9 fitting. This means that it is ‘designed’ by the manufacturer to meet a performance requirement – the allowable pressure rating is to match straight seamless pipe of equivalent material. The manufacturer will often use a heavier wall than the specified ‘nominal’ wall to achieve this pressure rating. In designing a piping system, because it is a B16.9 fitting we need do no further engineering on its pressure rating.
The moment that a hole is cut in this reducer it is no longer a B16.9 fitting, and the manufacturer is not responsible for its pressure rating. The pressure rating needs to be established by the piping designer.
Clearly some engineering judgement needs to be applied to this aspect of ‘modifying’ a B16.9 fitting. If the branch fitting being attached to the NPS 24 fitting is, for example NPS 1, then it is arguable that the impact of the branch on the pressure envelope of the fitting is minor, and that it will have little impact on longitudinal bending stresses in the fitting. At what size does the branch fitting become significant? This is where engineering judgement is required.
For the case discussed above with an NPS 6 weldolet on an NPS 24 x 20 eccentric reducer, the integral reinforcing of the weldolet may be adequate for the pressure envelope, but it cannot be assumed. Remember that the weldolet is sized for reinforcing a specific weight of run pipe wall, and so for this case, at minimum the weldolet would need to be specified for the actual wall thickness of the reducer (not the nominal wall).
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Ross Sinclair