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Is it possible to drill 3 threaded holes in the blind flange?


There exists a quandary I’m currently grappling with concerning the task of drilling 3 threaded holes into a piece of equipment specifically identified as an 8 inch 300# stainless steel 316 blind flange. Each aperture, once created, is intended to accommodate a threaded connection.

At present, I find myself uncertain about the applicable regulations and standards that ought to be adhered to for such an undertaking. Similarly, the proper method for computing the relevant metrics escapes me.

If you are in possession of the requisite knowledge or can point me in the direction of a reliable source, I would greatly appreciate any enlightenment you could offer on the matter.


In the realm of the B16.5 doctrine, integrating a solitary, symmetric aperture in the blind flange is permissible. This doesn’t meddle with the prescribed pressure-temperature ranking. A caveat, though – it imposes a constraint on the diameter of the aperture. Should you dare to venture beyond this stipulated size, the obligation falls upon you to opt for a hubbed flange. For reference, cast your eyes upon the illustrious Table 6 in the B16.5 manual. (For those of you who possess the antiquated editions, it’s Table 7).

As we dare to dream, envision a scenario of applying more than one aperture to the blind flange. This transgresses the boundaries of its original design, causing it to forfeit its status as a standard pressure component. Consequently, the conventional pressure-temperature ratings may dissolve into irrelevance. The responsibility of unraveling the complexities of the newly birthed flange falls under the purview of the fitting design code, be it piping, boiler, or pressure vessel codes.

Now, for those in pursuit of a more ‘grounded’ perspective, allow me to indulge you: On the assumption that the newly introduced apertures don’t exert significant moments on the flange and the collective area of these holes doesn’t eclipse that of a single aperture in a standard blind flange (according to our trusty B16.5), you find yourself on safe terrain, practically speaking. A crucial detail – the spacing of these openings must be meticulously orchestrated to facilitate proper weld attachment if such a necessity arises.

However, should you defy the prescribed area for a single aperture, you’re thrust into the need for a hubbed blind (Godspeed on your quest to acquire one!). I recommend performing an Appendix 2 calculation as per ASME VIII-1. Brace yourself, though, as the blind flange – rubbed or not – may not survive the scrutiny of Appendix 2. This could be attributed to the tendency of ASME B16.5 flanges to be over-bolted. In such a predicament, your options are to enhance the flange‘s thickness by creating a custom design or elevate the class of the flange if you decide to tread the path of Appendix 2.



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