With large pipe dimensions, it is customary, to form a TEE intersection by cutting a hole in the straight run (the header) and welding in the leg perpendicular (the branch), this is called a STUB-IN.
Similarly, nozzles are installed on pressure vessels by cutting a hole in the side of the vessel and welding in an appropriately sized pipe to form the nozzle. These intersections ensure a "hole weakening" on the vessel or piping system, due to the metal removed and the stress concentration created.
In critical systems, this weakness must be compensated, and can be restored with a Reinforcing Pad, to strengthen the piping branch connection or the pressure vessel nozzle.
For the dimensions of a Reinforcing Pad you can handle as rule:
However, it is customary for most companies a so-called Branch table handling. This document defined the methods, which must be applied, if a branch (with or without a Reinforcing Pad) in a run pipe must be made. It is also usually present a specification that defines own dimensions, because each company has their own criteria. (e.g. type of product, pressure and temperature, national regulations, local markets)
Below an example for the assembly of a Reinforcing Pad.
On a number of images, you might be have seen a round hole in the Reinforcing Pad. This is a "Weep Hole" and may have multiple functions. Sometimes also called Tell Tale hole or Vent hole.
Opinions about the use of this hole varies, and therefore I give 2 possible uses of a weep hole:
By threading the hole, a test gauge can be thread for a "air/soap" test to check for leakage. The thread, mostly1/4" NPT will be used.
Weep holes serves as a vent during welding for entrapped gasses and prevents the Reinforcing Pad from becoming a "jacketed" vessel.
Weep holes in Reinforcing Pads shall be sealed upon completion of pressure test.
NPT is the best known and most widely used connection where the pipe thread provides both the mechanical joint and the hydraulic seal. NPT has a tapered male and female thread which seals
with Teflon® tape or jointing compound.
ASME B1.20.1 covers dimensions and gaging of NPT pipe threads for general purpose applications.
A vent hole shall be provided at the side (not at the crotch) of any pad or saddle to reveal leakage in the weld between branch and run and to allow venting during welding and heat treatment.