What is a Reinforcement PAD?
Reinforcing Pad or RePAD or RF Pad (Fig. 1) is a donut-shaped pad that goes around the branch of a branch joint to add strength to the joint. It resembles a round metal washer that has been bent to conform to the curvature of the pipe.
With the increase in pipe size, the cost of branch connection fittings increases. Sometimes, such fittings are not readily available as standard pieces. So, it is a standard method to fabricate the tee by cutting a hole in the header and welding the branch in the pipe. But the section where the straight pipe is punctured becomes a weak section due to the presence of that hole. So to handle the pressure and reduce the stress concentration in that region the thickness is increased locally in the form of a reinforcement pad or RF Pad.
A similar situation arises for Pressure Vessel Nozzle Connections. To increase the pressure and load-carrying capability of the equipment nozzles, RF Pad is welded. These reinforcement pads provide additional strength and capability to the pipe.
Why is the Reinforcement Pad or Repad required?
RePADs or RF Pads are plates used to reinforce components and/or nozzles by increasing thickness local to the component in high-stressed zones. These are made from the same size and material as the pipe header to which they are welded. On pipes or Pressure vessels, holes are made in the form of nozzle or branch intersection and thus the parent pipe/ vessel is weakened and high-stress zones are created. Hence, It is obvious to compensate for this weakness with a Reinforcing Pad to reduce the possibility of failure, as it strengthens the piping branch connection or the pressure vessel nozzle.
Uses of Reinforcing PAD
1. Normally Reinforcing pads are used at stub-on and stub-in branch connections if required per the line list or if required per the branch chart in the piping material specification. By using reinforcing pads it is not required to strengthen the complete header pipe. Clause 304.3.3 of ASME B 31.3 provides equations to check if any welded piping branch connection requires reinforcement.
2. Support trunnions are provided with reinforcement when specified by piping stress engineers. When supports loads of trunnions are more than the bearing capability of the trunnion, reinforcing pads are welded at the parent pipe and trunnion junction to enhance its load-carrying capability.
However, please keep in mind that reinforcement on trunnions from elbows is not suggested as standard practice, So it should be avoided to the maximum extent possible. The requirement of reinforcement must be specifically mentioned in the piping isometric drawing for conveying to the construction team.
3. Equipment nozzle connections are normally reinforced so that nozzles can carry more loads and moments from the piping side.
Design Features of RF PAD
Normally, the maximum thickness that is used in engineering companies as reinforcing pad thickness is 1.5 times the parent pipe thickness. Standard practice is to use the same thickness as the parent pipe.
The reinforcement material must be compatible with the parent and branch pipe material. Normally same material as the header pipe is used as RF Pad.
Clause 328.5.4.g of ASME B31.3 mentions that Reinforcing pads and saddles shall have a good fit with the parts to which they are attached.
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 the branch and run and to allow venting during welding and heat treatment. Normally 2 number of vent holes are provided which must be sealed with mastic or silicone gel to restrict the water flow inside the RF Pad. Vent holes are also known as weep holes or Tell-tale holes. The normal size of the weep hole is 6 mm. If the reinforcement pad is made of multiple pipe cuts, then a vent hole should be provided in each cut piece.
A pad or saddle may be made in more than one piece if joints between pieces have strength equivalent to pad or saddle parent metal, and if each piece has a vent hole.
Repad Symbol | Reinforcing Pad symbol in Drawings
Fig. 2 shows the normal RF Pad symbol that is used in piping drawings.
The reinforcing pad is a ring cut from a steel plate that has a hole in the center equal to the outside diameter of the branch connection. It is slipped onto the branch pipe before the branch pipe is welded to the header. Once the branch has been welded to the header, the reinforcing pad is slid down the branch to cover the weld connection. The reinforcing pad is then welded to both the branch and the header.
Part of this article is prepared by Mr. Satish Atmanathan, a senior oil, and gas professional with extensive work experience. For a more detailed explanation of all the above parts and visualization, listen directly to him in the following video:
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