Pipe Support – Small Bore Connection

A challenging task regarding pipe support design is supporting of small bore connections (SBC) with a lump mass at the end (e.g. pipe vent/drain, instruments connection etc.). With this article, Pipe Support Verification’s aim is to give the basic thoughts of doing a good SBC support design.

We can divide the supporting of SBC in two categories:
Static effects – meaning the lump mass needs supporting because of large static loads.
Dynamic effects – meaning vibrations in the SBC caused by flow induced vibration (FIV), acoustic induced vibration (AIV) or equipment induced vibration (EIV).

The most challenging is the supporting due to dynamic effects. To find the SBC that require bracing, we need to evaluate where it is likely that vibrations can occur. Regarding FIV, the piping stress group usually do a FIV screening to find the LOF parameter (likelihood of failure) for the pipe system. If this parameter is above the limit set for the project, bracing should be provided for all SBC in that system. A typical limit can be a LOF>0,5 for medium stiff system. In addition to FIV we need to evaluate other sources of vibrations such as AIV (typ. flare system) or EIV (Equipment that causes vibrations in connected pipe systems). This to find which SBC that need bracing.

When the SBC that need bracing has been concluded, the next question is how? There are several types of bracing. Clamped, Welded sections, welded plates (knee-braces) etc. How they are attached to the SBC does also vary.

Pipe Support Verification’s opinion is that welded bracing connected directly to the lump mass (flange, valve etc.) is the best solution. Connecting the bracing to the small bore pipe of the SBC will usually not give the required protection against rupture. Possible rupture might then occur between the small bore pipe and the flange/valve rather than between the small bore pipe and the header pipe (see fig. 1)

Fig. 1: SBC rupture between small bore pipe and SBC flange

Welded bracing using e.g. angle sections directly from the header pipe to the flange of the SBC (see fig. 2), gives a good support that also prevent rupture in the SBC/header in a good way.
Important: This method requires that the material of the brace (angle) is made of the same (or compatible) material as the pipe/SBC.
This type also requires that the bracing is welded on during prefabrication of the pipe.

Fig. 2: Welded bracing using angle section connected to the SBC flange

Fig 2. shows a welded bracing on a 3″ header. If the header is of larger size (e.g. 12″), a bracing in the transverse direction should be provided.

Pipe Support Verification thinks this is the most effective way to protect a SBC from rupture caused by dynamic or static effects. However there may be situations where such a design is not possible to achieve (e.g. after installation start-up). Then clamped or bolted solutions should be evaluated.

If a bolted solution using the flange bolts is selected, it should be fastened on the outside of the flange nut. A longer flange bolt with an extra nut may be required. This to be sure that the bracing do not affect the flange bolt tensioning (see fig. 3)

Fig. 3: SBC bracing using flange bolts

If a clamped solution is selected, it is recommended to clamp the SBC flange rather than the SBC pipe (see fig. 4). This to avoid the rupture shown in fig. 1

Fig. 4: Clamped SBC bracing using SBC flange

This should cover the basic points that needs evaluation to get a good SBC support design. There are of course other topics and special cases that need attention (e.g. fully welded knee-brace plates on gas export systems etc.), but a large part of the SBC should be covered with the evaluations in this article.

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