Support and Considerations for Small-bore Piping

Small-bore piping is defined as pipes with a diameter of 2 inches or less. It is a vital component of many piping systems and plays an important role in the efficient functioning of these systems. However, due to its small size, small-bore piping often receives less attention and support than it needs. This article provides an overview of the considerations and guidelines for supporting and designing small-bore piping. Followings are outlines of this article:

Definition and common sizes of small bore piping.
Guidelines for supporting and designing small bore piping.
Considerations for small bore pipe connections connected to high-temperature large-bore pipes.

What is Small-bore Piping?

Small-bore piping is defined as pipes that are 2 inches or less in size. In the piping industry, pipes are categorized into two groups; small-bore pipes and large-bore pipes. Any pipe having a size of more than 2 inches is termed a large bore pipe. While piping systems carrying pipes of two inches or less are considered small-bore piping systems. Some organizations consider a 2″ pipe size as large-bore.

Common Small-bore Pipe Sizes

The usual pipe diameters that are frequently used in small bore pipe connections are:

1/8″
1/4″
3/8″
1/2″
3/4″
1″
1-1/4″
1-1/2″ and 2″

Most of the time these lines are instrument connections, drain connections, sampling connections, or vent connections. Almost all large bore piping systems will contain some small bore pipe connections. In general, small-bore piping is considered non-critical and usually does not require detailed pipe stress analysis. Small-bore pipe sizes usually branch off from the large-bore main pipes.

Small-bore Piping Supports

Small-bore pipes are generally site-supported. In general, they are supported by taking support from nearby large-bore lines. Clamp supports are the most frequently and widely used small-bore piping supports. In absence of nearby parallel large-bore piping, simple steel members are used for support. As small bore pipes do not impose large support loads, the supports are not designed specifically. The following guidelines can be applied for supporting small-bore piping systems:

Overhang and unsupported spans for small-bore pipe connections need to be minimized.
Support shall be provided near the mass and stress concentration points to reduce the fatigue failure potential.
Supporting from the main pipe is preferred such that the small-bore connection moves along with the parent pipe during start-up, shutdown, thermal transients, etc.
Neoprene pad, PTFE pad, etc can be used to dampen vibration when supporting vibration-prone lines. • Large vertical runs should be avoided and a guide needs to be provided to reduce vibration tendency.
The bracing connections if provided between the small bore pipework and the parent line should be in two planes. Also, the bracing shall never be taken from local structures.
If the geometry of small bore piping is difficult to support, it must be re-routed to provide support easily.

Small-bore Pipes Connected to High-Temperature Large-bore Pipes

Small-bore pipe connections that are connected to high-temperature large-bore main pipes must be stress analyzed. For example, small-bore drain piping from high-pressure steam lines needs to be stress analyzed to provide proper flexibility that will be arising from the high temperature of the main pipe. Enough flexibility needs to be provided so that the branch connection interface does not fail due to excessive stress generation.

Vibration in Small-bore Pipe Connections

Due to their geometry and mass, small-bore piping connections are prone to vibration. Even a vibration of a very low amplitude vibration on the main piping can lead to excessive vibration in the small-bore branch connection and eventually may break due to fatigue failure.

Design Considerations for Small-bore Pipe Connections

Considering the flexibility and vibration potential of small-bore pipe connections, their sizes are sometimes limited during application as branch connections. The following guidelines can be used in general:

Process team to review if unnecessary small-piping can be avoided in the plant.
As the failure tendency for small bore piping increases with an increase in cantilever arm length, small bore piping must be routed to have an as small arm as possible.
Larger piping is preferable for mechanical strength consideration instead of using support bracing for small bore piping.
For direct mounted instrument connections, the minimum size of branch connections to run pipes shall be 3/4″ from 3/4″ through 2″ run pipe, 1″ from NPS 3 through 12, and NPS 1-1/2 for run pipes greater than NPS 12.
The location of small bore tappings shall be decided as close to rigid supports on the main pipe as possible.
To reduce vibration tendency and increase the mechanical strength of instrument connections bracing can be provided to the run pipe.
Non-direct mounted instrument connections from small-bore piping need to be minimized as possible. • Pipes with long branch connections are required to be braced.
Small bore pipe connections shall be avoided in piping directly connected to compressors, nozzles, or pulsation bottles.
The small bore piping connected to the flare tip must be designed to withstand the flare radiation temperature and flare tip thermal movement.
While providing stiffeners in small bore piping the differential expansion must be checked. For example, while supporting small bore piping that is connected with pipes or equipment with large thermal movement the differential thermal movement and stress generated must be evaluated.
Sufficient rigidity must be provided for small bore piping systems which are prone to vibration.
It is preferable to avoid socket welded fittings for small bore connections as those weld points create a weak link.
After installation of the plant, a site review must be planned to check all small bore pipe connections to find any problems with support.

Small-bore piping is a crucial component of many piping systems and needs to be supported and designed properly to ensure the efficient functioning of these systems. Adequate attention needs to be given to the design and support of small-bore piping systems to avoid any unnecessary failures. The guidelines and considerations mentioned in this article should help ensure that small-bore piping is designed and supported properly.