Sway Brace Assemblies - CAESAR II - Reference Data

CAESAR II Applications Guide

Reference Data

The sway brace is commonly used to allow unrestrained thermal movements while tuning the system dynamically to eliminate vibration. A sway brace resembles a spring in that it can be pre-loaded in the cold (installed) position. After pipe thermal growth, it reaches the neutral position and the load on the system in the operating condition is zero or negligible.

The sway brace is composed of a single compression spring enclosed between two movable plates. The spring is precompressed a full inch, which provides an initial force that instantaneously opposes vibration. Any movement from the sway brace neutral position is opposed by a load equal to the pre-load plus travel from the neutral position multiplied by the sway brace spring constant. After maximum allowed travel (usually 3-inches in either direction) is reached, the sway brace locks, preventing additional movement.

Manufacturers typically recommend a specific size sway brace for a given pipe nominal diameter.

A more specific sway brace selection is possible when the exact restraining force required to control the piping vibration is known. The energy necessary to control the piping is proportional to the mass, amplitude of movement, and the force causing the vibration. From this relation, the exact restraining force required to control the piping vibration can be calculated and an appropriate sway brace size selected.

After it is selected, the sway brace can be modeled in CAESAR II using a combination of a bi-linear restraint and a translational restraint:

In the event that the sway brace is to be installed in the operating condition (or the neutral position is to be adjusted in the operating position), the modeling in CAESAR II is a little more complex. In this case, before modeling the sway brace, you must analyze the piping system without the sway brace to obtain displacements from the cold to neutral operating position:

Run an analysis on the system without the sway brace to obtain the displacements from cold to operating condition. In the example illustrated below, the assumed CAESAR II-calculated displacement from cold to operating position is 0.5 inches

Sway Brace Installed in Sustained Condition

In the SUS case, the displacement D2 (vector 2) represents the pre-load in cold position. Under shutdown conditions, the pipe returns to its cold position and the brace exerts a force as previously described.

Sustained case restraint loads on sway brace = Pre-Load + Hot Deflection * Spring Rate

Sway Brace Installed in Operating Condition

In OPE, the displacement allows thermal expansion, and the sway assumes neutral position exerting zero or negligible load on the pipe.

Operating case restraint loads on sway brace =~ 0.0 (does not restrain thermal expansion)

Sway brace opposing compression force (movement occurs after pre-load is overcome).

  • Spring Rate: 150 lb../in.

  • Initial Loading: 150 lb.

  • Allowed Movement: 3 in.

  • Calculated Displacement: .5 in.

Be sure to include D2 in the sustained and operating cases.