Piping support stiffness

The calculated static and dynamic forces and moments of a piping system depend essentially on the stiffness of the fixed-point and support constructions. The pumps, valves, tanks, support constructions and other components usually are not modeled for piping calculations. Their elastic properties will be summarized in resultant stiffness matrices or spring rates which will be introduced into the calculation model at the respective connection points, either as fixed points or spring elements.

In the most of the cases, the exact stiffness matrices of the fixed points and supports are not available when the piping systems are to be calculated, because the design of these structures depends reasonably on the loads which, in turn, come from the piping systems. Therefore, one is forced to assume approximated values for the calculation.

https://inis.iaea.org/search/searchsinglerecord.aspx?recordsFor=SingleRecord&RN=14772430#

An industry-wide problem exists due to incorrect assumptions about pipe support stiffness. The current practice by many engineering firms is to assume a pipe support is rigid (e.g., 1E12 lb/in). In reality, the clamp and support system has some flexibility (e.g., 1E5 - 1E6 lb/in). This means the piping engineer’s assumptions can be over 1,000,000 times stiffer than the actual installation.

Inaccurate stiffness assumptions create (expensive) consequences, such as:

Higher pipe natural frequencies than would be measured in the field, which may lead to high pipe vibration.
Thermal loops that may not be required
Removal of pipe clamps (to solve thermal problems), which can result in high pipe vibrations.
Failed pipe clamps and equipment nozzles due to higher than expected pipe deflections and loads.

This problem exists because of these two issues:

There is a lack of understanding and information on this issue; and
The engineering firms do not have the experience/capability to determine the actual stiffness values. For example, specialized finite element analysis (FEA) software is required (e.g., ANSYS) as well as an understanding of how to properly model the stiffness.

GMPC

http://www.betamachinery.com/wiki/pipe-support-stiffness

The stiffness of pipe supports in the direction of applied restraint should be included in the pipe stress analysis model unless the supports can be qualified as rigid according to the following criteria:

a) 12” and larger pipe: minimum support stiffness of 100 Kips/inch in the direction of restraint;

b) 3” to 12” pipe: minimum support stiffness of 10 Kips/inch in the direction of restraint; c) 2” and smaller pipe: minimum support stiffness established by technical judgment.