Cold Spring - CAESAR II - Help

Cold spring is a method where you introduce pipe strain in the installed state to modify the resulting strain in the operating state. Adding this preload is commonly used to adjust (reduce) equipment load in the operating state. In CAESAR II, you can indicate a cold spring into your static load cases by using the CS variable in the load case definition.

A cut short (also called a cold pull) describes an intentional gap in the pipe assembly requiring an initial tensile load to close the final joint. A cut long (also called a cold push) describes an intentional overlap in the pipe assembly requiring an initial compressive load to close the final joint. This initial gap or overlap is modeled as a cut short material or a cut long material, respectively. CAESAR II reduces the cut short to zero length and doubles the cut long in any load case that includes the CS load in the load case definition.

This initial cold pull is difficult to implement with any accuracy—certainly in systems operating in the creep range where long term effect is difficult to control or even predict. Due to the difficulty of properly installing a cold spring system, most piping codes recommend that, when evaluating equipment loads, you model only two-thirds of the design cold spring for equipment load calculations. B31.3 also places an upper bound of four-thirds of the design cold spring for equipment load evaluation.

In simple, linear systems without intermediate restraints, you can calculate the cold spring element length (ignoring equipment growth) by using the following equation:

Ci = xLi a dT

Where:

Ci = length of cold spring in direction i; where i is X, Y, or Z (inches)

Li = total length of pipe subject to expansion in direction i (inches)

a = mean thermal expansion coefficient of material between ambient and operating temperature (in/in/°F)

dT = change in temperature (°F)

x = percent cold spring

When x = 0%, there is no cold spring and there will be no reduction in the thermal strain found in the operating load. When x = 100%, the operating load will have no thermal strain as all the expected pipe strain will be realized in the installed state of the piping system. If x = 50%, the pipe strain will be shared equally by both the installed load and operating load. This percent cold spring (x) is not the same term as the two-thirds check mentioned above.

No credit can be taken for cold spring in the stress calculations, because the expansion stress provisions of the piping codes require the evaluation of the stress range, which is unaffected by cold spring, except perhaps in the presence of non-linear boundary conditions, as discussed below. The cold spring adjusts installed and operating loads and the stress mean, but not the stress range used in most expansion stress calculations.

Cold Spring Considerations

You must consider several factors when using cold spring:

• Verify that the cold reactions on equipment nozzles due to cold spring do not exceed nozzle allowables.

• Verify that the expansion stress range does not include the direct effect of the cold spring (in other words, do not calculate the expansion stress range as the difference between the operating state with cold spring and the installed state without cold spring).

• Verify that the cold spring value/tolerance is much greater than fabrication tolerances. This is related to the two-third and four-third checks mentioned previously.

• For elevated temperature cases, where cold spring is used to reduce operating equipment load, using the hot modulus of analysis may also have a significant effect on the load magnitude.

Remember, however, that the software does not consider the hot modulus in the stress calculations for expansion stress ranges. These additional load cases, which you can use to evaluate equipment load (not system stress), can include the modulus of elasticity for the temperature under consideration.

Modeling design cold springs

Specify the cold gaps or overlaps as elements defined as cut short or cut long materials (CAESAR II materials 18 and 19, respectively). There are two approaches for this:

1. Model the whole length of the design cold spring.

   1. Reset the material property on the element following the cold spring element.

   2. Model the whole length of the design cold spring length for the cold spring element.

   3. Analyze the cold spring system by running the following load cases:

      Load Case 1 (OPE)	W+T1+P1+CS includes all of the design cold spring
      Load Case 2 (OPE)	W+P1+CS includes all of the design cold spring but not the temperature.
      Load Case 3 (SUS)	W+P1 standard sustained case for code stress check
      Load Case4 (EXP)	L1-L2 expansion case for code stress check.

   4. To check equipment operating loads considering actual cold spring variation (both two-thirds and four-thirds checks here), use the following load cases. You can define additional load cases for installed load variations.

      Load Case 1 (OPE)	W+T1+P1+CS includes all of the design cold spring
      Load Case 2 (OPE)	W+P1+CS includes all of the design cold spring but not the temperature.
      Load Case 3 (SUS)	W+P1 standard sustained case for code stress check
      Load Case4 (EXP)	W+T1+P1+0.66 CS (use hot modulus)
      Load Case 5 (OPE)	W+T1+P1+1.33 CS (use hot modulus)
      Load Case 6 (EXP)	L1-L2 expansion case for code stress check.

2. Model 2/3 of the design cold spring.

As a change from the previous design, model two-thirds of the design cold spring length, and use the following load cases:

Other Applications for Cold Spring

While often used to reduce the magnitude of loads on equipment and restraints (see above), you can also use cold spring to accelerate the thermal shakedown of the system in fewer operating cycles.