Cold-springing is a fabrication of piping to an actual length shorter than its nominal length so that it is stressed in the installed condition, thus compensating partially for the effects produced by expansion due to an increase in temperature. In using the cold springing, it is necessary to take into account the following circumstances:
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cold spring reduces the loads on equipment in the hot state and increases them in the cold state;
during installation of the piping system it is difficult to provide the required tolerance of the cold spring and further, in the course of operation of the piping system, it is impossible to control it. Taking into account this factor, a number of international stress codes for piping systems (ASME BPVC NB 3600, ASME B31.3), require to perform allowance for only 2/3 of the cold spring design value in determination of loads on supports and equipment in the working (hot) state and the full value of the cold spring in determination of loads in the cold state;
the cold springing does not affect the stress range between the hot and cold states, and, therefore, it shall not be taken into account for computation of the corresponding stress categories;
the relaxation of stresses due to thermal expansion in high-temperature piping systems leads to the cold springing of the piping system in the cold state irrespective of the use of the cold spring. The effect of use of the cold spring shows itself only at the initial stage of the piping system operation.
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. A cut short describes an intentional gap in the pipe assembly requiring an initial tensile load to close the final joint. A cut long 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 and, being used in systems that operate in the creep range, their 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 you only use two-thirds of the specified cold spring for equipment load calculations.
A piping system may be cold spring or prestressed to reduce anchor forces and moments. Cold spring may be cut short for hot piping and cut long for cryogenic piping. The CDt short is accomplished by shortening overall length of pipe by desired amount but not exceeding the calculated expansion. Cut long is done by inserting a length (making a length longer than required).
The amount of cold spring is expressed as percentage of thermal expansion. Credit for cold spring is no.t allowed for stress calculation. Different codes state the same.
- Sets forth the engineering requirements deemed necessary for safe design and construction of pressure piping
- Safety is the main consideration
- The above alone will not govern the final specification for any piping installation.
- Code is not a designs hand book.
- It does not do away with the need of designer or competent engineering judgment.
· basic design principles
· formulas
· supplemented by specific requirements to assure uniform
application of principles and guide selection of piping materials code prohibits designs and practices known as unsafe and contains warnings where caution, but not prohibition is warented.
Code Section Includes
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(a) Reference to acceptable material spec. and compo. std including dimensional std 8 pr./ temp rating.
(b) Requirement of or design of component, assemblies, supports
(c) Requirement and data for evaluation and limitation of stresses, reactions and movements. Dug to pressure changes, temp. changes and other forces.
(d) Guidance and limitation on selection of materials
(e) Req. of fabrication, assemblies, and erection of piping
(f) Requirement for examination, inspection and testing. of piping.
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 effect of the cold spring.
Verify that the cold spring value/tolerance is much greater than fabrication tolerances.
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.
Other Applications for Cold Spring
While often used to reduce the magnitude of loads on equipment and restraints (see below), you can also use cold spring to accelerate the thermal shakedown of the system in fewer operating cycles.
http://www.pipestress.com/technical-papers/ColdSpring.pdf
Cold springing of axial expansion joints is made to increase the allowable axial displacement. It doesn't affect anchor loads and stresses much because the expansion joint is much more flexible than piping.
To model the expansion axial joint cold springing, insert the node near the expansion joint and add the prestretch deformation into this node.
Below is shown the result of example model analysis without cold spring.