• Author / Uploaded
• Yakulito

• Categories
• Fluencia (deformación)
• Estrés (Mecánica)
• Redox
• Carga estructural
• Deformación (ingeniería)

Citation preview

A literature on “Cold Springing (Cold Pull) In Piping Systems” Definition of Cold Spring: 1. Cold spring is the process of intentional deformation (usually accomplished by cutting short or long the pipe runs between two anchors) of piping during assembly to produce a desired initial displacement and stress. 2. Cold spring is the intentional stressing and elastic deformation of the piping system during the erection cycle to permit the system to attain more favourable reactions and stresses in the operating condition. Why Cold Spring? Cold Spring can help to – · · ·

Reduce the hot stresses to mitigate the creep damage. Reduce the hot reaction forces on connecting equipment. Control the movement space.

Actual use of Cold Spring – · · ·

The general belief is that the additional creep damage caused by the initial thermal expansion stress (due to cold pull) is insignificant if the total expansion stress range is checked within the allowable limit. Also code does not allow us to take advantage of cold spring in reducing stresses. The control of the movement space is secondary. Thus the real gain of cold spring has become the reduction of the hot reaction on the connected equipment.

System without Cold Spring Vs System with Cold Spring:

Fig. 1: System without Cold Spring Vs System with Cold Spring

End Reaction at Design Temperature (as per B31.3):

Fig. 2: End Reaction at Design Temperature (as per B31.3) End Reaction at Installation Temperature (as per B31.3):

Fig. 3: End Reaction at Installation Temperature (as per B31.3) Cold Spring as per B31.3: 1. Use of the above equations is valid for a two-anchor piping system without intermediate restraints 2. For multi-anchor piping systems and for two-anchor systems with intermediate restraints above equations (21 and 22 as per B31.3) are not applicable. 3. Each case must be studied to estimate location, nature, extent of local overstrain, and its effect on stress distribution and reactions. 4. Code allows us to apply cold spring credit in calculating the thrusts and moments where actual reactions as well as their range of variations are significant. 5. Code restricts us form applying cold spring credit in stress range calculations. This is because, the piping system is affected more by the range of stress variation than by the magnitude of stress at given time. 6. Where cold spring is used in the piping system, experience has shown that it cannot be fully assured. Therefore, the reactions shall be computed for two cases. First, with the assumption that only two-thirds of the design cold spring is present, and second, with the assumption that fourthirds of design cold spring is present. That effectively suggests us to calculate and compare the end reactions from couple of Caesar runs. First being one with only two-thirds of the cold spring and another using four-thirds. The results in both cases should be satisfactory. This is especially valid for computer analysis.

Both the sustained loads and the operating loads should be within manufactures allowable for the particular piece of equipment. Cold Spring factor (C): Following procedure explains how to get cold spring factor – 1. Understand the system and decide in which direction application of cold spring will give us maximum benefit. 2. Get the maximum displacement corresponding to the direction in which cold spring is planned. This can be found out from displacement report using Caesar II. 3. The factor is then decided by the amount of cold spring applied to the system with respect to the expansion. 4. A 100 percent cold sprung system will have zero load in expansion case. For Example:- If we have 80mm displacement during hot condition in say Y direction and we decide to give 40 mm of cold spring then it shall be a 50% cold sprung system. Hence cold spring factor will be 0.5.

How Cold Spring is applied on Site: 1. System is fabricated short as specified by stress engineer. 2. System is erected with a gap at some final closure weld, equal to the “cut shorts” in specified direction. 3. Forces and moments are then applied to one or both ends, as necessary, to bring the final joint into alignment. 4. Pipe displacement can also be achieved by displacing supports in the vicinity of cold spring using hoists or adjustable supports. Stress engineer shall provide the movement of each of the restraint as required. This must appear on isometric or support drawings. · ·

One of the ways of deciding the restraint displacement is, using Caesar displacement report. The other is using the following formula :- D = Dc + (C x Dh)

Where, D = restraint disp. to be adjusted during cold springing. Dc = Cold spring displacement without the restraint. C = Cold Spring Factor. Dh = Hot displacement without the gap and the restraint. 5. Anchors are provided to preserve alignment during welding, post weld heat treatment and final examination. 6. Restraints are then removed, the resulting reactions are absorbed by the terminal points and the line is in a state of stress. 7. During start-up the line expands and the levels of stress and terminal reactions will decrease as envisaged Few Important Points related to Cold Spring: 1. Use of cold spring shall be avoided. 2. Cold springing shall only be used to limit the magnitude of forces and moments on the nozzle and not to reduce stresses.

3. Use of cold spring in the piping connected to rotating equipments is prohibited. 4. Cold spring shall not be used in the vicinity of the nozzle. 5. Cold springing shall be limited to line design temperature of 300 0C maximum. Drawbacks: The effectiveness of cold spring is generally considered to be questionable. It should not be used indiscriminately. Although it may provide an easy way out for an analyst to solve an equipment load problem, there are a number of considerations while adopting cold spring in a piping system. Following points shall shed light on these factors:1. Extra anchor points, to hold the cold sprung line in position till start up, are required. Also extra hoists are required to move supports in desired direction during installation. These all arrangements make cold springing a costly act. 2. Cold spring applied to low temperature lines does not provide great benefit as it does in high temperature lines. 3. Theoretical cold spring gap, which can be very helpful in low temperature lines, is very difficult to measure and control in the field. 4. Due to small displacements, the effectiveness of the cold spring is unpredictable and might even produce a load which is damagingly high to the equipment. 5. After the plant has been operated, deliberately installed cold spring can be misunderstood to be piping misalignment when disconnected during shut down and “corrected”. 6. During repair, caution should be exercised when cutting into such lines as the line will be in a state of stress when cold. Proper anchoring on both sides of cut have to be planned to prevent possible accidents. 7. Installation procedure for every type of support adjacent to cold spring is different. Excessive care should be taken during stress analysis about knowing exact restraint displacements to achieve cold spring. Same care has to be replicated at site during construction. Any erroneous interpretation can be a starting point for disastrous situation. 8. Most of the times, the support adjustment/displacement can extend to few more supports than just one adjacent support. The same has to be considered and understood both at analysis and construction stage. 9. Special support drawing showing necessary arrangements to achieve the cold springing during erection (with the help of nearest structural member) shall be made and the loads on the respective members shall be informed to civil through piping load data. 10. It is required to write installation instructions of cold springing on the special support drawing and same shall be supervised at site.