In the installation of thermal piping systems, it is common to hear that "the compensator needs to be pre-stretched" or "the corrugated compensator needs to be cold tightened". So, why does the compensator stretch? What are the consequences of not stretching? How to determine the amount of stretch? This paper will systematically explain the principle, function and operation method of compensator pre-stretching, and help engineers and technicians to correctly understand and execute this key process.
I. Basic concept of compensator pre-stretching
The question of why the compensator stretches, first of all, we need to understand the working state of the compensator in the pipeline system. The compensator is installed between two fixed brackets, and its core function is to absorb the thermal expansion and contraction of the pipe due to temperature changes.
When the pipe is elongated by heat, the compensator is compressed; When the pipe cools and contracts, the compensator is stretched. In the installed state, the compensator is in an initial position, and the selection of this position directly affects its working performance.
Pre-stretching (also called cold tightening) refers to the application of a deformation amount opposite to the direction of thermal expansion to the compensator in advance during pipeline installation, so that the compensator is in a pre-compressed or pre-stretched state when cold。 In this way, when the pipe is put into operation and subjected to thermal expansion, the amount of deformation of the compensator will be reduced, thus reducing its working stress.
Second, why does the compensator stretch? Three core roles
1. Reduce the stress on pipes and equipment supports
The primary reason why the compensator is stretched is to reduce the load-bearing capacity of the supports on the pipes and equipment。
When the corrugated compensator is deformed, an elastic reaction force is generated that is proportional to the amount of deformation-the greater the deformation, the greater the reaction force. This reaction force will act on the pipe holding bracket and the equipment interface. If pre-stretching is not carried out, the deformation of the compensator in the hot state is the largest, the reaction force is also the largest, and the load requirement on the support and equipment is the highest.
Core principle: The pre-stretching amount is generally 1/2 of the design compensation amount (the maximum thermal elongation of the pipe). In this way, when the pipe is heated to the rated temperature, the compensator is compressed from the pre-tension state to the intermediate position, the deformation amount is only half of the maximum deformation amount, and the generated reaction force is only half of the maximum value。
In short: pre-tensioning allows the compensator to be in a better stress range during operation, the fixed bracket can be designed to be more economical, and the equipment interface is less stressed.
| Installation Status | Compensator state | In-operation deformation | Thrust against bracket |
|---|---|---|---|
| Not pre-stretched | Natural state | Maximum (from 0 to + Δ L) | maximum |
| Pre-stretched 50% | Pre-stretched state | Halving (from +0.5 Δ L to-0.5 Δ L) | halve |
2. Extending the fatigue life of the compensator
Another important reason why the compensator stretches is to extend the fatigue life of the bellows.
The fatigue life of bellows is closely related to the stress amplitude when it works. The greater the stress amplitude, the greater the damage to the material per cycle, and the fewer cycles allowed. The fatigue life can be increased exponentially by reducing the working stress amplitude by half by pre-stretching。
According to industry experience, pre-stretching of compensator is a necessary process to increase the number of fatigue uses. Generally, the pre-stretching measure is one half of the compensation amount。 This is the core value of the compensator that needs to be pre-stretched.
3. Compensate for deviation of installation temperature from design zero
The design of the compensator is based on the "zero temperature" (i.e., the midpoint of the maximum and minimum temperature of the design). When the installation ambient temperature is not equal to the zero temperature, pre-stretching or pre-compression is required to compensate for this deviation。
Rules:
- When the installation ambient temperature is equal to zero temperature: No pre-tension or pre-pressing
- When installation ambient temperature is above zero temperature: shall be pre-compressed
- When the installation ambient temperature is below zero temperature: shall be pre-stretched
For pipes with medium temperatures above 400°C, the amount of stretch may be greater than Δ L/2; When the medium temperature is above 500°C, the amount of stretch may be Δ L。 The higher the temperature, the greater the thermal expansion of the pipe, and the more prominent the significance of pre-stretching.
3. Operation methods and key points of pre-stretching
1. Determination of pre-stretch amount
The understanding of why the compensator should be stretched should ultimately be implemented to the operational level. The amount of pre-stretching shall be determined in accordance with the design requirements and the ambient temperature at the time of installation, and the allowable deviation shall be ±10mm。
The amount of pre-stretching is not the larger the better, nor the smaller the better, but it is accurately calculated according to the actual working conditions of the pipeline. Within the allowable compensation range, pre-stretching will not affect the compensation amount of the compensator and its own life。
2. Construction method of pre-stretching
Typical construction steps for on-site pre-stretching are as follows：
Step 1: Leave the required clearance in the pipe for pre-stretching. Usually on the straight pipe sections at both ends of the compensator, 2-2.5m away from the compensator, a gap of Δ L/4 is left.
Step 2: Fixly connect one end of the compensator to the pipe (welded or flanged).
Step 3: Using machines such as pipe drawers, jacks or chain cranes, stretch the pipe in the opposite direction of thermal expansion until the reserved gap is filled.
Step 4: Maintaining the tensile state, welding and fixing the live port and the other end of the compensator.
Step 5: Loosen the stretching implement and check that the compensator is in the correct pre-stretched state.
3. Construction precautions
- Uniform force: during pre-stretching, the force should be gradually increased to ensure that the circumferential surface of each node of the bellows is uniformly stressed, and the deviation should be less than 5mm
- Concentricity: The waveform compensator must be concentric with the pipe and must not be skewed
- Temporary fixation: It should be fixed immediately after stretching in place to prevent rebound
- Safety protection: For large and medium-sized compensators, the nodes and joints should be protected from damage when using hoisting machinery
4. What are the consequences of not pre-stretching?
After understanding why the compensator is stretched, it is natural to ask: What happens if it is not pre-stretched?
- Excessive force on the bracket: the fixed bracket and the equipment interface bear the maximum thrust, which may cause deformation of the bracket and damage to the equipment interface
- Shortened fatigue life of compensator: Bellows are prone to early fatigue cracking when working at maximum stress amplitude
- Compensator pull-off risk: In extreme cases, if the fixing bracket is not properly set or the tie rod device is not adjusted in place, the compensator may be pulled into a straight cylinder, completely losing the compensation function
According to the failure analysis, the common causes of compensator pulling failure include: improper setting of fixed bracket, non-adjustment of tie rod device in place, restricted pipe shrinkage, improper design and use, etc. 。 And correct pre-stretching is an important measure to avoid these problems.
V. Pre-stretching requirements of different types of compensators
VI. Clarification of common misunderstandings
Myth 1: Pre-stretching can increase the amount of compensation
Positive solution: Pre-stretching can not increase the maximum compensation amount of the compensator, it only translates the available working area of the compensator, so that the working deformation amount in the hot state is reduced。
Myth#2: All compensators require pre-stretching
Positive solution: The requirements of pre-stretching depend on the type of compensator and installation conditions. When the installation ambient temperature is equal to the zero temperature, the waveform compensator may not be pre-stretched。
Myth 3: The larger the amount of pre-stretch, the better
Correct solution: The amount of pre-stretching must be accurately controlled according to the design requirements. Too large or too small may affect the compensation effect and even cause damage to the bellows.
Myth 4: The transport tie rod is not removed after installation
Correct solution: The compensator has a temporary fixed tie rod during transportation and installation, which must be removed after installation, otherwise the compensator cannot expand and contract freely, and will be pulled out in hot state。
sum up
The core answer to why the compensator stretches can be summarized as three points:
| effect | Principle | Effect |
|---|---|---|
| Reduce bracket load | Reduce the amount of thermal deformation from Δ L to Δ L/2 | Halving the thrust of the bracket, safer equipment interface |
| Extended fatigue life | Reducing the working stress amplitude of bellows | Exponentially increased fatigue life |
| Adapt to installation temperature difference | Compensation for deviation of installation temperature from zero temperature | Ensure that the compensator operates in the optimum range |
Operation Points:
- The amount of pre-stretching is generally 1/2 of the design compensation amount, and the allowable deviation is ±10mm
- Bear the force evenly and maintain concentricity during construction, and fix it immediately after stretching into place
- Installation ambient temperature should be pre-compressed when above zero and pre-stretched when below zero
- The transport tie rod and temporary fixtures must be removed after installation
A compensator that performs pre-stretching correctly can be in the optimum stress state during hot operation, which both reduces the load on the pipeline system and extends its own life. This is why pre-stretching is regarded as an indispensable and critical process in the thermal pipeline construction code.