In the installation of flue system, the pre-stretching (also called pre-deformation or cold tightening) of expansion joint is a key process that directly affects the operation safety and service life of pipe. However, many field technicians lack a clear understanding of how to determine and set the pre-tension value of the flue expansion joint. If the pre-tension value is too large, the bellows may be damaged by excessive tension, while if the pre-tension value is too small, the due compensation effect will not be achieved. This paper will systematically expound the definition, calculation method, setting standard and operation key points of pre-tension value of flue expansion joint, and provide professional guidance for engineering design and installation personnel.

I. What is pre-stretching of flue expansion joint?

Before answering how to set the "pre-tension value of flue expansion joint", it is first necessary to understand the concept and purpose of pre-tension.

Pre-stretching refers to the operation of pre-stretching the expansion joint by a certain length in the cold state according to the temperature difference between the working temperature and the installation temperature of the pipeline system when installing the expansion joint, so that it can be in the best compensation position in the hot state。

The main functions of pre-stretching include:

1. Reduce the amount of deformation of bellows during operation: make the expansion joint avoid extreme tension or compression position under hot state, and prolong its fatigue life
2. Reduce the thermal stress of pipeline system: Rationally distribute the thermal displacement and reduce the stress burden of fixed support
3. Improve the compensation efficiency: make the expansion joint work effectively within the set compensation interval

Simply put, pre-stretching is equivalent to reserving a "buffer space" for the expansion joint so that it will not be damaged by excessive stretching or failed by excessive compression during thermal expansion of the flue。

2. Calculation method of pre-tension value of flue expansion joint

The pre-tension value of flue expansion joint needs to be determined by engineering calculation, rather than arbitrarily set by experience. Here is the standard calculation:

1. Basic formula

The calculation of the amount of pre-tension (or pre-compression) is usually related to the operating temperature and installation temperature of the pipe. For expansion joints that require full compensation of axial thermal displacement, the amount of pre-tension can be determined according to the following principles:

When the installation temperature is lower than the operating temperature (most common operating conditions):

• The pipe is elongated after being heated, and the expansion joint shall be pre-stretched
• It is recommended that the amount of pre-stretching be 50% of the total axial compensation amount

When the installation temperature is higher than the operating temperature:

• The pipe is shortened after cooling, and the expansion joint should be pre-compressed
• The amount of pre-compression is also 50% of the total amount of compensation in the axial direction

2. According to the provisions of different standards

Different standards and specifications have different requirements for setting pre-pull values:

According to the Metal Pipe Construction Code:

Before installation, the bellows expansion joint shall be pre-stretched or pre-compressed according to the design documents, and the force shall be uniform

As per Rectangular Flue Compensator Standard:
Taking the standard half-high expansion joint as an example, the basic expansion amount of a single wave is ±11mm without cold drawing; Considering cold drawing, the allowable amount per wave can be expanded up to 2 times (i.e. ± 22mm)

This means that the compensation capacity of the expansion joint can be approximately doubled by pre-stretching (cold drawing).

3. Calculation Example

Assume that the total axial thermal displacement to be compensated for a flue system is 50mm:

It can be seen that reasonable pre-stretching can effectively reduce the wave number of expansion joint and reduce the engineering cost。

3. Setting standard of pre-tension value of flue expansion joint

In addition to calculating according to the 50% principle, the pre-tension value of flue expansion joint must meet the following standard requirements:

1. Allowable deviation range

According to the Construction Code of Industrial Metal Pipeline Engineering GB50235-2010:

The allowable deviation of pre-tension or compression is ±10mm

This deviation requirement applies to most industrial flue systems and should be strictly controlled during installation.

2. Recommended values by working conditions

3. Special case: 100% pre-bias of SCR system

In the SCR denitrification flue system, in order to ensure the uniformity of flue gas flow field and the use effect of ammonia injection grid, there are special requirements for the installation of expansion joints:

The expansion joint shall be 100% pre-offset during installation, that is, the flue on both sides of the expansion joint shall be offset during installation, and the axis of the flue on both sides shall be on a straight line during operation

This 100% pre-bias is a special pre-deformation method, which is different from the conventional 50% principle and needs to be implemented according to the design documents.

4. Operation methods and key points of pre-stretching

After mastering the pre-tension value of the flue expansion joint, the correct operation method is equally important.

1. Operation Procedures

1. Confirmation before installation: Check the pre-pull value specified in the design document
2. Temporary constraint: the expansion joint is temporarily fixed in the pre-tensioned state with the transport tie rod
3. Pipe in place: Connect the pipe to both ends of the expansion joint
4. Fixed bracket installation: After the fixed brackets at both ends are firmly installed
5. Release temporary constraint: After pre-tensioning is completed, remove the transport rod or loosen the set nut
6. Check and confirm: Confirm that the expansion joint is in pre-tensioning working condition

2. Operation precautions

• It is strictly prohibited to adjust the installation deviation by bellows deformation: the installation error of pipeline shall not be adjusted by means of compensating for pipeline deformation or expansion joint
• The force should be uniform: the force should be applied symmetrically and uniformly during pre-stretching to avoid unilateral force
• Irreversible after pre-stretching: Once pre-stretching is completed and installed and fixed, do not operate in reverse direction
• Record pre-tension value: The actual pre-tension amount should be indicated in the installation record for subsequent maintenance

3. Pre-tension operation of non-metallic expansion joint

For non-metallic expansion joints, if pre-tensioning or pre-compression is required, it can be operated by adjusting the nut on the transport rod。 For expansion joints that need to be transversely pre-biased, after one end is fixed, hoist the other end, loosen the positioning nut on the transport rod, and adjust the interface before fixing。

V. Clarification of frequently asked questions and misunderstandings

Myth 1: The larger the pre-stretch value, the better

Correct solution: Too high pre-stretching will cause the bellows to be in an over-stretched state when it is cold, which will accelerate fatigue failure. It shall be strictly controlled according to the design calculation value, and the allowable deviation shall be ±10mm。

Myth#2: All expansion joints need to be pre-stretched

Correct solution: When the installation temperature is close to the working temperature (temperature difference ≤50℃), pre-stretching is not necessary. The design documents will specify whether the amount of pre-tension and pre-deformation is required 。

Myth 3: The temporary restraint can not be removed after pre-stretching

Positive solution: The temporary restraint device after pre-stretching must be removed after the pipe is installed and fixed, otherwise the expansion joint will not work properly。

Question: Pre-tensioning vs. transport tie rods

The transport tie rod is a temporary component used to protect the expansion joint at the factory. For general-purpose expansion joints, the transport tie rod is usually removed after installation; For expansion joints requiring pre-stretching, the transport tie rod can only be removed after pre-stretching is completed。

VI. SUMMARY

The pre-tension value of flue expansion joint is a key technical parameter in the installation of flue system, which directly relates to the service life of expansion joint and the safe operation of pipeline. The core conclusions can be summarized as follows:

1. The pre-tension value is calculated according to the 50% principle: normally, the pre-tension (pre-compression) is measured as 50% of the total axial compensation. Pre-stretch when installation temperature is lower than operating temperature, vice versa pre-compress
2. Allowable deviation ±10mm: The deviation between the actual value and the design value of the pre-stretching operation shall not exceed 10mm, which shall be strictly controlled
3. Cold drawing can improve the compensation capacity: through pre-stretching, the compensation amount per wave of the expansion joint can be expanded by about 2 times, effectively reducing the wave number and cost
4. SCR flue must be 100% pre-deviated: special working conditions (such as SCR desulfurization system) have special requirements for pre-deformation, which should be strictly implemented according to the design documents
5. The operating specification should not be ignored: the design value should be confirmed before pre-stretching, the force should be uniform during operation, the temporary restraint device should be removed after installation and fixation, and the expansion joint should be strictly prohibited to compensate for pipeline installation deviation

For engineers and technicians, the importance of pre-tension value calculation and setting should be fully understood, and a clear pre-tension value should be determined according to the design documents before installation. During the construction process, the operation should be strictly followed by the specifications, and records and acceptance should be made. Correct pre-stretching operation is one of the key links to ensure the long-term stable operation of flue expansion joint.