In modern urban central heating system, the pipeline expansion joint of heating network is the key component to ensure the safe operation of pipeline network and absorb heat displacement. The heating pipeline transports high-temperature hot water or steam, the operating temperature is usually between 120-150℃, the design pressure can reach 2.5MPa, and the thermal elongation of a 100-meter-long carbon steel pipeline can reach about 140 mm when it rises from normal temperature to 120℃. If there is no expansion joint to absorb the heat displacement, the huge thermal stress will lead to pipeline twisting, weld cracking and compensator damage, which seriously threatens the heating safety。 This paper will systematically explain the professional technical knowledge of pipeline expansion joint of heating network from applicable standards, structure types, type selection calculation to installation and maintenance.
1. Applicable standards and scope of pipeline expansion joints in heating network
The design, manufacture and inspection of the expansion joints of the heating network pipes shall comply with the special standards for municipal heating pipes. According to CJ/T 402-2012 "Bellows Compensators for Urban Heating Pipelines" issued by the Ministry of Housing and Urban-Rural Development, this standard is applicable to the manufacture and inspection of bellows compensators for urban heating pipelines with design pressure less than or equal to 2.5MPa and design temperature less than or equal to 350℃。
Scope of application of standard:
- Design pressure: ≤2.5MPa (usually 0.6-1.6MPa)
- Design temperature: ≤350℃ (about 120-150℃ for hot water pipeline)
- Adapted pipe network: primary network, secondary network hot water pipe and steam pipe
2. Main types of pipeline expansion joints in heating network
1. Axial type expansion joint
Axial expansion joint is the most basic compensation form in thermal pipeline, which is mainly used to absorb the axial thermal displacement of pipeline. Common ones are:
- Internal pressure axial expansion joint: the inner cavity of the bellows bears the medium pressure, the structure is simple and the cost is low
- External pressure axial expansion joint: the outer cavity of bellows is subjected to medium pressure, which can provide greater compensation amount
2. Hinged corrugation compensator
The hinge type corrugated compensator in the expansion joint of heating network pipeline is specially used for the compensation of angular displacement. It is composed of bellows, end pipe, hinge assembly and flange. The device absorbs multi-plane angular displacement through the combination of universal ring and hinge plate, the compensation amount ranges from ±4 degrees to ±9 degrees, and can withstand 0.1-2.5MPa pressure。
Core advantage: The hinge assembly can withstand the thrust generated by internal pressure on the expansion joint and is suitable for thrust-sensitive equipment connections。
3. Universal hinge expansion joint
The universal hinge expansion joint has a universal ring and two pairs of mutually perpendicular hinge structures in its circumferential direction. The designed universal ring, hinge and pin can withstand the thrust formed by internal pressure on the expansion joint, so it can absorb the angular displacement of any plane。
Application scenarios: L-shaped pipe bend, Z-shaped horizontal pipe, heating pipe gallery with limited space.
4. Sleeve compensator (packing box type)
The sleeve compensator is composed of inner and outer sleeves capable of axial relative movement, and the inner and outer sleeves are sealed by packing box. It has the advantages of small friction resistance to fluid flow and compact structure; The disadvantage is that the sealing packing needs to be replaced regularly.
3. Failure Reasons and Prevention of Expansion Joints of Heating Network Pipelines
1. Environmental corrosion-a potential "invisible killer"
According to the failure analysis case, the stainless steel bellows of a heating pipeline suddenly leaked after only 13 years of operation, and its design pressure and temperature were lower than the design value (actual 0.8MPa vs design 1.6MPa, actual 120℃ vs design 150℃), and the material was also qualified。 Finally, it is found that the root cause of the failure is that the sewage containing high concentration of chloride ion (Cl⁻¹) is poured into the installation well in rain and snow weather, which makes the inner surface of the bellows in the alternating environment of soaking and dry and wet for a long time。
Failure mechanism:
- Pitting initiation → stress corrosion cracking propagation → corrosion fatigue accelerated penetration
- Chloride ion (Cl⁻¹) is the core inducer of pitting and stress corrosion cracking in stainless steel
Precautionary measures:
- Ensure that the installation environment of the expansion joint is kept dry and waterproof and drainage is done well
- The chloride ion content of water used for hydraulic test shall not exceed 25ppm
- After the test, drain the accumulated water in time and blow dry the inner surface of the bellows
- It is strictly prohibited to contain chloride ions in thermal
2. Improper design selection
When selecting the type of expansion joint of heating network pipeline, it should be comprehensively determined according to the pipeline diameter, compensation requirement and support condition. The arrangement rules of different types of expansion joints are different. The hinge rotation plane of hinge expansion joints should be consistent with the displacement rotation plane。
Precautionary measures:
- The complex pipe system is divided into "straight", "L" or "Z" pipe segments, and the compensation amount is calculated respectively
- Follow the principle of "one fixation, one compensation": an expansion joint is arranged between a fixed bracket
- Select the corresponding expansion joint structure according to the displacement type (axial, transverse, angular)
4. Selection and Calculation of Expansion Joint of Heating Network Pipeline
1. Calculation formula of thermal displacement
Δ L = α × L × Δ T
Among them:
- Δ L: thermal elongation (mm)
- α: line expansion coefficient of pipeline (carbon steel 12×10⁻⁶/℃)
- L: length of pipe section between two fixed brackets (m)
- Δ T: Difference between operating temperature and installation temperature (℃)
2. Rated compensation requirements
When selecting the model, ensure that the rated compensation amount of the expansion joint is ≥1.2× the calculated thermal displacement
3. Applicable scenarios of different types of expansion joints
| Pipe arrangement | Recommended expansion joint types | Compensation direction |
|---|---|---|
| Long straight pipe section | Axial expansion joint | axial |
| L-elbow | Hinged Expansion Joints (2) | Single plane angular direction |
| Z-pipe section | Universal Hinge Expansion Joints (2-3) | Multiplane angular direction |
| Underground pipeline | Welded expansion joint | axial |
| Thrust-sensitive equipment | Hinged/Large Tie Rod | Angular/transverse |
V. Installation specification of expansion joint of heating network pipeline
1. Check before installation
- Check whether the model, specification and pipe configuration of expansion joint meet the design requirements
- Confirm flow direction identification: Expansion joint with inner sleeve, the direction of inner sleeve is consistent with the direction of media flow
- Check whether the hinge rotation plane is consistent with the displacement rotation plane (hinge type)
2. Critical controls during installation
It is strictly prohibited to use expansion joint deformation to adjust pipeline deviation: it is strictly prohibited to use the method of deforming bellows to adjust pipeline installation overtolerance, so as not to affect the normal function of compensator。
Cold tightening/pre-stretching treatment: When "cold tightening" of the expansion joint is required, the auxiliary components used for pre-deformation should be removed after the pipeline is installed 。
Welding protection: No welding slag is allowed to splash onto the bellows surface during installation, and no other mechanical damage is allowed to the bellows。
Stent Configuration:
- Guide bracket: the first guide bracket is ≤4 times the tube diameter from the expansion joint
- Fixed bracket: Set at pipeline end point, branch point, valve, must bear blind plate force and elastic reaction force
3. Transportation Tie Rod Handling
After the pipe system is installed, the auxiliary positioning members and fasteners used for installation and transportation on the compensator should be removed as soon as possible, and the limiting device should be adjusted to the specified position according to the design requirements, so that the pipe system has sufficient compensating ability under environmental conditions。
This is a highly overlooked part of installation-if the transport tie rod is not removed, the expansion joint will lose its compensation ability.
Maintenance and Monitoring of Expansion Joints of Heating Network Pipelines
1. Regular inspections
| Check Items | cycle | Contents |
|---|---|---|
| Appearance inspection | Monthly | Whether there are cracks, corrosion and deformation on the bellows surface |
| Environmental inspection | Quarterly | Whether the installation well has accumulated water and environmental humidity |
| Stent inspection | per annum | Whether the fixed bracket and guide bracket are loose or not |
| Insulation inspection | Every overhaul | Whether the insulation layer is damaged or damp |
2. Intelligent monitoring technology
The field of heating pipeline has begun to apply intelligent monitoring technology to feedback the running status of expansion joints in real time through online condition monitoring system。 Some high-end expansion joints adopt double-layer bellows design, and an alarm device is set between the layers to alarm in time when the inner layer leaks to avoid sudden failure。
VII. Summary of failure prevention
The reliable operation of the expansion joint of the heating network pipeline needs to be controlled from the whole chain of design, selection, installation, operation and maintenance:
| Link | Core Essentials |
|---|---|
| Design selection | According to CJ/T 402-2012 standard, select type according to pipe diameter, temperature, pressure and compensation amount |
| Environmental control | Keep the installation environment dry and prevent sewage immersion; Hydraulic Test Control Cl⁻¹ ≤25ppm |
| Installation Specifications | Consistent flow direction, strictly prohibited deformation adjustment, removal of transport tie rod, reasonable configuration of guide bracket |
| Bracket matching | The fixed bracket shall be subjected to blind plate force, and the distance between the guide bracket and the expansion joint shall be ≤4D |
| Intelligent monitoring | Consider the double-layer belt alarm structure to realize early warning |
sum up
The pipe expansion joint of heating network is an indispensable flexible connection component in urban central heating system. The following core principles should be followed in the correct selection and specification installation:
- Standard first: Strictly implement CJ/T 402-2012 standard, design temperature ≤350℃, design pressure ≤2.5MPa
- Type matching: axial type for straight pipe section, hinge type for elbow (±4°-±9° angle compensation), universal hinge type for complex pipe system
- Environmental anti-corrosion: the surface of corrugated pipe is prohibited from contacting chloride ion sewage; Cl⁻¹ content ≤25ppm in hydraulic pressure test, drain the accumulated water and blow dry after pressure test
- Installation points: the flow direction identification is consistent with the flow direction of the medium; The hinge rotation plane is consistent with the displacement plane; It is strictly prohibited to use expansion joint deformation to adjust the installation deviation; Transport tie rod must be removed after installation
- Stent configuration: follow the principle of "one fixation and one compensation"; The distance between the guide bracket and the expansion joint is ≤4 times the pipe diameter
An expansion joint of heating network pipeline with reasonable design and standard installation can run stably for a long time under high temperature alternating working conditions, and provide reliable guarantee for urban heating safety. It is recommended to carry out a detailed pipeline stress analysis at the design stage and ensure that construction codes are strictly followed during installation.