In coal-fired power plant and industrial flue gas desulfurization system, flue expansion joint is the key flexible compensation element connecting absorption tower, flue and fan. Because desulfurization flue gas has the characteristics of low temperature, high humidity and strong corrosion, the performance requirement of expansion joint is much higher than that of ordinary flue. Compiling a complete and rigorous technical specification for desulfurization flue expansion joint is not only the basis for ensuring the long-term stable operation of equipment, but also the core basis for bidding and project acceptance. This paper will systematically explain the main points of the compilation of technical specifications for desulfurization flue expansion joints from the scope of application, type selection and classification, material requirements, performance indexes to installation acceptance.
I. Overview of technical specifications for desulfurization flue expansion joints
Desulfurization flue expansion joint technical specification is a technical document prepared for the procurement and acceptance of flue expansion joint equipment in wet or dry desulfurization system. It is used to specify the technical requirements of the functional design, structure, performance, materials, manufacture, installation and testing of the expansion joint。 This specification is applicable to the original flue, clean flue and bypass flue in limestone-gypsum wet desulfurization and circulating fluidized bed dry desulfurization processes.
The following basic principles should be followed in the preparation of the specification:
- Working condition pertinence: Fully consider the particularity of low temperature, high humidity and strong corrosion of desulfurization flue gas
- Standard applicability: Based on the current standards such as GB/T 12777 and JB/T 12235-2015, the strictest requirements shall be implemented in case of contradiction
- Full-cycle management: covering the whole process requirements from design, manufacturing, inspection to installation, operation and maintenance
Scope of Application and Project Overview
The technical specification of desulfurization flue expansion joint should first specify the following scope of application:
1. Applicable Systems
- Original flue of desulfurization system (before the inlet of absorption tower, usually high-temperature untreated flue gas)
- Clean flue of desulfurization system (outlet of absorption tower to chimney, low temperature saturated wet flue gas)
- Bypass flue and booster fan inlet and outlet
2. Differentiation of working conditions
The technical specification should clearly distinguish the different working conditions requirements of the original flue and the clean flue. The original flue gas temperature is usually 120~180 DEG C, and can reach 350 DEG C instantaneously; After the clean flue is washed by desulfurization, the temperature is as low as 45-80 DEG C, and it is saturated wet flue gas, which is extremely corrosive。
3. Expansion joint type selection
According to the working conditions of different sections of the desulfurization system, the technical specification of the desulfurization flue expansion joint should specify the type selection requirements of each position:
3.1 Non-metallic expansion joints
It is suitable for most desulfurization flues, especially the clean flue section. Its features include:
- Can simultaneously absorb axial, lateral and angular displacement
- Good vibration isolation and noise reduction effect, suitable for fan inlet and outlet
- Excellent corrosion resistance, anti-corrosion by composite skin material
For wet desulfurization flue, the non-metallic expansion joint with fluororubber composite skin is recommended, because its acid resistance and temperature resistance are superior to silicone rubber。 For dry desulfurization system, due to the weak acidity of flue gas, silicone rubber composite skin can meet the requirements。
3.2 Metal expansion joint
It is suitable for flue in high temperature section (such as the front section of desulfurization tower inlet and bypass flue), and the temperature resistance can reach above 600℃. Rectangular metal expansion joints are commonly used in low pressure pipelines with design pressure ≤0.05MPa。 However, under the working conditions of high dust content or strong corrosion, metal corrugated pipe is susceptible to erosion, wear and acid etching, and its application is limited.
IV. Requirements for technical parameters
The core of the technical specification is to clarify various technical parameters. When preparing technical specifications for desulfurization flue expansion joints, the following parameters shall be filled in one by one according to the equipment list：
V. Material and structural design requirements
5.1 Non-metallic expansion joint skin structure
The skin of non-metallic expansion joint of desulfurization flue should adopt multi-layer composite material structure, and each layer should have clear functions：
| horizon | Materials | Thickness | function |
|---|---|---|---|
| Outer layer (weather resistant layer) | Fluorine rubber/silicone rubber composite cloth | 2.5mm | Corrosion resistance, aging resistance, sealing |
| Intermediate layer | F4 cutting film + PTFE cloth | 0.3+0.25mm | Chemical isolation, anti-penetration |
| Reinforcement layer | Alkali-free fiberglass cloth (2 layers) | Single layer 0.88mm | Resistance to tension and pressure |
| Inner layer (thermal insulation) | Aluminum silicate fiber cotton + stainless steel mesh | ≥80mm | Thermal insulation, reducing skin temperature |
| Hemming cloth | Fluorine rubber cloth | 2.0mm | Edge seal enhancement |
5.2 Requirements for metal components
- Frame: Q235-A channel steel, thickness ≥6mm, should have sufficient stiffness
- Pressing plate: thickness ≥6mm, material Q235-A, compression bolt spacing ≤100mm
- Inner baffle/guide tube: Prevent dust abrasion bellows, the material is 1Cr18Ni9Ti stainless steel for original flue section, and Q345 can be used for air duct section
- Bolts and nuts: galvanized parts or stainless steel, minimum bolt size 16mm
5.3 Special requirements for corrosion resistance
Wet desulfurization flue is very corrosive, traditional non-metallic expansion joints and 316L, 2205 and other metallic expansion joints have the problems of short life and frequent leakage in practical use。 For net flue expansion joints, focus should be on:
- Take reliable anti-corrosion measures for flange end faces (e.g. glass flake lining)
- The connection between the skin and the frame should be sealed to prevent the acid from penetrating
- A water-phobic hole (DN100) is set at the bottom of the horizontal expansion joint to discharge the condensate in time
VI. Performance Assurance and Acceptance
The technical specification of desulfurization flue expansion joint shall clearly specify the performance guarantee value and acceptance criteria:
6.1 Performance Assurance
- Sealability: 100% airtight, air leakage rate ≤2%
- Pressure tolerance: can withstand maximum design positive/negative pressure plus 10mbar margin li>
- Displacement compensation: Meet the design compensation requirements without jamming or damage
- Service life: Overall life of non-metal expansion joint ≥10 years, metal frame ≥30 years
6.2 Citation Criteria
The specification should explicitly cite the following main standards:
- GB/T 12777 General Technical Specifications for Expansion Joints of Metal Bellows
- JB/T 12235-2015 Non-Metallic Compensator
- EJMA American Association of Expansion Joint Manufacturers Standards
- D-LD2000 "Typical Design Manual for Smoke, Air and Pulverized Coal Pipeline Parts"
6.3 Quality Certification Documents
The Seller shall provide:
- Product Certificate
- Material Inspection Report (Material Certificate)
- Product inspection report (with official seal)
- Non-destructive testing report (if applicable)
INSTALLATION AND MAINTENANCE
7.1 Installation Specifications
- Flow direction confirmation: Check the media flow direction arrow on the expansion joint before installation to ensure that the direction of the guide tube is consistent with the direction of flue gas flow
- Welding protection: Take measures to prevent weld slag from splashing onto the skin surface during welding operations
- Pre-tensioning/cold tightening: If pre-deformation is required, its auxiliary components shall be removed after installation
- Transportation tie rod: After installation, the positioning tie rod for transportation protection must be removed
7.2 Maintenance Requirements
- Full bolt tightening shall be carried out once in 1 month and once in 3 months after operation
- Check the skin surface quarterly for damage, aging and bulging
- Check bolts for looseness or corrosion
- Clean the dust accumulated at the bottom of the expansion joint regularly, and check whether the drainage hole is unblocked
VIII. Summary
Compiling complete and rigorous technical specifications for flue expansion joints in desulfurization system is the key to ensure the safe operation of flue in desulfurization system. The core points are summarized as follows:
- Zone selection: metal or high-temperature resistant non-metal expansion joints can be selected for the high-temperature dusty section of the original flue; Anti-corrosion non-metallic expansion joints must be selected in the low temperature and strong corrosion section of the clean flue, and fluororubber composite skin structure is recommended.
- Complete parameters: The technical specification shall specify key parameters such as interface size, temperature and pressure, media composition, compensation amount, etc., and indicate the installation position (original flue/clean flue) per device.
- Strict material control: the skin adopts multi-layer composite structure, and the materials and thickness of each layer should be clear; The thickness of metal frame shall be ≥6mm, the pressure plate shall be ≥6mm, and the bolts and washers shall be galvanized parts or stainless steel.
- Clear performance: specify core guarantee values such as sealing (100%), pressure margin (+10mbar), service life (skin ≥10 years), etc.
- Installation specifications: clarify the installation requirements such as flow direction confirmation, welding protection, transportation tie rod removal, etc., and stipulate the regular maintenance and inspection plan after commissioning.
It is suggested that the engineering and technical personnel should refer to the relevant design guidelines of DL/T desulfurization project when preparing the specification, and adjust the parameters according to the specific project working condition data. Before the specification is issued, it must be countersigned and confirmed by professional thermal control and anticorrosion, so as to ensure the reasonable selection and reliable operation of the expansion joint of desulfurization flue.