How to select clad tubesheet for heat exchanger?
Clad Tubesheet for Heat Exchangers & Boilers
In the chemical, petroleum, power generation, and marine engineering industries, the design life and operational reliability of heat exchangers and boiler equipment directly impact plant safety and economics. As a key pressure-bearing component in these equipment, the selection and design of clad tubesheet has become a core technology that engineers must master. This article provides an in-depth analysis of the technical advantages, selection criteria, and industry applications of clad tubesheets from an engineering practice perspective.
1. Economic and Technical Necessity of Clad Tubesheets
Under harsh operating conditions where high temperature, high pressure, and corrosive media coexist, single-material tubesheets often face the dilemma of "over-design" or "under-design." Taking a refinery hydrogenation unit as an example, the heat exchanger tube side medium is an oil-gas mixture containing hydrogen sulfide (temperature 320°C, pressure 12MPa), and the shell side is boiler feed water. If a full stainless steel tubesheet is used, the material cost alone would be as high as $250,000, while using a clad tubesheet (base layer 16MnIII, cladding layer 321 stainless steel) reduces the cost to approximately $90,000, saving more than 60%.
Engineering Practice Data:
According to ASME BPVC Section VIII and NB/T 47002 standards, the design of clad tubesheets must simultaneously meet the pressure-bearing strength of the base material and the corrosion resistance of the cladding material. In actual engineering, the cost advantage of clad tubesheets becomes more obvious as the size increases:
- Diameter ≤ 800mm: cost savings of approximately 30-40%
- Diameter 800-1500mm: cost savings of approximately 40-55%
- Diameter > 1500mm: cost savings can reach 55-70%
2. Material Combinations and Performance Comparison for Clad Tubesheets
The performance of clad tubesheets depends not only on the respective characteristics of the base and cladding materials but also crucially on the quality of the interface bonding. The following are common clad tubesheet material combinations and their applicable working conditions in the chemical industry:
| Material Combination | Base Material | Cladding Material | Max. Service Temperature | Main Corrosion Resistance | Typical Applications |
|---|---|---|---|---|---|
| CS/304L | Q345R | 304L | 400°C | Resistant to oxidizing acids, organic acids | Atmospheric and vacuum distillation units, chemical intermediate heat exchangers |
| CS/316L | Q345R | 316L | 400°C | Resistant to pitting, crevice corrosion | Media containing chloride ions, seawater coolers |
| CS/2205 | 16MnIII | S31803 | 300°C | Resistant to chloride stress corrosion | Offshore platforms, harsh media in chemical plants |
| CS/Ti | Q345R | Gr.2 Ti | 300°C | Resistant to seawater, chlorides | Coastal power plants, seawater desalination units |
| CS/Ni-Cu | 16MnIII | Alloy 400 | 480°C | Resistant to hydrofluoric acid, alkaline media | Fluorochemical industry, alkali liquor evaporators |
| CS/Hastelloy | SA516 Gr.70 | C-276 | 550°C | Resistant to strong oxidizing media | PTA units, hydrometallurgy |
Technical Point: The interfacial shear strength of clad tubesheets is a key indicator for evaluating bonding quality. According to ASTM A263/A264/A265 standards, the interfacial shear strength of explosion-clad and roll-bonded clad plates should not be less than 140MPa, while actual high-quality products can reach 210-280MPa, ensuring long-term reliability of equipment under thermal cycling conditions.
3. Comparison of Cladding Technologies
Different cladding processes determine the microstructure, interface characteristics, and final performance of tubesheets. The following is a technical and economic comparison of four mainstream cladding processes:
| Process Type | Bonding Mechanism | Interface Strength | Suitable Material Combinations | Production Efficiency | Relative Cost | Applicable Size Range |
|---|---|---|---|---|---|---|
| Explosive Cladding | Metallurgical bonding formed by high-speed impact | Very High (200-350MPa) | Almost all combinations, especially for dissimilar metals | Low | High | Diameter ≤ 5000mm |
| Hot Roll Bonding | Atomic diffusion under high temperature and pressure | High (180-250MPa) | Stainless steel/steel, nickel alloy/steel | High | Medium | Width ≤ 4000mm |
| Weld Overlay Cladding | Metallurgical bonding formed by deposition welding | Medium (140-200MPa) | Stainless steel/steel, nickel-based alloy/steel | Low | Medium-High | Almost unlimited |
| Explosive-Roll Bonding | Dual process of explosion + hot rolling | Very High (220-320MPa) | All combinations, especially thick plates | Medium | High | Width ≤ 4500mm |
Process Selection Decision Matrix:
In actual engineering, selecting a cladding process requires comprehensive consideration of the following factors:
- Equipment Criticality: Core equipment (such as nuclear power plant steam generators) should prioritize explosive cladding or explosive-roll bonding
- Material Combination: Dissimilar metals such as titanium/steel, zirconium/steel should first consider explosive cladding
- Production Cycle: For urgent projects, consider hot roll bonding or weld overlay cladding
- Cost Control: For large-volume standardized products, hot roll bonding is preferred
- Size Limitations: Extra-large size (diameter > 4000mm) tubesheets can only use explosive cladding or weld overlay cladding
4. Design Points and Manufacturing Quality Control
The design of clad tubesheets is different from single-material tubesheets and requires special attention to the following technical points:
Design Key Parameters:
- Minimum Cladding Thickness: Determined based on corrosion allowance, typically 3-8mm, ASME standard requires not less than 3mm
- Minimum Base Thickness: Based on strength calculation, considering ligament efficiency and design pressure
- Tube Hole Machining: Must ensure the cladding completely covers the tube hole inner wall to avoid base material exposure to media
- Heat Treatment: Stress relief heat treatment temperature must consider both base and cladding material characteristics
During the manufacturing process, quality control is key to ensuring the long-term reliable operation of clad tubesheets:
| Inspection Item | Inspection Method | Acceptance Standard | Inspection Ratio | Remarks |
|---|---|---|---|---|
| Interface Bonding Rate | Ultrasonic Testing | ≥98% (ASME requires ≥95%) | 100% | Unbonded area ≤50cm², spacing ≥100mm |
| Interface Shear Strength | Mechanical Property Test | ≥140MPa | 1 set per batch | ≥200MPa for critical equipment |
| Cladding Chemical Composition | Spectrum Analysis | Complies with material standard | Once per heat number | Focus on key elements such as Cr, Ni, Mo |
| Cladding Thickness Uniformity | Ultrasonic Thickness Measurement | Tolerance ±10% | At least 4 points per m² | Minimum thickness not less than 90% of nominal thickness |
| Bending Performance | Side Bend Test | No cracks | 2 pieces per batch | Bend angle 180°, D=4T |
5. Industry Application Cases and Best Practices
Case 1: Titanium-Steel Clad Tubesheet for PTA Unit
A purified terephthalic acid (PTA) unit oxidation reactor feed heat exchanger, with tube side medium being acetic acid solution containing bromides, and shell side being steam. The original design used a full titanium tubesheet with a unit cost as high as $580,000. After optimization design, a titanium-steel clad tubesheet (base layer 16MnIII, cladding layer Gr.2 titanium) was adopted, reducing the cost to $230,000. The equipment has been operating stably for 6 years, with periodic inspection showing a cladding corrosion rate of <0.05mm/year.
Case 2: Duplex Steel Clad Tubesheet for Power Plant Seawater Cooler
A coastal power plant seawater cooler, with tube side medium being seawater (chloride ion content 18000ppm), and shell side being steam. A 2205 duplex stainless steel/carbon steel clad tubesheet was adopted, replacing the original 316L stainless steel tubesheet, not only reducing costs by 45% but also solving the pitting problem of 316L in seawater. The equipment design life was extended from 10 years to over 20 years.
Case 3: Weld Overlay Tubesheet for Hydrotreater Feed-Effluent Exchanger
A refinery hydrocracking unit reactor feed-effluent heat exchanger, with design temperature of 420°C, design pressure of 18MPa, and medium containing high concentration H₂S. An SA336 F22 base layer + Inconel 625 weld overlay clad tubesheet was adopted, which not only met the high temperature strength requirements but also provided excellent resistance to hydrogen sulfide corrosion. The equipment has been safely operating for over 12 years.
Clad tubesheet
Clad tubesheet successfully resolves the contradiction between cost and performance faced by heat exchanger and boiler equipment under harsh operating conditions through optimized material combinations and advanced manufacturing processes. In the context of carbon neutrality goals and high-quality development, the rational selection of clad tubesheet can not only significantly reduce equipment investment but also improve equipment reliability and extend service life, making it one of the key technologies for achieving safe, environmentally friendly, and economical operation of plants. With the continuous development of new materials and processes, the application prospects of clad tubesheets in equipment for extreme conditions will be even broader.