Assessing Tube-to-Tubesheet Joints in Heat Exchangers
Evaluating Tube-to-Tubesheet Joints in Heat Exchangers
Tubesheet joint configurations, designs, and procedures greatly influence the reliability and performance of shell and tube heat exchangers. The wrong configuration can lead to leaks, downtime, and damage to the exchanger. Let’s discuss how to evaluate tube-to-tubesheet joint configurations in shell and tube heat exchangers and how to get field or shop repair services.
Tubesheet Joint Design Considerations
While selecting a tubesheet joint configuration for shell and tube heat exchangers, the following design considerations must be taken into account:
1. Design Pressure Rating: The operating and design pressure rating of the heat exchanger should be considered as leakage between streams is more likely to occur at higher operating pressures. Typically seal or strength-welding is considered.
2. Criticality of Contamination Across Streams: Some processes absolutely cannot allow for any leakage from one stream to another. In such an instance, adding a seal or strength-weld should be considered.
3. TEMA Type: Mechanical stresses are induced into the tubesheet joint due to pressures and differential thermal expansion between the tubes and shell. Additional tube-rolling expansion depth and/or tube-welding may be required for fixed-bundles (NEN, BEM, AEL, etc.) and floating head types (AES, BET, etc.).
4. Metallurgy: The tube material should be compatible with the tubesheet material to ensure a secure connection. Tubes made of a harder material than the tubesheet cannot be rolled/expanded into a softer tubesheet material. Further, dissimilar welding will be required for welding stainless tubes to a carbon-steel tubesheet. Copper and brass tubes are typically brazed instead of welded. Titanium requires special welding in an inert environment and may not be possible at your manufacturer of choice.
5. Expansion Joints: Bellows or flanged & flued expansion joints should be considered on fixed-bundle heat exchangers when thermal expansion is such that mechanical stresses are too high at the tube-to-tubesheet joint.
Types of Tubesheet Joint Configurations
The most common types of tubesheet joint configurations used in TEMA and API 660 shell and tube heat exchangers include:
1. Mechanical Expanded: Typically double-ring-groove (2RG) and hydraulically, pneumatically, or roller expanded 2 inches from tube-side face. High-pressure exchangers may require expansion throughout the full thickness of the tubesheet.
2. Seal- Welded: A single-pass TIG weld (with filler metal) to reduce the possibility of leakage between process streams. This is in addition to mechanical expansion.
3. Strength-Welded: A two-pass TIG weld (with filler metal). This is usually performed with a light expansion (no ring-groove) and provides both strength and sealing. Typically used for fixed-bundle (NEN, AEL, BEM) type and high-pressure heat exchangers.
4. J-Groove: A welded tubesheet joint that uses a J-groove weld-prep to penetrate deeper into the tubesheet joint for thicker tubes at higher pressures. Can be found on waste-heat boilers.
5. Inner-Bore Welded: The tube is butt-welded to the back side of the tubesheet. This can be found on some waste-heat boilers and sulphur condensers. The benefit is that it puts the weld into the `cooler’ shell-side process rather than the incoming hot-gas tube-side. The downside is that it is time-consuming, difficult to inspect, and more costly overall.
Evaluating Tubesheet Joint Configurations
When evaluating tubesheet joint configurations, the following factors should be considered:
Cost
Only some heat exchangers require a welded tube-tubesheet joint configuration, and the additional cost may not be cost-effective. It is essential to compare the benefits and costs of the different joint configurations to determine which one is the most cost-effective for your application.
Durability
The joint configuration must withstand the operating conditions of the heat exchanger. The joint should be able to handle the pressure, temperature, and stresses it will be exposed to in the exchanger.
Reliability
The joint configuration must be reliable and provide a secure connection between the tube and tubesheet. The joint should be clean and free of burrs that could compromise the joint and create leakage paths.
Prepping the Tubesheet and Tubes
Every joint configuration should be cleaned appropriately before assembly. The tubesheet and tubes should be free of debris, dirt, and other contaminants that could reduce the effectiveness of the joint. Welded joints should have the tubes lightly expanded into the tubesheet without lubrication before welding.
Inspection and Testing Process
A process for inspecting and testing the joint must be followed. This should include visual inspections to check for any irregularities or damage. Non-Destructive Examination (NDE) such as Liquid Penetrant Inspection (LPI) is typically used to inspect tubesheet welds. Hydrostatic pressure testing will ensure the joint can withstand the operating conditions of the heat exchanger under regular operation and up to the required design and hydro-testing pressure ratings. Helium leak testing can be performed as a costly option.
Get Reliable Leak-Free Tubesheet Joints with Altex Industries Heat Exchangers
A reliable tube-to-tubesheet joint configuration is essential for uptime in shell and tube heat exchangers. At Altex, we offer shop and field repair services to make sure tubesheet joints are secure and reliable. Contact us to learn more.
