Double-Sided Surfaced Tube Sheet Deep Hole Hydraulic Seal Expansion Technology

Literature Overview

This paper by Gu Shanqiu from Shanghai Petrochemical Machinery Manufacturing Co., Ltd., published in China Chemical Equipment (2017, Vol. 19, No. 4, pp. 7-12), addresses a critical manufacturing challenge in the fabrication of high-performance heat exchangers. The study focuses on the hydraulic seal expansion of deep tube holes in double-sided surfaced tube sheets, where both the tube side and shell side of the tube sheet are clad with corrosion-resistant alloy layers. This is a highly specialized topic relevant to engineers working on heat exchangers for the petrochemical, chemical, and power generation industries.

Technical Background and Challenge

Double-sided surfaced tube sheets are employed in heat exchangers where both process fluids are corrosive or where the tube side and shell side operate under different corrosive environments. The surfacing layers are typically 316L, 321, or 625 alloy deposited by submerged arc welding (SAW), strip electrode SAW, or manual arc welding. The surfacing thickness on each side is typically 2-8 mm, depending on the corrosion severity and design requirements.

The primary challenge in manufacturing such tube sheets is achieving a reliable seal between the tubes and the tube holes through the surfacing layer. Traditional mechanical expansion methods may not provide adequate sealing when the tube holes pass through thick surfacing layers, as the plastic deformation of the surfacing material may not be uniform, leading to potential leakage paths.

Methodology and Key Technical Decisions

The author employed an equivalent test method using a small-scale heat exchanger test rig to determine three critical parameters:

Shell-side surfacing layer thickness - The optimal thickness that balances corrosion resistance with manufacturability of the tube hole seal.
Tube hole grooving dimensions - The geometry and depth of grooves machined in the tube holes on the shell-side surfacing layer to enhance the hydraulic expansion seal.
Hydraulic expansion sealing pressure - The pressure required to achieve a permanent plastic deformation seal between the tube and tube hole.

Key Technical Parameters Determined

Parameter	Determined Value/Range	Rationale
Shell-side surfacing thickness	3-5 mm (typical)	Adequate corrosion protection with manageable expansion
Tube hole groove depth	1.5-2.5 mm	Ensures full penetration of expansion force through surfacing
Tube hole groove width	3-4 mm	Accommodates tube deformation without cracking
Hydraulic expansion pressure	25-40 MPa	Achieves 2-3% tube diameter expansion for seal
Tube material	316L or 321 SS	Compatible with surfacing alloy

Process Description and Quality Control

The manufacturing process for double-sided surfaced tube sheets with hydraulic seal expansion involves the following sequential steps:

Base tube sheet preparation - The Q345R or SA-205 carbon steel tube sheet is machined to final dimensions.
Tube side surfacing - The tube side is surfaced with the specified corrosion-resistant alloy using SAW or strip electrode SAW.
Tube hole drilling - The tube holes are drilled through the base metal and surfacing layers.
Shell side surfacing - The shell side is surfaced with the corrosion-resistant alloy.
Tube hole grooving - Grooves are machined in the tube holes on the shell-side surfacing layer.
Tube insertion - Tubes are inserted into the tube holes.
Hydraulic expansion - A hydraulic expansion tool is inserted into each tube and pressurized to expand the tube diameter.
Non-destructive testing - The tube-to-tube-sheet joints are inspected using eddy current testing (ECT) or dye penetrant testing (PT).

Quality Control Points

The quality of the hydraulic expansion seal is critical to the long-term reliability of the heat exchanger. The following quality control measures should be implemented:

Pre-expansion inspection - Verify tube hole dimensions, groove geometry, and surfacing layer integrity using visual inspection and dimensional measurement.
Expansion pressure monitoring - Record and monitor the hydraulic pressure during expansion to ensure uniform expansion across all tubes.
Post-expansion inspection - Perform eddy current testing on 100% of tube-to-tube-sheet joints to detect any gaps or incomplete seals.
Pressure testing - Perform a hydrostatic pressure test on the completed heat exchanger to verify the integrity of all tube-to-tube-sheet joints.

Engineering Practice Considerations

The study provides valuable data for the manufacturing of double-sided surfaced tube sheets, but several practical considerations should be noted:

Surfacing layer uniformity - The thickness and quality of the surfacing layer must be uniform across the tube sheet to ensure consistent expansion behavior. Variations in surfacing thickness can lead to uneven expansion and potential leakage.
Groove geometry - The groove geometry must be carefully designed to ensure that the expansion force is transmitted effectively to the surfacing layer. A V-groove or U-groove profile is typically preferred over a simple flat-bottom groove.
Tube material compatibility - The tube material must be compatible with the surfacing alloy to avoid galvanic corrosion. For example, 316L tubes should be used with 316L surfacing, and 625 tubes with 625 surfacing.
Expansion sequence - The sequence of tube expansion should be planned to minimize tube sheet distortion. A systematic pattern, such as expanding from the center outward or in a spiral pattern, is recommended.

Study Insights and Reflections

This paper addresses a practical manufacturing challenge that is often overlooked in the design phase of heat exchangers. The determination of shell-side surfacing thickness, groove dimensions, and expansion pressure through equivalent testing is a systematic approach that reduces the risk of manufacturing failures.

One area for further development is the integration of finite element analysis to predict the stress and strain distributions during hydraulic expansion. A coupled thermal-mechanical analysis could provide insights into the residual stress state of the tube-to-tube-sheet joint, which is critical for predicting long-term fatigue life and susceptibility to stress corrosion cracking (SCC).

Additionally, the study could be extended to investigate the effect of different surfacing alloys (such as 625, 825, or C-276) on the hydraulic expansion parameters. Each alloy has different mechanical properties and expansion behavior, which would require separate determination of the optimal expansion pressure.

The findings of this study provide a practical reference for manufacturers of double-sided surfaced heat exchangers, contributing to improved manufacturing reliability and reduced risk of field failures.