Hydrogenation Reactor Tube Sheet Cladding Process and Anti-Deformation Measures

Literature Overview

Published in 2024 in the journal China Chemical Equipment, this technical paper by Jia Na and Wu Jingjing from Xi'an Aerospace Huawei Chemical and Bioengineering Co., Ltd. addresses one of the most challenging fabrication problems in hydrogenation reactor construction: the cladding of large-diameter tube sheets while controlling dimensional deformation within tight tolerances. Hydrogenation reactors operate under extreme conditions (300 to 450 degrees Celsius, 100 to 200 MPa hydrogen partial pressure), demanding clad tube sheets with corrosion-resistant overlay layers (typically Inconel 625, 316L, or 347H stainless steel) on high-strength low-alloy steel or Cr-Mo steel base plates.

Technical Challenge Analysis

The fundamental difficulty arises from the mismatch between the geometric requirements of the tube sheet (thousands of precisely spaced tube holes, tight flatness tolerances typically within 0.5 to 1.0 mm per meter) and the thermal distortion inherent in multi-pass weld overlay processes. A typical hydrogenation reactor tube sheet may have a diameter of 1500 to 3000 mm and a thickness of 80 to 150 mm, requiring overlay thicknesses of 3 to 8 mm on one or both faces.

Weld Overlay Process Selection

The paper discusses the selection of welding processes based on component geometry and thickness requirements:

Process	Applicable Thickness	Typical Heat Input	Distortion Level	Equipment Complexity
SAW (Submerged Arc)	3-10 mm	30-80 kJ/cm	Moderate	High
ESW (Electroslag)	5-15 mm	50-120 kJ/cm	Low	High
GTAW (TIG)	1-3 mm	5-15 kJ/cm	Low	Medium
GMAW (MIG)	2-6 mm	15-40 kJ/cm	Moderate	Medium
PTA (Plasma Transfer Arc)	1-4 mm	10-25 kJ/cm	Low-Moderate	High

For hydrogenation reactor tube sheets, the preferred approach is typically a combination of SAW for the bulk of the overlay (providing high deposition rates of 5 to 10 kg/h) followed by GTAW or PTA for the surface finishing pass to ensure metallurgical quality and eliminate surface defects.

Anti-Deformation Measures

The paper presents a comprehensive strategy for deformation control, which can be organized using the PDCA framework:

Plan Phase - Pre-Welding Design Controls

Symmetrical welding sequences: Overlay welding is performed in a balanced, symmetric pattern from the center outward, or using a "back-step" method where welds are applied in segments of 100 to 150 mm advancing in opposite directions on alternating sides.
Restraining fixtures: The tube sheet is clamped to a rigid backing plate using high-strength bolts at 150 to 200 mm spacing, with the clamping pressure calculated to resist the expected thermal contraction forces without causing permanent deformation.
Backing plate design: A rigid backing plate of equivalent or greater thickness than the tube sheet is used, with a gap of 0.3 to 0.5 mm between the tube sheet and backing plate filled with a high-temperature refractory material to allow controlled contraction.

Do Phase - Process Parameter Control

Preheating: The entire tube sheet is preheated to 200 to 300 degrees Celsius (depending on base material) using induction heating or gas flames, with temperature uniformity within plus or minus 30 degrees Celsius across the surface.
Interpass temperature control: Maintained at 150 to 250 degrees Celsius, monitored at multiple points across the surface using infrared thermometers.
Heat input limitation: For each pass, the heat input is controlled below 40 kJ/cm for SAW and below 15 kJ/cm for GTAW to minimize the thermal gradient.
Welding direction rotation: For circular welds, the direction is rotated by 90 degrees every quarter revolution to distribute thermal stress uniformly.

Check Phase - In-Process Monitoring

Real-time deformation measurement: Using laser displacement sensors or dial indicators at 8 to 12 points around the circumference and across the diameter, with measurements taken before and after each welding segment.
Temperature mapping: Infrared thermography to identify hot spots and adjust the welding sequence accordingly.
Post-weld cooling rate control: Controlled cooling using ceramic blankets or heated backing plates to maintain the cooling rate below 5 degrees Celsius per minute.

Act Phase - Post-Weld Correction

Shot peening: Applied to the overlay surface to introduce compressive residual stresses and improve dimensional accuracy.
Mechanical machining: Final machining to achieve dimensional tolerances (flatness within 0.3 mm per meter, thickness tolerance plus or minus 0.2 mm).
Stress relief: Post-weld heat treatment at 620 to 680 degrees Celsius for 2 to 4 hours (for Cr-Mo base materials) or 400 to 450 degrees Celsius (for austenitic overlay layers) to relieve residual stresses.

Metallurgical Considerations

The overlay material selection for hydrogenation service requires consideration of hydrogen embrittlement resistance, high-temperature strength, and corrosion resistance in the presence of hydrogen sulfide and other contaminants. Inconel 625 (UNS N06625) is the most commonly specified overlay for severe hydrogenation service due to its excellent resistance to hydrogen embrittlement, high-temperature strength retention, and resistance to sulfidation. The weld overlay procedure must comply with NB/T 47014 and ASME IX qualification requirements, including impact testing at the service temperature minus 20 degrees Celsius.

Study Reflection

This paper exemplifies the integration of process engineering and manufacturing technology in solving a real-world fabrication challenge. The systematic approach to deformation control—combining design, process parameters, in-process monitoring, and post-processing—provides a transferable methodology applicable to other large-diameter clad components such as flanges, heads, and reactor shells. The emphasis on real-time monitoring and adaptive process adjustment reflects the maturity of Chinese chemical equipment manufacturing in addressing complex fabrication challenges.