CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay of Tube Sheets for Phenol Distillation Column Heaters
Background and Service Conditions
The phenol distillation column heater is a critical piece of equipment in the cumene process for phenol production. The tube sheet in such a heater is subjected to a highly corrosive environment containing phenol, acetone, hydrocarbons, and trace amounts of water at elevated temperatures, typically between 180 and 260 degrees Celsius. The base material of the tube sheet is generally a carbon steel or low-alloy steel such as 16MnR or Q345R, which provides the necessary structural strength for pressure containment. However, these materials offer insufficient resistance to the aggressive phenol-acetone-water medium. The weld overlay layer must therefore provide a corrosion-resistant barrier while maintaining adequate mechanical integrity under cyclic thermal loading and internal pressure.
From a design perspective, the tube sheet must comply with GB/T 150.2 or ASME VIII Div.1 requirements for pressure boundary components. The overlay thickness is typically specified at 2 to 3 mm on the process-side face, with a minimum of 1.5 mm remaining after machining. The selection of overlay material is governed by the severity of the corrosive environment and the required service life. In my experience, 304 stainless steel (S30408) is commonly used for moderate phenol service, while 316L (S31603) or duplex steel (2205) may be specified for more aggressive conditions where chloride concentrations are elevated.
Welding Process Selection and Parameters
The choice of welding process for tube sheet overlay is a critical decision that affects both quality and productivity. For large-diameter tube sheets with diameters exceeding 1000 mm, submerged arc welding (SAW) is the preferred method due to its high deposition rate and excellent penetration characteristics. However, SAW requires precise flux management and generates significant heat input, which can lead to distortion on thin-walled tube sheets.
Gas tungsten arc welding (GTAW) is preferred for smaller tube sheets or for the first pass where precise control of the dilution rate is essential. The dilution rate between the base metal and the overlay is a critical parameter, as excessive dilution from carbon steel into the stainless steel overlay can reduce the corrosion resistance below acceptable levels. The dilution rate should be kept below 30 percent for 304 overlay on carbon steel.
| Parameter | GTAW Overlay | SAW Overlay |
|---|---|---|
| Wire diameter | 1.6 - 2.4 mm | 2.0 - 3.0 mm |
| Current | 120 - 180 A | 400 - 600 A |
| Voltage | 16 - 22 V | 28 - 34 V |
| Travel speed | 30 - 60 mm/min | 200 - 400 mm/min |
| Heat input | 0.5 - 1.5 kJ/mm | 2.0 - 5.0 kJ/mm |
| Typical passes | 3 - 5 | 2 - 3 |
The interpass temperature should be maintained below 150 degrees Celsius to prevent excessive grain growth in the overlay and to minimize distortion. Preheating is generally not required for carbon steel base materials, but the overlay should be deposited in a way that minimizes residual stresses. A recommended welding sequence starts from the center of the tube sheet and works outward in a symmetric pattern to distribute heat input evenly.
Quality Control and Inspection
The quality of the overlay on tube sheets is verified through multiple inspection methods. Ultrasonic testing (UT) in accordance with NB/T 47013.3 or ASME V Article 4 is performed to detect lack of bond, cracks, and porosity at the interface between the base metal and the overlay. The acceptance criteria typically require no cracks, no lack of fusion, and porosity not exceeding the limits specified in the applicable code.
Visual inspection and magnetic particle testing (MT) are used to detect surface and near-surface defects. A critical aspect of inspection is the verification of overlay thickness after machining. The thickness is measured at multiple points across the tube sheet, and the minimum thickness must meet the design specification. In practice, I have found that measuring at a grid pattern of at least 25 points across the tube sheet provides a reliable assessment of thickness uniformity.
Corrosion testing is also an important verification step. A salt spray test in accordance with ASTM B117 for 48 to 96 hours can provide an indication of the overlay's corrosion resistance. Additionally, intergranular corrosion testing per ASTM A262 Practice E or Practice A can verify that the overlay has not suffered from sensitization during the welding process.
Engineering Practice Insights
In my experience working on phenol distillation column heaters, several practical lessons have emerged. First, the tube sheet holes for tube insertion must be drilled and reamed after the overlay is completed, as welding before drilling can introduce stresses that may cause cracking around the holes. Second, the transition zone between the overlay and the tube sheet edge is a common location for defects, and extra care must be taken to ensure complete coverage at the edges. Third, the welding procedure specification (WPS) must be qualified per NB/T 47014 or ASME IX, with attention paid to the dilution rate and the mechanical properties of the overlay in the as-welded condition.
A notable challenge is the management of distortion during multi-pass overlay welding. For tube sheets with a thickness of 20 to 30 mm, the distortion can be significant if the welding sequence is not carefully planned. Using a back-plate with clamping or employing a welding sequence that alternates between opposite sides of the tube sheet can substantially reduce distortion. Post-weld stress relief is generally not recommended for overlay applications because it can promote sensitization of the stainless steel overlay and increase the risk of intergranular corrosion.
Summary
The successful application of weld overlay on phenol distillation column heater tube sheets requires careful attention to process selection, parameter control, and quality verification. The key to success lies in maintaining a low dilution rate, controlling heat input to prevent distortion, and ensuring thorough inspection at both the interface and the surface. The selection of overlay material should be based on the specific corrosion environment, with 304 stainless steel being adequate for most phenol service conditions and 316L or duplex steel reserved for more aggressive environments. With proper planning and execution, overlay tube sheets can provide reliable service for 10 to 15 years in phenol distillation column heaters, offering a cost-effective alternative to solid stainless steel tube sheets while maintaining the structural integrity of the carbon steel base material.