Machining of Deep Holes in Stainless Steel Forged Tubesheet

In 2023, our company made the tube sheet of reflux condenser for a company in Jiangsu. The outer diameter of the tube plate is 1960mm, the thickness 150mm, material 316L. The drawing requires the tube sheet to be manufactured and accepted by JB4728-2000 “Stainless Steel Forgings for Pressure Vessels,” which stipulates Class II forgings. The equipment has a total of 2 pieces of pipe plate, nine pieces of baffle plate, and baffle plate size ϕ1790mm × 20mm – 2576 holes on each pipe plate, pipe hole size as shown in Figure 1. The holes are arranged in a square triangle, as shown in Fig. 2.

Processing of Deep Hole in Tube Sheet

The drawing requires that the deep hole in the tube plate be processed, 96% of the hole bridge width must be ≥ 5.77mm, the minimum allowable hole bridge width is 3.48mm; the pipe hole should be strictly perpendicular to the sealing surface of the pipe plate, and its perpendicularity tolerance is 0.08mm. As the length-to-diameter ratio of the pipe hole is 6, it belongs to the category of deep hole processing. Our company had no CNC or deep hole drilling machine at that time, so we could only use the existing Z3080 rocker drilling machine. And the accuracy of the tube sheet processing, especially the tube hole spacing and diameter tolerance, verticality, and surface roughness, greatly affects the assembly and use of heat exchanger performance. Therefore, if we use the ordinary rocker drilling machine drilling, there is a considerable risk, and a little careless control may cause the size of people with low incomes. We analyzed the processing difficulties and developed a detailed processing plan to complete the task of drilling the tube sheet with high quality under the existing equipment.

20230330002900 42193 - Machining of Deep Holes in Stainless Steel Forged Tubesheet
Figure.1 Pipe bore

20230330003015 66333 - Machining of Deep Holes in Stainless Steel Forged Tubesheet
Figure.2 Pipe plate arrangement

1. Analysis of the difficulties of deep hole machining of pipe plates

  • (1) The number of holes is large, and the bridge’s width is strict, so the scribing accuracy is rigorous.
  • (2) The perpendicularity of the tube holes is very strict when drilling, and the tube sheet is thick, so if the holes are slightly skewed, the error at the back of the tube sheet will be magnified, which will eventually cause the position size and perpendicularity to exceed the standard.
  • (3) The number of baffle plate layers, the concentricity of the tubesheet, and baffle plate hole requirements are strict; once the concentricity cannot be guaranteed, the equipment production will significantly increase the difficulty of the heat exchanger tube through the tube.
  • (4) The plate material is 316L, forgings, high material plasticity, cutting resistance, and the material organization is denser in the forging process, further enhancing the material’s toughness; the material itself has a hardening process phenomenon. Therefore, the drilling bit adhesion wear, coupled with the poor thermal conductivity of stainless steel, drilling depth, and intense cutting heat, is not easy to dissipate, so the drill bit wears the life of the drill bit is very low.
  • (5) Stainless steel material chips are discharged in ribbons, not easy to break, and even entangled in the drill bit, scratching the inner wall of the pipe hole and affecting the surface roughness of the pipe hole.

2. Deep Hole Machining Scheme and Precautions for Tube Sheet

Given the above analysis of the processing difficulties, we decided to use the first drilling and expansion of the processing technology as follows.
(1) Scribing: To ensure the concentricity of the tube sheet and folded plate after drilling, we use a scribe on a folding plate; after passing the inspection and drilling the positioning holes with a f6mm drill bit, use the folding plate as a template to drill two pieces of tubeplate and other eight pieces of folding plate processing technology. The folded plate is to be scribed in the vertical car, first processing the outer circle, then leveling, and then scribing. When scribing, the folded plate will be placed horizontally, and the surface will be colored to make the scribing part clear and eye-catching. Manual scribing is due to the scribing needle lines with a certain width, so there is inevitably a certain amount of cumulative error. As the tube holes are arranged in a positive triangle, if you start scribing from the center outward, with the positive triangle cross line outward, the original error will gradually be diffused and magnified, resulting in the external hole center offset and center distance error over the difference.
We use the scribing method from the outside to the inside to avoid this situation. The specific approach is first to use the scribing gauge to find out the center of the baffle plate, draw a cross line, and pay attention to ensure the cross line is accurate and perpendicular. Then the center of the folding plate as the center, the center of the outermost vertex of the pipe hole from the center of the folded plate as the theoretical value of the radius of the circle, make a circle inside the cut hexagon, first scribe the center of the pipe hole located on each side of the hexagon, and then by the drawings on each side of the pipe hole center point and the corresponding side of the pipe hole center point scribed line connected to the intersection of the connecting line. The connecting line is the center of the other pipe hole. After scribing the line, first check the center distance error of each pipe hole; after passing the test before punching the sample, drilling the positioning hole. Using this scribing method, the position error of the external pipe hole can be controlled no more than 0.2mm, and the position error of the internal pipe hole is controlled within 0.2mm. To minimize the error, note that you must ensure that the scribing needle and sample punch are sharp.
(2) Drilling: Before drilling, fix the baffle plate as a drilling die on a tube sheet to be drilled, check the concentricity and fit of the baffle plate and the tube sheet, and use a f6mm drill bit to drill the positioning hole on the baffle plate to the tube sheet at a drilling depth of 8-10mm. Drill the holes with a tapered shank twist drill. Note that the tube sheet must be fixed on the table when drilling to ensure the tube sheet’s level and, thus, the hole’s verticality after drilling. In addition, when drilling the bottom hole of f23mm, 2-3 times chip return should be carried out in each hole, which is very beneficial to the smooth discharge of chips and the cooling of the cutting-edge part of the drill bit.
(3) Reaming: After the first drilling with a f23mm twist drill, we ream the holes with a f25.4mm reaming drill. Since the first drilling has made the pipe hole size reach f23mm, thus the cutting volume is relatively small during the reaming process, which makes the surface quality of the pipe hole have a more excellent guarantee. When reaming, pay attention to checking the drill bit’s wear and keep the drill bit’s cutting edge sharp.
(4) The choice of cutting fluid: in the processing of stainless-steel deep hole pipe holes, lubrication is not the central conflict; the main conflict is the tube sheet and the heat dissipation of the drill bit. We use 10% emulsion as the cutting fluid and increase its flow rate into the cutting-edge cutting heat as much as possible to take away, extend the drill bit’s life, and improve the pipe hole’s surface quality.
(5) The choice of cutting dosage: a reasonable cutting dosage is conducive to the improvement of the accuracy of the pipe hole. When drilling, the speed is set to 105r/min, and the feed is 0.32-0.4mm/r; when reaming the hole, the speed is 200r/min, and the feed is 0.45-0.6mm/r. The practice has proved that choosing such a cutting amount can improve efficiency and ensure the quality of the drilling.

3. Conclusion

The pipe holes processed by the technology mentioned above scheme are checked to meet the technical requirements of the pipe plate. The following points should be noted when processing.

  • (1) To meet the drilling depth requirements, the length of the drill bit should be shortened as much as possible to obtain a higher rigidity of the drill bit.
  • (2) The drill bit must be mounted correctly, kept sharp, and resharpened in time after being blunt.
  • (3) Observe the chip discharge condition in time during the drilling process, and if you find the chips messy and rolled up, you should immediately withdraw the tool for inspection to prevent chip blockage.

Source: China Tube Sheet Manufacturer – Yaang Pipe Industry (www.steeljrv.com)

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