Deep hole machining of large tube plate assembly

Through the development of large heat exchangers, focusing on the deep hole processing test, deep hole processing of tube plate assembly and processing process of processing difficulties in the control method, and ultimately solved the deep hole processing of tube plate assembly process problems.

Yaang for an enterprise contracted a large heat exchanger equipment; a large size, complex structure characterizes the equipment. In particular, the deep hole processing of the tube plate assembly has a certain degree of difficulty. To develop a rigorous process program, the use of a CNC three-axis deep hole drilling machine and special BTA drill, after a large number of process tests, strict control of the key points of the process, overcome the manufacturing difficulties in the process of deep hole machining and ultimately make the manufacturing quality of the product to meet all the requirements of the design and related technical conditions.

1. Structure and characteristics of tube plate assembly

1.1 Product structure and specifications

The structure of the tube plate assembly is shown in Figure 1.
1.2 Mechanical properties of the forged tube plate (see Table 1)
20230926012803 74982 - Deep hole machining of large tube plate assembly
Figure.1 Schematic structure of the tube plate assembly
1.3 Specification of the pipe plate assembly
One side of the surface of the tube plate is overlaid with a layer of nickel-based stainless steel, tube plate diameter ∅ of 3148 mm and a thickness of 520mm.
Table.1 Mechanical Properties of Tubes and Plates

Project σS(MPa) σb(MPa) δ5(%) Impact energy (J)
Parameter 375 550 34.5 283,202,236

2. Main features of the tube plate assembly
The pipe plate assembly consists of a short section of the cylinder body and pipe plate, as shown in Figure 1. Pipe plate assembly technical requirements are as follows:

  • (1) The number of holes and hole diameter: 4958 – ∅ 22.3 ± 0.05 mm;
  • (2) Hole pitch requirements: surfacing adjacent holes: 31 ± 0.17 mm, any hole: 31 ± 0.30 mm; non-surfacing surface: 31 ± 0.32 mm, any hole: 31 ± 0.50 mm;
  • (3) Each hole axis relative to the axis of the pipe plate parallelism is 0.26mm.

3. Drilling test

As can be seen from Figure 1, the front end of the tube plate with a section of the barrel length of 390mm, plus the thickness of the tube plate 520 mm, so that the total length of the drill pipe extends to 910mm, almost reaching the limit of deep hole drilling. In order to ensure the smooth implementation of the tube plate assembly drilling, through the drilling test piece, in the tube plate drilling the actual position of the upper, lower, left, and right four typical areas, simulating the position of the tube plate drilling test, through the test tooling, tooling and with the accuracy of the standard selection, and finally determine the optimal drilling process parameters.

4. Tube plate assembly drilling

4.1 Drilling tooling

In order to ensure the smooth implementation of the pipe assembly drilling work, the deep hole drilling machine drilling system has been modified, see Figure 2.
20230926013414 89829 - Deep hole machining of large tube plate assembly
Figure 2: Schematic of deep hole drilling drill bit mounting
As can be seen from Fig. 2, the length of the reinforcing sleeve is 500 mm, and the length of the reinforcing sleeve is 500 mm, which is increased by the reinforcing sleeve, reinforcing sleeve, and spring sleeve.
In the drilling test, due to the rigidity of the reinforcing sleeve, the top of the workpiece is not a regular swing, and the reinforcing sleeve top of the workpiece, when the drill bit, oscillates only 0.02 mm to meet the needs of drilling accuracy.

4.2 Deep hole drilling inspection items and qualification standards

To ensure the drilling accuracy of the tube sheet assembly, through the test to find out the deep hole drilling inspection items and qualification standards, see Table 2.
Table.2 Inspection items, qualification standards

Item Inspection items Qualification criteria for inspection items
1 Concentricity of guide sleeve, spindle hole, etc., at the front, middle, and rear three points of the equipment Coaxiality (static and dynamic) not greater than 0.06 mm
2 Inner diameter of guide sleeve Inner diameter of guide sleeve ≤ 22.40 mm
3 Clearance between reinforcement sleeve and reinforcement sleeve Not greater than 0.04 mm
4 Clearance between support ring and drilling pipe Inner diameter of support ring ≤ 20.06 mm
5 After tightening the BTA drill bit Radial runout ≤ 0.07 mm

4.3 Tube plate assembly drilling

To ensure the accuracy of the tube plate hole pitch and diameter in the drilling process through the following ways to control.
(1) Cutting parameters (see table 3)
Table.3 Cutting parameters

Project Start drilling and surfacing welding Drilling the base layer
Drill bit feed rate (mm/min) 50 120
Spindle speed (rpm) 1000 1300

(2) The ambient temperature is not less than 10℃.

(3) Pipe plate assembly in the X-X′ vertical direction and Y-Y′ horizontal direction with jack adjustments to find the right, the verticality of finding the right with a percentage table in the pipe plate cylinder side of the same circumference, in the four center line to find the right 4 points and the center of the circle, and then use the percentage table to find the right four center line of the pipe plate to ensure that the four center line tolerance within the range of ± 0.20 mm.

(4) Tube plate components are adjusted after passing the clamping V-shaped frame, then U-type pressure plate components, while the chain can assist the pressure.
(5) To ensure the correctness of the drilling program, deep hole drilling before drilling the tube plate to retrieve the program, qualified after the drill guide sleeve placed on the surface of the tube plate, one by one pressure holes, according to the program to go again, and finally by the inspector according to the tube plate drawings to check all the holes one by one the number of holes, holes, and holes and hole spacing and so on without error and then proceed to the formal drilling.
(6) The surface of the tube plate should be flat, and the plane accuracy should not exceed 0.05 mm/m.
(7) Pipe plate components should be clamped, relative immobility when drilling.
(8) Formal drilling before the first warm-up, pre-drilled holes in the specimen method, and then formal drilling.
(9) Comprehensive testing of deep hole drilling indicators, checking the gap between the components (static and dynamic), and whether the fasteners are loose.
(10) The drilling process to keep the working environment does not have a vibration process that exists so as not to affect the drilling.
(11) To avoid damage to the BTA drill bit during the drilling process, which may affect the drilling accuracy, replace the BTA drill bit with a new one after drilling a certain amount of pipe holes (to be determined through tests).

5. Inspection results

A total of 4958 holes were drilled in the tube sheet assembly, and the measured data are as follows.

  • (1) The diameter of all holes ranges from ∅22.33 to ∅22.36 mm, with a pass rate of 100%.
  • (2) Beginning and end pitch tolerance is less than or equal to 0.3 mm, pass rate 100%.
  • (3) Hole verticality requirements for 0.12-0.25 mm, pass rate 100%.
  • (4) hole surface roughness < 3.2 μm, pass rate 100%.

6. Conclusion

The test proves that the process measures taken are reasonable: start drilling with low feed to ensure stable drilling centering; low-speed idling 20-30 min before drilling, and pre-drilling a few holes for the warm-up; strengthened quality management in the drilling process, from the human, machine, material, law, environment and other aspects of control, to avoid the impact of the human factor on the drilling, strengthened monitoring of the equipment and the environment, the stability of the equipment is good! The precision of the tooling, such as the extension sleeve, meets the drilling precision requirements, thus ensuring that the deep hole drilling equipment fully meets the drilling needs of the pipe plate. To summarize, the process measures taken are practical and reasonable, fully meet the requirements for deep hole processing of large and complex tube sheet components, and provide a reference basis for the future manufacture of similar equipment.
Author: Li Guoji

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