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TUBE-TO-TUBESHEET JOINTS: THE MANY CHOICES
B. J. Sanders
Consultant
307 Meyer Street
Alvin, Texas 77511
ABSTRACT
After a decision has been made to u zirconium as the material of construction for a shell and tube heat exchanger, one must take into account the process design parameters and other considerations in order to obtain the optimum mechanical design for the specific application. One of the most important steps in the mechanical design process is determining the method of attachment for the tube-to-tubesheet joint. Many of the problems which have been experienced with zirconium shell and tube heat exchangers could have been avoided by giving more attention to the lection and design of
the tube-to-tubesheet joint. Some of the many factors to consider are tubesheet thickness and what type of tubesheet will be ud, i.e. solid, explosion clad or loo lined. Other factors to consider are whether or not to al weld, expand only, strength weld or u some combination of the and what should be quence of events during proof testing and inspection.
KEYWORDS
Seal welding, tube-to-tubesheet welds, heat exchanger, mock-up, tube expansion
INTRODUCTION合肥一对一
Heat exchangers are widely ud in many ways and are en in many types of industrial, commercial and residential applications. Heat exchangers come in many configurations, sizes and materials of construction. Some of the various types of heat exchangers are shell and tube, plate coil, pipe coil, bayonet and air finned tube. This paper deals with the shell and tube heat exchanger which is the most commonly ud type in the Chemical Processing Industry (CPI), See Figure 1.
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Figure 1.
DISCUSSION
Rules for designing and fabricating the pressure containing parts of shell and tube heat exchangers
are contained in the Boiler and Pressure Code of the American Society of Mechanical Engineers (ASME Code) and the general rules for design and fabrication are contained in a publication known as Standards of the Tubular Exchanger Manufacturers Association (TEMA)
The major parts of a shell and tube heat exchanger are the shell, heads, tubes, tubesheet and baffles. It is difficult to highlight the importance of one heat exchanger component above any of the others, but due to the complexities of the design and fabrication of a tubesheet and the tube-to-tubesheet joint, there are more potential leak paths at this location than with any of the other components. When zirconium is employed as the construction material the design and fabrication of the tubesheet and completing the tube-to-tubesheet joint becomes even more complicated due to the mechanical properties of zirconium and the necessary controls needed to produce a quality weld that has good ductility, corrosion resistance and strength.
OVERVIEW OF DESIGN DETAILSbevel
Process Design
During the process design pha for a heat exchanger, veral factors must be considered and the are shown in Figure 2.
美发连锁店TUBE-TO-TUBESHEET JOINT
PROCESS DESIGN
•Specify: Media, Flows, Temperature, Pressure
•Specify Materials of Construction
•Determine shell diameter / length
•Determine type of head(s)
•Determine shell & head nozzle size / location
•Determine tubing diameter / gauge (if needed)
•Determine required heat transfer surface
•Determine tubesheet hole pattern (pitch)
服装设计图稿
•Process design sketchtitanic什么意思
Figure 2.
Mechanical Design
The major steps carried out during the mechanical design stage are shown in
Figure 3. The weld joint for the tubesheet may be one of either a strength weld or a al weld. Several acceptable designs for the welds are shown in Figure 4. When a strength weld is employed and properly tested it is not necessary to perform the tube end expansion step. However if only al
welding is done then expansion of the tube end is needed in order to achieve the required mechanical strength at the joint.
TUBE-TO-TUBESHEET JOINT
MECHANICAL DESIGN
•Calculations to determine thickness for: shell,
head(s), nozzles, tubing, tubesheet
•Number and location of rolling grooves •Baffle details
西安的托福学习机构•Expansion joint requirements
•General weld joint details •Flanged joints and bolting details
•
Tube-to-tubesheet joint details
Acceptable weld joints where a is not less than 1.4t
生活大爆炸第九季ACCEPTABLE TUBE-TO-TUBESHEET WELDS
Acceptable weld joints where a is less than 1.4t
Thermal Rating and Vibration Analysis
Thermal performance rating calculations are required in order to show that the heat exchanger will perform as expected.
A vibration analysis is a must for zirconium due to its notch nsitivity and low modulus of elasticity.
Figure 3.
Figure 4.
Quality Plan / Inspection and Test Plan
After the process design, mechanical design, thermal rating and vibration analysis are completed and approved, a complete and thorough quality plan and an inspection test plan must be prepared a
nd agreed to between the purchar and fabricator. The two documents must include all quality control steps, witness points, hold points and inspection approval points for construction and asmbly of the entire heat exchanger but more especially the steps involved with the manufacture of the tubesheet and the asmbly and testing of the tube-to-tubesheet joint.
TUBE-TO-TUBESHEET JOINT
Quality Plan
Quality plans can be stated in many ways and Figure 5 contains a typical one.
TUBE-TO-TUBESHEET JOINT
QUALITY PLAN
•Tube-to-tubesheet mock-up (approval)
•Shop traveler
•U of sub-vendors
•Welder qualifications
•Welding procedures
•Cleaning procedures
•Step quence for tube-to-tubesheet joint
completion including welding
•Special clofit per TEMA table RCB-7.41网络推广培训
Figure 5.
Inspection and Test Plan
Like quality plans, inspection and test plans can vary with heat exchanger types but major steps can be en in Figure 6.
TUBE-TO-TUBESHEET JOINT
INSPECTION & TEST PLAN
•Inspect tubesheet blank (explosion clad)
•Inspect finish machined tubesheet
•Witness attachment of tubesheet (if applicable)
•Witness cleaning of tubesheet holes and tube ends
•Witness inrtion of tubes and expansion of tube ends
•Witness welding of tube ends
•Witness air test
•Witness helium leak test
•Witness hydrostatic test
Figure 6.
Fabrication-Inspection-Testing of the Tube-to-Tubesheet Joint
The fabrication of a heat exchanger involves many operations of forming, machining, drilling and welding in order to manufacture all of the parts, which are to be asmbled to produce the final heat exchanger. Emphasis here is on fabricating the tubesheet, inrting the tube ends and curing them in the tubesheet holes whether by expanding only, welding only or a combination of expanding and welding. Tube end expansion may be done either by hydraulic means or by u of a conventional three roller expander. Welding may be done manually, with or without filler metal, or by u of miautomatic welding equipment.
Before the fabrication step is begun, a mock-up tube-to-tubesheet joint should be asmbled using the same materials and design that is propod for the full size heat exchanger. See Figures 7 and 8 for examples of a mock-up tube-to-tubesheet joint. The propod tube end expansion method and welding procedure should be employed and the mock-up cross-ctioned for macro examination in order to proof test all the parameters and establish the weld quality and weld penetration. In some unusual cas micro examination may also be employed to check weld quality.
