DESC:TUBESHEET ATTACHMENT IN HIGH PRESSURE HEAT EXCHANGER AND METHOD OF CONSTRUCTION THEREOF

Field of the invention:

The present invention relates to heat exchangers and more specifically, to a tubesheet attachment in high pressure screw plug or breech lock heat exchanger used in refinery and other applications of process plant equipment.

Background of the invention:

High pressure heat exchangers are widely used in critical services in process industries such as Hydrocracking unit, Hydrotreating unit, Hydrowaxing unit, Hydrofining units etc. These are known as Breech Lock or Screw Plug Exchangers. These are basically U-tube exchangers with channel side operates at high temperature and pressure and it has a threaded end closure. In some designs, the shell side may also be at high pressure. In all these types of applications, there is a need for removal of tube bundle for maintenance and cleaning of tubes at regular intervals to maintain the heat transfer efficiency. Such heat exchangers can be of following three types:-

1) HH Type: In HH Type of high pressure screw plug or breech lock heat exchanger as shown in fig. 1, shell and channel are integral and bundle is inserted inside shell or channel. Gasketted joint is provided below the tubesheet to achieve separation of shell side (16) and channel side (17) fluid. Referring to Figure 1 for the constructional details, the tubesheet (1) and tubes (2) are generally designed for differential pressure and have internal apparatus with a channel box (10), a split ring (6), an internal flange (7), a push bolt (8) for sealing the tubesheet (1) against the shoulder of the channel (4) using a tubesheet gasket (11). The bundle is removable type for maintenance purpose.

The drawbacks associated with HH type of heat exchangers are:
• Intermixing between shell side and tube side fluid has been observed in few such heat exchangers.
• This intermixing results in deterioration of quality of final product which is found not acceptable.
• Recent or proposed environmental norms or laws require the products quality to be much more stringent. Thus even minor intermixing between shell or channel side found is not likely to be acceptable.
• Though there is provision in the design to externally tighten the gasketted joint, yet leakage once initiated makes it very difficult to achieve the leak tightness. Also every upset operating condition encountered during plant operation initiates the leak which requires frequent external tightening and eventually leading to shutdown.

2) HL Type: HL Type of high pressure screw plug or breech lock heat exchanger as shown in fig.2 has tubesheet (1) welded or integral with channel and shell is flanged which is bolted to the back side of tubesheet (1). Referring to Figure 2 for the constructional details, tubesheet (1) is welded or integral with channel (4) and bolted with shell flange (14). Sealing of shell side fluid against the outside atmosphere is achieved by shell flange gasket (12). Shell assembly is removable type to expose tube bundle for maintenance or cleaning purpose.

The drawbacks associated with HL type of heat exchangers are:
• In this type of equipment, shell side fluid is sealed by shell flange gasket (12) which is exposed to the atmosphere. In case of leakage through this gasketted joint, shell side fluid will directly leak to the atmosphere. Since these equipment contains high pressure, high temperature flammable fluids (such as H2), any leakage to the atmosphere may have safety hazards.

3) Tubesheet welded with the shell or channel side: This construction can be used for both HH and HL type of design. Referring to Figure 3 for the constructional details, the tubesheet (1) is welded with shell (13) as well as with channel (4).
The drawbacks associated with welded type of heat exchangers are that:
• Construction of such equipment having weld overlay on shell side would require closing weld (between shell and tubesheet or between shell and head) to be of dissimilar material which are not recommended due to potential failure.
• Due to welded construction of tubesheet, tube bundle will not be accessible for maintenance, cleaning or inspection purposes and hence may pose issues in thermal performance.

Accordingly, there exists a need to provide a method of construction tubesheet attachment in the high pressure heat exchanger which would be alternate to above options and overcome above-mentioned drawbacks.

Objects of the invention:

An object of the present invention is to provide various design configurations for attaching tubesheet to pressure boundary shell or channel by means other than gasketted joints. On such configurations in present invention is to provide tubesheet with skirt welded to shell or channel.

Another object of the present invention is to provide method of construction of the tubesheet with expansion bellows assembly and internal apparatus to counter pressure thrust.

Summary of the invention:

Accordingly, the present invention provides tubesheet attachment in high pressure heat exchanger and method of construction thereof. The high pressure heat exchanger includes a tube bundle with the tubesheet having two sides a shell side and a tube side, the tube bundle having a plurality of tubes filled with tube fluid therein, a shell filled with shell fluid therein, a channel header, an expansion bellows and internal apparatuses including a split ring, an internal flange and a push bolt arrangement. The tubesheet attachment comprises a skirt shell welded to the tubesheet at one end and to the channel header at other end. The skirt shell is preferably in cylindrical shape acting as a seal to isolate the shell fluid from the tube fluid. A first weld is formed between the skirt shell and the channel header. A second weld is formed between the tubesheet and the skirt shell depending upon design configurations of the skirt shell. While attaching the tubesheet with the expansion bellows and the internal apparatuses, a stub is welded to the channel header at one end and to the expansion bellows that is welded with tubesheet at other end. The method eliminates external gasketted joint, intermixing of fluids and permits removal of a tube bundle for maintenance or cleaning.

Brief description of the drawings:

The objects and advantages of the present invention will become apparent when the disclosure is read in conjunction with the following figures, wherein

Figure 1 shows the constructional details of HH type of high pressure screw plug or breech lock heat exchanger, in accordance with the prior art;

Figure 2 shows the constructional details of HL type of high pressure screw plug or breech lock heat exchanger, in accordance with the prior art;

Figure 3 shows the constructional details of tubesheet welded with the shell or channel side which can be used for both HH / HL type of design, in accordance with the prior art;

Figures 4 (a) to 4 (f) show various design configurations of a tubesheet attachment with a skirt shell, in accordance with the present invention; and

Figure 5 (a) and 5 (b) shows the attachment of the tubesheet with expansion bellows and internal apparatuses, in accordance with the present invention.

Detailed description of the invention:

The foregoing objects of the present invention are accomplished and the problems and shortcomings associated with the prior art, techniques and approaches are overcome by the present invention as described below in the preferred embodiments.

Accordingly, present invention provides a method of construction of tubesheet having skirt which is attached to pressure boundary shell or channel of heat exchanger by welding instead of gasketted joint. This tubesheet system is used for tube bundles in high pressure screw plug or breech lock heat exchanger in refinery and other applications, wherein intermixing between shell and tube side fluid through the gasketted joint can take place during operation especially due to upset conditions which results in degradation of quality of final product. The present invention ensures tubesheet attachment to the channel header to provide leak tightness of the joint yet permits removal of the tube bundle for maintenance purpose by removing the attachment weld.

This present invention is illustrated with reference to the accompanying drawings, throughout which reference numbers indicate corresponding parts in the various figures. These reference numbers are shown in bracket in the following description and in the tables below.

Part No Part Name
1 and 51 Tubesheet (as per prior art and present invention respectively)
2 Tubes
20 Skirt shell
30 First weld
40 Second weld
4 and 45 Channel Header (as per prior art and present invention respectively)
50 Expansion Bellows
55 Stub
6 and 60 Split Ring (as per prior art and present invention respectively)
7 and 70 Internal Flange (as per prior art and present invention respectively)
8 and 80 Push Bolt (as per prior art and present invention respectively)
9 and 90 Push Rod (as per prior art and present invention respectively)
10 Channel Box
11 Tubesheet Gasket
12 Shell Flange Gasket
13 and 53 Shell (as per prior art and present invention respectively)
14 Shell Flange
15 Annular Plate
66 Shell side
77 Channel Side
100 High Pressure Heat Exchanger

Now referring to figures 4 (a) to 4 (f) and 5 (a) to 5 (b), a tubesheet attachment in a high pressure heat exchanger (100) and a method of construction thereof, in accordance with the present invention is shown and described herewith.

The high pressure heat exchanger (100) includes a tube bundle with the tubesheet (51), a shell (53), a channel header (45), an expansion bellows (50) and internal apparatuses. The tube bundle includes a plurality of tubes filled with tube fluid therein. The shell (53) is filled with shell fluid therein. The tubesheet (51) has two sides a shell side (66) and a tube side (77). The internal apparatuses includes (60), an internal flange (70) and a push bolt (80) arrangement. The tubesheet attachment in a high pressure heat exchanger (100) comprises a skirt shell (20) welded to the tubesheet (51) at one end and to the channel header (45) at other end. The skirt shell (20) acts as a seal to isolate the shell fluid from the tube fluid. There are two welds. A first weld (30) is formed between the skirt shell (20) and the channel header (45). A second weld (40) is formed between the tubesheet (51) and skirt shell (20) depending upon the design configurations of the skirt shell (20). The tubesheet attachment further comprises a stub (55) welded to the expansion bellows that is welded to the tubesheet (51) at one end and to the channel header (45) at other end. This type of the tubesheet (51) attachment eliminates the gasketted joint and intermixing of the shell side (66) and the tube side (77) fluid observed in HH type (figure 1) or leakage of fluid to atmosphere in HL Type (figure 2).

The method comprises steps of welding the tubesheet (51) to the channel header (45) with the skirt shell (20). In an embodiment, the welding can be carried out by any of welding processes including Gas Tungsten Arc Gas Welding (GTAW/TIG), Shielded Metal Arc Welding (SMAW), Submerged arc welding (SAW) and like. There are two welds. The method includes forming the second weld (40) at first step and forming the first weld (30), at next step.

The first weld (30) is formed between the skirt shell (20) and the channel header (45). In the embodiment, the first weld (30) is small removable fillet weld (30). During shut down, the first weld (30) can be removed and entire tube bundle can be pulled out of shell (53) or channel header (45) assembly for cleaning of tube bundle.

The second weld (40) is between the tubesheet (51) and skirt shell (20) depending upon the design configurations of the skirt shell (20). In the embodiment, the second weld (40) is full penetration weld (Corner/Butt). In the embodiment, shape of the skirt shell (20) depends on the application of the high pressure heat exchanger (100). In a preferred embodiment, the skirt shell (20) is cylindrical shape. The various embodiments for the attachment of the tubesheet (51) to the channel header (45) with the skirt shell (20) with its attachment to tubesheet (51) in the heat exchanger (100) are shown in Figures 4 (a) to 4 (f) for various design configurations of the skirt shell (20).

Selection of various types of weld joint configuration and location of the second weld (40) is very much crucial and it is depended on the selection of tubesheet (51) material and skirt shell (20) material. Depending upon the type of service in which equipment will be operated, the tubesheet design is carried out as shown in fig 4(a) to fig 4(f). The location of the second weld is finalized from various joint configurations as shown in Figs. 4(a) to 4(f). Based on various considerations such as,
a) available space in channel header (45),
b) tubesheet (51) design configuration,
c) skirt shell (20) length and
d) feasible and appropriate Nondestructive testing (NDT) technique to ensure weld soundness
Since, the second weld joint (40) is subjected to severe stress condition, it is essential to select the correct weld joint configuration.

In an alternate embodiment, the proposed invention can also be applied in outer body (the shell (53) or the channel header (45)) of existing heat exchangers on case to case basis.

The method further comprises the attachment of the tubesheet (51) with the expansion bellows (50) and the internal apparatuses as shown in Figures 5 (a) and 5(b). The stub (55) of the expansion bellows (50) is welded to the tubesheet (51) at one end and to the channel header (45) at other end. Fig 5(a) and Fig 5(b) show various joint configurations for the stub (55) welded to the channel header (45). As shown in Fig 5(a), the expansion bellows (50) can be attached as fillet weld. Alternatively, as shown in Fig 5(b) the expansion bellows (50) can be placed in groove made in the tubesheet (51) and the stub (55). In this design, the pressure thrust from the shell side (66) is resisted by internal apparatus with a split ring (60), an internal flange (70) and a push bolt (80) arrangement and sealing between the shell side fluid and the tube side fluid is achieved using the expansion bellows (50) and the stub (55). The pressure thrust from the channel side (77) is directly transmitted through the tubesheet (51) to a step (not numbered) created in the channel header (45) and the shell assembly (53). During shut down after removing internal apparatus, a first weld (30) between the stub (55) and the channel header (45) can be removed and entire the tube bundle can be pulled out of the shell (53) or the channel header (45) assembly for cleaning of the tube bundle.

Advantages of the invention:

1. The tube bundle with a tubesheet (51) as per proposed invention provides a solution to replace the gasketted joint between the shell side fluid and the tube side fluid by welded construction to eliminate the risk of intermixing of the fluids compared to HH Type (Figure 1).
2. The tube bundle with the tubesheet (51), as per proposed invention provides a solution to eliminate external gasketted joint as compared to HL type (Figure 2).
3. The tube bundle with tubesheet (51), as per proposed invention is removable (by cutting the small weld) to permit its removal for maintenance or cleaning, which is not possible in all welded construction (Figure 3).
4. The tube bundle with tubesheet (51), as per proposed invention can be easily fitted in existing outer body (the shell (53) / the channel header assembly (45)) of the heat exchanger and hence capital expenditures for new equipment are avoidable.
5. The method of the present invention can be used for U tube / fixed tubesheet / floating tubesheet construction.

The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the present invention and its practical application, to thereby enable others skilled in the art to best utilize the present invention and various embodiments with various modifications as are suited to the particular use contemplated. It is understood that various omission and substitutions of equivalents are contemplated as circumstance may suggest or render expedient, but such are intended to cover the application or implementation without departing from the scope of the present invention. ,CLAIMS:We claim:

1. A tubesheet attachment in a high pressure heat exchanger (100), the high pressure heat exchanger (100) having,
a tube bundle with the tubesheet (51) having two sides a shell side (66) and a tube side (77), the tube bundle having a plurality of tubes filled with tube fluid therein,
a shell (53) filled with shell fluid therein,
a channel header (45),
an expansion bellows (50) and
internal apparatuses including a split ring (60), an internal flange (70) and a push bolt (80) arrangement,
the tubesheet (51) attachment comprising:
a skirt shell (20) welded to the tubesheet (51) at one end and to the channel header (45) at other end, wherein the skirt shell (20) is cylindrical shape acting as a seal to isolate the shell fluid from the tube fluid;
a first weld (30) between the skirt shell (20) and channel header (45); and
a second weld (40) between the tubesheet (51) and the skirt shell (20) depending upon design configurations of the skirt shell (20),
wherein, the first weld (30) between the skirt shell (20) and the channel header (45) is removed and entire tube bundle is pulled out of the shell (53) or channel header (45) assembly for cleaning of the tube bundle, thereby eliminating a gasketted joint and intermixing of fluids from the shell side (66) and the tube side (77).

2. The tubesheet attachment in the high pressure heat exchanger (100) as claimed in claim 1, wherein weld joint configuration and location of the second weld (30) depends on
• selection of tubesheet (51) material and skirt shell (20) material, available space in channel header (45),
• design configuration of the tubesheet (51),
• length of the skirt shell (20) and
• feasible and appropriate Nondestructive testing (NDT) technique to ensure weld soundness.

3. The tubesheet attachment in the high pressure heat exchanger (100) as claimed in claim 1 further comprises a stub (55) welded to the expansion bellows that is welded to the tubesheet (51) at one end and to the channel header (45) at other end.

4. The tubesheet attachment in the high pressure heat exchanger (100) as claimed in claim 1, wherein the stub (55) is welded to the expansion bellows in any one of ways selected from:
• the expansion bellows (50) is attached as fillet weld, and
• the expansion bellows (50) is placed in groove made in the tubesheet (51) and the stub (55).

5. A method of construction of a tubesheet (51) in a high pressure heat exchanger (100), the high pressure heat exchanger (100) having,
a tube bundle with the tubesheet (51) having two sides a shell side (66) and a tube side (77), the tube bundle having a plurality of tubes filled with tube fluid therein,
a shell (53) filled with shell fluid therein,
a channel header (45),
a skirt shell (20) acting as a seal to isolate the shell fluid from the tube fluid,
an expansion bellows (50) having a stub (55) and
internal apparatuses including a split ring (60), an internal flange (70) and a push bolt (80) arrangement,
the method comprising steps of:
welding the tubesheet (51) to the channel header (45) with the skirt shell (20);
forming a first weld (30) between the skirt shell (20) and the channel header (45); and
forming a second weld (40) between the tubesheet (51) and the skirt shell (20) depending upon design configurations of the skirt shell (20), thereby eliminating a gasketted joint and intermixing of fluids from the shell side (66) and the tube side (77).

6. The method of construction of the tubesheet (51) in the high pressure heat exchanger (100) as claimed in claim 1, wherein the first weld (30) is small removable fillet weld (30) allowing to pull out of the shell (53) and/or the channel header (45) for cleaning of the tube bundle.

7. The method of construction of the tubesheet (51) in the high pressure heat exchanger (100) as claimed in claim 1, wherein the second weld (40) is full penetration weld (Corner/Butt).

8. The method of construction of the tubesheet (51) in the high pressure heat exchanger (100) as claimed in claim 1 further comprises attaching the tubesheet (51) with the expansion bellows (50) and internal apparatuses, wherein the stub (55) is welded to the expansion bellows that is welded to the tubesheet (51) at one end and to the channel header (45) at other end such that the pressure thrust from the shell side (66) is resisted by the internal apparatuses and sealing between the shell side fluid and the tube side fluid is achieved using the expansion bellows (50) and the stub (55).

9. The method of construction of the tubesheet (51) in the high pressure heat exchanger (100) as claimed in claim 1, wherein the pressure thrust from the channel side (77) is directly transmitted through the tubesheet (51) to a step created in the channel header (45) and the shell assembly (53).

10. The method of construction of the tubesheet (51) in the high pressure heat exchanger (100) as claimed in claim 1, wherein the welding is carried out by any of welding processes including Gas Tungsten Arc Gas Welding (GTAW/TIG), Shielded Metal Arc Welding (SMAW), Submerged arc welding (SAW) and like.

Dated this 28th day of January, 2019

Madhavi Vajirakar
(Agent for applicant)
IN/PA-2337