Double Tubesheet Heat Exchangers - Necessity and Challenges
Purushottam M Misal

Among all types of heat exchangers shell and tube types are in huge demand for industrial applications. This paper specifically talk about double tubesheet exchangers with covered connected shroud shell arrangement and it presents special precautions, guidelines for manufacturers during fabrication, testing and assembly of double tubesheet heat exchangers. It also presents two different assembly sequences which by implementing manufacturer can get a quality product.

These shell and tube heat exchangers are suitable for highly corrosive operating fluids and also stand for wide-spread of pressure and temperature conditions. However, all these heat exchangers are equipped with single partition between shell side and tube side fluid which is popularly known as tubesheet. Typically, there can be leakage through tube-to-tubesheet joint which are generally the weakest points in heat exchangers. This leakage can contaminate the other side with lower operating pressure adversely affecting the process parameters. These leakages can not be avoided even after properly designing the tube-to-tubesheet joint by using a strength welded and light expanded joint with appropriate mock up in the fabrication stage.

Applications
Double tubesheet heat exchangers are used for applications wherein mixing of tube side and shell side fluid must be avoided. For instance, Chlorosilanes, while being either on shell/tube side leaks through tubeto-tubesheet joint and mixes with water. It readily reacts with water to form corrosive hydrogen chloride gas and hydrochloric acid along with heat. Many Chlorosilanes evolve flammable gaseous hydrogen gas during exposure to water. Such scenario demand for non-mixing of shell side and tube side fluids.

Another example can be, Condensers in power plants. In the condenser application, water is used as a cooling medium. The cooling water (raw water ) can be sea water, river water, tank or pond water. As many a times, cooling water is brackish with lot of contaminants and since the steam side is under vacuum, this water can find a way into the steam condensed water through tubeto-tubesheet joint. Potential for leakage of cooling water arises from tube failures caused by a variety of factors. Mixing of cooling water contaminates the feed water, leading to its unacceptable chemistry.

Since, this condensate further goes to the hot well and from hot well the water is again pumped to the boiler with the help of boiler feed pump. The cooling water mixing with condenser water leads to many problems on the boiler side. The conductivity and pH level of the boiler water gets affected affecting the performance of boiler.

Thus, the primary concern is prevention of contamination of treated and demineralized water due to the leakage of circulating cooling water into the condenser steam space. To overcome this possibility, provision of double tubesheet construction has been made mandatory in couple of countries for power station Condensers.

So far, there is not any known method of joining tube-to-tubesheet which completely eliminates the possibility of leakage. With double tubesheet type construction, any leaks occurring through tube-to-tubesheet joint will accumulate in the space between two tubesheets instead of leaking and contaminating the fluid on the other side. Therefore, even though double tubesheets will not nullify the leakage, they will eliminate mixing of shell side fluid with tube side or vice-a-versa.

Construction
The conventional double tubesheet exchanger has two tubesheets at both ends of tubes. In general scenario, adjacent tubesheets are connected with each other with the help of tubes. Alternatively, shroud shells can be used to cover the gap between two tubesheets. In this case, leaked fluid from either side is collected in shroud shell.

Shell side tubesheet of double tubesheet U-tube unit, can be constructed with any of the attachment method suitable for removable bundle construction . In case of fixed-tubesheet arrangement, shell side tubesheet is welded with shell, whereas tube side tubesheet may be bolted or welded with channel. In case of drastic difference in Mean Metal Temperature of shell side and tube side and different metallurgy used for shell side, tube side tubesheet, shroud may be provided with expansion bellow as shown in Figure 1


Figure 1. Shroud with expansion below

Typically Tubular Exchangers Manufacturers Association (TEMA) covers three types of double tubesheet constructions. Integral (Figure 2), Connected (Figure 3) and Separate (Figure 4) double tubesheet type constructions.


Figure 2. Integral type double tubesheet construction


Figure 3. Connected type double tubesheet construction


Figure 4. Seperate type double tubesheet construction

In all three types of constructions care should be taken while designing the tube-totubesheet joint. Primarily, tube side tube-totubesheet joint needs to be strength welded with light expansion (Figure 5).


Figure 5. Tube to tubesheet (tube side) Joint

This is to nullify the possibility of leaking the tube side fluid through tube-to-tubesheet joint. On the other side shell side tube-to-tubesheet joint shall be grooved with minimum two grooves and expanded to the full length (Figure 6). This grooved expansion joint need to be selected considering the fact that, on shell side tube-to-tubesheet joint welding is practically impossible.


Figure 6. Tube to Tubesheet (shell side) joint

DESIGN:
In connected (Figure 3) and integral (Figure 2) double tubesheets, axial load distribution is taken care by interconnecting element/shroud shell (in case of connected double tubesheet) or integral portion of the tubesheet (in case of integral double tubesshet). In the integral double tubesheet construction, interconnecting element is so rigid that it distributes thermal and mechanical radial loads between the tubesheets and prevents the individual radial growth of tubesheets. For both constructions, tubes can mutually transfer all mechanical and thermal axial loads between the tubesheets.

In general, various types of stresses originated in the construction can be listed as:
  • Differential pressure stresses due to difference in operating pressures between tube and shell side fluids.
  • Axial stresses resulted due to tension or compression of the tubes. Differential thermal expansion between shell and tubes is another parameter that induces axial stresses.
  • Shear stresses induced due to differential thermal expansion between the tubes and tubesheet in radial direction.
  • Thermal and pressure stresses induced due to upset conditions.
Interconnecting element and tubes between tubesheets of the connected double tubesheet needs to be designed for below listed parameters:
  • Interconnecting element - Radial shear stress at the junctions due to differential thermal expansion of the tubesheets.
  • Interconnecting element - The combined stresses due to bending and axial tension induced due to differential thermal expansion of tubesheets and thermal expansion of tubes respectively.
  • Tubes - Axial tensile or compressive/buckling stresses acting because of operating pressure and thermal expansion.
Fabrication & Testing
Tube-to-tubesheet leak tightness is directly affected by how the double tubesheet exchangers are manufactured. A good quality of tubesheets, baffles and tube supports are produced by drilling the holes with the help of CNC Machines (Computerized Numerically Controlled) either individually or in stack. The CNC machines assures that holes in tubesheets, baffles and support plates are concentric and precise enough to allow them to be occupied by tubes easily. If tubesheets and baffles/support plates are stacked and drilled on conventional radial drilling machines, there is drift as drill penetrates the stack.

During assembly, hole-to-hole positions may also be displaced if tubesheet main center lines are not maintained congruently. Additionally, major difficulties may also be created, if tubesheets are not kept parallel with each other. For the above cited reasons, it is highly important for purchaser to review manufacturer's equipment/tools and techniques used for drilling and assembly.

Below are couple of guidelines for manufacturers to assure proper assembly:
  • Tube side and shell side faces of tubesheet shall be machined flat and perpendicular to tube (and bolt) holes. Adjacent faces of tubesheets shall also be machined in the similar fashion from just outside of OTL till tubesheet periphery.
  • Suitable number of Spacers either made from pipe, rod or plate shall be prepared preciously machined to the specified gap distance between the tubesheets.
  • Match marking/punching on tubesheet shall be done.
  • Align these match marking points on both tubesheets of each pair.
  • The spacers shall be placed equally on the periphery between the pair of tubesheets. Clamping of these aligned tubesheet pair shall be done. These clamping shall be kept in place until all tubing, tube-to -tubesheet joining, tubesheet to shell/channel assembly has been completed.
  • A GO gauge machined from a rod for a length somewhat longer than the distance between outer faces of tubesheets shall be prepared. Diameter of gauge shall be 0.05 mm less than the recommended TEMA standard drilled hole size with over tolerance of 0.00 mm and under tolerance of TEMA permitted hole under tolerance. The GO gauge is to ensure free entry of tubes in tube holes of both tubesheets. Before tubing the assembly, check randomly in each quadrant of tubesheet layout that the gauge is entering freely. Since non concentric holes in adjacent double tubesheets induce bending and shear forces on tubes and tubesheet ligaments, their concentricity is ensured with this GO gauge.
  • Tubesheet ligament tolerances shall be strictly ensured as per TEMA Table RCB 7.22 or RCB 7.22M. Since we are dealing with double Tubesheet constructions, these tolerances can be further made tighter based on manufacturer's capability and confidence.
Careful selection of type of tube-to-tubesheet joint, sequence of welding and expansion within the tubesheet is of utmost importance. Displaced holes and ligament distortions make it very difficult to produce tight expanded joints. The outer tubesheet joints can be made tight by welding. However, problem remains at the inner tubesheet, where joints can be only made by process of expanding as there is no access for welding.

In general, tube end rolling (expansion) within tubesheet shall always be done after welding of tube-to-tubesheet joint. This is mainly because of below reasons -
  • Tube expansion (rolling) before welding may leave lubricant from the tube expander in the tube holes. Lot of other fabrication impurities also gets accumulated at tube ends. Satisfactory welds are rarely possible under absence of extreme cleanliness.
  • During tube expansion before welding, expander pushes tubes against inside surface of tubesheet in the tube holes creating uneven gap between outer periphery of tube and tube hole within tubesheet. Successful welding with uneven weld gap is very difficult.
  • Tube-to-tubesheet joint welding after expansion creates uneven tube movement within tubesheet because of tube thermal expansion. This leads to non-uniform tube tightness with tubesheet surface within tube holes which was already achieved by rolling operation.
  • Tube-to-tubesheet joint welding after expansion will trap the welding gases in the space between outer tube surface and tubesheet hole.
During tube expansion care should be taken in such a way that the expanded portion should never extend beyond the shell-side face of the tubesheet, since removal of such a tube is extremely difficult.

In addition to this, tube expansion for inner tubesheets shall be done before welding to outer tubesheets.

Consequently, correct sequence of assembly and testing is very important while fabricating the double tubesheet construction. Especially in fixed tubesheet like TEMA L, M, N and outside-packed floating head (P type rear heads) where number of tubesheets become 4 considering double tubesheet arrangement. In such cases, insertion of tubes through all 4 tubesheets becomes very critical and many a times becomes a challenge. In the factory. U tube double tubesheet constructions are relatively easy in assembly.

Fabrication and assembly sequences has been presented below for fixed double tubesheet heat exchanger:

Method 1:
  • In case of small diameter shells - tubesheet/baffle/tie rod/spacer skeleton shall be made outside the shell considering inaccessible shell inside area. The same can be made inside Shell in case of bigger diameter Shell where operator can enter inside and work.


  • First bundle skeleton shall be made with tie-rod end tubesheet pair in place along with spacers and clamping as discussed in above paragraph. (see Figure A )
  • Insert above skeleton into the main shell. Nontie rod end tubesheet pair (along with spacers and clamping) shall also be kept in line. Tack welding of shell with shell side tubesheets shall be carried out. (see Figure B).
  • Tubes shall be inserted from tie rod end tubesheet pair through the skeleton and guided through the holes of nontie rod end tubesheet pair. Guiding rod typically very small diameter (less than tube inside diameter) shall be used from the opposite end (non-tie rod end) for enabling tube entry through holes in tubesheets and baffles/support plate. (see Figure C)
  • Tubesheets to main shell welding and NDE shall be carried out. (see Figure C)
  • Both ends shell side tube-to-tubesheet joint expansion in grooves shall be carried out. Length of mandrel shall be suitable for tube expansion inside the tubesheet. (see Figure D)
  • Both ends channel side tube-totubesheet joint strength welding and light expansion shall be carried out. (see Figure D)
  • Tube to shell side tubesheet joint leak testing (with helium or air) shall be carried based on project specific requirements. (see Figure D)
  • Tube-to-tubesheet joint on shell side shall be tested for shell side hydrotest pressure. Any leakage can be found out with necked eyes from the free space between pair of tubesheets. (see Figure D)
  • Channel assembly which has been made ready in parallel shall be connected and bolted with main shell assembly. (see Figure E)
  • Tube-to-tubesheet joint along with other tubeside joints shall be tested for tube side hydrotest pressure and any leaks can be cited from the free space between pair of tubesheets. (see Figure E)
  • Shroud shell shall be rolled separately in two pieces and match fitted to ensure perfect roundness.
  • After completion of all the tests, tubesheet spacers and clamping arrangement shall be removed. Shroud shell shall be inserted in the space between pair of tubesheets in two different parts. It shall be then welded along the length with root run by TIG. Shroud shell shall then be welded with tubesheets. (see Figure F)
  • Shroud shell hydrotest is not required.
Basically, in Method-1, shroud shell is fitted at the very end. This gives the scope for visibility through the space between pair of tubesheet especially during hydrotest.

Method 2
Method-1 may have difficulty in inserting the shroud shell in two parts, welding along the length and then with tubesheet. To overcome this difficulty, Method-2 has been presented below. All the steps in Method-1 shall be followed except below:
  • Shroud Shell shall be first made ready and it shall be tack welded with one of the tubesheet on tie-rod end side and pair of tubesheet shall be made ready. In this arrangement tubesheet clamping is still required however, tubesheet spacers can be avoided as shroud shell will now act as spacers. In the similar fashion Non-Tie rod end tubesheet pair shall also be made ready.
  • Tube-to-tubesheet joint on shell side shall be tested for shell side hydrotest pressure. Any leakage can be detected with drop in pressure, as now with presence of shroud shell there is no visibility in the space between pair of tubesheets.
Manufacturer can alter the intermediate fabrication and testing sequences based on shop facilities, experience and individual technical capability.



Demerits of Double Tubesheet Heat Exchangers
  • Although exchanger total surface area is more, effective surface area reduces significantly due to the fact that effective tube length is measured between inside faces of shell side tubesheet. Tube length surface area in the shroud area is not considered as a heat transfer area. This increases required tube length and in turn overall length of exchanger, further increasing cost of the heat exchanger.
  • Addition of two more tubesheets in double tubesheet construction increases the cost further.
  • As discussed in previous sections there are many criticalities and difficulties involved in tubesheet/baffle/support plate drilling and machining specially to achieve tube hole concentricity and tubesheet surface parallelism. In addition to this, there are challenges in correct sequence of assembly which makes it difficult to produce quality product.
  • Maintenance of these heat exchangers can be very difficult especially tube removal since the tube has been fixed with tubesheets at 4 places.
  • The arrangement is only possible in fixed tubesheet, U tube and outside-packed floating head.
Conclusion
A well planned fabrication and assembly sequences can be useful while manufacturing double tubesheet heat exchangers.

Apart from Power Plants, these heat exchangers are also required in Pharmaceutical Industry for Sanitary applications and are designed to meet high quality requirements and hygienic standards of Pharmaceutical industry. These types are also required in Polysilicon manufacturing plants which then used in Solar Power Plants.

This paper presents reasons for selection of such type of heat exchangers, various types of stresses in tubes, tubesheet and shroud shell, fabrication, assembly sequences and testing methodologies. Out of two proposed assembly sequence methods, any one method can be adopted by the manufacturer.

References
1. Standards of the Tubular Exchanger Manufacturers Association (TEMA), 9th Edition, New York

2. Perry's Chemical Engineers Handbook, Seventh Edition, Mc-Graw Hill Publications

3. A Working Guide to Shell and Tube Heat Exchangers, Stanley Yokell, Mc -Graw Hill, 1990.