| Double Tubesheet Heat Exchangers - Necessity and Challenges | |
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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:
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:
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 -
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:
 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:
 Demerits of Double Tubesheet Heat Exchangers
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. |
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