April 2022

Heat Transfer

Uncommon lessons: Shell-and-tube heat exchangers—Part 1

Shell-and-tube heat exchangers (STHEs) are the most common heat transfer process equipment in all types of industrial plants.

Shell-and-tube heat exchangers (STHEs) are the most common heat transfer process equipment in all types of industrial plants. Due to the variety of available construction options, an STHE can be suitably designed for most processes and design conditions and can be constructed from different materials or combination of materials. STHEs are typically manufactured in accordance with international or national codes or standards, with the American Society of Mechanical Engineers (ASME) Code¹ being the most popular, along with associated Tubular Exchangers Manufacturers Association (TEMA)² and American Petroleum Institute (API) Standard 660³ requirements.

According to the ASME Code, an STHE is a multi-chamber vessel, with two chambers for most constructions that are commonly addressed as shell-side and tube-side or channel-side. In an STHE, the tubesheet, tubes, floating head (if applicable) and internal expansion bellow (if applicable) are the common elements. Depending upon process needs, the STHE may have a variety of external and internal components, such as cylinders, concentric and/or eccentric cones, dished heads, thick or thin expansion bellows, tubes (straight, u-tube, low fin, twisted, etc.), girth flanges, tubesheets, bolted covers, nozzles, multi-purpose nozzles, baffles, tie-rods, impingement rods, sealing strips, sliding strips, bundle pulling eye bolts, saddle support, lug support and skirt support (for large and long exchangers). FIG. 1 illustrates a floating head type heat exchanger and its major parts/components.

Apart from designing components to comply with applicable pressure and coincident temperature conditions and evaluating thicknesses or stresses in different components, there are specific requirements in the ASME Code, TEMA Standard and the API Standard 660 that are often not complied with in design and early fabrication stages. Based on the authors’ opinion, failure to identify these during design reviews leads to costs that must be corrected at later stages in fabrication.

This article focuses on specific types of lessons and design limitations that none of the commercial design programs (software, Excel spreadsheets, etc.) will identify with their “help” menus or warnings. Merely checking drawings with respect to design calculations is insufficient to ensure thorough compliance and reliable product design that will operate (thermally and mechanically) as intended.

The authors’ purpose is to shed light on those uncommon but valuable lessons that are not discussed in reference manuals or design guides. These lessons will be helpful in engineering and design activities.

Maximum allowable working pressure (MAWP)

The authors encountered shell-side MAWP calculation for an exchanger with a thick (flanged and flued) expansion bellow. This expansion bellow was designed (separately) using finite element analysis (FEA) with shell-side design pressure; therefore, the MAWP for the expansion bellow is the shell-side design pressure. Surprisingly, a higher shell-side MAWP obtained from a design report generated via a computer program was indicated on the drawings and accordingly was carried over in the ASME forms and equipment nameplate. The higher MAWP was based on the design of other parts of the shell-side chamber, excluding the expansion bellow.

Note: When a part of a pressure chamber (in this example, the bellow) is designed using an FEA program, the pressure used to design the part becomes the MAWP of that part, unless a higher MAWP is used in the FEA. This should not be missed when evaluating the governing MAWP of the chamber. The MAWP of the chamber is the lowest MAWP of all the parts that make up the chamber. Due to the complexity of expansion bellow calculations, the computer program may not be able to model the equipment inclusive of the expansion bellow to determine the shell-side MAWP. When any part of an exchanger is designed using an FEA, the pressure used in the FEA should be considered for MAWP calculation; otherwise, a higher pressure may get stamped on the nameplate and ASME forms.

A similar situation may arise for tube-side, single-pass, TEMA-type AES exchangers that are provided with tube-side internal expansion bellows. The bellows are designed considering shell-side and tube-side design conditions separately—it is possible that the internal bellows are not verified to shell-side and tube-side MAWPs.

Design of common elements

Regarding design pressure for common elements, such as tubes, tubesheet and floating head assembly, when a full vacuum exists on the shell-side or tube-side (or both sides), vacuum pressure on one side should be added to the design pressure of the opposite side to arrive at the final design pressure for the applicable side, according to UG-21 of the ASME Code.¹

The authors have encountered many heat exchangers in which the floating head cover and flange, and tubes were designed without considering the full vacuum on the shell side.

Coefficient of thermal expansion (TE)

The coefficient of thermal expansion (TE) is required to verify the shell-side differential thermal expansion stresses. The TE values in Tables TE-1 to TE-5 of ASME Section II-D⁴ provide the values of TE coefficients for temperatures down to 20°C (68°F).

When evaluating the need for the expansion bellow for heat exchangers in low-temperature and cryogenic service, the TE at the mean metal temperature (MMT) of the materials should be used. It is obvious that at lower temperatures, materials will experience contraction rather than expansion. A common understanding among code users is to apply the proper value of TE from ASME Section II-D (Material Properties) to perform mechanical design. However, TE at MMT below 20°C (68°F) is not contained in ASME Section II-D because the Materials Code only provides TE down to 20°C (68°F), while for this type of design condition it is necessary to use a TE value of far below 20°C (68°F).

If the TE at a sub-zero MMT is unavailable, other references should be explored to source TE at a sub-zero MMT to use in the analysis rather than using the TE at 20°C (68°F). This critical information is available in ASME B 31.3 (Piping Code)⁵ or any other reference book. This was clarified by ASME Code¹ in the following interpretation number 14-1240.

Standard designation: BPV Section VIII Div. 1

Edition 2010/Addenda 2011

Paragraph/Figure/Table No.: UHX-13

Subject description: Axial differential thermal expansion between tubes and shell

Date issued: 04/08/2015

Record Number: 14-1240

Question (1): Are the rules in Part UHX valid if the MMT of the shell or tubes, or both, are below 20°C (68°F)?

Reply (1): Yes

Question (2): In accordance with paragraph U-2(g) and the Introduction to Subpart 2 Physical Properties Tables in Section II, Part D, may other sources be used for values of TE when the material or temperature is not listed in Tables TE of Section II, Part D?

Reply (2): Yes.

In one project that contained fixed tubesheet exchangers in cold service, it was observed that the design calculation report mentioned a TE value at 20°C (68°F), even if the value of MMT was in negative single or double digits because the computer program referred to ASME Section II, Part D database in which the lowest temperature for TE is 20°C (68°F), and the same was used for developing the expansion bellow design. This led to a bellow design with an incorrect number of bellow convolutions.

Length of tube expansion for UHX calculations

ASME and TEMA codes^1,2 specify the tube expansion details with or without grooves; however, light expansion is not mentioned in either of these codes. API Standard 660³ mentions light expansion as one where tube wall thinning is < 2%. In many instances, the exchanger data sheet specifies strength welded tubes to be lightly expanded for a tube-to-tubesheet joint (TTSJ).

The purpose of light expansion is to establish metal-to-metal contact between tube and tubesheet. Light expansion of tubes in tube holes does not, in any capacity, provide mechanical strength to overall TTSJ capacity except by centering the tubes in the tube holes and helping to reduce crevices (large gaps) between the tube hole and the tube outside diameter (OD). Mock-up tests for such a TTSJ do not include expansion related parameters (torque, tube wall thinning, etc.), and the joint is purely qualified considering the weld for load carrying capacity.

ASME Code Part UHX calculations for tubesheets require the length of tube expansion in the calculations to be considered in those equations. The description of ℓtx in nomenclature of UHX-5.1 emphasises various factors to be considered for expansion length, which is clearly shown in this excerpt from ASME Code:¹

ℓtx = expanded length of tube in tubesheet (0 ≤ ℓtx ≤ h) [see Figure UHX-11.3-1, sketch (b)]. An expanded tube-to-tubesheet joint is produced by applying pressure inside the tube such that contact is established between the tube and tubesheet. In selecting an appropriate value of expanded length, the designer shall consider the degree of initial expansion, differences in thermal expansion, or other factors that could result in loosening of the tubes within the tubesheet.

If the length of light expansion is considered in the strength calculation of the tubesheet, the tubesheet calculation will provide significantly reduced required thickness, as the equations are written so that the expansion length will add to the strength of the tubesheet; however, the equations do not differentiate between light and heavy expansion and only heavy expansion will aid in strengthening the TTSJ.

The industry uses different terminology for expansion, one being light expansion, or light-rolled and expanded with or without groves. Light expansion is considered when tube thinning is 1%–3%; mechanically, this aids the tube OD to contact the tubesheet hole inside diameter (ID).

Conversely, tube expansion carried out for strength purpose has a minimum wall thinning of 5%—depending on the material of the tube and tubesheet. Often, such expansion is provided with grooves in the tubesheet where tube material will flow during the expansion process. It is the engineer’s choice to take advantage of tube expansion in the tube hole. As discussed earlier, light expansion does not contribute to the TTSJ capacity. If the TTSJ is lightly expanded tubes, the expansion length of the tubes should not be considered in the calculations (as its effect towards strengthening the tubesheet will be nearly “zero”). FIG. 2 shows different types of commonly used tube-to-tubesheet joints in the industry.

The authors noted tubesheet calculations that were using the lightly expanded length of tubes in the equations and arriving at thinner tubesheets to save material thickness and cost, jeopardizing the strength and long-term reliability of the TTSJ. In those examples, the tubesheets were calculated 20 mm thinner than the actual required thickness computed without considering the lightly expanded length of tubes in the calculations.

Incorrect flange finish

Kammprofile, camprofile or “grooved” gaskets are made of a grooved, solid metal core with soft coating of suitable non-metallic sealing layer(s) [graphite, polytetrafluoroethylene (PTFE), etc.] on each side. According to API Standard 660, the gasket contact surface required for a kammprofile gasket is 125 micro-in.–250 micro-in. However, the authors noted that for some heat exchangers, a 63 micro-in. surface roughness finish was used for gasket faces. A flange gasket surface roughness of 63 micro-in. is appropriate for solid metal gaskets, while a kammprofile gasket uses solid metal (core) but has a soft covering on the top and bottom faces; therefore, a higher surface roughness is recommended. This type of inaccuracy will lead to a non-performing body flange and is likely to jeopardize the exchanger.

Incorrect gasket factor “m” and seating stress “Y” for a kammprofile gasket

The value of “m” and “Y” for a kammprofile gasket varies from manufacturer to manufacturer. To perform the mechanical design of a girth flange using a kammprofile gasket, the correct gasket factor “m” and seating stress “Y” values must be obtained from the manufacturer to ensure the production gasket specifications match the values used in the design of the girth flange for production. In one case, the mechanical design of a girth flange with a kammprofile gasket was performed using “m” and “Y” values from internet sources; when the actual gasket manufacturer’s certificate was received, the “m” and “Y” values were larger compared to those used in the design and construction of the girth flange. The larger “m” and “Y” values per the gasket manufacturer’s certificate overstressed the flange, causing the already-manufactured girth flange to fail ASME Code requirements.

Shell-side vent and drain nozzle for vertical fixed tubesheet heat exchanger

All chambers of shell-and-tube heat exchangers should be capable of being vented and drained independent of process piping. A fixed tubesheet heat exchanger is often provided with vent and drain connections. For a horizontal exchanger, there are many location choices for the vent and drain nozzles; however, for a vertical fixed tubesheet exchanger—if provided on the shell—there is always some dead space left out without complete venting and draining. The preferred method is to provide vent and drain on the tubesheet rather than on the shell: this allows complete venting and draining of shell-side fluid. Tubesheets are drilled vertically through half of the thickness and then further drilled horizontally to vent or drain out to the atmosphere, as shown in FIG. 3; and a similar arrangement can be applied to drain nozzle.

Lack of baffle support (to the tube-bundle) due to a flanged and flued expansion joint

A flanged and flued expansion joint is typically provided with an internal sleeve to assist the shell-side flow; however, a sleeve may not be necessary in a vertical exchanger. An internal sleeve is necessary for process reasons to minimize the frictional losses or pressure drop and ensure smooth shell-side flow.

For horizontal exchangers with a flanged and flued joint, an internal sleeve is purposely required for mechanical reasons and to support the baffle (as well as prevent the tube bundle from sagging) because a possibility exists that one or two baffles will most likely be located along the length of the expansion joint where a portion of cylindrical shell will be unavailable due to placement of the expansion joint. Without a sleeve or liner, a baffle (or baffles) that falls within the ends of the flanged and flued joint will lack shell support. The sleeve acts as the shell and provides mechanical support to the tube bundle to resist gravity.

Without the sleeve, maldistribution of shell-side flow, resonant vibrations or sagging of the tube bundle—or all these combined—can ultimately cause the exchanger to fail. The authors encountered some horizontal exchangers with flanged and flued expansion joints in which internal sleeves were not provided.

Consider FIG. 4C: a sleeve is still required where baffles are clearing the expansion joint, which is not designed to share part of the bundle weight (acting as a point load). For the situation shown in FIG. 4B, a sleeve is necessary to provide baffle support. Providing a sleeve will also give more flexibility with respect to baffle location. FIG. 4A illustrates an acceptable construction.

Design review experience is acquired by continuous practice and improvement, and the authors believe that the feedback of uncommon lessons is helpful. 

Part 2 

Part 2 of this article will appear in the June issue. HP

 

LITERATURE CITED

1. American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code, Section VIII, “Rules for construction of pressure vessels—Division 1,” New York, New York, July 2019.
2. “Standards of the Tubular Exchangers Manufacturer’s Association Inc. (TEMA), 10th Ed., 2019.
3. American Petroleum Institute (API) Standard 660, “Shell-and-tube heat exchangers,” 9th Ed., Washington, D.C., May 2020.
4. American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code, Section II-D, “Material properties,” New York, New York, July 2019.
5. American Society of Mechanical Engineers (ASME), Boiler and Pressure Vessel Code, B31.3, “Process piping,” New York, New York, July 2018.

The Authors

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