Heat Exchanger Standards for Shell & Tube Equipment Part II
API 660 has a process focus. It is not a manufacturers’ standard, but rather an end-user standard. As such, it hosts more extensive requirements for the manufacturer, particularly with regard to quality and fabrication documentation for the equipment.
API 660 is used extensively in petroleum-refinery applications and includes drawing and information requirements. In addition to its refinery focus, the standard also targets chemical and LNG (liquefied natural gas) plants.
API 660 references TEMA as a base standard. In particular, TEMA R standards are incorporated. In addition, API 660 references ASME, EJMA (Expansion Joint Manufacturers Association) and NACE (National Association of Corrosion Engineers) standards.
While all the standards discussed here provide a list of issues that the end-user should address, API 660 provides the most complete set. It also provides a list of information that should be provided along with a proposal. This proposal information includes specification sheets and detailed information on the design and materials of construction.
After a contract award, API 660 recommends extensive drawing and document submittals. The drawings to be submitted provide end users with information that can be useful for future maintenance and rebuilding.
Spare parts are also specifically addressed by API 660. Since many plants maintain their own equipment, spare tube plugs and gaskets can be important parts of the shipment.
Quality assurance documentation requirements are also provided. Documentation, such as material certifications, heat treatment and the types of NDE (non-destructive examination), can be important if failures occur. Such documentation can provide assistance in diagnosing problems.
Since API 660 is a refinery standard, it deals with a wide variety of fluids. It recognizes the need to reference NACE standards for addressing corrosion issues, such as wet hydrogen sulfide.
With the emphasis on the corrosiveness of the fluids, it is often desirable to have carbon steel clad with corrosion-resistant alloys. API 660 provides some guidelines in the use of the cladding material. This cladding can be used for tubesheets and other pressure-boundary components.
For end-users focused on specification, there is also a substantial amount of information in API 660 on reporting requirements. These requirements include welding details and procedures. The extent to which the reporting goes back to the end user is greater in API 660 than in most other relevant standards. Many of the fabrication procedures used by the manufacturers are referenced for review and approval. Such review and approval requires a certain level of expertise by the end user in order to assume responsibility over performance and reliability.
The full review of the mechanical calculations by the end user is similar to that in the ASME requirements. The primary difference is that the manufacturers’ third-party authorized inspection agency (AIA) only reviews the calculations to ensure conformance with the ASME requirements and does not look at design limitations that may be imposed by other standards or specifications. The AIA does not check that the design meets API, HEI or TEMA standards or the specifications used when the units were purchased. This would also include nozzle loads and support loads that may be required by the standards or by the user specification.
API 660 has several pages dedicated to inspection and testing. This includes extensive quality control requirements. Many of the requirements are options in the ASME BPVC but are selected by the customer. In general, the requirements are good design practices.
Requiring all Category A and B welds (defined at the beginning of the standard) to be full penetration and internal welds that all ground-flush to the shell ID is good practice, but may increase the cost. The strength welding of the tubejoint requirements are also good practice, and the standard allows for the user to accept other high-integrity ASME allowable tubejoint welds, such as explosive bonding (as referenced in TEMA).
There are also other areas where additional costs will accrue to the customer. The need for expertise in the calculation and procedure evaluation can be significant. Additional thicknesses on the tubesheets, flanges and other pressure-boundary components can also add cost. Reducing the use of reinforcement pads may also increase the cost of the heat exchanger.
The use of hydraulic stud tensioning for bolting 2-in. and larger components may also impact cost on larger and higher-pressure heat exchangers. It should be noted that the end user of a larger plant will most likely have such tensioning equipment, and a 2-in. bolt may provide a slightly smaller flange.
There are also adjustments to the girth flange designs. The cost added for the flange and gaskets can be significant.
HEI STANDARDS FOR SHELL-AND-TUBE HEAT EXCHANGERS
The HEI Standards for Shell-and-Tube Heat Exchangers (HEI) is a standard developed to address the needs of other exchangers used in power generation and similar industrial operations. Like TEMA, it is a manufacturers’ standard. While the HEI Standard covers much of the same ground as the TEMA Standards, it has additional information on heat-exchanger surface protection. It also provides for the ability to evaluate different proposals within the context of a published pressure-drop calculation.
Shell-and-tube heat exchangers found in the power-generation and affiliated industries can vary significantly in size and complexity. Everything from liquid metal sodium-loop units and nuclear-fuel pool coolers to small lubrication-oil coolers are included in the HEI standard. HEI also maintains a useful set of technical briefs on its website. Some tubing vendors provide additional details on questions of material of construction.
HEI provides a more specific set of performance and design standards, as well as calculations on hydraulic performance. The scope of the heat exchangers is less inclusive than TEMA. In evaluating flow-induced vibration, TEMA provides a well-established and extensive approach. HEI’s approach is not as extensive, but is certainly adequate. Most criteria are evaluated by computer programs.
The HEI Standard complements the complete set of HEI standards, including the Feedwater Heater Standard and Steam Surface Condenser Standard. Feedwater heater units can operate from a vacuum to supercritical pressures. Steam surface condensers are very large and operate at high vacuum.
Complex large one-, two- and three-zone feedwater heaters in vacuum up to ultra-critical applications (6,000 psi) are also covered in the Feedwater Heater Standard. The main steam surface condensers are covered in the HEI Standard of the same name. These units operate at high vacuum and there are additional standards in HEI covering the vacuum equipment as well. At times, these standards may help provide guidance on shell-and-tube heat exchangers that may be outside the limits of TEMA, API or even ASME BPVC.
There are similarities and crossovers between the HEI and TEMA. The HEI Standard incorporates a unique nomenclature set that is a bit more descriptive, but also more complex. While HEI is more complete in the design equations, it is also more limited in its discussion of heat-exchange requirements. Much of the services covered in HEI are related to water and steam, which do not carry the same corrosion issues as those found in fluids addressed in API 660. However, the mechanical issues may be more extreme, due to the large pressure and temperature differences in their operation.
HEI is also used as a requirement in critical services, such as nuclear-fuel pool coolers and as such, can encompass portions of the ASME BPVC that are outside other standards. The standard includes ASME Section III, Division 1 Class 1, 2 or 3, as well as Section VIII Division 1 or 2 heat exchangers.
In the context of shell-and-tube applications, HEI provides additional information for customers to communicate to the manufacturer. Nozzle sizing in HEI is an important issue and is addressed specifically. In this regard, the inlet area and impingement protection is well defined and is similar to the TEMA recommended good practices. Relief-valve sizing is also directly addressed in HEI, and there is a section on heat exchanger protection that includes cathodic protection, painting and in-service inspection.
ASME BPVC
In the context of shell-and-tube heat exchangers, the ASME BPVC is usually employed to ensure that the design and manufacture of the physical components of the heat exchanger are designed and built to provide a mechanically safe unit. The BPVC provides for standard calculations to determine the minimum thicknesses of the pressure-retaining envelopes. Adherence to the BPVC is very important in the insurance of the plant that the exchanger is servicing.
The BPVC has a number of sections covering the design and fabrication of the various pressure parts of the shell-and-tube heat exchanger. Section I of the BPVC covers power boilers, Section III covers nuclear components, and Section XII covers transport tanks. Section IX covers welding. The section most often used in the context of shell-and-tube heat exchangers is Section VIII Division 1, which covers unfired pressure vessels.
This code provides the allowable material stresses for most materials for design purposes. The BPVC relies heavily on voluntary consensus standards developed by ASTM International (West Conshohocken, Pa.; www.astm.org) for establishing tests necessary to vet the integrity of the materials.
International boiler codes, such as the Japanese MHLW, European Standard (PED), Malaysia (DOSH) or Chinese SQL License, may have similar requirements or equivalence to ASME BPVC.
In general, equipment manufacturers provide quotations that must meet a user’s thermal and quality requirements by providing the necessary heat-transfer surface in a pressure boundary. The design must be in accordance with the rules and criteria established by the ASME BPVC. The various flanges need to be designed and fabricated according to the calculations in the BPVC. The tubesheet(s) also needs to be designed in accordance with the BPVC unfired heat exchanger criteria. The shells and channels are often of welded construction, and the thicknesses, weld design, NDE and heat treatment need to be performed in accordance with the BPVC.
The fabrication and welding of shell-and-tube heat exchangers need to be in accordance with the BPVC. This means all the welding needs to be completed according to certified procedures. The materials and processes for the welding need to be in accordance with the BPVC. To ensure that these requirements are met, an authorized inspection agency is employed to provide third-party certification that the design and fabrication have met the requirements. A vessel that meets the BPVC is affixed with a stamp that provides the design temperature and pressure, along with a registration number from the National Board of Boiler and Pressure Vessel Inspectors (National Board). Such certification is often a requirement by those who insure the plant. It is also required by law in most U.S. states. The ASME at its base is a safety code.
The BPVC does not guarantee that the unit will meet the heat-exchange requirements. It ensures the vessel will not create a safety issue if operated in accordance with the specification. It does not guarantee that the materials will not fail from corrosion. The choice of materials for corrosion resistance is the user’s responsibility, since they best understand the corrosion potential of the fluids. The BPVC cannot guarantee that the vessel supports and nozzles will take unspecified loads imposed.
When using the BPVC for shell-and-tube exchangers, the UHX requirements for tubesheet design have been incorporated. There are number of limitations to UHX, such as the percentage of the tubefield that is tubed. In such situations, UHX references back to section U2(g), which is basically a fallback for the design engineer to use good practices. FEA is normally used to satisfy the UHX requirements.
FINAL RECOMMENDATIONS
When an engineer is either buying heat exchangers for a new plant or specifying a replacement shell-and-tube exchanger to meet new process conditions, the starting point is choosing the base-level standards for the manufacture of the exchanger. Once the standards are selected, they must be reviewed to ensure that purchasers are confident that they can respond to the manufacturer in its efforts to meet the requirements.
The manufacturer must be specific with the customer on the information they will provide. They also need to support the user with appropriate options in materials and designs, which may provide additional lifetime or cost savings. In addition, they may meet operational limits, such as off-load or cyclic designs, that may not have been specified. Manufacturers should be considered as resources in this process.
The most reliable and efficient units are built when the user accurately communicates the needs of the end user and selects a manufacturer with a high level of engineering expertise who properly utilizes and applies the codes and standards referenced in the user specification. A high level of understanding of the codes and standards, along with engineering, can result in cost savings and increased efficiency for the end unit.
by Thomas Muldoon
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