This section provides general background on exchanger materials. It summarizes major factors that must be considered in selection of materials for exchanger components and in the exchanger design. contents: 810 Major Component Materials 811 Tubes 812 Tubesheets 813 Baffles 814 Shell 820 Minimum Pressurizing Temperature 830 Sacrificial Anodes 840 Insulation
800 Materials Considerations Abstract This section provides general background on exchanger materials It summarizes major factors that must be considered in selection of materials for exchanger components and in the exchanger design Chevron Corporation Contents Page 810 Major Component Materials 800-2 811 Tubes 812 Tubesheets 813 Baffles 814 Shell 820 Minimum Pressurizing Temperature 800-6 830 Sacrificial Anodes 800-7 840 Insulation 800-7 800-1 December 1989 800 Materials Considerations Heat Exchanger and Cooling Tower Manual 810 Major Component Materials This section suggests materials for components of shell and tube heat exchangers, including tubes, tubesheets, baffles, and shell The table at the end of this section (Figure 800-3) provides a list of ASME materials commonly used for these components 811 Tubes Tube Material There often is no single correct material for a given service Although the choice of tube material is generally dictated by temperature and corrosion conditions, how well a material performs is greatly influenced by actual service conditions and the corrosion control measures in use Keep this in mind when reading Figure 800-1, which lists common exchanger tube materials The information is not meant for materials selection For information about specific corrosives or specific types of process plants refer to the Corrosion Prevention Manual Selecting the right tube material is only one way to ensure good performance Often one can control conditions in the exchanger by altering the nature of the process fluid or by controlling exchanger design Also, corrosion-inhibiting chemicals may be added to the process fluid Tube Wall Thickness Tube wall thickness is chosen for temperature-pressure and corrosion considerations Except at high pressures (usually over 1000 psi) where the strength of thicker tubes is needed, anticipated corrosion rates will determine wall thickness The tube thicknesses given below fulfill 95% of typical requirements, although thicker tubes may be used where high corrosion rates are expected We try to standardize sizes to simplify maintaining a stock of materials for maintenance Carbon steel tubes: 14 gage minimum (13 gage is average) 12 or even 10 gage is sometimes called for Alloy tubes: 16 gage (except titanium) Titanium tubes: 18 or 20 gage Effect of Exchanger Design on Corrosion Maximum Allowable Tube Velocity In most common hydrocarbon services, exchanger design sets the maximum velocities to accommodate conditions other than corrosion, such as pressure drop However, there are some services for which a velocity beyond some critical threshold may initiate rapid corrosion One example of this is the “end impingement” failures of copper alloy tubes in sea water Some other examples are given in Figure 800-2 Effect of Shell-side Water on Tube Material In cooling water service, tube life is greatly affected by where the water is put Cooling water on the shell side creates December 1989 800-2 Chevron Corporation Heat Exchanger and Cooling Tower Manual Fig 800-1 800 Materials Considerations Common Exchanger Tube Materials (This table is illustrative only It is not suitable for materials selection See Section 811.) Service Typical Materials Fresh Water Comments Carbon Steel Short life unless water is good quality and chemical treatment is carefully controlled Admiralty(1) Very few problems 304 Stainless Steel Only for low chloride waters under nonscaling conditions Admiralty(1) Suffers end impingement at high velocity 70—30 Cupro-Nickel More resistant to end impingement than Admiralty Titanium Essentially corrosion-proof to 250°F Special grades OK to 450°F Hydrocarbons-Sweet Carbon Steel Very sensitive to trace H2S over 500°F Hydrocarbons-Sour Carbon Steel Limited to about 550°F maximum Chrome—1/2 Moly Where too hot for carbon steel Hydrocarbons-Naphthenic 316 Stainless Steel Above 1.5 neutralization number Hydrogen-Sweet Carbon Steel, C-1/2 Mo, 1-1/4 CR1/2 Mo, 2-1/4 CR-1 Mo Choice depends on temperature and hydrogen partial pressure See API Publication 941 Hydrogen-Sour Same as above; also 321 Stainless Steel Materials choice depends on stainless steel temperature and on hydrogen and H2S partial pressure Steam Carbon Steel CO2 corrosion may demand better materials in a condensing environment Sea Water (1) Inhibited grades only Use ASTM B111 Grades C44300, C44400 corrosion problems that may be hard to overcome For example, in well-treated cooling water, carbon steel exchanger tubes have good life when the water is tube side However, it is virtually impossible to obtain good tube life on carbon steel with water on the shell side, no matter how well the water is treated If one tries to compensate by upgrading tube material to Admiralty, for example, a galvanic corrosion problem is created where the alloy tubes join the carbon steel baffles And if the baffles are upgraded to brass, a new galvanic cell is made where brass baffles touch the steel shell Shell-side water is also a poor choice for stainless steels Local boiling may occur in low flow areas, especially adjacent to tubesheets, with resultant concentration of chlorides and stress corrosion cracking of the tubes Chevron Corporation 800-3 December 1989 800 Materials Considerations Fig 800-2 Heat Exchanger and Cooling Tower Manual Some Typical Tube Velocity Limits Service Material Sea Water (Avoid velocities below fps) Maximum Velocity Admiralty fps 70—30 Cupro-Nickel fps Titanium Effectively no limit Fresh Water All Materials No hard data available; seldom limits exchanger design Hydrocarbons All Materials Depends on service but seldom limits exchanger design Concentrated H2SO4 Carbon Steel fps Ammonium Bisulfide Solutions (3% +); in wastewater treatment and hydroprocessing plants Carbon Steel 20 fps in air-cooled exchangers; generally avoid water cooling Stainless Steel, Incoloy 800 30 fps Titanium Effectively no limit;do not use titanium if high-pressure hydrogen is also present Tube Quality and ASTM Specifications While specifying the proper alloy to get the required corrosion resistance, tube quality is ensured by ordering tubes to the appropriate ASTM specification An ASTM specification not only covers chemical composition, but also the method of tube manufacture and quality control tests ASTM specifications cover a variety of end uses Some of the ASTM specifications commonly used for tubes are: • A179 Seamless carbon steel • A214 Welded carbon steel • A199 Chrome-moly steel • A249 Welded stainless steels • A213 Seamless stainless steels • B111 Copper alloys Except for the carbon steel, all of these specifications cover a number of related alloys For example, B111 covers four kinds of Admiralty, several cupro-nickels, aluminum brass, and aluminum bronze When specifying materials, cite both the ASTM specification and the grade For example, a welded Type 304 stainless tube would be specified as ASTM A240-TP304 December 1989 800-4 Chevron Corporation Heat Exchanger and Cooling Tower Manual 800 Materials Considerations Welded vs Seamless Tubes In general, seamless alloy tubes are ordered instead of welded, but carbon steel tubes may be either seamless or welded With carbon steel, there is little or no sacrifice in life by using welded tubes, and the cost is much lower However, welded tubes should be purchased only from Company-approved suppliers who have a proven track record on quality control One exception to the above generality is the use of welded titanium tubes; the cost differential between welded and seamless here is significant, and tube manufacturers have demonstrated their ability to produce defect-free tubing 812 Tubesheets Materials Tubesheets are usually made of the same material as the tubes One major exception is with copper alloys General practice is to use naval-rolled brass (NRB) tubesheets with Admiralty tubes and Monel or 70-30 cupro-nickel tubesheets with 70-30 cupro-nickel tubes Cladding When constructing an exchanger using alloy tubes in which the corrosive fluid is on the tubeside, it may be economical to use alloy-clad rather than solid alloy tubesheets If clad tubesheets are used, cladding thickness should be 0.5 inch, so that the first serration is entirely within the cladding When the tubes are rolled in place, this will allow an alloy-to-alloy seal at the first serration Such a seal prevents corrosive fluid from entering the crevice between tube and tubesheet to cause galvanic corrosion where alloy and carbon steel are in contact See Section 520 and Specification EXH-MS-2583 for more information on cladding Galvanic Attack In places where tubesheet and tube materials differ take the following precautions: Chevron Corporation • Consider differential thermal expansion that can loosen rolled joints • Consider the galvanic relationship between tube and tubesheet when handling a corrosive aqueous (i.e., electrically conductive) fluid such as sea water Do not use materials that are far apart in the galvanic series, and especially not have the tubesheet be the more noble metal (See the Corrosion Prevention Manual, Section 200.) • Using the wrong materials combination may result in accelerated corrosion or end impingement For example, in sea water service use of a Monel rather than naval brass tubesheet with Admiralty tubes can reduce tube life by an order of magnitude • Know that there are many fluids in which the probability of galvanic attack may not be obvious The answer here is to search for relevant experience with that fluid 800-5 December 1989 800 Materials Considerations Heat Exchanger and Cooling Tower Manual Note that galvanic attack is not a problem in hydrocarbons and usually is not severe in fresh water Galvanic attack can sometimes be prevented through the use of sacrificial anodes Tube Rolling There is no problem in rolling soft tubes into a hard tubesheet But rolling hard tubes into a soft tubesheet can result in the enlargement of tubesheet holes, without the joint becoming tight See Section 520 for information on the tube-to-tubesheet joint 813 Baffles Baffles are usually made of the same material as the exchanger shell, carbon steel being the most common The most important consideration in choosing baffle material is corrosion resistance; it is poor economy to have to rebuild an exchanger because the baffles have corroded while the tubes are still in good condition Baffles should be designed to last at least as long as the tubes One refinery had to replace four expensive titanium bundles after years because the steel carcass had corroded Seal strips on longitudinal baffles (so-called “lamiflex baffles”) have some special problems They are very thin and have little tolerance for corrosion In addition, bending stresses render them susceptible to stress corrosion cracking in certain services The 300 Series stainless steels are the most common seal strip materials, but special alloys are required for some services such as hydroprocessing plants Seek the advice of corrosion or materials engineers before choosing seal strip materials for new services 814 Shell An exchanger shell is nothing more than a pressure vessel and is designed according to the same criteria (typically ASME Code, Section VIII, Division 1) Materials suitable for pressure vessels are also acceptable for exchanger shells One important materials limitation is that it is seldom practical to use more than 1/4inch corrosion allowance on an exchanger shell If corrosion is deeper than this, bypassing around the baffles will cause a major degradation in exchanger performance 820 Minimum Pressurizing Temperature Minimum pressurizing temperature (minimum design metal temperature) is a critical design factor for pressure vessels Exchanger shells and channels are pressure vessels, and must be designed accordingly This subject is covered in detail in the Pressure Vessel Manual In brief, we establish a minimum pressurizing temperature to avoid a catastrophic brittle fracture Ordinary carbon steels, for example, become brittle at low temperatures The ductile-to-brittle transition temperature may range from well above December 1989 800-6 Chevron Corporation Heat Exchanger and Cooling Tower Manual 800 Materials Considerations ambient to well below ambient, depending on the grade and thickness of steel used A material must be chosen that will not suffer brittle fracture under the conditions in which an exchanger is expected to operate This includes hydro-test, which must be done at a temperature above the minimum pressurizing temperature 830 Sacrificial Anodes The purpose of sacrificial anodes is to extend the life of critical heat exchanger parts by the application of cathodic protection The subject is discussed in detail in Section 1600 of the Corrosion Prevention Manual The Company does not often use cathodic protection for heat exchangers Although sacrificial anodes can be installed in exchanger channels or water boxes to protect tubesheets, tube ends, and the channel section itself, anode life usually is not long enough to justify their installation While such anodes can protect tube ends from corrosion (as long as the anodes last) protection generally does not extend more than one or two tube diameters down the inside of the tube One application that did prove to be reasonably successful was the installation of carbon steel sacrificial anodes in water boxes in the Borco Refinery sea water desalination plant This was to protect the 90-10 cupro-nickel tubesheets from galvanic corrosion caused by contact with the titanium tubes Another successful anode installation was the use of aluminum anodes in exchanger channels at Richmond They prevented galvanic attack of Monel tubesheets caused by dissimilar metal contact with titanium tubes in a sea water environment 840 Insulation Heat exchangers are basically pressure vessels and are insulated as such This subject is covered in the Insulation and Refractory Manual The large flanged connections on heat exchangers cause the major problem with insulation For more information on the criteria for insulating large flanges and the design of the flanges and bolting, see Section 550 of this manual Removable Insulation Removable covers can be removed for exchanger maintenance, or to look for flange leaks after startup, and then reinstalled after inspection Large covers, however, are hard to handle, particularly those used on very hot equipment For design information on removable covers, see the Insulation and Refractory Manual, Section 100, and IRM-EG-4197 Chevron Corporation 800-7 December 1989 Commonly Used Materials for Components of Shell-and-Tube Heat Exchangers (1 of 2) Low Alloy Steels Components Carbon Steel C-1/2Mo 1-1/4Cr-1/2Mo SA-285-C SA-515 and 516 (All grades) SA-204-A SA-204-B or SA-204-C SA-387-11 Class or SA-106-B or SA-53-B SA-335-P1 SA-105 or SA-181 (Class 60 or 70) 2-1/4Cr-1/2Mo High Alloy Steels 18Cr-8Ni Stabilized 5Cr-1/2Mo 12 Cr 18Cr-8Ni-3Mo 18Cr-8Ni SA-387-22 Class or SA-387-5 Class or (Tubesheets and baffles only Use 12Cr clad for shells and channels.) Do not use 12Cr for pressure containing parts (except tubes) Use 5Cr1/2Mo tubesheets with 12Cr tubes SA-240-TP316 or SA-240-TP316L SA-240-TP304 or SA-240-TP304L SA-240-TP321 or SA-240-TP347 SA-335-P11 SA-335-P22 Not used Do not use SA-312-TP316 or SA-312-TP316L (Seamless or welded) SA-312-TP304 or SA-312-TP304L (Seamless or welded) SA-312-TP321 or SA-312-TP347 (Seamless or welded) SA-182-F1 SA-182-F11 SA-182-F22 SA-182-F5 or SA-182-F5a (For forged tubesheets and covers only) Do not use SA-182-F316 or SA-182-F316L SA-182-F304 or SA-182-F304L SA-182-F321 or SA-182-F347 SA-209-T1 SA-199-T11 SA-199-T22 SA-199-T5 SA-268-TP405 or SA-258-TP410 (Seamless or welded) WELDED: SA-249-TP316 or SA-249-TP316L SEAMLESS: SA-213-TP316 or SA-213-TP316L WELDED: SA-249-TP304 or SA-249-TP304L SEAMLESS: SA-213-TP304 or SA-213-TP304L WELDED: SA-249-TP321 or SA-249-TP347 SEAMLESS: SA-213-TP321 or SA-213-TP347 PLATES: (For rolled and welded shells, shell covers, channels and nozzle necks, heads flat covers, tubesheets and baffles) 800 Materials Considerations December 1989 Fig 800-3 PIPE: (For pipe sized shells and nozzle necks) 800-8 FORGINGS: TUBES: WELDED: SA-214 SEAMLESS: SA-179 Chevron Corporation Heat Exchanger and Cooling Tower Manual (For body and nozzle flanges, blind flanges, couplings and forged flat covers and tubesheets) Commonly Used Materials for Components of Shell-and-Tube Heat Exchangers (2 of 2) Low Alloy Steels Components Carbon Steel C-1/2Mo 1-1/4Cr-1/2Mo 2-1/4Cr-1/2Mo High Alloy Steels 5Cr-1/2Mo 12 Cr 18Cr-8Ni-3Mo 18Cr-8Ni 18Cr-8Ni Stabilized BOLTS: SA-193-B7 SA-193-B7 SA-193-B7 or SA-193-B16 SA-193-B7 or SA-194-B16 SA-193-B5 Do not use SA-193-B8M SA-193-B8 SA-193-B8T or SA-193-B8C SA-194-3 Do not use SA-194-8M SA-194-8 SA-194-8T SA-194-8C Caution: For hydrogen service, verify that Cr and Mo content are high enough to resist H2 attack NUTS: SA-194-2H SA-194-2H SA-194-2H SA-194-2H Heat Exchanger and Cooling Tower Manual Chevron Corporation Fig 800-3 800-9 800 Materials Considerations December 1989 ... Fig 800-1 800 Materials Considerations Common Exchanger Tube Materials (This table is illustrative only It is not suitable for materials selection See Section 811.) Service Typical Materials Fresh...800 Materials Considerations Heat Exchanger and Cooling Tower Manual 810 Major Component Materials This section suggests materials for components of shell and... seal strip materials, but special alloys are required for some services such as hydroprocessing plants Seek the advice of corrosion or materials engineers before choosing seal strip materials