EXH800 materials considerations

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

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

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 compo-nents

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 consider-ations 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 stan-dardize sizes to simplify maintaining a stock of materials for maintenance

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

Carbon steel tubes: 14 gage minimum (13 gage is average) 12 or even

10 gage is sometimes called for

(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 corro-sion 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

Fig 800-1 Common Exchanger Tube Materials (This table is illustrative only It is not suitable for materials

selec-tion See Section 811.)

Admiralty(1)

304 Stainless Steel

Short life unless water is good quality and chemical treatment is carefully controlled

Very few problems Only for low chloride waters under nonscaling conditions

70—30 Cupro-Nickel

Titanium

Suffers end impingement at high velocity More resistant to end impingement than Admiralty

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

5 Chrome—1/2 Moly

Limited to about 550°F maximum 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

CR-1/2 Mo, 2-1/4 CR-1 Mo

Choice depends on temperature and hydrogen partial pressure See API Publi-cation 941

Hydrogen-Sour Same as above; also 321

Stain-less Steel

Materials choice depends on stainless steel temperature and on hydrogen and

H2S partial pressure

mate-rials in a condensing environment

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:

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

Fig 800-2 Some Typical Tube Velocity Limits

Sea Water (Avoid velocities below

3 fps)

Admiralty 70—30 Cupro-Nickel Titanium

5 fps

6 fps Effectively no limit

limits exchanger design

limits exchanger design

Ammonium Bisulfide Solutions

(3% +); in wastewater treatment

and hydroprocessing plants

Carbon Steel

Stainless Steel, Incoloy 800 Titanium

20 fps in air-cooled exchangers; generally avoid water cooling

30 fps

Effectively no limit;do not use titanium if high-pressure hydrogen

is also present

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 sacri-fice 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 manufac-turers 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 excep-tion 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:

• 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 do 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

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 sacri-ficial 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 mate-rial 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 4 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 mate-rials 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 1/4-inch corrosion allowance on an exchanger shell If corrosion is deeper than this, by-passing around the baffles will cause a major degradation in exchanger perfor-mance

820 Minimum Pressurizing Temperature

Minimum pressurizing temperature (minimum design metal temperature) is a crit-ical 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 tempera-tures The ductile-to-brittle transition temperature may range from well above

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

informa-tion on removable covers, see the Insulainforma-tion and Refractory Manual, Secinforma-tion 100,

and IRM-EG-4197

Components Carbon Steel C-1/2Mo 1-1/4Cr-1/2Mo 2-1/4Cr-1/2Mo 5Cr-1/2Mo 12 Cr 18Cr-8Ni-3Mo 18Cr-8Ni

18Cr-8Ni Stabilized PLATES:

(For rolled and

welded shells,

shell covers,

channels and

nozzle necks,

heads flat

covers,

tubesheets

and baffles)

SA-285-C SA-515 and 516 (All grades)

SA-204-A SA-204-B or SA-204-C

SA-387-11 Class 1 or 2

SA-387-22 Class 1 or 2

SA-387-5 Class 1 or 2 (Tubesheets and baffles only Use 12Cr clad for shells and channels.)

Do not use 12Cr for pres-sure containing parts (except tubes) Use 5Cr-1/2Mo 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

PIPE:

(For pipe sized

shells and

nozzle necks)

SA-106-B or SA-53-B

SA-335-P1 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)

FORGINGS:

(For body and

nozzle flanges,

blind flanges,

couplings and

forged flat

covers and

tubesheets)

SA-105 or SA-181 (Class 60 or 70)

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-214 SEAMLESS:

SA-179

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

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 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 SA-194-3 Do not use SA-194-8M SA-194-8 SA-194-8T

SA-194-8C

Components Carbon Steel C-1/2Mo 1-1/4Cr-1/2Mo 2-1/4Cr-1/2Mo 5Cr-1/2Mo 12 Cr 18Cr-8Ni-3Mo 18Cr-8Ni

18Cr-8Ni Stabilized