Materials

Updated: 2016-12-20th MCT

ASME Section VIII materials is a very broad topic. Properties used in section VIII are found in Section II-D and is fairly extensive, through a range of temperatures. Below we discuss the topics dealing with temperature and pressure ratings in old and new vessels. As well as provide guidance in sorting through the numerous materials in Section II-D and how to ensure the proper one is used for calculations.

Dual Certified Vessels for Low Temperature Service

PVE-5920, Last Updated: Aug. 21, 2012, By: LRB

Background:

Many existing carbon steel vessels built before modern code rules do not meet current code requirements for low temperature service. The service history on these vessels is good resulting in an attitude that the current code rules are too restrictive. However, there is an alternate way to understand these vessels: many are in service conditions like propane storage where the pressure at lower temperatures can never get very high. Many of these vessels can never experience situations of combined high pressure and low temperature.

Propane Pressure Temperature Curve:

A propane storage vessel typically has a pressure rating of 250 psi. In Canada it can also have -50°F MDMT (Minimum Design Metal Temperature) depending on the service location. What it does not experience is 250psi at -50°F. The pressure temperature curve for Propane is shown below. Some data points have been highlighted on the propane P-T curve. At -50°F, a common design temperature for exposed locations in Canada, the pressure in a propane storage vessel is a small vacuum of -2 psig – the tank will not be able to generate any useful pressure.  At -20°F, a common minimum temperature for older design codes, the pressure is only 11 psi. At 250 psi, a common propane design pressure, the temperature is 127°F. Once the contents of the vessel reach this temperature, a 250 psig relief valve will open allowing the release of some gas reducing the temperature of the remaining contents through boiling.

Propane pressure vs temperature curve.  Data source

A Typical Vessel Design:

A sample vessel was designed with a minimum wall thickness to just pass 250psi service. The material category was changed from Curve A to B to C to D. As the pressure was reduced the minimum allowed temperature reduced depending on the material curve used. A crude generalization is that Curve A represents coarse grained materials with poor low temperature impact (toughness) properties. Curve D materials have the best properties obtained through methods like normalization or quench and tempering. Curve B and C are intermediate materials. This applies only to carbon and low alloy steels. The list of materials and their curve is found in Fig UCS-66. Through application of ASME rules built into all commercial code calculators, the curves below were generated:

Sample vessel P-T ratings for four different curves of steel used in pressure vessel design

In this case, all material curve variants for this sample vessel have pressure ratings above the propane PT curve, so all could be used safely, but Curve A in this case could not reach -50°F. The Curve A material can be seen to have the poorest pressure rating at low temperature, and the Curve D the best. When a vessel is designed for new construction it is possible to combine the selection of material with the appropriate testing to obtain MDMT of -50°F, even at full pressure. However, for this sample vessel, no material combination provides a full pressure rating at -50°F.

Continuing this sample assuming Curve B material: ASME has additional rules like UG-20(f) that allow higher pressures to be used for some materials down to -20°F service. With UG-20(f) applied, the Material pressure temperature curve for a Curve-B vessel now looks like:

Just looking at curve B material

The design has been dual rated: 250 psig at down to -20°F and 115 psig for down to -50°F. This is recorded according to the rules of UG-116(a(5) footnote 37 which does not restrict the number of minimum temperature pressure combinations used. One full calculation set is required for each P-T combination. Two full calculation sets are required for this vessel: the first is calculated at a pressure of 250 psig and shows a minimum temperature of -20°F; The second is calculated at 115 psig and shows a minimum temperature of -20°F.

As an alternative to using these curves, it is possible to impact test materials and welds. See the comment from ABSA at the end of this article. In general it does not pay to be optimistic about coarse grained materials and welds passing impact tests prior to seeing the actual test results.

More on SA-212 material:

Source: www.krrao.com, removed from web site

ASTM SA-212-39 (S-55) was put into Section II in the 1940 edition of the Code. There were two grades in S-55: A and B, each with two different minimum tensile strength requirements controlled by carbon content.

In 1952… it was required in SA-212 that plates intended for low-temperature service must meet the impact requirements in SA-300. SA-212 could be purchased to a fine-grain-melting practice-and subsequently normalized and tempered-for low temperature service, or purchased to a coarse grain-melting practice; the single specification permitted the manufacture of both plate grades. The SA-212 Specification continued up to 1962 as the carbon steel plate material of choice for low-temperature service for boiler drums and pressure vessels.

[In the] 1968 edition of Section II … the SA-212 Specification was deleted …it was replaced with two specifications. The SA-212 steel plate melted to coarse-grain practice was replaced with SA-515 (Specification for Pressure Vessel Plates, Carbon Steel, for Intermediate and Higher Temperature Service) and the SA-212 steel plate melted to finegrain practice was replaced with SA-516 (Specification for Pressure Vessel Plates, Carbon Steel, for Moderate and Lower Temperature Service). These two specifications-along with SA-299 (Specification for Pressure Vessel Plates, Carbon Steel, Manganese-Silicon), which has slightly higher room-temperature strength-were first published in the 1949 Edition of Section II. They continue to be used today as the carbon steel plate materials of choice for boiler and pressure-vessel applications.

SA-212 in older vessels being recalculated could be either coarse or fine grain. Either Curve A or a better curve. Proving that it was built to fine grain practice and impact tested to SA-300 could be difficult for an old vessel. Sometimes it is only possible to assume that it was made to Curve A. There are also some concerns that special care is required for hydrotesting coarse grained pressure vessels. See TSSA and National Board (search page for SA-212)

When a used vessel moves to a new location in Canada a new CRN registration number is usually required. The CRN calculations are based on the inspected wall thickness. Three possible calculation methods are used: 1) calculations to be based on current code rules (see note from ABSA below); 2) Calculations based on code rules at time of construction or; 3) Calculations based on both the time of construction and the current code rules, the most conservative to be applied. Typically a calculation set to one of the above methods is prepared to create a submission package and get a review engineer assigned. At that point the calculation method can be changed to the assigned reviewer’s requirements. Also note that some Canadian reviewers/jurisdictions do not allow vessels to be dual rated. Some of our customers place reserve bids on used vessels and do not complete the transactions until the CRN has been obtained. It is important to get this sorted out before moving the vessel! More info from ABSA:

1.Q2. Is it permissible to bring into and operate a used pressure vessel that was manufactured of SA-212 Grade B steel? The vessel was not impact tested when it was manufactured.

1.R2. A used pressure vessel made of SA-212 Grade B steel may be brought into and registered for operation in provided that its proposed design conditions meet the intent of the current ASME Pressure Vessel Code. Since the current Code requires a minimum design metal temperature (MDMT) for a pressure vessel, such an MDMT must be established for the used vessel using the current Code methodology. SA-212-B material would be considered a Curve A material for the purposes of Code paragraph UCS-66. Therefore, a maximum allowable working pressure (MAWP) that supports the MDMT without impact testing would have to be established. It is assumed that it is not feasible to impact test all the shell and head plates and weld joints to support an MDMT lower than that without impact testing.

Unlisted Materials

Background

The Canadian CRN registration system requires that all fittings used on registered vessels* or included in a registered piping system carry CRNs. To register the fittings, design validation based either or code calculations, finite element analysis or proof testing is required.

When a design is based on code listed materials, the code of construction provides allowable operating stress levels. If the design of the pressure containing item is simple, the regular code rules can be used and will supply a pass/fail judgement. If no code rules exist for a complex or unusual shapes, Finite Element Analysis (FEA) can provide the stresses which can be compared with the listed allowables for a pass/fail judgement.

An alternate to FEA is to proof test the item at stress levels far above operating. The items actual and guaranteed minimum tensile strengths are required for the proof test. The formula used is from VIII-1 UG-101(m):

Where B is the burst test pressure and P the allowed operating pressure. The burst test has to be at least 4 times the operating pressure. E is the welding efficiency if the item is welded – typically between 0.7 and 1.0. Two more pieces of information are required – Su – the specified minimum tensile strength of the material and Suavg – the tensile test results from the item under test. Typical proof test pressures are 5-6x operating pressure, a requirement in many cases more conservative than regular code calculations or FEA.

For code listed materials, all of the required information is available for either calculations/FEA or for burst testing.

Unlisted Materials

Codes B31.1 and B31.3 are useful for registering fittings because they allow unlisted materials to be adopted and because they provide fewer restrictive design rules. Be aware that ABSA has a ruling that requires items that look like vessels (even slightly) to be registered under VIII-1 where adoption is not permitted. This requirement was put into writing in 2008. As of 2014 no other province is in agreement. A significant number of fittings are available Canada wide but not in Alberta due to this one requirement.

An unlisted material made to a specification can be adopted if the material’s guaranteed minimum yield and tensile strength are available at the operating temperature. The code adopted strength is based on a formula using these two inputs resulting in allowable design strength. Or the minimum tensile strength can be used in the proof test. Using this process, almost any IID listed material can be adopted for use in B31.1 or B31.3.

This is a typical formula for adopting unlisted material in B31.3. Sy and St are the materials guaranteed minimum strength. More complex methods are used at higher temperatures where the materials creep properties need to be taken into account. Availability of elevated temperature material properties can severely limit the adoptability of unlisted materials. Caution: see Unlisted Material Registration Problems below.

Unlisted Materials With No Specified Strength

Many fittings materials are not code listed and have no guaranteed minimum tensile or yield strength information. Two common examples: SAE1010 is a carbon steel and B85 A380 is a die cast aluminium. Both are made to chemical only specifications and both are used in fittings.

To use either of these materials in Canadian registered fittings, the purchaser has to agree with the mill/foundry what minimum tensile and yield strength level is acceptable. A specification referenced or written into each material batch purchase order is required. Chosen strength levels are obviously important. Set too high and excessive batches will be rejected upon physical testing. Set to low and the parts will not pass code calculations. Also note that the ratio between the actual and minimum tensile strength impacts the required burst test pressure. The lower the minimum specified strength, the higher the required proof test. A sample purchase order or a copy of the specification would be required with the CRN application. Caution: see Unlisted Material Registration Problems below. Setting appropriate guaranteed minimum stress levels commonly causes confusion, an example follows.

Example: A manufacturer who is investigating a new unlisted material gets some pull test results. 4 tensile test results at ambient [ksi] 47, 46, 44, 48. 4 yield stress results [ksi] 25, 26, 23, 28. The results are at ambient only, and the product will only be used at ambient so elevated temperature testing is not required. What should the guaranteed minimum yield and tensile be? Each material batch will be tested, so setting the specified minimum too high will results in batches being rejected. For example, a specified minimum tensile of 45 ksi would cause the 3rd specimen to be rejected. Some number around 40 ksi tensile and 20 ksi yield might be reasonable as is shown in this graph.

The unknown materials test results after specified minimum tensile and yield strengths are chosen.

What happens if the guaranteed minimum is set too low? If the product is to be burst test, from the top equation, the required burst test is increased by the ratio of Su/Sur, where Su is the specified minimum burst test, and Sur the test results from the item under test. If samples 1, 2 and 3 are taken from the test object, Sur = average(47,46,44) = 45.6. If the specified minimum is 40 ksi, then the burst test ratio is 4 x 45.6/40 or 4.56x. However if the specified minimum was set way low to 20 ksi, then the ratio would be 4 x 45.6/20 or 9.12x.

If the product will not be used at ambient, then elevated materials properties are required. For CRN applications, temperatures above 100ºF are considered elevated (source unknown). Additional elevated temperature material testing is required to cover the design conditions.

The manufacturer needs to document the minimum specified properties and other characteristics of the unlisted material with no specified strength per B31.3:

B31.3 2010 323.1.2 Unlisted Materials. Unlisted materials may be used provided they conform to a published specification covering chemistry, physical and mechanical properties, method and process of manufacture, heat treatment, and quality control, and otherwise meet the requirements of this Code. See also ASME BPV Code Section II, Part D, Appendix 5. Allowable stresses shall be determined in accordance with the applicable allowable stress basis of this Code or a more conservative basis.

Alberta requires that this document be published on the manufacturers web site available for unrestricted access.

Caution: Although B31.1.2(C) states “Unlisted materials shall be qualified for service within a stated range of minimum and maximum temperatures based upon data associated with successful experience, tests, or analysis; or a combination thereof.”Applicants should consider that the use of experience to register fittings in the CRN system is practically impossible.

Unknown Materials

If all of the above fails, many Canadian reviewers will allow a fitting to be registered with “unknown” materials if it can be proof tested to 10x operating pressure (no tensile strength testing required, no guaranteed minimum specification provided). This category includes many plastics that are not covered by the piping codes, glass, ceramics and steels that cannot be adopted by the above methods.

Clearly 10x operating is a severe test not possible with many products. This method is reserved for products that are highly overdesigned.

* See Do I Need A Fitting With a CRN? for some common items that only require CRN registration if they are in a piping system, but not if they are on a vessel.

Selection of the Correct IID Listed Stainless Steel Stress Values

File: PVE-8296, Last Updated: Apr 8 2015, LB

This is an extract from one of our in house training topics, we thought our customers would find it useful.

Lots of IID Entries

The IID book provides many rows of stress properties for the 304 grade of stainless steel pipe. These 12 line items cover SA-312 304 stainless steel in welded, seamless, low, high and regular carbon and high and low stress. Quite a lot – here are the 3 variables covering the 12 lines.

1. Seamless or welded
2. Low or high strength
3. The amount of carbon in the grade: L, regular or H.

This is further confused by Dual Certified stainless (304/304L). Other variations like N, LN also exist with different chemistry not discussed here.

For a simple 304 grade stainless pipe, the different combinations here provide for 12 different IID table 1A lines with different allowed stress/temperature curves. You only want one.

12 lines of IID material properties for 304 stainless steel pipe.

The table notes are required to choose the correct line. For Section VIII-1 use, if we do not care how the material properties are calculated, the following notes can be ignored:

G21 and W13 apply to section I only, W12 is for Section III use

T4, T6, T7 and T8 explain how the high temperature properties were calculated

Removing the notes that are not helpful

These are the useful notes:

G3 These stress values include a joint efficiency factor of 0.85.

G5 Due to the relatively low yield strength of these materials, these higher stress values were established at temperatures where the short-time tensile properties govern to permit the use of these alloys where slightly greater deformation Is acceptable. The stress values in this range exceed 66% but do not exceed 90% of the yield strength at temperature. Use of these stresses may result in dimensional changes due to permanent strain. These stress values are not recommended for the flanges of gasketed joints or other applications where slight amounts of distortion can cause leakage or malfunction. For Section Ill applications, Table Y -2 lists multiplying factors that, when applied to the yield strength values shown in Table Y -1, will give allowable stress values that will result in lower levels of permanent strain.

G12 At temperatures above 1000°F, these stress values apply only when the carbon is 0.04% or higher on heat analysis.

G24 A factor of 0.85 has been applied in arriving at the maximum allowable stress values in tension for this material. Divide tabulated values by 0.85 for maximum allowable longitudinal tensile stress.

W14 These S values do not include a weld factor. For Section VIII, Division 1 and Section XII applications using welds made without filler metal, the tabulated tensile stress values shall be multiplied by 0.85. For welds made with filler metal, consult UW-12 for Section VIII, Division 1, or TW-130.4 for Section XII, as applicable.

Welding

Note G3 or G24 is applied to all welded pipes. The longitudinal efficiency for an ERW (Electric Resistance Welded or fillerless welded) pipe is set at 0.85. Note W14 indicates that a weld efficiency of 0.85 has not been included but should be included if the product is ERW.

It is obvious that the table would be simpler if the ERW and seamless product forms were separated. As a suggestion, try using the Smls. & wld. pipe grade only for seamless product, and Wld. pipe for welded grade. The additional 0.85 efficiency will not need to be applied resulting in less confusing calculation sets.

Even if an ERW efficiency factor of 0.85 has been applied, additional reduction in efficiency might be required according to the rules of UW-12(d). For example, pipe caps welded on the end of a vessel made out of ERW pipe with no radiography will require an efficiency of 0.85 to be applied to the pipe long seam, this is in addition to the 0.85 already taken off for the ERW efficiency. This is easier to do if the welded material line has been chosen from table IID.  Here is an interpretation.

Standard Designation: BPV Section VIII Div 1
Subject Description: Section VIII, Division 1, UW-12(c)
Date Issued: 02/18/1988
Record Number: BC88-043
Historical Interpretation numbers : VIII-1-86-218
Question(s) and Reply(ies): Question: Is it the intent of the new stress multiplier rules that, for a vessel consisting of an ERW pipe shell with seamless ellipsoidal or torispherical dished heads and no radiography of the Category B seams, the stress values from Table UCS-23 for ERW pipe be multiplied by E = 0.85 for calculations involving circumferential stress in the shell?

Many customer drawings do not specify if the product form is seamless or welded. If this cannot be clarified on the drawing then the lower strength welded must be assumed.

High Strength or Low Strength

Half of the listing have note G5, indicating that the strength level of the material is set above the customary 66% yield limit. The use of these values is not recommended for flanges, but not prohibited. Our experience indicates that ASME VIII-1 Appendix 2 and Y flange designs are highly conservative. It is our policy to use high strength materials for these applications except when registering in the province of Alberta which has an undocumented requirement to use the low strength values, or when the customer prohibits it.

The difference between high strength and low strength values for stainless steel at temperature

The Chemistry of the Grade (L, Regular or H)

Usually it is the job of the customer to specify the correct grade of stainless to use, but sometimes we have to ask for changes based on the operating temperature. The regular grade (TP304) has a maximum carbon content of 0.08%. TP304L has a maximum content of 0.035% and TP304H ranges form 0.04-0.010%

Carbon content ranges of plain, L and H grade 304 stainless steel

304L is not listed for applications above 1200°F. Note G12 prohibits the use regular grade 304 above 1000°F unless the carbon content is above 0.04%. (There is no explanation for the difference between 1200°F for the 304L grade and 1000°F for 304 without the extra carbon.) 304H always has 0.04% or greater carbon so no note is required for high temperature use.

Dual Certified

Dual certified stainless is produced by the mill to meet the requirements of both 304 and 304L. From the above graph, the carbon content must be below 0.035%, and from the top table, the higher 75,000 tensile and 30,000 yield must be met. This is not a challenge for modern mills.

Sometimes customers use a confusing 304/304L designation on the bill of materials. Does the customer mean materials that meet both specifications (dual certified) which would allow higher strength levels to be used in the calculations, or is the customer giving themselves a choice between using the stronger or the weaker material, in which case the weaker would have to be calculated. Usually the customer means the first interpretation, but where this cannot be correctly interpreted, the weaker material must be calculated.

Conclusion

With these 3 variables understood, the most appropriate IID listed line can be selected. Other product forms like plate are simpler because the welded or seamless variable does not exist, but the method is the same.