Selection and Use of Pipeline Materials

 Selection and Use of Pipeline Materials

When one thinks of materials for use in the piping codes the usual thought is about the materials that make the pipe, fittings, and supporting equipment—the materials that the codes address. However, there are more materials than that to be considered.The material that the piping will be immersed in is important. In above-ground piping, that is usually just air, and is not always significant. Even then one has to consider the environment—for example, the humidity levels and whether the location has extreme weather such as temperature and wind. If the location is earthquake prone, that has bearing on the design calculations and the construction.Buried piping has another set of concerns. One has to know the topography and soil conditions that the pipeline is routed through. 

Usually there is need for some kind of corrosion protection. Does the route cross rivers, highways, canyons, or other things that can cause special problems?

All these questions must be considered, and they are not usually spelled out in the piping codes. They may be mentioned as things that must be considered; however, there is often little guidance. There is a whole new set of code requirements for offshore and underwater pipe-lines. The pipeline codes explain those requirements in detail.One also needs to consider the fluid or material that the pipe system will be transporting. Often, the code’s title is the only indicator of the fluid. B31.8 is specifically for gas transmission. That code does have specific requirements in it for sour gas. As mentioned before, B31.1 Power. Piping is primarily involved with steam-water loops. In each of the codes the scope gives some more information regarding these transport materi-als. B31.3, because of its broad range of application to a variety of process industries, has the most information about transport fluids. It defines four types of fluid:

1. Category D service. These must meet certain requirements and are basically low pressure, not flammable, and not damaging to human tissue.

2. Category M service. This is the opposite of Category D fluids and therefore must be treated by separate requirements.

3. High-pressure fluids. These are fluids that have extremely high pressures as designated by the owner and have independent requirements.

4. Normal fluid service. This is not your everyday normal Category D fluid service, but it does not meet the requirements in 1, 2, or 3, and is generally called the “base code.” One can use that base code for Category D fluids, as it is sometimes simpler when Category D service is over the entire project.This gives a flavor of what the various transport fluids can be.

 

Selection of Materials

By and large what the fluid a project is for comes as a given. The speci-fier or designer then chooses an appropriate material to handle that fluid under those conditions. In general, codes do not have within their scopes which material should be used in which fluid service. However, they may limit which materials can be used in certain system operation conditions, like severe cyclic conditions or other effects that must be considered. Many of these do not give specific ways to make those considerations. Some methods are discussed later in this chapter.At this point, given a fluid and the need to calculate which piping material should be used, there comes a little bit of interaction with regard to sizing the pipe. This is especially true when there is the opportunity to have more than one operating condition in the life of the system. In those multiple-operation situations, a series of calculations must be made to find the condition that will require the thickest pipe and highest compo-nent pressure rating. For instance, it is possible that a lower temperature and a higher coincident pressure may result in use of heavier pipe than a higher temperature and a lower pressure. This combination may not be intuitively obvious. Such considerations will be discussed and demon-strated in much more detail in Part II and the Appendix.The sizes required may have an effect on the materials of selection. All components may not be available in materials compatible with pipe materials. This conundrum was common when higher-strength, high-temperature piping was developed in the late 1990s for high temperature service. Material to make components out of similar material was not readily available for several years.It is also true that when newer materials are developed the fabrication skills and design concerns take a little time to develop. New techniques are often required for a result in the same net margins one is used to with the older materials. That and similar problems explain why the adoption of new materials proceeds at a less-than-steady pace.

Having explained generically some of the material problems, we can turn our attention to the materials of construction for a pipe system. Each code has what is generally called listed materials. These are materials that the various committees have examined and found to be suitable for use in systems for the type of service that that book section is concerned with. It stands to reason that those books that work with a wider variety of materials have more types on their “preferred” list.

ASTM and Other MaterialSpecifications

In piping these are most usually ASTM grades of materials. For ferritic steels, they usually are ones from ASTM Book 1.01. In many instances, it also lists API 5L piping materials. One major exception is boiler external piping, listed in B31.1, which requires SA materials rather than ASTM.It is basically true that one can substitute SA for ASTM materials of a similar grade. The SA materials are often the same as ASTM materials of the same grade, as in SA-515 or A-515. Section II of ASME’s Boiler and Pressure Vessel Code (BPVC) is the materials section, which reviews the ASTM materials as they are developed for applicability to the boiler code.

There is a little hitch that always occurs when one standards-writing body adapts or references another’s standard for their purposes—a time lapse problem. If standard group A issued a change to their standard, the adopting group B cannot really study it for adoption until after the publication date. And then they can’t necessarily get it adopted in time for their next publication date, which is most likely to be out of sync by some amount of months or possibly years with the change. So the lag exists quite naturally.

 In addition, sometimes the change made by group A is not necessarily totally acceptable to group B. Specifically for the SA/ASTM problem there are some SAs that say this is the same as the ASTM of a specific edition with an exception. Or they might just keep the earlier edition that they had adopted.

Because of this inherent lag, standards groups spend a fair amount of effort letting you know which edition of a standard they have accepted is the one that is operative in that code. Typically, B31 and other standards will list the standard without an edition in the body of their code. Then they will offer an appendix to the code that lists the editions that are currently approved. Every attempt is made to keep the inherent lag in timing to a minimum.In addition to these listed materials, sometimes unlisted materials are accepted with certain limitations. Also, some discuss unknown materials and used or reclaimed materials. Table 3.1 shows what each B31 book section generally will say.

Other standards have materials requirements that often point back to ASTM or an acceptable listing in another standard. This helps to eliminate duplication of effort and the lag problem is again minimized. Some stand-ards develop their own materials. The most notable of these is MSS SP-75, which has a material called WHPY that has a defined chemistry and other mechanical properties.

Listed and Unlisted Materials

The listed materials are those in the B31 books, which list the allowable stresses at various temperatures for the materials that they have listed.So, because in their applications there is a wide range of temperatures utilized in their systems, they need these tables. Over a wide range of temperatures the yield and ultimate strengths will go down from ambient temperatures. In addition, at some temperature, time-dependent proper-ties, such as creep and creep rupture, become the controlling factor.To establish the allowable stresses at a specific high temperature could require expensive and time-consuming tests. The ASME determined a method that, while it doesn’t completely eliminate the tests, reduces them to an acceptable level. It uses them to establish the allowable stress tables.In cases where the material one wants to use in a project is not listed in the particular code, the first step is to determine whether that code allows the use of such a material. Some guidelines of where to look are in Table 3.1.B31.3 is the most adaptable to unlisted materials, so a brief discussion of that procedure is given. It is important to note that the code does not give one license to use it in compliance with other codes; however, it is a rational method to determine acceptable stresses for temperatures where there isn’t a published table of allowable stresses.The nonmathematical part is to select a material that is in a published specification. This is quite probable because of the proliferation of national or regional specifications that for one reason or another have not been recognized by the codes in either direction. That is to say, the code from one country does not specifically recognize another country’s or region’s material specification. There is progress in the direction of unifying these different specifications, however slow.

To be useful, they must specify the chemical, physical, and mechanical properties. They should specify the method of manufacture, heat treat, and quality control. Of course, they also must meet in all other respects the requirements of the code. Once the material is established as accept-able, the next priority is to establish the allowable stress at the condi-tions, particularly temperatures in which the material is intended to be used.

This discussion assumes one is intending to use that material at a tem-perature that is above the “room” temperature or temperature where normal mechanical properties are measured. Measuring mechanical prop-erties at higher temperatures is expensive and can be very time dependent if one is measuring such properties as creep or creep rupture. The ASME code, recognizing that this process is difficult, developed a trend line concept to avoid requiring such elevated-temperature mechanical tests for each batch of material made, as is required for the room temperature properties. This is called the trend curve ratio method.

The method is relatively straightforward. Some of the difficult extended temperature tests have to be made. While as far as is known there is no set number of tests, it stands to reason that there should be more than two data points to ensure that any trend line that is not a straight line will be discovered from the data points. It also stands to reason that the tempera-ture range of the tests should extend to the higher temperature for which the material is used. This eliminates extrapolating any curve from the data and limits any analysis to interpolation between the extreme data points, which is just good practice.

Obviously, if the intended range extends into the creep or creep rupture range, those tests should be run also. This decision becomes a bit of a judgment call. As a rule of thumb the creep range starts at around 700°F or 371°C. However, depending on the material, that may not be where those temperature-dependent calls control the decision.So now one has a set of data that includes the property in question at several different temperatures. For purposes of illustration, we make an example of a set of yield stresses. This is not an actual material but an example. The data for listed materials can be found in ASME Section II, Part D, and these are already in tables so there is no need to repeat that data here. We will call this material Z and the necessary data to establish the trend curve ratio are listed in Table 3.2.Given these tables, a regression on the temperature versus the com-puted ratios can then be established. It should be noted that the original data might be in the same degree intervals that the table is intended to be set up in, but in general this is not the case. Therefore, a set of data that ranges from the room or normal temperature to the highest intended temperature can then allow a regression that is basically interpolative rather than extrapolative. It is unlikely that the material supplier has test data at the exact temperature at which one is going to use the material.One might note in delving into Section II of the boiler code, which is the basic material and stress section, that these yield temperature charts rarely go above 1000°F. This is accompanied by the general fact that this is a temperature that is usually within the creep range and that yield is the less dominant mechanical property. Yield above that temperature is not as critically needed.Regardless, the regression yields formulas that allow one to predict the yield at any intermediate temperature. For the previously presented data one regression is a third-degree polynomial that has a very high correlation coefficient. That formula is

Ratio at temperature Ry = 1.00361-(2.08E-0.06)T - ((9.5E - 0.07)T^2) -(1.58E - 10)T^3

One might think that the latter terms might be ignored, but if one thinks of, say, a temperature of 500, that 500 is cubed; therefore, that small constant changes the yield by over 500 psi in the current example, and that is a significant change in stress.This explanation applies to the method ASME has developed to avoid the requirement for each batch of material to go through extensive high-temperature testing.

A test of tensile and yield at room temperature (generally defined as 70°F or 20°C) satisfies the requirement. The temperature values is that room temperature value multiplied by the appropriate temperature, Ryor Rt. The same general technique is used for both yield and tensile properties.

Allowed Stress Criteria forTime-Dependent Stresses

The other criteria for establishing allowable stresses are that of creep and creep rupture. The criteria involve a percentage of creep over a length of time. These have been standardized in ASME as the following values:

1. 100% of the average stress for a creep rate of 0.01% per 1000 hours. 

This can be described as causing a length of material to lengthen by 0.01% in 1000 hours when a steady stress of a certain amount is applied at a certain temperature. Obviously this requires many long tests at many temperatures and many stresses.2. 67% of the average stress for a rupture at the end of 100,000 hours. 

Once again, many stresses at many temperatures are tried until the part breaks or ruptures.

3. 80% of the minimum stress for that same rupture. Again, many stresses at many temperatures are tried.These criteria are basically the same over all the ASME codes. The double shot at the rupture criteria (2 and 3) comes about to eliminate any possibility of having a test that gives a wide variability of highs and lows. It is essentially an analogy for having a rather tight standard deviation in the data. One can also assume that there are expedited testing methods for the creep-type tests. A full-length test of 100,000 hours would last over 11 years and several different stresses would have to be tested. Even a full 1000-hour test would take over 41 days.Having assembled all that data, the decision for any given tempera-ture is then made to allow the lowest stress. The tensile stress has a percentage applied to it that is set, as much as possible, to ensure that the material has some degree of ductility. The main stress factor is yield stress. The percentage of yield that is allowed is dependent on the code section. Generally, the two most often used criteria are 67% of yield and a divisor of 3.5 on the ultimate tensile stress, all at the desired design temperature. The creep criteria are included in this survey, and the one that yields the lowest stress is established as the allowable stress at that temperature.This is not true in the books where the applications have a limited range of operating temperatures, mostly in the pipeline systems. In those, they simply set the specified minimum yield of the material as the base allowable stress. Their calculation formulas then have a few variable constants based on the pipeline’s location class and the temperature and any deviation for the type of joint that is employed in making the pipe. It is noted that the temperature range for pipe containing natural gas, for instance, would be quite small. On the other hand, that pipeline can go through a wide variety of locations.

Stress Criteria for Nonmetals

When one comes to nonmetals the presentation of stresses is consider-ably different. Nonmetals have a much wider set of mechanical properties with which to contend. There are several types of nonmetallics. Those recognized by the various codes are thermoplastic, laminated reinforced thermosetting resin, filament-wound and centrifugally cast reinforced thermosetting resin and reinforced plastic mortar, concrete pipe, and borosilicate glass. One doesn’t need to be an expert to recognize that they represent a wide range of reactions to stress or pressure. The allowable stresses are set this way as well. For instance, B31.3 refers to five different stress tables for the above-mentioned materials. A brief listing of how those tables vary is as follows:

1. The thermoplastic pipe table lists several ASTM designations and allowable stresses over a limited temperature range for each ASTM designation. It is the most like the metal tables.

2. The laminated reinforced thermosetting pipe table lists an ASTM specification with a note stating the intent is to include all of the possible pipes in that specification. That specification gives allowable usage information.

3. The filament-wound materials (e.g., fiberglass piping) table lists several ASTM and one American Water Works Association (AWWA) specification with the same note as that in item 2.

4. The concrete pipe table lists several AWWA specifications and one ASTM, and it states the allowable pressure for each pipe in the specification. The specification itself defines the controlling pressure-resisting dimensions and attributes, eliminating the need for any wall thickness calculation.

5. The borosilicate glass table lists one ASTM specification and an allowable pressure by size of pipe.

This is the way ASME has chosen to handle the nonmetal materials that they list.B31.3, which for now is the only high-pressure design for pipe code, has a separate allowable stress table for the limited number of metals that are recognized for use at those high pressures. Those tables do have an unpredictable difference in allowable stress values for common tempera-ture. Like everything in the chapter, they are mandatory to comply with the code once a piping system has been defined by the owner of the system as a high-pressure system. Many times it is asked: What is high pressure? The general requirements are that it can be anything, with no specific lower or upper limit. It is high pressure only if the owner specifies it as so. For purposes of writing the chapter the committee used the defini-tion as any pressure and temperature that are in excess of the pressure at that temperature for the material as defined in the ASME B16.5 pressure-temperature charts as Class 2500.

Corrosion and Other Factors

A main remaining consideration in material selection is what is called the material deterioration over time, commonly referred to as corrosion allowance. That corrosion can occur on the outside of the pipe due to the environment the pipe is in, and can come from the inside due to the fluid and the velocity and temperature of that fluid.

The amount of corrosion allowance to be allowed is dependent on the rate the corrosion will occur over time and the expected lifetime of the particular system. The calculation effort, after the corrosion allowance is set, is addressed in Chapter 5 to calculate pressure thickness. Setting that allowance is outside the scope of the codes. There is a suggestion in B31.3, 

Appendix F, Precautionary Considerations, that points the reader to publications such as the National Association of Corrosion Engineers’ 

“The Corrosion Data Survey.” This would help guide the setting of corrosion allowance.

The Appendix contains a list of common materials from the U.S. ASTM Book 1.01, which by far lists the vast majority of the materials used in piping. As was mentioned, the ASME has its Division 2 listing of materi-als, which have an SA or SB designation. By and large, they are ASTM materials that have been adopted. Some have restrictions on elements like the chemistry, or some other portion of the current ASTM material may be invoked when adopting them. Those restrictions are noted in the listing. The primary purpose of these materials is for use in the boiler code sections; therefore, they are not treated in this piping-related book more than they have been already.

There are materials standards from other geographical sections of the world. Many of them are similar to ASTM materials, but some are quite different. It appears on cursory examination that often these standards have a greater number of micro-alloyed materials. The mélange of materi-als has not been resolved into some simple—“these are the materials of the world”—standard. There is considerable work going on in that area, but it might take a long time to get to the finish line in that effort. For those who feel the need, there are books that attempt to be conversion sources to compare world materials—for example, Stahlschussel’s Key to Steel. It is quite expensive and most detailed, and works primarily with European steels but lists many regional steels. I have used it with success in untangling the web of various steels.There is a little more to consider in preparing to do the calculations required by or suggested by the codes: the business of sizing the pipe for a particular system. This includes the flow in the system and the attendant pressure drops, which, as mentioned, are not really a code-prescribed concern. However, a basic understanding of the methods employed in this process is background for the user of the codes and as such is addressed in Chapter 4. A description of the calculations and examples with certain parameters are given rather than an explanation of the development of those parameters.

The reader will note that the metals listed as acceptable are often ASTM standards. One of the interesting things about ASTM steels is that they are segregated into different forms. The steel might have almost exactly the same chemistry, and therefore in the casual reader’s eye be the same material. This could be considered true. Certainly, it is true if the various elements in the steel are within the chemical tolerance of the specification for the particular form being reported. However, the chemistry is not the only thing that ASTM and other standards would specify. The major forms of the same material would most likely have different mechanical properties and minimum stresses. Those things depend to an extent on things like the method of manufacture and postmanufacture treatment, as well as the chemistry. It is true that chemistry is the main ingredient; however, the other factors will make a difference and that is why the same chemical material would have a different number depending on the form the material takes—pipe, plate, or forging or casting.