efore we move into the second part of our discussion regarding supply piping materials, let’s back up a step and review the general comments we made in Section II. In outlining the criteria for proper specification of supply piping materials, we included the following general considerations:

1. Potability—Can the material safely transport drinking water?
2. Strength—Is the material strong enough to withstand the supply pressure of the application involved?
3. Size and Capacity—Are we providing the proper diameter piping to meet the requirements of the installation?
4. Resistance to Corrosion and Scaling— (Especially a consideration in “bad water” areas.)
5. Temperature Characteristics—Any limitations?

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"Continuing our study of supply (pressure) piping, this Section deals specifically with the varieties of plastic materials available for such applications today."

Each of these criteria involves a great deal of technical depth and detail that is properly the concern of the people who engineer and specify piping systems. Our goal in this course, however, is not to train you to become a specifying engineer,but simply to give you some basic understanding of the materials that such an engineer would commonly select, and that you as wholesalers would likely handle as a result.

Unlike our coverage of steel and copper piping materials, which are “singular” product categories, the field of plastic pipe is plural; that is, it involves many different specific sub-categories.To the average person, the subject of plastics, in general, is a bit of a mystery.

For example, while most folks today would likely regard stainless steel as a higher grade metal than tin for most applications, they probably lack a comparable understanding of the specific materials within the plastics field. But specifying the proper specific material is just as essential in plastics as it is in metals. (If Tony Dorsett’s football helmet had been made of polystyrene rather than ABS, for example, there’s a good chance he might stand a few inches shorter today. While the first material is relatively brittle, the second can take a pounding all day.)

We’ll be getting into some specific types of plastic materials shortly, but before we do, let’s quickly establish the two most basic categories into which all plastics fall.

Thermoplastics: This category of materials is formed into shape by means of elevated pressures and temperatures, a process that is reversible. In other words, once molded or extruded, it can be returned to its original, or moldable, state through subjection to the same factors—heat and pressure.

Reversibility is important not because it is likely that, once molded, a product would be melted down and formed into something else, but because applications of the product that involve excessive heat and pressure could cause the material to partially revert, resulting in a loss of desired properties (such as strength). Each individual type of thermoplastic material has its own characteristics in this regard, involving recommended levels of heat and pressure to which it can safely be subjected.

In regard to the joining of certain types of plastic piping materials (i.e., pipe and fittings), this reversible characteristic becomes an advantage in that a partial breaking down of the piping surface permits a strongly bonded joint through one of several welding processes.We will discuss these processes later in this article.

Thermoset Plastics: Unfortunately, the name for this other basic category is not very different from the first one, and this can sometimes cause confusion.Thermoset plastics can also be formed by means of elevated temperatures and pressures, though there are some specific types that essentially can be cast at room temperature. The significant difference in thermosets as a broad category, however, is that the process in this case is irreversible. Once formed, such materials cannot be returned to a “moldable” state again.

It might be helpful to remember the tail end of thermoset—the three letters, “set” —as a means to establish the definition in your mind. Once this material is formed, it is set that way for good. One of the most familiar examples of a thermoset material for most of us is the unbreakable dinnerware made of a material called Melamine.

Most of the specific types of materials used for plastic piping today fall within the first category—Thermoplastics—and all of those covered in any detail in this

Plastics is the fastest growing category within the field of piping today. Originally, applications were limited to irrigation, lawn sprinkling and certain industrial uses. But today plastic piping is used, in one form or another, in justabout every type of piping application. There are several reasons for the success of such materials in recent years. Overall, producers of plastic piping products claim that increased popularity of their products is due to the following advantages:

1. Resistance to corrosion and chemical attack (latter point varies, depending on specific material involved);
2. Smooth inner ‘walls, that provide a lower friction factor;
3. Self-insulating (will not conduct heat as readily as metals);
4. Lightweight, easy handling;
5. Availability of coils in some cases;
6. Ease of installation and versatility of method;
7. Relatively low cost;
8. Savings on labor.

Following is a brief description of most of the plastic piping materials used for supply purposes today:

PVC (Polyvinyl Chloride) — one of the first materials used for indoor supply piping, this has high strength, good weathering characteristics and high resistance to chemicals such as acids and alkalis. It loses some of its properties when used with high temperature fluids, however, and for that reason,it is limited to use as cold water lines. PVC piping is rigid, and comes in straight lengths only.

A CLOSER LOOK AT PEX PEX is the newest and fastest-growing material in common use for supply piping today. PEX, short for crosslinked polyethylene, has become the most widely used flexible plastic piping material suitable for both hot and cold supply applications. Originally developed and used in Europe (since way back in the 60's), its use in this country was pretty much limited to non-plumbing applications until the mid-90's. Those initial uses included hydronic radiant, snow-melting, ice rink and refrigeration systems. Until that time, polybutylene (PB) had been the flexible plastic piping material of choice, but when massive field failures finally forced that off the market, the door swung wide open for an alternative. The succeeding heir, PEX, is composed of high density polyethylene which undergoes a physically or chemically induced molecular change by one of several processes. Manufacturers explain the result as a three-dimensional "bridging" or "networking" of molecules that form a thermoelastic material, stable at high temperatures while retaining flexibility and resistance to chemical attack. Translation for the rest of us: the stuff is stronger and more durable than regular PE under a wide range of temperature extremes (up to 180ºF for plumbing supply applications and 200ºF for others). PEX is produced in copper tube sizes ranging from 1/4" to 2", available in coils as long as 1,000 feet, as well as in straight 20' lengths. While the material is sometimes joined with compression fittings, by far the most common method of joining is by means of insert fittings. There are two basic components involved with this approach: the insert body itself, and the crimp rings. The crimps rings, usually copper, are first slid over the ends of the tubes to be joined. Next, males ends of the insert fitting body are pushed into the ends of the tubes to be joined. Finally, the rings are positioned into their correct alignment over each tube end and a special tool is used to crimp the assembly together. The crimping squeezes the compressible wall of the tubing onto the rigid insert inside, providing a secure mechanical connection and a leak-free seal.

CPVC (Chlorinated Polyvinyl Chloride) — this material has properties that are generally the same as PVC, with the exception that it has higher temperature capabilities, and thus can be used with hot water systems. Where installers once used PVC for cold water and CPVC for hot, the general practice today is to use CPVC for both.

PEX (Cross-linked Polyethylene) — one of the fastest growing options today, this material is popular because it is both flexible (furnished in coils) and able to handle hot and cold water temperatures. It has largely replaced polybutylene in the market.

PEX-AL-PEX — is a variation of PEX that is made with a thin layer of aluminum buried in the wall section (PEX on the inside, aluminum in the middle, PEX on the outside). Like plain PEX, this type bends easily, but unlike PEX, it retains a precise bend when released (most flexible plastic piping materials have a mind of their own when it comes to their exact bent form — this type acts more like copper tube).

PB (Polybutylene) — largely discontinued today, this flexible hot/cold material has been the subject of widespread problems over the past 20 years. While a portion of these failures were the fault of the acetal fittings and the over-crimping of retaining rings, the material has nonetheless fallen from favor and use.

PE (Polyethylene) — this is used primarily for outdoor applications involving the connecting of buildings to mains, farm irrigation, lawn sprinkling, etc. It is flexible, has high impact resistance and is resistant to stress cracking and weathering.

Plastic pipe is manufactured through a process called “extrusion.” When you extrude something, you force it through a precisely defined opening, so that the result is a length of material with the exact profile of that opening. A simple example of this would be a toothpaste tube. When you squeeze, you produce a length of paste, the diameter of which is determined by the opening at the end.Then, if you were able to place a fat pencil lead in the opening of the tube, your next length of paste would be hollow on the inside, and you would suddenly find yourself in the peppermint pipe business.

The extruding of pipe today is accomplished on a large piece of machinery, rather predictably called an extruder. Here’s how this works: The thermoplastic raw material is furnished to the pipe manufacturer in pellet or powder form. This is then dumped into the receiving hopper on top of the extruder, which feeds into a chamber where a large screw-like shaft moves it in the direction of the opening, called the “die,” that determines the ultimate shape. As the material is moved, it is also compressed (providing pressure) and heated to the required levels until it turns into a semi-solid state, the optimum moldable condition. Finally, it passes through the precise configuration of the extrusion die, emerging with the exact outside and inside diameters determined by that die.

Unlike the primary forming processes involved in the various types of seamless metal pipe production,there is no limitation on the length which can be produced by an extrusion process. This is because there is no external mandrel (center forming pin) beyond the point where the material emerges from the die. In extruding, the part that parallels the mandrel in function, called the “pin,” is positioned inside the die opening, secured from behind,and therefore does not limit the

How plastic pipe is joined depends largely on the specific material involved. Not all systems can be used with all types.

Solvent Weld — PVC and CPVC: By applying a coating of solvent cement to the male-female surfaces of the pipe end and fitting, and inserting these parts together, the joint fuses together into a hard, solid state. This is not a glue or filler

— the mating surfaces flow together and harden as an actual “weld.” The compound contains both solvent and a portion of the piping material being joined. It is recommended that hard surface piping materials such as these be “primed” first with straight solvent, to give the mating surfaces a head start on “melting” properly.

Mechanical Joining — PEX and PE: The two basic methods of connecting PEX and PE are called insert and compression. An insert fitting (usually metal) is pressed into the connecting tube ends, and a metal ring is then crimped over the assembly to make a secure, leak-free joint. A compression fitting is similar to the type used with copper tube, involving the use of a ferrule (grip ring) that is compressed into the wall of the piping material by the tightening of a nut. Prior to assembly, a metal stiffener is inserted into each tube end to assure a uniformly round profile for sealing the joint. Compression joints have the advantage of allowing disassembly, if needed.

Thermal Weld — PE: This involves the use of a special heating tool that essentially melts the mating surfaces of the pipe and fitting before their assembly. Here again, the flowing of like materials together results in an actual weld.

There are a number of approaches used in joining plastic piping today, though not all systems of connection can be used with every type of material. For example, solvent welding, commonly used with PVC pipe, will not work with polyethylene pipe. And insert fittings, commonly used with certain types of polyethylene pipe, are not suitable for PVC. So it’s not enough to know the various ways to connect plastic pipe—you have to recognize “what works with what.”

The following are the most common systems used in joining plastic supply piping. (There are one or two others that are used with plastic DWV piping, which we’ll discuss later in the series.)

Following are more detailed descriptions of the joining processes used today:

Solvent Welding: By far the most commonly used system of joining plastic pipe today is the solvent weld approach. By applying a coating of solvent cement to the male-female surfaces of the pipe and fitting, and inserting these parts together, the joint fuses together in a hard, solid state.

Each solvent cement actually contains an element of the material for which it is intended to be used. For example, the solvent cement used with PVC piping is made of PVC material dissolved in a chemical called tetrahydrofuran. The solvent welding bond is—as the name states—an actual weld, not just a filler or “glue.” The application of the solvent causes the pipe and fitting surfaces to become somewhat soft and tacky,allowing them to flow together with the plastic of the cement to unite in a tight bond.

Just as there are specific types of plastic piping materials that can be solvent welded, there are specific solvents designed for. use with each. This is important to understand, since such solvents are not interchangeable among plastic piping materials. Here are the specific steps involved in making a good solvent welded joint:

1. Cutting: Plastic pipe can easily be cut with either a fine-tooth saw, or a special pipe or tubing cutter with a cutting wheel specifically designed for this application. If a saw is used, it is a good idea to use it with a miter box to assure a square cut of the pipe.
2. Deburring, Beveling: As in the preparation of other types of pipe, it is important to remove burrs that result from cutting.This can be accomplished with a common knife, a coarse half-round file, or a special deburring tool. In addition, most manufacturers recommend a beveling of the outside end of the pipe to act as a “lead”for easy insertion into the fitting socket, and to prevent “wiping” of the solvent during insertion. Here again, this can be accomplished with a knife, file, or special tool designed for this purpose.
3. Priming: With certain types of plastic pipe, it is recommended that the surfaces be coated with a straight solvent, called a “primer,” before the actual solvent cement is applied.This is especially needed on some of the hard-finished, high-gloss products now being produced. The primer penetrates and softens the mating surfaces to a gummy consistency, assuring that they will flow into a true weld once the cement is applied.
4. Applying The Solvent Cement: Using a brush or dauber, the specified solvent cement is applied to the end of the pipe, and to the inside of the pipe fitting socket. As mentioned, it is essential that the solvent cement be the proper type for the specific type of plastic pipe involved. With some types of plastic pipe, a second application of solvent cement is recommended before insertion of the parts.
5. Joining: As soon as the solvent cement is applied, the pipe is inserted into the socket as far as it will go, with the pipe or fitting being given a quarter-turn in the process. (The rotation assures an even distribution of the cement on the mating surfaces.) Given an adequate amount of solvent cement application, the insertion will force a portion of excess material out at the entrance of the socket. This is carefully wiped off, leaving a small bead or “fillet” around the circumference.
6. Drying or “Curing” Time: The length of time a solvent welded joint must set before it can be handled, and then used under pressure, involves several variables. One of these has to do with the specific type of material itself, and what the manufacturer recommends for drying time. Other factors include the size of the pipe, and temperature and humidity factors in the location where the welding is being done. Generally speaking, drying times are faster with smaller diameter pipe, and when temperatures are relatively high, and humidity low. On the subject of temperatures, it is usually not recommended that solvent weld joints be made when ambient temperatures are below 40°F, or above 90°F when exposed to direct sunlight.
7. Thermal Welding: Another means of welding certain types of plastic pipe involves the use of special heating tools that essentially melt the mating surfaces of the pipe and fitting before their assembly. Heated by either gas or electricity, one such tool is designed with male and female metal “tool pieces” that mate in relationship with the pipe and fitting. After a short period of insertion in this tool, the surfaces of the pipe and fitting are softened to a gummy state, at which time the parts are removed and assembled together. As the plastic surfaces flow together, they cool into a hard, solid joint. Like the solvent approach,this results in a true weld;that is, the parent materials actually flow together and unite.
8. Threading: Though not used as frequently as welding methods, threading of plastic pipe is an accepted alternate approach on certain types of material. It should only be attempted on heavier grades, however, such as Schedule 80 and higher.
   
   The process involved here is basically the same one we described in our coverage of steel pipe, with the following additional considerations. With plastic pipe, it is extremely important to use cutting dies that are clean, sharp, and in good condition. Ideally,such dies should not be the ones used in cutting metal piping materials. Worn dies, which might still do an adequate job on metal piping, will not produce a good quality thread on plastic.
   
   In addition, it is necessary to insert a tapered plug into the end of the plastic pipe when threading, in order to prevent distortion. (Distortion, or “ovaling,” could result in nonuniform depth of thread cutting, or even gouging and tearing of the pipe wall.) Because of the somewhat resilient nature of plastic material, mating threads have a tendency to form a better seal than in the case of metal, and in some cases, screwed joints can be made without the application of “pipe dope.”
   
   Important Note: Even though the thread dimensions between plastic and metal pipe are the same, it is usually not recommended that they be mated directly together.The tapered nature of pipe threads would tend to stress a plastic material if it were mated with a more rigid material such as metal. In addition, there are differences in thermal expansion, which means that plastic would typically expand more under elevated temperatures, than would metal. This does not mean, however, that plastic piping material cannot be used in the same piping system with metal. The solution is a rather simple one; the use of adapter fittings make the transition from one type of piping material to the other. If you were going from PVC to steel, for instance, one end of the appropriate adapter would be plastic, with the other end metal.
9. Fillet Welding (Hot Gas Method): This form of welding plastic pipe is usually intended as a repair measure, since it tends to give a more superficial bond than the other approaches. In cases where there has been a faulty joint involving another welding approach, it can sometimes be corrected by melting a bead (called a “fillet”) of the same type of plastic around the circumference of the pipe where it enters the fitting.
   
   After cleaning the surface thoroughly, a rod of plastic (the same material as the pipe itself) is melted into the “seam” by means of a jet of high temperature gas.This is not a flame, it is simply gas that has been heated to temperatures in the 500°F to 600°F range.
10. Insert Connections: Certain types of plastic piping—most notably, varieties of polyethylene—are joined using a system of plastic inserts and clamps. The approach is really quite simple and easily installed. The plastic insert (usually a material such as nylon) slips inside the pipe end, and is secured in place by means of a screwed clamp (similar to the kind often used on hoses under the hood of your car). As is the case with other systems of connecting plastic pipe, there are special adapter versions of insert fittings that permit connecting with metal piping systems.
11. Flare Connections & Compression Fittings: Finally, we have two additional methods of connecting certain types of plastic pipe or tube that are quite similar to approaches covered in our earlier discussion of copper tube. Flare connections are made by either forming the end of the plastic pipe by means of a heated tool, or by cold forming the flare with another type of tool. The connection then made is identical to the copper tube version.
    
    In the case of compression fittings, one type is identical to the one used with copper, with the exception that inserts are usually pressed into the tube end to prevent collapsing when the ferrule is compressed. (Ferrules used with plastic piping are sometimes plastic, rather than brass.) In addition, there is a type of compression fitting that seals with a gasket, rather than a ferrule.

The subject of plastic piping materials is a bit of a challenge to communicate, because unless you are a graduate chemical engineer,you may have a little difficulty appreciating the fundamental points of distinction here. Without getting in over our heads at this point, let’s just say that plastics experts evaluate the suitability of various materials for given applications on the basis of criteria such as (just to name a few of the categories): tensile strength, flexural strength, impact strength, thermal conductivity and thermal expansion. Based on these criteria, the plastics industry has found certain materials most ideally suited for use in specific piping applications.

Given a material that is structurally sound for such applications, it must also be judged suitable for use with potable water, if it is to be used in installations that provide drinking water. Manufacturers of the base plastic materials must submit samples to NSF (National Sanitation Foundation) for testing and approval. NSF approval is still not the whole story, however, since local plumbing approval agencies maintain the final voice over what is acceptable in their respective communities.

And now you know why it’s not possible to buy a short piece of plastic pipe!

Like metal varieties, most types of plastic pipe are offered in more than one weight grade (meaning there is a selection of wall thicknesses available for various pressure requirements).The subject of weight classes isn’t quite as simple in this case, however, since there are two different systems by which plastic pipe can be designated —by schedule number,or, by standard dimension ratio.

Schedule System: With many types of plastic pipe, the graduated differences in weight (wall thickness) are designated by “schedule number.” The common schedule designations for plastic pipe are 40, 80 and 120. Under this system, Schedule 40 is the lightest weight grade (thinnest walls), Schedule 120 is the heaviest (thickest walls). It is important to understand that while the system makes provision for all these classifications, this does not necessarily mean that each manufacturer offers its piping materials in all such schedules.

These weight grades of plastic pipe roughly parallel those of steel pipe covered last month (Standard, Extra .Strong, Double Extra Strong). As with steel pipe, the graduated weight schedules in plastic become thicker toward the inside, maintaining a consistent outside diameter in each of the nominal sizes involved. For example, all schedules of 1/2” nominal plastic pipe have an outside diameter of .840”, but the inside diameters become increasingly smaller as you move from the lightest to the heaviest grades (40 to120).

Standard Dimension Ratio System (Sdr): Plastic pipe that is not designated according to the schedule system is assigned a weight classification based on what is called a "standard dimension ratio," or "SDR." This is a system based directly on a pressure rating value, rather than on an arbitrary dimensional factor. As a result, all pipe within a particular SDR series has the same pressure capabilities, regardless of diameter. (This is not true of the schedule number system, in which pressure capability generally drops with increased diameter.)

There are some types of plastic pipe that fall within this SDR system of weight classification which, unlike those in the schedule system,maintain a consistent inside diameter through the various weight grades of a nominal size. In this case, for example, all SDR classifications of 1"nominal pipe will have an inside diameter of 1.049", while the outside diameter becomes increasingly larger as you move from the lightest to the heaviest grades.

Plastic piping materials that are joined by means of insert fittings use this system, since it is necessary that the inside diameters relate consistently to the dimensions of the inserts, regardless of weight.

The first thing to determine regarding plastic pipe size is whether you are dealing with IPS (iron pipe size) or CTS (copper tube size) basic classifications. Most plastic pipe is made to conform to one of these two basic sizing systems. (There are exceptions with specialized types of plastic tubing.) Properly labeled plastic pipe will always indicate which basic sizing system is involved, with an appropriate "IPS" or "CTS" included in the data given.

Once this basic sizing classification is determined,the specific nominal sizes of plastic pipe available are essentially the same as those covered on our tables for steel and copper piping last section. In other words, IPS plastic pipe is available in most of the nominal sizes listed for steel pipe, while CTS plastic pipe is available in nominal sizes comparable to copper tube. This is a general statement, however, and does not mean that each manufacturer of plastic pipe makes its material in all possible sizes.

Check with the manufacturer catalog in each case to determine specific information on weight