Last updateFri, 11 Jun 2021 4pm

Beyond Valves

Putting the “P” in PVF

Of course, the “P” in PVF stands for pipe, those long round things with the hole down the middle! Learning a little bit more about the pipes that connect to your valves will help you as a valve professional.

First, we need to understand how pipe is sized. Pipe is measured by utilizing a dimensionless number known as nominal pipe size or “NPS”. These sizes run from ¼ inch to above 72 inches. If you take a caliper to a 1 inch NPS pipe you will be surprised to see that it is not 1 inch in diameter, rather, it is actually 1.315 inches in diameter. This oversize dimensioning applies to pipe through NPS 12 inches. For diameters 14 inches and above, the NPS is the actual outside diameter. When it comes to dimensions, pipe should not be confused with tubing. Tubing size is equal to its outside diameter.

Pipe Schedules

Since the strength of pipe is proportional to its wall thickness, pipe is also categorized in this way. Initially pipe dimensions were fairly standardized. In fact the wall thickness was called “standard” (STD), or “standard wall.” As pressures increased, the designations were amended to include thicker walled pipe called “extra-strong” (XS) or “extra-heavy” (XH). Eventually the term “double extra-heavy” (XXH) or “double extra-strong” (XXS) was added to the range of pipe wall thicknesses as they grew thicker to accommodate even higher pressures. The three pipe thicknesses STD, XS and XXS were inadequate to handle an ever-increasing range of fluid pressures, so a new system based upon numbers called “pipe schedules” was devised.

These schedules are the approximate expression of 1000 P/S, where P is the service pressure and S is the allowable stress, both measured in PSI. Pipe schedules begin with 5 and currently run all the way up to 160. The most common schedules in use today are 40, 80 and 160. The old terms—standard, extra-heavy and double extra-heavy—are still used and they correspond to various pipe schedules, depending upon the NPS of the pipe. Schedules with an “S” suffix, such as SCH 20S, refer to stainless-steel pipe, dimensioned in accordance with ASME B36.19M. Steel pipe is dimensioned in accordance with ASME B36.10M.

Materials and Specifications

Today, the predominant material for industrial piping is steel. However, that was not always the case. The oldest piping systems on record, in ancient Rome, 2000 years ago, were made of lead. The industrial revolution and the Iron Age ushered in the era of iron pipe, which peaked in the 1920s. Although steel is the primary choice for industrial applications, non-metallic piping materials have made huge headway in the water and wastewater industry where iron once was king.

Steel pipe material specifications are detailed in ASTM documents, and some of the more common ones are listed in the table below:


Material Notes

Seamless & welded steel pipe


Seamless carbon steel pipe


Alloy steel pipe (Cr/Mo, etc.)


Stainless-steel pipe


Line pipe for petrol­eum pipelines


How It’s Made

Another important identifier in steel pipe is the process by which it is made, either welded or seamless. Welded pipe is made from plate, which is rolled into a circle and then welded down the length of its longitudinal seam. Seamless pipe is made by squeezing and rolling round bar around a pointed mandrel that pierces the inside of the bar, creating the pipe bore. Due to the equipment costs involved in seamless pipe manufacturing, it is primarily used in the smaller sizes. For critical applications, however, seamless pipe is almost always specified, and it is commonly manufactured in sizes through NPS 24.


Pipe can be provided with plain ends, threads or mechanical connections. Before the perfection of pipe welding techniques 75 years ago, most piping was supplied with threads on each end. Through the use of couplings and threaded pipe fittings, pipelines and piping assemblies were created. Making a leak-free connection on a 40-foot joint of 6-inch threaded pipe took quite a bit of muscle power, a bucket of “pipe dope” and a giant-sized pipe wrench!

Today, virtually all industrial piping is welded. Properly welded pipe joints have high integrity and they are much faster to make than by threading fittings to each piece of pipe. In the waterworks industry mechanical connections are used to join most large diameter water and wastewater lines. These mechanical connections employ a bell and spigot or grooved-end design, combined with a resilient seal and bolting mechanism to lock the joint in place. Mechanical joints are also popular because they allow for a slight misalignment to permit pipe runs to dodge around obstacles without the use of curved pipe or excessive use of elbow fittings.

From Homes to Industry

If you have walked through a house under construction or made a wrong turn down one of the big box store aisles, you have probably seen white plastic PVC piping. While not allowed in most industrial plants, the plastic pipe forms the basis of nearly all residential piping. PVC pipe as used in home construction is usually schedule 40, and it is welded to PVC fittings and valves using a plastic solvent. For some low-pressure, highly corrosive industrial applications, plastic pipe made of exotic materials such as Kynar and Kalrez is used.

Industrial facilities and power plants require a maze of piping that twists and turns in all directions. Small-diameter pipe is welded to fittings to achieve direction changes in the pipe runs, while larger pipe is bent. Steel pipe of virtually any size can be bent to any angle. This process creates what is called “fabricated” piping that is formed to the exact geometry called for in the piping blueprints. The prefabricated pipe sections are then shipped intact to the job site where they are joined with other fabricated sections. This off-site fabrication saves time and money as well ensuring more accurate construction.

Greg Johnson is president of United Valve (www.unitedvalve.com), a valve service company in Houston, TX. Reach him at greg1950@ unitedvalve.com.

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