JFAPBC (2004) 1:8-11 DOI: 15298150418024

© ASM International 1547-7029 / $19.00

• E • of •L •U • T(continued) •I•O•N S •the O Perfect InRPursuit Pipe

by R. John Phillips

In Pursuit of the Perfect Pipe Clean water and pipes that last forever? Sounds good to me. Water and waste removal piping systems have existed for centuries—advancing in technology in response to the demand from growing metropolises. Through the centuries, the quest for clean water and the need to transport water greater distances have been the primary motivators for improving water piping systems. As of late, however, water pipe system design has taken a more obvious step toward increased functionality and durability.

Roman Aqueducts The Roman aqueducts were laboriously constructed at great expense using materials (stones and mortar) that have lasted for thousands of years. The aqueducts allowed communities to become cities, and cities to become an empire, growing beyond the constraint of a local water supply. Cities were no longer forced to be built a reasonable distance from rivers and lakes or be limited to the water available from cisterns. At the height of its use, the aqueduct system serving the city of Rome provided 300 gallons per day of water for each citizen, as well as a means for the removal of waste from the city. Aqueducts also allowed the siting of new towns along advantageous trade routes or defensive locations without the necessity of an immediately adjacent water supply. Despite the remarkable expanse and construction of the Roman systems, limited capacity ultimately made aqueducts obsolete, sometimes soon after their completion. The open construction of the system could lead to contamination of the water supply, either through natural means or sabo-

tage. The aqueducts were easily damaged by earthquakes and war. Even minor repairs could take years. A community dependent on an aqueduct was

helpless if the aqueduct was damaged or destroyed. Even so, some Roman aqueducts and sewers have escaped damage and remain in service today.

Roman-built aqueduct pillars that have withstood the passing of time

Reprinted from The Stress Point, with permission from Engineering Design & Testing Corp.

8

Volume 4(1) February 2004

Journal of Failure Analysis and Prevention

Through the ages, mankind has tunnelled through mountains, dammed up rivers, constructed huge edifices and buried millions of miles of pipe in search of a good glass of water.

Wooden Piping Systems

A ready source of timber and a little of the famed Yankee ingenuity resulted in the unlikely but effective wooden piping systems constructed in the Boston area. Significant water piping systems appeared again in the eighteenth and nineteenth centuries when municipal water supplies in the United States were constructed using logs hollowed by burning or drilling. Advanced wood piping systems remained in use well into the twentieth century. Wood piping systems were effective because the material of construction was readily available, the wood itself did not seriously contaminate the water, and the wood had a relatively long service life—features that metal piping systems of that time lacked to various degrees.

Journal of Failure Analysis and Prevention

An unusual benefit of the wood piping system was that firefighters could excavate the water main in front of a burning structure and auger a hole in the wood piping to create a water supply for bucket brigades or fire pumps. Removable plugs placed in these augured holes were called fireplugs, a term still used for fire hydrants today. The joints between the logs were often secured and sealed only by the swelling of the wood when water was introduced to the systems. Metal was sometimes wrapped around the wood

to add strength and increase the ability of the wood pipe to withstand pressure. However, the metal wrap had the tendency to corrode over time, weakening the pipe. Wood piping could also be damaged by soil movement, insects, and decay. The wood piping was also limited in volume and pressure capacity, making it unsuitable as growing cities required increased water supply. Surprisingly, wood water supply piping is still occasionally found in service, particularly in the northeastern United States.

Volume 4(1) February 2004

9

In Pursuit of the Perfect Pipe (continued)

Modern Piping Systems Modern piping systems use a variety of metals and plastics. Metal pipes have a considerable ability to withstand pressure, temperature, and environmental degradation. However, new plastic piping systems are continuously being introduced with improvements in function, service life, and ease of installation. And, while the material and labor costs vary depending on the materials, the cost of plastic piping is generally the lower for material and labor.

Copper and Stainless Steel Piping Copper and stainless steel are “passive” metals. Passive metals corrode rapidly but quickly form a tight, protective corrosion layer that serves as a thin film to protect the metal from the environment and prevent further corrosion. The dull coating that forms on a shiny penny is a good example. In the right environments, pipes made from passive metals can have very long lives. For this reason, copper has long been a preferred material for residential water supply piping. However, pitting corrosion on the inside of the pipes and the resulting pinhole leaks in copper piping have become widespread problems in many areas of the United States, coinciding with the introduction of requirements for cleaner drinking water. The more aggressive corrosive features of cleaner water, the use of chemicals to flush water systems, particles that are freed and then deposited on the pipe walls by the flushing, or a combination of these factors are generally thought to contribute to the corrosion. In any case, the end result is randomly spaced corrosion cells where the protective film on the pipe wall is breached. These corrosion cells rapidly progress into pinhole leaks. Galvanized Steel Piping Galvanized pipe has been popular because of its low cost and relatively

10 Volume 4(1) February 2004

long life in most, but not all, water supplies. Galvanized piping relies on a zinc coating that corrodes before the pipe base material. Ultimately, however, the base material will corrode once the zinc coating is consumed. Selective corrosion of the zinc coating produces thick layers of loose deposits that can reduce the inside diameter of the pipe, sometimes to the point of obstructing the flow through the pipe.

tubing is inexpensive, easy to install, nonreactive, and does not contaminate most fluids. Vinyl tubing has little ability to resist pressure at elevated temperatures and must be reinforced if it is subjected to significant pressures even at room temperature. Vinyl tubing also degrades rapidly when exposed to ultraviolet (UV) light (sunlight or strong artificial lights).

Iron Pipe Iron pipe is often used for large water transmission lines. It is relatively inexpensive and can be cast in large and complex shapes. Iron piping is subject to pitting corrosion and also to graphitization. Graphitization is an insidious form of corrosion in which the iron is preferentially consumed, leaving behind just a weak, brittle graphite structure having the same appearance and dimensions as the original pipe. Iron pipe can be protected with “cathodic” systems that apply an electrical voltage to prevent corrosion; however, these systems are prohibitively expensive for small piping systems and can leave the piping unprotected if the cathodic systems malfunction.

Polyvinyl Chloride Piping Polyvinyl chloride (PVC) piping systems are some of the oldest plastic piping systems and are well known for significantly reducing the cost of plumbing. Thin wall PVC piping is frequently used for unpressurized drain systems. Polyvinyl chloride piping is also used for pressurized systems, but it has a limited temperature range and is relatively bulky because of the wall thickness required to resist pressure. At elevated temperatures, PVC piping loses its ability to resist pressure, and it also becomes brittle at lower temperatures. In addition, PVC piping is easily damaged by external forces and must be well supported to prevent the weight of liquid from breaking the pipe. Chlorinated polyvinyl chloride (CPVC) is a newer variation of PVC piping that has an improved ability to resist temperature variations and is often used in residential hot water systems.

Vinyl Piping Vinyl tubing is used in industrial and some residential applications, such as icemaker supply lines. Vinyl

Journal of Failure Analysis and Prevention

Polybutylene Piping Polybutylene (PB) piping was popular for residential use 10 to 20 years ago. This piping was easy to install, less bulky than PVC piping, and suitable for both hot and cold water service. The piping was installed with barbed fittings and compression rings that were crimped onto the piping to secure (or “swage”) it to the fittings. The plastic fittings used with this piping system degraded over time and frequently resulted in leaks and consequent property damage. The later PB systems used metal fittings, but leaks at fittings persisted. PB piping has been taken off the market because of the leak problems, and homes with this piping are being repiped.

Polybutylene pipe with a leaking plastic fitting

Polyethylene Piping Cross-linked polyethylene (PEX) piping is a relatively new product that is gaining popularity for water supply systems. It is formulated by several different processes that result in piping that consists of only one long, crosslinked molecule. Cross-linked polyethylene piping is installed in a similar method to PB piping and is suitable for both hot and cold water use. Unlike other plastic piping, PEX piping is not as readily damaged when the water in the piping freezes. The earlier PEX piping was translucent and would degrade in UV light, making it unsuitable for prolonged exposure to sunlight or strong artificial light. The newer PEX piping has added carbon black to increase resistance to UV light. The carbon black makes the pipe opaque. The pipe is now available in several different colors (red pipes for hot water and blue pipes for cold water). There is some documentation indicating that PEX piping can become brittle over time due to chlorine additions to water or other contaminants that come in contact with the pipe. It is too early, however, to judge the effective service life of this type of piping.

PEX pipe with a brass fitting

Conclusion With today’s complex systems having mastered the art of moving large amounts of water, tomorrow’s technical challenge remains a combination of time and the elements. Here’s a “glass-half-full” wish that maybe one day leaks or repairs will be a thing of the past.

Journal of Failure Analysis and Prevention

R. John Phillips, P.E., Engineering Design & Testing Corp., Charleston District Office, P.O. Box 848, Mt. Pleasant, SC 29465. Contact e-mail: [email protected].

Volume 4(1) February 2004

11