What are the types and sizes of pipes used for water transmission and distribution?

Answer 1

A Comparison of Corrosion Protection Systems and Requirements for Water Transmission Pipe

Three separate studies in the last five years have indicated that the direct costs of external corrosion of distribution and transmission pipe for U.S. utilities is $4.4 to $5.0 billion per year. Large diameter pipe used for water transmission lines likely represents 20% to 25% of this total, or more than $1.0 billion per year. The three most commonly used pipe materials for water transmission, concrete cylinder pressure pipe (CCP), welded steel pipe (WSP) and ductile iron pipe (DIP) all contain ferrous materials that need to be protected from external corrosion. Each of these three styles of pipe provides entirely different methods to prevent external corrosion.

The causes of external corrosion are directly related to the environment around the pipe and the nature of the pipe itself. Because of the variability in soil conditions from region to region, or even along a pipeline route, and the differences between the three styles of pipe, universal or generic solutions to this problem are not practical.

In developing solutions to combat possible external corrosion of water transmission lines, three basic and common sense concepts should be addressed.

First is the impact on the community if the line were to fail after being put in service. Most transmission lines serve large populations and many are the sole source of water supply. If this is the case, more care is necessary to build in corrosion protection during the initial installation.

Second is the local experience with external corrosion. Each municipality has extensive knowledge based on its existing system which will identify the severity or lack of severity of possible external corrosion. The history of maintenance work on all systems, including small diameter distribution pipe, will indicate if the materials used for these pipelines are suitable for the more critical applications of large diameter transmission pipelines.

Third is a realistic assessment of the capabilities and workload of local municipal employees to monitor, repair or replace pipe that may deteriorate due to external corrosion. For example, some municipalities have no experience with managing cathodic protection systems that are generally always necessary with WSP.

What causes external corrosion?

Corrosion of a metal is its physical degradation due to the electro-chemical reaction between the metal and the environment surrounding the metal. There are two common forms of corrosion: galvanic and electrolytic, also known as stray current corrosion.

Galvanic Corrosion

Galvanic corrosion is by far the most common form of external corrosion. The galvanic corrosion process is similar to what takes place in an ordinary battery as shown below.

For galvanic corrosion to occur, four separate components are required:

an anode;

a cathode;

an electrolyte;

a metallic connection between the anode and cathode.

When the four elements are connected, ions leave the anode, causing a loss of metal at the anode. The ions move through the electrolyte (e.g. soil) to the cathode.

Electrolytic (Stray Current) Corrosion

Direct currents from man-made electrical systems may use moist earth as an electrolyte to complete their circuit. An example of this is a rectified cathodic protection system which is intended to protect a buried steel gas main from galvanic corrosion. When the "stray" current is unintentionally picked up by a buried metallic pipeline, loss of metal will occur at any point where the current discharges from the unintended pipe (see drawing).

For galvanic and stray current corrosion, the method of preventing corrosion is to interrupt the circuit and stop the flow of ions. For galvanic corrosion, this is generally accomplished by covering the pipe with a barrier coat that prevents current from entering or leaving the pipe. Each of the three types of pipe used for water transmission have different options for breaking the circuit, causing galvanic corrosion. Stray current corrosion may be prevented by relocating the source, providing a "safe" exit from the pipe (e.g. an electrical "drain") or applying a good barrier coating to prevent pick-up of current.

Concrete Cylinder Pressure Pipe (CCP)

Of the three pipe materials commonly used for water transmission, concrete pressure pipe with over 70 years of experience, has the longest and best record for preventing external corrosion. It is also the only pipe material that includes corrosion protection in the "as shipped" product. The cement-rich mortar or concrete exterior of CCP provides both chemical and mechanical protection to the structural steel elements in the pipe. The mortar creates a high pH environment (generally 12.5 to 13.5) that passivates the steel and prevents galvanic corrosion in most soils. Where additional protection is necessary to counteract very adverse soil environments, solutions such as barrier coats and/or cathodic protection are available. The CCP industry association, the American Concrete Pressure Pipe Association (ACPPA) has published recommended practices for protecting CCP in adverse environments. All of its recommendations are consistent with the published practices of the corrosion engineering profession. For example, bonding, monitoring and application of cathodic protection on CCP are detailed in a specific NACE paper on recommended practices.

Welded Steel Pipe (WSP)

WSP normally relies on a combination of a bonded barrier (dielectric) coating and cathodic protection to prevent corrosion. It is generally recognized that all bonded coatings have some pinholes, voids or damage caused by in-plant application or handling during transport and installation. The cathodic protection system is necessary to protect these bare and exposed areas of the pipe.

There are a number of potential problems with the corrosion protection systems on WSP:

Tape coat bonded systems often have air voids between the tape and the steel surface, particularly at weld beads, tape laps and on fittings. Oxygen and moisture can migrate to these voids and initiate corrosion. Cathodic protection cannot protect these areas due to the shielding effect of the dielectric coating. For this reason, tape coat systems are no longer used on oil and gas transmission lines.

Bonded paint systems will deteriorate over time. Gas and oil pipeline companies have elaborate methods to rehab or replace coating systems on buried pipe every so many years. There is no easy way to replace or rehab these systems on buried water lines.

Cathodic protection systems (CP systems) require periodic (often annual) monitoring, inspection and maintenance. Complete replacement of the CP system after 20-25 years is a common practice due to consumption of anodes and expected life of other components. Many municipal water department maintenance staffs are not trained or equipped to do this work.

Ductile Iron Pipe (DIP)

Ductile iron pipe is essentially a WSP without the corrosion protection system used by WSP. DIP is supplied by the manufacturer with a 1 mil shop applied cosmetic coating that is intended to keep the pipe from showing visible rust before being installed. The concept is that the wall of the pipe is thick enough to allow loss of wall thickness from the galvanic corrosion process for the expected service life of the pipe. That may have been true when cast iron pipe was common and had a wall thickness of 1 inch or more. Today, a typical 36 inch ductile iron pipe has a wall thickness of 3/8 inch, compared to 1 inch or more for cast iron pipe. The reduction of about 2/3 of the wall thickness, the only "barrier" to corrosion, reduces the expected life proportionately. It is not surprising that DIP is failing in 15-20 years compared to 50-70 years of life for cast iron pipe. Analysis of the cost of corrosion on water distribution and transmission lines, estimated at $4.4 to $5.0 billion per year in the U.S., is 90%-95% caused by repair and replacement of cast and ductile iron pipe.

Where environments are more corrosive, the DIP industry recommends use of loose polyethylene sleeves (also called baggies). That system has several potential problems.

The baggies are almost impossible to install without creating tears and rips. Each tear or open space is a location for potential corrosion. It is generally accepted that cathodic protection cannot be used on non-bonded encasement systems, such as baggies.

In addition, the DIP industry has stated that it will not supply pipe with bonded coatings (paint or tape) and that its warranty of performance is void if others apply these systems to their pipe. Of the three types of pipe available for water transmission, DIP is the only one to offer systems that are not consistent with recommendations of the corrosion engineering community for good practice to avoid external corrosion.

Conclusion

Water transmission lines are critical assets for all municipalities. External corrosion is the leading cause of premature failure of all water pipelines and costs municipalities in excess of $4.4 to $5.0 billion per year. Each style of water transmission pipe offers different solutions to combat external corrosion. Concrete cylinder pressure pipe and welded steel pipe solutions are consistent with corrosion control technology; ductile iron pipe solutions are not. Concrete cylinder pressure pipe has the best record of performance of the three available materials and is the only product that comes from the factory with the total corrosion prevention system built into the pipe.