SUBSTITUTE MATERIALS FAQS

Q: What effect do scratches have on various piping materials?

A: Plastic pipe: Compared with Ductile Iron pipe, plastic pipe is a very soft material and is consequently much more vulnerable to abrasions, scratches, and other damage during shipping and installation. This includes pvc, pvco, and hdpe pipe. In fact, C900 (4-inch through 12-inch pvc), C905 (14-inch through 36-inch pvc), and C909 (4-inch through 12-inch pvco) state that "pipe surfaces shall be free from nicks and significant scratches." C906 (4-inch through 63-inch hdpe) states that "the walls shall be free from cuts, cracks, holes, blisters, voids, foreign inclusions, or other defects that are visible to the naked eye and that may affect the wall integrity." Both of these statements are arguably impractical stipulations relative to many rugged construction sites.

According to Hucks, tests performed on plastic pipe have shown that a scratch 0.01 inches in depth and 10 inches in length on a 1-1/2-inch, 160 psi pressure rated pipe reduced the cycles to failure from 52,000 to 9,600. J-M Manufacturing Company recommends in its "Installation Guide for PVC Water Pipe" that "gouges which have a depth greater than 10 percent of the wall thickness of the pipe should be repaired." This critical depth is about 0.02 inches for 6-inch Class 150 pvc pipe. According to J-M, the damaged section should be repaired with a clamp or removed.

The AWWA Committee Report "Design and Installation of Polyethylene (PE) Pipe Made in Accordance with AWWA C906" states "gouges deeper than 10 percent of the pipe wall thickness should be corrected by removing the affected portion of pipe and subsequently rejoining the remaining pipe ends by an approved joining method."

Serious consideration should be given to the above stipulations when specifying pipe materials for trenchless pipe-bursting applications. In such operations, long-term reliability of the pipe could be compromised by scratches and gouges from sharp shards of the broken pipe that end up in the backfill.

Steel pipe: Although steel pipe might be perceived as being as resistant to scratches as Ductile Iron pipe, the difference in the two materials' design procedures makes Ductile Iron pipe much less susceptible to any loss in design strength because of scratches.

In accordance with ANSI/AWWA C150/A21.50, once the controlling net thickness required for internal pressure and external load for Ductile Iron pipe has been determined, a nominal "service allowance" of 0.08-inch is added. Additionally, a casting allowance depending upon pipe size (0.05-inch to 0.09-inch) is added. This gives the "total calculated thickness" for Ductile Iron pipe design and is the thickness one would use to select the appropriate pressure class. Additionally, required weight tolerances ensure that effective wall thicknesses are always greater than calculated net wall thicknesses. The additional thicknesses above the controlling net thickness (0.08-inch to 0.17-inch) offer an additional factor of safety and added durability and provide more than adequate allowance for scratches and abrasions that can result from handling.

AWWA C200 for steel pipe states "...For plate, the maximum allowable thickness variation shall be 0.01-inch under the ordered thickness. For sheet, the maximum allowable thickness variations shall be [0.005-inch to 0.009-inch, depending on nominal thickness] ..." In other words, the plate or sheet used to make the pipe can be 0.005-inch to 0.01-inch thinner than the nominal thickness for that gauge of plate or sheet. This means that any scratch, no matter how small, may compromise the safety factors of steel pipe. Additionally, steel pipe design allows safety factors to be compromised by scratches. Section 1.5.1 of AWWA C200 states "...The finished pipe shall be free from unacceptable defects. Defects....will be considered unacceptable when the depth of the defect is greater than 12.5 percent of the nominal wall thickness."

Concrete cylinder pipe: The long-term effect that coating disturbance might have on corrosion is more important than the initial effect scratches have on concrete cylinder pipe. Concrete cylinder pipe relies primarily on a rigid exterior cement-mortar coating. The hydrated cement-mortar provides an alkaline environment with an initial pH of approximately 12.5 that is in contact with the steel bar and cylinder. This alkaline environment generates an oxide film on the steel, a process known as passivation. The passivating film protects the steel from galvanic corrosion and will generally do so as long as the coating is intact and not exposed to environments that are corrosive to the mortar or its underlying steel. However, should any condition develop that results in cracks or damage to the cement-mortar coating, the pipe is then at risk of corrosion failure. The coating can be damaged by an outside mechanical force (such as adjacent construction or rough handling) or from corrosion.

Ductile Iron pipe: Because of Ductile Iron's great strength, durability, and additional wall thickness above the controlling net thickness (0.08-inch to 0.17-inch), there is no practical loss of strength from scratches and gouges from normal handling.
(Issue: Spring/Summer 2002)

Q: Is the head loss (and power cost) greater in high-density polyethylene (hdpe) pipe than in Ductile Iron pipe for the same nominal size pipe?

A: Yes. In all normally specified sizes, cement-mortar-lined Ductile Iron pipe manufactured in accordance with ANSI/AWWA C151/A21.51 has an internal diameter that is larger than the nominal pipe size. For high-density polyethylene (hdpe) pipe manufactured in accordance with ANSI/AWWA C906, the inside diameter is normally much less than the nominal pipe size. Also, hdpe pipe, which is manufactured to the same outside diameter as Ductile Iron pipe, has an inside diameter that is always less than that of cement-mortar-lined Ductile Iron pipe due to the necessary difference in wall thickness.

Using 16-inch pipe as an example, cement-mortar-lined Pressure Class 250 Ductile Iron has an inside diameter of 16.61 inches while SDR 9 (pressure rated 200) hdpe pipe (DIOD) has an inside diameter of only 13.30 inches. Thus, for these 16-inch pipes, Ductile Iron has a flow area that is 1.56 times larger than the flow area of hdpe pipe (216.7 in² versus 138.9 in²). Even if one utilizes a Hazen-Williams coefficient of 155 for hdpe pipe (which is touted by hdpe manufacturers but not proven in long-term service as has a conservative coefficient of 140 for Ductile Iron pipe) and 140 for cement-mortar-lined Ductile Iron pipe, and assuming a flow rate of 1,500 gpm, the 16-inch hdpe pipe would have a head loss of 2.35 feet per 1,000 feet of pipe compared to only 0.96 feet for Ductile Iron pipe. Therefore, the difference in actual inside diameters results in significantly lower head loss (and pumping cost) for Ductile Iron pipe. This should be accounted for in engineering economic design.

For more information, request our technical article "Hydraulic Analysis of Ductile Iron Pipe."
(Issue: Fall/Winter 1999)

Q: How does the smoothness of cement-mortar-lined Ductile Iron pipe compare to that of hdpe?

A: When engineers ask this question, they are normally trying to relate the head loss of Ductile Iron pipe to hdpe pipe. Since hdpe pipe manufacturers advocate a Hazen-Williams coefficient of 155, and the Ductile Iron pipe industry advocates 140, some engineers may wrongly conclude that Ductile Iron pipe will result in higher head loss. To accurately determine the head loss resulting from fluid flowing through any pipe, both wall roughness and the actual inside diameter have to be considered. Because hdpe pipe manufactured to ANSI/AWWA C906 has a smaller outside diameter than Ductile Iron pipe and requires a thicker wall, its inside diameter is always less than that of cement-mortar-lined Ductile Iron pipe. Also, nonstandard hdpe pipe, which is manufactured to the same outside diameter as Ductile Iron pipe, has an inside diameter that is always less than that of cement-lined Ductile Iron pipe due to the necessary difference in wall thickness. This difference in actual inside diameters results in significantly lower head loss for Ductile Iron pipe. For more information, request our technical article "Hydraulic Analysis of Ductile Iron Pipe."
(Issue: Spring/Summer 1998)

Q: Is it true that steel pipe does not include an explicit surge allowance but that pressure class Ductile Iron pipe has a 100 psi surge allowance incorporated above its pressure class rating? If so, for an installation involving 200 psi working pressure plus 100 psi surge, should I specify pressure class 300 psi steel pipe vs. pressure class 200 psi Ductile Iron pipe?

A: The answer to the first question is Yes. Concerning the second question: In steel pipe design, surge pressure is treated one of two ways. It is either totally ignored, and the allowable stress in the pipe wall is limited to 50% of the yield strength of the material; or it is included, but the allowable stress in the pipe wall is then limited to 75% of the yield strength of the material.

For steel pipe, if the anticipated surge pressure is less than or equal to half the working pressure, it is ignored, and the allowable stress is limited to 50% of the yield strength of the material. This results in a safety factor of 2.0 for no surge and of only 1.33 if the surge pressure is half the working pressure.

If the anticipated surge pressure is greater than half the working pressure, steel pipe design includes the surge pressure, but the allowable stress is increased from 50% to 75% of the yield strength. This results in a safety factor of only 1.33 for all installations.

No such games are played in the design of Ductile Iron pipe. In Ductile Iron pipe design, a 100 psi surge pressure allowance is always incorporated above its pressure class ratings. Also, the allowable stress in the pipe wall is always limited to less than 50% of the yield strength of the material.

Another example of Ductile Iron pipe's more conservative design compared to steel is that a service allowance and casting tolerance are added to the net thickness, resulting in higher actual safety factors. Steel pipe incorporates a negative tolerance and additionally allows defects up to 12.5% of the nominal wall thickness.

Therefore, for an installation involving 200 psi working pressure plus 100 psi surge, Ductile Iron pipe design would require pressure class 200 Ductile Iron pipe, which has an additional 100 psi surge allowance, resulting in a minimum safety factor of more than 2.0. Steel pipe design would result in specifying pressure class 200 psi steel pipe with a safety factor of only 1.33. For an equal performance design in this installation, pressure class 300 psi steel pipe with a safety factor of 2.0 should be specified.
(Issue: Fall/Winter 1994)