DIFFERENCES BETWEEN DUCTILE IRON (DI) PIPE AND STEEL PIPE
In North American snowmaking systems, corrosion of steel pipe is becoming more prevalent and severe. Ductile Iron (DI) pipe has been used extensively in Europe, because there is no doubt that DI pipe has a longer lifespan than steel pipe. But, the unknown was whether DI systems could support the higher operating pressures used in snowmaking in North America.
In 2006, special production DI pipe and fittings began to be imported; to replace corroded steel pipe in snowmaking systems in the North American Ski Industry. Since this time, the North American Ski Industry has been flooded with information comparing the pros/cons of this specialized DI system with the pros/cons of the welded steel system.
The purpose of this document is to try to present some “main street” information, understandable by all parties, which will help owners and managers better understand the pros/cons of each system.
One point is very clear: ALL PIPE MATERIALS ARE NOT EQUAL.
DI pipe has inherent properties not available with steel pipe:
- An annealing oxide layer, formed during the annealing process in the normal production cycle;
- A powdered zinc coating on pipe from European manufacturers, applied as a normal production step;
- An external shop-coat applied over the annealed oxide layer, and the powdered zinc coat;
- A centrifugally applied specialty cement interior lining; and,
- Thicker pipe walls to satisfy design standards, and normal production requirements.
Steel pipe has none of the foregoing characteristics, so other methods are used to address the corrosion problem:
- Heavier wall thicknesses can be used to achieve a longer depletion cycle;
- Special coatings, fusion bonded epoxy (FBE), can be used to provide the pipe with additional protection; and/or,
- Costly and maintenance-intensive electro-galvanic corrosion protection systems can be installed.
Of all the issues considered in this area, comparing DI pipe with steel pipe, there are six (6) topics which have generated significant interest:
- Pipe Wall Thickness / Design Standards;
- Hydraulic Flow Characteristics;
- Causes of Corrosion;
- Corrosion Mechanisms; and,
- Corrosion Protection – Polyethylene Encasement and Zinc Coating.
CAVEAT: DI pipe manufactured in Europe contains zinc coating, applied as a normal production step. Zinc coating is not a standard production step used by any North American manufacturer. North American DI pipe can be supplied with zinc coating; but this requires a separate manual process, which significantly increases the cost of the DI pipe.
In the interest of brevity, important information on the six (6) foregoing topics is summarized below. If more indepth information is desired, an additional EXECUTIVE SUMMARY has been completed and is available, by contacting firstname.lastname@example.org.
- WALL THICKNESS: For 12” pipe with a normal operating pressure of 1,500 psig: DI pipe wall is 0.390 inch; for this same operating pressure API 5L steel pipe wall is 0.250 inch.
- LONGEVITY: No pipe material is infallible: all pipe materials corrode; controlled by the corrosivity of the soil. In one soil environment the corrosion rate for standard DI pipe is 0.8 mil/year; in exactly the same soil environment the corrosion rate for standard API 5L steel pipe is 3.0 mils/year.
- HYDRAULIC CHARACTERISTICS
- POWER: Power can be reduced. The Roughness Coefficient (RC) of the internal surface of any pipe has a significant affect on friction losses. Mechanical Engineering Textbooks recommend an RC of 140 for cement lined DI pipe; and, an RC of 115 for clean new steel pipe. As steel pipe ages, internal scale forms/accumulates, which decreases the RC of steel pipe even further, to a range of 100 to 105. To pump 4000 USgpm through 2500 ft of 12” pipe: the HP required for DI pipe will be 19.2% less than the HP required for steel pipe.
- PIPE SIZE: Pipe size can be reduced. If the same pumping equipment is used to pump 4000 USgpm through 2500 ft of pipe, with the same residual operating pressure at the top of the line: a DI pipe system will require 1,500 ft of 12” and 1,000 ft of 10” pipe; a welded steel pipe system will require 2,500 ft of 12” pipe. This represents a significant reduction in capital cost for the DI system.
- FLOW: Water flow can be increased. If the same pumping equipment is used to pump water through 2500 ft of 12” pipe, with the same residual operating pressure at the top of the line: the DI pipe system can deliver 4,900 USgpm; the welded steel pipe system can deliver 4,000 USgpm. This represents a significant improvement in production efficiency for the DI system.
- INSTALLATION: Field experience has shown that installation time is less for DI pipe. Material costs are always higher for DI pipe; but, the total installed costs are frequently lower for DI pipe systems.
- CAUSES of CORROSION: Many different factors influence the corrosion process, making it hard to generalize on this subject. Based on many studies which have been completed in this area, especially relating to DI pipe; the major influence is the corrosivity of the soil environment surrounding the pipe. In 1964 the 10-Point Soil Evaluation System was developed; and can be reviewed in detail, in Appendix “A” of the ANSI/AWWA C105/A21.5 Standard. In this Standard there are three (3) levels of soil corrosivity:
- <10 Points: Passive non-corrosive soils;
- ≥10 Points: Active corrosive soils; and,
- >20 Points: Uniquely severe corrosive soils; including coal, cinders, muck, peat, mine wastes, and landfills. It is very unusual to encounter these conditions in alpine environments like ski areas.
As shown above, all soil environments are not corrosive. In the alpine environment, active soils are most prevalent at lower elevations. It is wise to analyze soils along any pipeline, to identify “hot spots” of high corrosivity.
- CORROSION MECHANISMS: Corrosion in both systems requires oxygen; but, each material corrodes in a different way. For DI pipe the dominant corrosion mechanism is uniform surface oxidation, forming a continuous oxide layer on the external surface. For steel pipe the dominant corrosion mechanism is localized pitting, in which loose scale of iron oxide, sulfates and nitrates forms on the external surface; and, periodically these layers of loose scale drop off; and are replaced continuously by new layers, as corrosion continues.
- CORROSION PROTECTION: In any active corrosive soils (≥10 Points), additional corrosion protection should be used for any pipe system. Many types of additional protection have been used; some better known methods include:
- Polyethylene encasement (PE);
- Electro-galvanic protection;
- Impressed current protection;
- High performance bonded exterior coatings, fusion bonded epoxy (FBE) on steel pipe; and,
- Sacrificial coatings, zinc on DI pipe.
Of all these methods, the PE system is the most widely used in DI piping systems, because:
- It is very effective;
- It is the most economical;
- It is easy to install;
- It requires no maintenance or monitoring; and
- It has no operating costs.
Because of installation methods used for steel pipe, i.e., long runs of welded steel pipe, the PE system is not frequently used in steel pipe systems.
- BENEFITS OF THE PE SYSTEM:
- Usage / Acceptance: High usage of the PE system has resulted in broad acceptance; National and International Standards exist in many countries.
- Oxygen Depletion: When properly installed, the PE system isolates the pipe from oxygen in the surrounding environment.
- Flexibility: PE is not a bonded coating; it can be used when required; or not used if not required. This reduces total cost.
- Zero Handling Damage: Because PE is not a bonded coating, no damage is incurred during shipping/handling.
- Dielectric Strength: PE has a high dielectric strength; and, this characteristic provides excellent corrosion protection in soils where the incidence of stray currents is high.
- Installation Costs: Historical experience shows that PE protection adds 0.4 – 0.6% to overall project cost. For 12” DI pipe, the cost of an impressed current galvanic protection system is approximately 5 times the cost of a PE system. For 12” DI pipe, the cost of a sacrificial anode protection system is approximately 30 times the cost of a PE system; not including the on-going maintenance expenses for the impressed current system and for the sacrificial system.
- BENEFITS OF ZINC COATING: Zinc provides corrosion protection in two different phases:
- In storage, the zinc coating reacts with carbon dioxides and moisture in the air; forming zinc carbonates in the pores of the exterior polyurethane (PUR) coating. These carbonates seal off the pores of the PUR coating, which significantly inhibits further reaction while in storage.
- When pipe is installed and backfilled, zinc reacts with chemicals in the soils to form a dense and uniform layer of crystals consisting of zinc oxides, zinc hydrates and other zinc salts of different compositions. Because of the partially blocked pores in the PUR coating, this crystallizing process is slowed down; but not completely suppressed. So in the confined region around the surface of the PUR coating, a slow continuous conversion process occurs, in which zinc oxides, zinc hydrates and other zinc salts continue to form.
Other benefits of zinc coating include:
- Sacrificial Repair: When there is damage to the exterior coatings, exposing bare cast iron; an electrochemical cell forms at the bare cast iron. Zinc is more negative than iron and the electrochemical reaction causes the zinc ions to migrate to the damaged area, and form a “scarring” layer on the exposed metal. The zinc ions continue to migrate and form this “scarring” layer over the bare metal, until the exposed area is covered.
- Installation: Zinc coating does not require any additional steps in the installation phase.
CAVEATS WITH ZINC COATING:
- Availability: Zinc coating is not offered as a normal – emphasize NORMAL – step in production, by any manufacturer in North America.
- Lack of Data: Because zinc coated pipe is not used extensively in North America; very little research has been completed in North America, comparing zinc coated DI pipe with normal DI pipe and/or steel pipe. For this reason, accurate field data is minimal regarding the benefits of zinc coating. But this lack of field data in North America does not affect the facts. The performance of DI pipe systems in Europe, since the early 1960’s, indicate, without a doubt, that DI pipe has a longer lifespan than non-zinc coated DI pipe and/or steel pipe, in the same soil environment.
- European Pipe: High pressure zinc coated pipe is available from Austria and France. Based on ISO Standards, the thickness of zinc coating ranges from 150 gm/cm² to 200 gm/cm². Coating thickness will influence longevity. For all European pipe, coating thickness should be certified.
- North American Pipe: Some US manufacturers can supply zinc coated pipe; requiring a separate manual process; usually completed at a different location. This manual process increases costs; and, there can be quality issues not encountered with production line processes. Costs for zinc coated pipe from US manufacturers are 30% to 40% higher than costs for normal asphalt coated pipe from these US manufacturers, depending on pipe size.
- Redundancy: Because zinc coating is a thermally bonded coating, it is not feasible to use this coating just where active corrosive soils occur. If zinc coating is used, it must be installed along the entire pipeline.
- Stray Electric Currents: Zinc coating does not provide any corrosion protection in stray current environments. This is a major advantage of the PE system compared with zinc coating.