Browse Our Archives

Over the past 40 years, most snowmaking systems have employed steel pipe to get water and air up (and sometimes down) the mountain. Many of these systems have grown and been modified several times, to accommodate new lifts and snowmaking technology, but now, the new technology involves the pipe itself. Ductile iron pipe, to be precise, which promises a longer lifespan and easier installation than steel.

Not surprisingly, ductile iron pipe is catching on. There were several U.S. resort installations last year, and the number of installations this year will approximately triple. As word about the ease of installation and its 50-plus year life span spreads, the use of high pressure ductile iron pipe for snowmaking will become as commonplace as steel.

Why did resorts initially use steel pipe? In snowmaking’s early days, steel was the simplest and least expensive available technology for transmitting snowmaking water and compressed air. In many instances, steel pipe was selected solely based on the test pressure specification of the pipe from the steel mill. Little attention was paid to such things as internal or external steel corrosion, due either to cost considerations or, possibly, ignorance.

That was then, this is now. Over the past ten years, the ski industry has experienced a steady increase in steel pipe failures—primarily in buried water lines, but in air piping also. The degradation in pipe wall thickness due to external and internal corrosion has become a massive and costly headache. Pipe failures seem to happen at the most inappropriate times, such as Friday afternoon prior to an important weekend. The failures can be embarrassing if, say, a 100-foot tall water geyser suddenly appears in the middle of a base area, or the buried line beneath the base lodge ruptures and floods the rental shop. The more insidious breaks flood freshly groomed trails and sap the water capacity of the snowmaking system as they give birth to new mountain streams.


What’s Happening To Our Pipes?

In the majority of instances, internal corrosion stems from residual moisture within an empty steel pipe, which combines with oxygen provided by the internal pipe air. Over the years, this process eats away at the steel pipe wall, producing rust flakes that are slowly ejected through snow guns (at the same time clogging and eroding water nozzles and jets—see related story, page 37). Normally, every time a snowmaking line is charged with water, the initial flow from a snowmaking hydrant is rusty orange and gritty with corrosion debris. Early-season startups can be nightmarish for small water nozzle/jet snow guns due to this debris.

External corrosion typically results from an electro-chemical process induced by a variety of causes, such as wet clay soils, high acidity, high alkalinity, and stray ground currents. This corrosion is difficult to predict in buried pipe; it is not uncommon for one section of buried steel pipe to exhibit minimal external corrosion while a short distance away the pipe is completely perforated with rust. Once the wall thickness is compromised by either process, the first major overpressure will cause the pipe to burst.

Corrosion limits the useful life span of buried steel snowmaking pipe to an average of about 20 years. This varies from as little as 10 years in aggressive soils to as much as 30 years in more benign soils. To limit the recurring capital expense and associated depreciation of installing pipe, the best long-term solution is to install long-lived pipe. Thus the new popularity of ductile iron pipe.


The Skinny on Ductile Iron Pipe

This is not a brand-new technology. Ductile iron pipe has been used in potable water and sewage systems throughout the world since its commercial introduction in the 1950s. Prior to that, the material of choice for 150 years was cast iron pipe. And cast iron has a history of long life for this use. One of the first cast iron water distribution lines, constructed in France to service the Garden of Versailles during the mid 1700s, is still in service today. Some 100- and 150-year-old cast iron piping systems are still in use today in major cities in the U.S. and Canada.

Ductile iron pipe superseded cast iron due to its superior strength and ductility, resulting in a thinner wall thickness and reduced overall weight, along with the ability to withstand higher pressures. Ductile iron is produced in a similar mill process as cast iron, but the addition of alloying elements in controlled quantities, in particular magnesium, increases the strength of the cast iron dramatically by altering the microstructure of the material. The strength of the ductile iron used in commercial piping (yield strength, 42,000 psi, and tensile strength, 60,000 psi) is comparable to one of the higher grades of steel pipe (X42 API 5L) commonly specified for snowmaking.

But that is a relatively recent development. For a long period of time, the maximum working pressure rating for ductile iron pipe was 350 psig (working pressure is the maximum recommended operating pressure, versus test pressure, which is the steel mill pressure test value for pipe integrity), thereby limiting its application in higher pressure situations. Ductile iron pipe is now manufactured in Europe by specialty manufacturers with specifications that allow a working pressure of 1,450 psi for the most common pipe diameters of 3 to 12 inches and 928 psi for diameters up to 20 inches. This high-pressure spec came about because of demand for potable water systems and hydroelectric penstock applications in mountainous Alpine terrain, and it is suitable for the most demanding snowmaking applications.

Several steps protect against corrosion. The outer surfaces of ductile iron pipe have a zinc layer, which is then covered by a coating of either polyurethane or epoxy. The external layer of zinc acts as a protective and sacrificial surface to protect the underlying ductile iron from normal corrosion, while the top coating acts as an additional protective layer. The inner surface of the pipe is coated with a thin layer of cement-mortar to eliminate internal pipe corrosion within the normal ranges of water pH. The entire pipe is, in essence, sealed from the normal influences of corrosion. This type of pipe is rated for a 50-year minimum lifespan in the normal range of soils, and is in compliance with U.S. AWWA and European ONORM ductile iron pipe standards.


Installing Ductile Iron Pipe

Modern ductile iron pipe is manufactured in a mostly automated, centrifugal casting process that yields pipe in 5- to 6.5-meter lengths for most common diameters. High-pressure ductile iron pipe meets tight dimensional tolerances, with bell and spigot ends that are easily assembled. These are then locked into place with ductile iron segments, creating a joint that will not pull apart under pressure. The sealing of the joint is accomplished by a circular rubber gasket within the bell end that tightly encases the spigot portion of the pipe. The gasket/joint design is such that further internal pressure completely seals the joint, yet will still allow 4º to 5º of lateral movement to the connecting part to aid in the positioning of the pipe. (See photo above.)

The bell and spigot joint design allows the use of various types of ductile iron connectors or adapters that can accommodate any conceivable situation and eliminates the need for any joint or connection welding, except for connecting to existing steel pipe. The connectors are coated internally and externally to minimize corrosion, and allow the same lateral joint flexibility as the bell and spigot pipe joints. The use of angled connectors in tight trail curves or hairpins eliminates the need for multiple welding of complex miter joints and angle joints.

Hydrant and lateral installations utilize the same type of coated pipe and connectors, so they yield the same 50-plus year life span as the rest of the system. Hydrant laterals are easily coupled to “Tees” in the main distribution piping, eliminating the need for any cutting or welding of the main pipe. The lengths of the laterals are adjusted with an assortment of short-length pipe adaptors. For steep slope direct-coupled hydrants, the “Tee” adapter is oriented for side hydrant mounting with a high-pressure galvanized street elbow, allowing the hydrant angle to be adjusted to the desired vertical position.

Ductile iron pipe is also a very flexible methodology for installing snowmaking pipe in steep and/or sensitive environmental areas, and it cuts installation time. The elimination of the welding process and sparks during periods of high fire danger is a major added benefit. Ductile iron pipe can be laid with a small crew, with a minimum of exposed trench for steep slope areas or public access areas, thereby eliminating long runs of open trench.
For these reasons, a ductile iron pipe system lends itself to in-house installation. The normal crew requirement is three laborers/installers and one excavator operator. An experienced crew can lay an average of 600 to 800 feet of pipe a day.

The major obstacle is the cost of the installation and, in some instances, the additional cost for removal of the old pipe. The total contractor-installed cost of a buried air and water ductile iron pipeline is roughly $65 to $110 per foot of trench, with the cost increasing as a function of pipe diameter and wall thickness. These figures include the cost of the pipe, unloading, joining, trenching of pipe and laterals, water bars, drainage, hydrants and laterals, fittings, isolation valves, revegetation/mulching along with area personnel wages and overhead.

But ductile iron pipe will continue to make inroads in snowmaking systems. It simply offers too many advantages to ignore.