Figure 1

Protection of Ductile Iron Pipe from External Corrosion

A Rubber Gasket Joint (no electrical bond) retains water provide uniform resistance to current flow.
B 8 mil thick polyethylene shields against stray loose wrap or tube.
C Wrap and contained water provide uniform environment along metal. Dielectric polyethylene shields against stray currents.
D Any sulfate reducing bacteria in water soon deplete life support (nutrient) and die off.
E Thin shoot of water loses corrosivity as a result of initial reactions.

Extensive burial tests and field experience have consistently demonstrated that ductile iron pipe resists soil corrosion. This is explained in part by the character of the products of initial corrosion. Graphite (14%) and the oxides of iron (47%) and silicon (25%) along with several minor compounds in the corrosion product form a tough matrix that remains tightly in place, inhibiting the progression of corrosion. The uniformity of the ductile iron microstructure also seems to contribute to corrosion resistance. In fact, long term comparative corrosion rate studies in the United States, France, and Great Britain have shown that the soil corrosion rate in ductile iron pipe is an average of 29% less than in its predecessor gray cast iron pipe. These studies included over 30 tons of pipe specimens in various corrosive soils.

Ductile iron pipe can be attacked in certain highly corrosive soils or concentrations of stray direct current. But because of its configuration, metallurgical structure, and the quality of corrosion product, ductile iron is the easiest of all metal pipes to protect.

For insurance against external corrosion the installaiton of loose, 8-mil thick polyethylene sheeting or tubing around the pipe has proven completely effective and costs a fraction of adherent coatings and/or cathodic protection. If the soil where ductile iron pipe is being installed is found to be severely corrosive, either by experience or analysis according to Appendix A of ANSI/AWWA C105/A21.5, loose polyethylene encasement should be specified. The function of this system is explained in Figure 1.

Loose polyethylene encasement for ductile iron pipe is also an effective shield against stray current. In addition it provides a uniform environment for the pipe and controls corrosion resulting from sulfatereducing bacteria. More than 10 million feet of polyethylene-wrapped ductile iron pipe is being used in the most severely corrosive soils in the United States. Some of this pipe is more than 25 years old. Yet, there have been virtually no corrosion failures to date. The few failures that have occurred were the result of neglect such as not re-wrapping the pipe after making a service tap or failing to repair large tears in the wrap during installation.

Ductile iron pipe is not especially vulnerable to stray direct current and not at all to long-line current because of the retained electrical resistance offered by rubber-gasket joints. However, in extreme applications, such as crossings with cathodically-protected pipelines or installations proximate to impressed current ground beds, 8-mil polyethylene shields against the assimilation of direct current, as shown in Figure 2.

Polyethylene Encasement Installation

Various installation methods have been developed for this corrosion-prevention System.

Installation procedures commonly used for wrapping ductile iron pipe with 8-mil polyethylene are as fallows:

Step 1. Cut polyethylene tubing about two feet longer than pipe length. Gather the polyethylene tube on the spigot end of the pipe being assembled

Step 2. Using shallow holes to accommodatesystem. the wrapped hell, pull the gathered tubing Installation procedures commonly used for over the bell and fix it in place with tape, hand or string.

Step 3. Take up slack in the tubing along the pipe barrel and secure in place. Step 4A, 4B. Overlap the tubing from the next pipe length and secure in similar fashion.

Step 4A, 4B. Overlap the tubing from the next pipe length and secure in similar fashion.



Polyethylene Encasement Precautions:
Regardless of the method chosen, there are a few precautions that are an important part of successful installation:

Make sure that no soil or other foreign material is allowed in the annular space between the polyethylene film and the pipe.

Bedding and backfill must be free of any debris that might damage the polyethylene.

Avoid damage to the polyethylene by tamping or other construction operations.

It is necessary that any metallic service connection be electrically insulated from the pipe to ensure the integrity of the system. This is accomplished by providing a dielectric bushing for the corporation stop connection and taping or wrapping the service line with polyethylene for a length of three feet from the tap. It is also important to resecure the polyethylene film around the tap.

Pacific States Cast Iron Pipe Company offers a variety of pipe linings capable of handling most water and waste water applications.

Drinking Water Transmission and Distribution

Due to the extremely aggressive waters distributed by some public water supplies, Pacific States has improved its Portland cement mortar linings to handle such water without difficulty. Not only are the highest quality water, sand, cement and seal coatings used, but each step of the lining process has been studied in detail and improved where possible. Certification of quality is required of all lining raw material suppliers, and leaching and seal coat bonding tests are completed routinely. Each step of the operation is inspected frequently: pipe interior preparation, mortar mixing, centrifugal distribution of mortar into pipe, lining smoothness, initial set, and seal coating. It is our intent that this lining exceed the requirements of ANSI/AWWA Standard C104/A21.4 and that it will serve its intended functions - prevention of tuberculation and red water and provision of smooth flow characteristics without difficulty even in the most severely aggressive soft waters.

Pipe Lining Production

Pacific States cement mortar lining system includes the following:

Photo 1. High quality sand and cement are stored in silos and moved pneumatically to the cement mortar mixing unit.

Photo 2. Sand, cement and prewarmed water are added to the mixer in carefully controlled ratios. Mixing time is closely controlled.

Photo 3. Completed mortar is distributed into rotating pipe through a lance. The pipe is prewarmed during cold weather.

Photo 4. The lined pipe is spun to provide sufficient centrifugal force to distribute and smooth the mortar. Vibrators applied to the pipe's exterior promote a compact and uniform mortar lining.

Photo 5. Bells are rinsed clean of any residual mortar and the lining is lightly flushed with warm water.

Photo 6. The lined pipe passes through an environmentally controlled chamber to enhance intial set and reduce moisture to the proper level for seal coating.

Photo 7. A seal coat is applied to the internal surface of the lining to aid hydration by holding moisture in the mortar.

Photo 8. Completed pipe is stored in temperature controlled bays. Seal coat bonding is checked after final cure.

Photo 9. Leaching tests are completed on water exposed to the lined pipe in accordance with ANSI/AWWA Standard C104/121.4. The exposed water also is subjected to flavor analysis.

Wastewater Piping

Pacific States also manufactures specific pipe for wastewater piping situations including gravity sewers, force mains and waste treatment plant piping. Detailed specifications for epoxy, Protecto 401 or aluminous cement lined pipe are available upon request.

Pacific States Protecto 401

Pacific States' primary wastewater lining material is the Protecto 401 ceramic epoxy lining. This novolac epoxy/ceramic quartz pigment lining is ideal for a variety of sewer and industrial waste applications. It has exhibited superior mechanical bonding strength and abrasion resistance while protecting ductile iron pipe and fittings against interior corrosion.

Application of the lining occurs in a low temperature controlled environment ensuring that the 40-mil lining will not develop any residual stresses. Adherence to strict specifications from surface preparation to inspection are observed. Upon completion of the lining process each pipe must pass a 2500 volt holiday detection test.

Unlike most other ductile iron pipe foundries, Pacific States applies the Protecto 401 where the pipe itself is produced. This gives Pacific States control of the quality of the workmanship and the production/delivery schedule.

Pacific States Coal Tar Epoxy

Ductile iron pipe lined with catalyzed coal-tar epoxy is proven by field experience to be especially suited to severe waste water service. The application of this epoxy lining to Pacific States pipe is quality controlled to exacting standards. The interior surface of the pipe wall is sand blasted smooth to assure uniform lining thickness. The lining is applied in 10 to 12 mil (dry film thickness) layers until the specified lining thickness is reached, usually 20 to 24 mils. The lining at 24 mils thickness must have a permeability rating of .13 perms, a direct impact resistance of 100 in-lb., an abrasion resistance of 20 liters of sand per mil, an elongation of 5% and dielectric strength of 250 volts per mil when fully cured. In addition, the lining material is subjected to immersion tests in corrosive media, including 25% sodium hydroxide at 140° F and 3% sulfuric acid at 125° F along with other corrosive compounds. No failure is permitted. The result is a tough, smooth, tightly bonded, holiday-free (passes 67 volt wet sponge detection test) lining in ductile iron pipe.

High Pressure Applications

iron pipe has been serving Manti, Utah at an internal pressure of 1,100 psi without a single failure. This is more than three times the rated working pressure of 350 psi stated in ANSI/AWWA Standard C151/A21.51. For high pressure applications please consult your Pacific States representative for information on maximum allowable pressure. In most cases, pressure capacity will greatly exceed pressure rating in the standard tables. Pressure ratings for ductile iron pipe are C151/A21.51 along with maximum depths of earth cover. With ductile iron, for example, a 12-inch Class 50 pipe with a nominal wall thickness of .31 inches can withstand internal pressures up to 1,610 psi (based on minimum yield strength with all minus tolerances removed from the thickness). This 12-inch pipe would not actually fail until the internal pressure exceeded 2,300 psi (based on minimum ultimate strength of 60,000 psi).

Prudent materials selection is critical to the success of any public water supply and must involve a study of long-term economy. Ductile iron pipe affords long-term economy unequalled by any other pipe material.

Low Maintenance Costs

Design engineers can predict ordinary earth loads, traffic loads and internal pressures to be encountered by underground pipe, but there are unpredictable influences on pipe loadings: swell pressures in certain clay soils; subsidence of support soil; uneven settlement; excessive water hammer; construction influences, i.e., removal of support by adjacent construction (beam loads); vibrations; seismic activity, and, in colder climates, frost penetration loads. Ductile iron pipe, because of its unusually high strength, ductility and impact resistance, has a proven history of handling all of these factors better than any other underground pipe material, thereby greatly reducing maintenance costs.

Flow

The larger than nominal diameters and internal lining smoothness of ductile iron pipe significantly reduce pumping costs. The comparisons of actual inside diameters of 24-inch pipe in Figure Three were developed by the Ductile Iron Pipe Research Association. The fact that more water can be moved through a ductile iron pipeline for a given amount of energy may make it possible to specify a smaller nominal diameter to accomplish a required flow. Cutting and Tapping Pacific States ductile iron pipe is completely field efficient. It can be cut to fit without loss of joint tightness and tapped directly for services without the use of tapping saddles, which means even greater savings.

Location

Underground ductile iron pipe can be easily located with standard pipe locating equipment - an advantage in emergency situations.

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