The procedure for installing gaskets is simple. However, a large part of the reliability of the seal depends on cleanliness of the joint at the time of installation. Considering the variety of conditions that may be encountered in transit or at the jobsite, it would not be possible to ensure joint cleanliness if the gaskets were pre-installed by the manufacturer. Pre-installation would also expose gaskets unnecessarily to ultraviolet exposure and even vandalism.
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For mechanical joints, the gland should be slipped some distance back from the plain end of the pipe with the lip of the gland facing the bell. The inside diameter of the mechanical joint gasket is smaller than the outside diameter of the pipe. Brush the plain end of the pipe and the gasket with an approved pipe lubricant as supplied by the manufacturer. The gasket must then be stretched over the plain end of the pipe with the thinner side of the wedge facing the bell. The lubricant allows the gasket to slide more easily into the bell and become equalized as the gland compresses it to achieve a reliable seal.
The TYTON JOINT® and FIELD LOK 350® gaskets have a stiff rim called the heel bonded to a circular cross section called the bulb. After correct installation, the heel will fit into the first groove just inside the bell. The bulb will enter the bell first and will be compressed between the inside of the bell at the gasket seat and the outside of the pipe to achieve a seal. The gasket diameter is larger than the bell opening, so a technique must be followed to allow the gasket to be properly fitted inside the bell.
For smaller pipe, up to about 20”, draw a loop of the gasket towards its center forming somewhat of a heart shape. While holding the loop with one hand, start fitting the gasket heel into the groove of the bell with the other. Gradually release the loop while pressing the gasket evenly into position around the inside circumference of the bell. It may be necessary to firmly seat the loop with the heel of the hand to ensure it is fully seated.
As pipe size increases, it will be necessary to use an increasing number of loops to facilitate gasket installation. In the largest sizes, it is not uncommon to have as many as eight loops, evenly spaced around the gasket. Regardless of pipe size, if the gasket has been properly installed, the leading edge of the rubber should be slightly below the smallest part of the bell opening all around the inside circumference. If any part of the gasket is sticking up, it must be worked until fully seated, or the gasket must be removed and re-installed.
Once the gasket is properly seated, continue with the assembly procedure to make up the joint.
ANSI/AWWA C600 “Installation of Ductile-Iron Water Mains and Their Appurtenances” requires that newly installed Ductile Iron water mains be hydrostatically tested at not less than 1.25 times the working pressure at the highest point along the test section and not less than 1.5 times the working pressure at the lowest point of testing.
After the air has been expelled and the valve or valves segregating the part of the system under test have been closed, pressure is then normally applied with a hand pump, gasoline-powered pump, or fire department pumping equipment for large lines. After the main has been brought up to test pressure, it is held at least two hours and the make-up water measured with a displacement meter or by pumping the water from a vessel of known volume. The make-up water is called the “testing allowance,” and the allowable amount is a function of length of pipe tested, nominal diameter of the pipe, and the average test pressure. The hydrostatic pressure test helps to identify damaged or defective pipe, fittings, joints, valves, or hydrants, and also the security of the thrust restraint system.
The “testing allowance” is not a “leakage allowance.” Properly installed Ductile Iron pipelines with properly assembled joints are bottle-tight and do not leak. The “testing allowance” is, however, a practical measure used to maintain the pressure, which might actually drop because of factors other than leakage, including trapped air, absorption of water by the cement lining, extension of restrained joints and other small pipe-soil movements, temperature variations during testing, etc.
Yes, you can. Ductile Iron pipe and fittings can be direct tapped for air release valves, sampling ports, service connections, etc. You do want to ensure that there is adequate thread engagement to provide both strength and a leak-free seal. Testing has shown that, with the use of a good thread sealant, as little as one full thread engagement will provide a leak-free tap. Following the conservative nature of our industry, we recommend that you choose at least two full threads of engagement.
The limiting factor in achieving adequate thread engagement for a given metal thickness is the relative curvature of the parent body as the size of the tap increases. There are tables in AWWA/ANSI C151/A21.51 which show the maximum size of tap that can be used on a given size of pipe, and thickness to achieve 2, 3, or 4 thread engagement.
Also, you can order fittings with a boss cast at the location of the desired tap. The flat surface of the boss, along with the increased metal thickness, provides for multiple thread engagement of tap sizes larger than could be accommodated on the curved surface of the fitting.
The advantages of using push-on fittings are the same as for using push-on pipe. Push-on fittings like U.S. Pipe’s TYTON JOINT® Fittings result in a more reliable joint, with much less labor. The reliability of a mechanical joint is very dependent on the skill of the installer, who must ensure that the bolts at the bottom of the joint in a muddy trench get the same uniform torque as the others.
When joints must be restrained, the use of mechanical joint retainer glands requires approximately twice as much labor to install as an unrestrained mechanical joint. Both require significantly more time and effort to install than U.S. Pipe’s restrained push-on joints: the TYTON JOINT® with FIELD LOK 350® Gaskets, and the TR FLEX® Joint.
Some contractors tell us that push-on fittings are more difficult to install than mechanical joint fittings. There is no question that the two joints require slightly different procedures to install. However, there is ample evidence to show that contractors who have become comfortable with the technique of installing push-on fittings spend more time laying pipe and less time chasing joint leaks.
No. Usually, a pipeline flowing constantly maintains a fairly even temperature. TR FLEX® Pipe should always be extended at installation to engage the locking segments which provide joint restraint. When the joint is properly extended, there is sufficient clearance between the face of the pipe plain end and the back of the adjoining bell to accommodate slight changes in length due to thermal effects.
If a TR FLEX Telescoping Sleeve is installed above ground, internal pressure will usually extend the sleeve to its maximum length. In the case of a bridge crossing where the ends of the pipeline are usually fixed, the increase in length often causes the pipeline to snake. A zigzag pipeline is not only aesthetically not pleasing, it can also result in over-deflection at the joints.
The exception to this recommendation is where a pipeline is to cross a lengthy bridge that has been constructed in sections such that the roadbed has expansion joints between sections. Provided the individual pipe on each section are given adequate vertical and lateral support, it may be desirable to add a TR FLEX Telescoping Sleeve at each bridge expansion joint so that the pipeline can accommodate significant movement fo the roadbed.
Yes, it is possible, but it must be done very carefully.
In a standard TR FLEX joint, the locking segment cavity and gasket seat are adjacent to each other. The joint should be retracted to engage the locking segments, a distance that will vary by size but averages less than 3/4″.
In a TR FLEX Telescoping Sleeve, the locking segments cavity and the gasket seat are separated by an appreciable distance (depending on size, this can be up to approximately one foot.) Similarly, the location of the weld bead on a plain end for Telescoping Sleeve is considerably further back from the face of the plain end than on a standard TR FLEX plain end. It is these design features working together that allow the collapse and expansion of the Telescoping Sleeve while maintaining a reliable seal throughout the range of longitudinal movement.
The TR FLEX GRIPPER® is used to accommodate field cuts of TR FLEX® Pipe. The Gripper Ring takes the place of both the locking segments and the factory applied weld bead, which would be cut off on a field cut pipe.
If it should be deemed necessary to install a Telescoping Sleeve with a Gripper Ring, it is important to consider two things:
1. Once the Gripper Ring is tightened around the pipe installed in a Telescoping Sleeve, the ability for significant longitudinal movement is eliminated.
2. Prior to tightening the Gripper Ring, it must be determined that the pipe has not been pulled out past the gasket, which would prevent achieving a seal.
One way to do this is to measure from the face of the Telescoping Sleeve to a point approximately one inch past the back of the gasket seat. Apply a band of duct tape around the circumference of the field cut pipe with the edge of the tape closest to the face of the plain end located at the distance measured. Insert the gasket and the Gripper Ring, lubricate the gasket and plain end of the pipe and insert the plain end until the edge of the duct tape is flush with the face of the bell. After ensuring the Gripper Ring is square on the pipe, tighten the torque-limiting bolts of the Gripper Ring until the heads snap off.
If at all possible, check to be sure that the gasket is in place and fully compressed between the inside of the Telescoping Sleeve and the outside of the plain end of pipe.
Cast iron is a generic name for any high carbon molten iron poured as a casting. When used to refer to pipe, cast iron (sometimes called gray iron) is a specific type in which the free graphite (Carbon) is in the shape of flakes. Cast Iron pipe were introduced into the United States in .
Ductile Iron is a specific type of cast iron in which the free graphite is in the shape of nodules or spheroids. (Other names for ductile iron are nodular iron or spheroidal graphite iron.) Ductile Iron Pipe were introduced to the market in .
Although nearly identical chemically, the two irons are quite different metallurgically. The now obsolete standard for Cast Iron Pipe (ANSI/AWWA A21.6/C106) required an iron strength of 18/40 (18,000 psi Bursting Tensile Resistance and 40,000 psi Ring Modulus of Rupture.) Although tensile testing was not a requirement of this standard, a tensile test of gray cast iron pipe would give a test result of approximately 20,000 psi Ultimate Tensile Strength, with no measurable Yield Strength or Elongation.
The current standard for Ductile Iron Pipe (ANSI/AWWA A21.51/C151) requires a minimum grade of 60-42-10 (60,000 psi Ultimate Tensile Strength, 42,000 psi Yield Strength, and 10% Elongation.) In addition, Ductile Iron Pipe manufactured under this standard are required to meet a minimum of 7 ft lbs impact resistance by the Charpy test. (Compare Gray Iron Pipe with an impact resistance of approximately 2 ft lbs or less.)
The difference in the physical properties of these two materials is attributable almost entirely to the difference in the shape of the free graphite. The shape of the graphite is determined at the instant of solidification and is made nodular by the addition of magnesium to the molten iron bath. Although Cast Iron was the best engineering material available for pipe production for nearly five hundred years, the development of Ductile Iron Pipe provides a far superior product.
U.S. Pipe’s primary method of thrust restraint are restrained joints.
A column of liquid moving through a pipeline has momentum or force that tends to separate the joints at changes in direction (bends and tees), stops (plugs, caps, or closed valves), and changes in size (reducers). Some means must be used to prevent joint separation to maintain the integrity of the pipeline. Three such means are thrust blocks, tie rods, and restrained joints.
Thrust blocks are usually poured-in-place concrete. They must be engineered with full knowledge of the pipeline operating characteristics and of soil type and bearing strength. They must bear against virgin soil, because thrust forces in the pipeline are transmitted through the thrust block to the soil. Depending on these conditions, thrust blocks can be quite massive. The use of thrust blocks can delay completion of the project to allow the concrete to cure adequately before applying test pressure to the pipeline. If future construction disturbs the thrust block or the surrounding soil, joint restraint and the integrity of the pipeline can be jeopardized.
Tie rods usually involve some sort of fabricated steel harness on either side of the joint held together by tie-rods. This type of joint restraint is generally labor intensive. A tie-rod type of joint restraint must be adequately protected against weakening by corrosion, or else the joint restraint and integrity of the pipeline can be jeopardized.
Restrained joints are designed to hold the joint together against a rated pressure while the pipeline transfers the thrust force to the surrounding soil envelope. In order to calculate the footage of restrained pipeline necessary for the thrust force to be fully dissipated to the soil, it is necessary to know pipe diameter, maximum anticipated internal pressure, depth of cover, soil type, and trench construction type, as well as the configuration (e.g., bend angle) requiring restraint. The calculated restrained footage must be installed on each side of the fitting. Since polyethylene encasement for external corrosion protection reduces the friction between the pipeline and the surrounding soil, the calculated restrained footage is usually multiplied by a factor of 1.5 for pipelines where polyethylene encasement is to be installed.
Mechanical joint retainer glands, both common and proprietary design, are available for use where such devices must be used (e.g., a special valve or meter). However, U.S. Pipe does not recommend their use. Restrained push-on joints manufactured by U.S. Pipe are less susceptible to external corrosion, offer appreciably more deflection, and are much less labor-intensive to install.
Because buried Ductile Iron pipelines are electrically discontinuous and are essentially grounded for their entire length, overhead AC power lines normally don’t impose corrosion or safety concerns.
A consequence of AC power lines and buried pipelines sharing rights-of-way is that AC voltages and currents can be induced by magnetic induction on the pipelines. The magnitude of the induced voltage and current on the pipeline is a function of a number of variables, including the length of pipeline paralleling the AC power line, the longitudinal resistance of the pipeline, and the resistance of the pipeline coating.
Ductile Iron pipe is manufactured in nominal 18- and 20-foot lengths and employs a rubber-gasketed jointing system. These rubber-gasketed joints offer electrical resistance that can vary from a fraction of an ohm to several ohms but nevertheless is sufficient for Ductile Iron pipelines to be considered electrically discontinuous. In effect, the rubber-gasketed joints normally segment the pipe, restricting its electrically continuous length, and prevent magnetic induction from being a problem. Also, in most cases, Ductile Iron pipelines are installed bare with only a standard 1-mil asphaltic coating and therefore are effectively grounded for their entire length, which further prevents magnetic induction on the pipeline.
During construction of Ductile Iron pipelines in the vicinity of overhead AC power lines, certain safety precautions should be followed, e.g., “limit of approach” regulations governing construction equipment, grounding straps, chains attached to rubber-tired vehicles to provide a ground, grounding mats, etc., especially if safety concerns are heightened due to the use of joint bonding and dielectric coatings.
To install the gasket correctly in the groove in the bell, it must be uniformly distributed around the interior of the bell circumference. To do this, the gasket must be looped as it is initially placed in the bell. As a general rule:
4″ through 12″ gaskets generally require one loop. In cooler weather it may be easier to install the 10″ and 12″ gaskets using two loops placed at the twelve and six o’clock positions.
14″ through 20″ gaskets generally require two loops but three may be necessary, placed at the twelve and six o’clock positions.
24″ through 36″ gaskets generally require four loops, spaced approximately 90° apart. Put the bottom loop in first to prevent debris from being introduced into the joint.
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42″ and 48″ gaskets generally require six loops, spaced equally around the circumference of the bell. Put the bottom loop in first to prevent debris from being introduced into the joint.
54″ through 64″ gaskets generally require eight loops, spaced equally around the circumference of the bell. Put the bottom loop in first to prevent debris from being introduced into the joint.
In cooler weather, it is usually a good idea to warm the gaskets before trying to install them or store them in a warm environment.
Never lubricate the gasket or gasket groove prior to installation of gasket into the bell.
Potable Water
This is by far the most common application for Ductile Iron Pipe. Because of its reliability and durability, it is the ideal choice for the transmission and distribution of potable water. The value of potable water is rapidly increasing. Water lost between the treatment plant and the customer’s meter is revenue lost. From the aspect of protecting the public health, it is vitally important to protect water quality from treatment to point of use. With the exception of some special linings for sewer service, virtually all of the products marketed by U.S. Pipe & Foundry are approved by the National Sanitation Foundation (NSF) for the conveyance of potable water.
Fire Protection
Historically, the primary purpose for developing a system to distribute water was for fighting fire. The concern for the protection of lives and property was paramount, even above that of providing water for drinking and sanitation. A fire protection system must be absolutely reliable and fully functional at all times. Factory Mutual is an insurance organization with a focus on risk management and the prevention of property loss. In that role, they are particularly interested in fire protection systems, most of which are 12″ and smaller. Most of the products 12″ and smaller marketed by U.S. Pipe and Foundry are approved by Factory Mutual for use in fire protection systems.
The National Fire Protection Association is a national organization that promulgates codes and standards dedicated to fire safety and prevention. It is common for one such body to recognize and accept standards written by another organization, and to incorporate them into their own. Many of the NFPA Standards for fire protection systems incorporate the same American Water Works Association Standards to which U.S. Pipe and Foundry manufactures its products.
Wastewater
Ductile Iron pipe and fittings are ideally suited for wastewater systems. Wastewater pipelines fall into two categories: gravity sewers and force mains. A leaking sewer line can spread contaminated wastewater to the groundwater system. Infiltration and inflow (I&I) can overburden the wastewater treatment plant, since every gallon flowing to the plant must be treated. Severe I&I also leads to extremely excessive treatment costs. It is important to specify and install a durable piping material with reliable joints. U.S. Pipe’s TYTON JOINT® and TR FLEX® Pipe joints are bottle-tight, preventing both I&I and exfiltration.
In a gravity sewer system, wastewater flows downhill through the force of gravity. Gravity systems generally do not flow full, which can lead to septic sewage transformations that can lead to hydrogen sulfide gas being converted to concentrated sulfuric acid, which is very aggressive toward cement mortar linings and Ductile Iron. In a properly designed and constructed gravity sewer system, there will be adequate slope to provide a self-cleaning velocity (generally accepted as 2 ft/sec.). Under these conditions, a standard cement mortar lining will provide adequate corrosion protection for the pipeline. For less than optimum conditions, PROTECTO 401™, a ceramic epoxy lining, is recommended.
Gravity sewers must often be installed at great depths in order to achieve adequate slope. The inherent strength of Ductile Iron enables it to withstand the external loads imposed by the earth at greater depths. As the gravity sewer increases in depth, it will reach a practical limit. At this point in the project, wastewater is collected at a pumping station. The discharge line from the pump station is then a force main, since the wastewater is pumped under pressure.
A sewer force main operates as a pressure line. It does not have to be installed to a precise grade. For maximum hydraulic efficiency it should flow full at all times. This generally requires air relief valves at all high spots in the pipeline. When the pipe is kept full, there is no opportunity for hydrogen sulfide gas to collect, which virtually eliminates the possibility of septic sewage transformations. A standard cement mortar lining is usually adequate to protect the pipeline. Other linings, such as PROTECTO 401™, are available if the designer so specifies.
Reclaimed Water
A reclaimed water pipeline conveys treated wastewater for beneficial re-use. Only products meeting the requirements for potable water should be specified for reclaimed water, since, ultimately, reclaimed water usually ends up in the potable water supply system.
Digester Gas – Not!
Ductile Iron pipe and fittings are not suitable for digester gas service; thus, U.S. Pipe will not knowingly supply products for such a project. At one time, the ANSI A21.52 and A21.14 standards governed the manufacture of Ductile Iron pipe and fittings (respectively) for gas service. These standards were withdrawn a number of years ago. Ductile Iron pipe for gas service was required to undergo special processing and testing. The equipment needed is no longer available at any U.S. Pipe facilities.
Appendix A of ANSI/AWWA C151/A21.51, Ductile Iron Pipe, Centrifugally Cast, for Water, contains the minimum metal wall thickness required for 2, 3, and 4 threads for different diameter threaded outlets and different diameter pipe. Information is given for both threads conforming to Standard ANSI/ASME B1.20.1 (a.k.a. National Pipe Thread (NPT), Iron Pipe Thread (IP), or Standard Taper Pipe Thread) and AWWA C800 (a.k.a. Mueller Thread, cc thread, Corp Stop Thread). To assure adequate metal thickness for a particular pipe diameter and Pressure or Thickness Class, it is necessary to subtract the casting tolerance found in the Table in Section 4.4.2 from the Nominal Metal Wall thickness found in Table 1 of ANSI/AWWA C151/A21.51.
Concerning the security of a two engaged threads engagement, the Ductile Iron Pipe Research Association (DIPRA) conducted a study of ¾-inch and 1-inch corporation stops direct tapped into 6″ Pressure Class 350 pipe. The tests were conducted on pipe sections with less than nominal metal wall thickness. After multiple corporation stops were installed in each piece of pipe under city line pressure, the installations were observed for leakage through the threads. The water pressure was then raised to 1,000 psi in an effort to fail the 6″ pipe and threaded connection. Leakage was not observed at the threaded connection. These tests were conducted with and without 3-mil thread sealing tape applied to the threads of the corporation stop. The installed corporation stops were then subjected to pull-out and cantilever load tests. In the pull-out tests, the corporation stop failed at loads in excess of 6,500 pounds of force. The pipe threads were undamaged in each of the three tests. In the cantilever load tests, the corporation stops failed at bending moments in excess of 385 foot-pounds of force. Again the threads in the ductile iron pipe wall were undamaged.
It can be clearly seen that work crews can direct tap service connections into Pressure Class Ductile Iron pipe under pressure, effecting structurally secure, watertight seals. It is recommended that two layers of 3-mil thread sealant tape be applied to the corporation stop threads to achieve a watertight service connection using a minimal tightening torque.
The results of this study have been published by the Ductile Iron Pipe Research Association under the title Direct Tapping of 6-inch Pressure Class 350 Ductile Iron Pipe and is available through the Web Site http://www.dipra.org.
Below we provide the answers to several frequently asked questions regarding Ductile Iron Pipe corrosion control.
A: Because buried Ductile Iron Pipelines are electrically discontinuous and are essentially grounded for their entire length, overhead AC power lines normally don’t impose corrosion or safety concerns.
A consequence of AC power lines and buried pipelines sharing rights-of-way is that AC voltages and currents can be induced by magnetic induction on the pipelines. The magnitude of the induced voltage and current on the pipeline is a function of a number of variables, including the length of pipeline paralleling the AC power line, the longitudinal resistance of the pipeline, and the resistance of the pipeline coating.
Ductile Iron Pipe is manufactured in nominal 18- and 20-foot lengths and employs a rubber-gasketed jointing system. These rubber-gasketed joints offer electrical resistance that can vary from a fraction of an ohm to several ohms but nevertheless is sufficient for Ductile Iron Pipelines to be considered electrically discontinuous. In effect, the rubber-gasketed joints normally segment the pipe, restricting its electrically continuous length, and prevent magnetic induction from being a problem. Also, in most cases, Ductile Iron Pipelines are installed bare with only a standard 1-mil asphaltic coating and therefore are effectively grounded for their entire length, which further prevents magnetic induction on the pipeline.
During construction of Ductile Iron Pipelines in the vicinity of overhead AC power lines, certain safety precautions should be followed, e.g., “limit of approach” regulations governing construction equipment, grounding straps, chains attached to rubber-tired vehicles to provide a ground, grounding mats, etc., especially if safety concerns are heightened due to the use of joint bonding and dielectric coatings.
(Issue: Fall/Winter )
A: Corrosion is the oxidation-reduction process by which metals are oxidized by oxygen in the presence of moisture. The Arrhenius equations show that reaction rates increase with temperature. The rule of thumb is that the rate of a reaction will double with every 18°F increase in temperature. Another factor is oxygen solubility. As temperature increases, the total solubility of oxygen in water decreases, and the rate of solution of oxygen increases. These lines cross at about 176°F, which is the temperature where corrosion is a maximum (available oxygen to fuel corrosion is at its maximum). For this reason, polyethylene encasement is recommended for any elevated temperature installation. The maximum operating temperature for linear low-density polyethylene is 180°F and 200°F for high-density cross-laminated polyethylene.
(Issue: Fall/Winter )
A: Yes. The use of select, non-corrosive material (such as sand or limestone) for bedding and backfill is referred to as “trench improvement.” It is recognized that trench improvement generally provides good structural support and helps delay the onset of corrosion activity. However, experience has shown that trench improvement does not provide long-term protection to the pipe, particularly in highly aggressive soil environments. Permeation of native soil and moisture into the select backfill over time tends to make the select material take on corrosive properties. Therefore, trench improvement should not be used as the only method of corrosion control. Polyethylene encasement remains the most effective method for corrosion prevention of Ductile Iron Pipe.
(Issue: Spring/Summer )
A: Cement-mortar linings. Cement-mortar linings have been successfully used to protect the interior of iron pipe and fittings since . Cement linings prevent tuberculation by creating a high pH at the pipe wall and ultimately by providing a physical barrier to the water.
(Issue: Fall/Winter )
A: Unless otherwise specified, Ductile Iron Pipe manufactured in accordance with ANSI/AWWA C151/A21.51 is supplied with an asphaltic coating approximately 1-mil thick. This coating, which is applied for aesthetic reasons, is used under normal conditions for both above and below ground applications. Typically, it’s used in aboveground applications such as pump stations, bridge crossings, and pipe on supports installations. For special aboveground conditions, other types of coatings – epoxies, for example – are available. Installations that might require such coatings are corrosive wet wells, chemical environments, etc. Furthermore, some installations require the pipe to be primed for finish paint coats. The type of coating specified by the purchaser might depend on several criteria such as resistance to a given environment, temperature resistance, impact resistance, resistance to sunlight, gloss retention, appearance, compatibility to finish coats, etc.
Although Ductile Iron and carbon steel are both ferrous metals, there are inherent differences between the two that preclude the use of the same surface preparation and application of coatings. Attempts to apply steel surface preparation specifications to Ductile Iron is inappropriate and may actually result in damage to the pipe surface with subsequent reduced coating effectiveness and life expectancy.
Most coating manufacturers require some type of surface preparation prior to application as a condition of warranty. Since their recommendation for surface preparation will vary depending on the type of paint and the ultimate service environment, the coating manufacturer’s technical data sheet should be consulted each time a special coating is used. Normally, recommendations given on coating manufacturer’s technical data sheets are for carbon steel and might not apply to Ductile Iron Pipe. Therefore, the pipe manufacturer should also be consulted regarding the type of coating, method of application, and type of surface preparation to be used.
(Issue: Fall/Winter )
A: Ductile Iron Pipe and fittings are normally furnished with a cement-mortar lining conforming to ANSI/AWWA C104/A21.4. Cement-mortar-lined Ductile Iron Pipe can be used for certain wastewater applications such as non acid-producing gravity sewers and sanitary sewer force mains that unquestionably flow full.
Microbiologically-induced corrosion, which is sometimes referred to as hydrogen sulfide (H2S) corrosion, can occur in gravity sewers. It occurs when bacteria in the anaerobic slime layer reduces existing sulfates to hydrogen sulfide. The H2S is liberated into the crown area of the pipe above the flow where Thiobacillus bacteria further metabolize the H2S into very low pH sulfuric acid. The sulfuric acid is corrosive to cement-mortar andiron.
This cannot occur in force mains where the pipe is flowing full. H2S cannot be liberated and sulfuric acid will not be produced. For such applications, cement-mortar linings are more than adequate. However, if the force main is acid-producing and air pockets are unavoidable in areas such as high elevations, special linings should be considered in those sections. Contact the DIPRA member companies regarding the most suitable lining.
(Issue: Fall/Winter )
A: Yes. Cement-mortar-lined Ductile Iron Pipe can be used for certain wastewater applications. Ductile Iron pipe and fittings are normally furnished with a cement-mortar lining conforming to ANSI/AWWA C104/A21.4. While originally developed to prevent tuberculation in water mains, cement-mortar lining is also highly suitable for non-septic gravity sewers and sanitary sewer force mains. Long-term testing and experience of cement-mortar-lined iron pipe, both in the field and in the laboratory, have proven its effectiveness for these applications. In addition to the uses mentioned above, cement-mortar-lined Ductile Iron Pipe can also be used for seawater applications. Special linings are often recommended for Ductile Iron Pipe used to transport septic sewage where hydrogen sulfides create a corrosion-related problem. Please contact the DIPRA member companies regarding the most suitable lining for this condition or other special services.
(Issue: Spring/Summer )
A: No. Because buried Ductile Iron Pipelines are electrically discontinuous and are essentially grounded for their entire length, underground electrical cables normally do not impose corrosion or safety concerns for Ductile Iron Pipelines. A consequence of underground electrical cables and buried pipelines sharing the same right-of-way is that AC voltages and currents can be induced on the pipelines by the expansion and contraction of magnetic fields. The magnitude of the induced voltage and current on the pipeline is a function of a number of variables, including the length of pipeline paralleling the underground electrical cable, the longitudinal resistance of the pipeline, and the resistance of the pipeline coating. Ductile Iron Pipe is manufactured in nominal 18- and 20-foot lengths and employs a rubber-gasketed jointing system. These rubber-gasketed joints offer electrical resistance that may vary from a fraction of an ohm to several ohms, but nevertheless is sufficient for Ductile Iron Pipelines to be considered electrically discontinuous. In effect, the rubber-gasketed joints segment the pipe and prevent magnetic induction from being a problem. Also, in most cases, Ductile Iron Pipelines are installed bare and are therefore essentially grounded for their entire length which further prevents magnetic induction on the pipeline. If, for some reason, a bonded-joint Ductile Iron Pipeline parallels an underground or overhead high-voltage AC power line, additional investigation may be warranted depending on the pipe coating (if any), length of parallelism, etc. AC voltages induced on a pipeline pose a shock hazard rather than a corrosion concern. Studies have concluded that AC current may cause corrosion at a rate that is only 1 percent or less than that of a similar electrical quantity of direct current. NACE RP-95, “Mitigation of Alternating Current and Lightning Effects on Metallic Structures and Corrosion Control Systems,” considers 15 volts AC open circuit to constitute an anticipated shock hazard.
(Issue: Fall/Winter )
A: The physical properties of Gray and Ductile Iron Pipe do not change with time; therefore, corrosion (both external and internal) is the main factor that can affect the structural integrity of the pipe. Cement-mortar linings and special linings have effectively eliminated concerns with internal corrosion; however, older installations may not have been installed with these linings. The first source of information regarding the in-place condition of a pipeline is its break record. This record can supply information such as the number of breaks per unit length of pipeline, location of problem areas along the pipeline, and the type of failures (corrosion, structural, joint, etc.). A soil survey along the pipeline can determine if the soils are considered corrosive to Gray or Ductile Iron Pipe, and can locate areas that may be more corrosive than others. From the soil survey and the break records data, a general assessment of the pipeline may be determined. This information may also help locate problem areas. Visual inspection is another method of assessing the condition of a pipeline. When utilizing this method, excavations are normally made along the pipeline at locations that are believed to be the most corrosive. The pipe is then inspected with a geologist’s hammer, scratch awl, or other tool in order to identify any graphitization present. If graphitization is present, it is then removed and pit depths measured. Coupons can also be cut from the pipe and sandblasted in order to record possible pit depths on both the outside and inside diameters. Pipelines can also be internally inspected by remote control cameras. Non-destructive-evaluation (NDE) of Gray and Ductile Iron Pipelines is a developing field offering potential in the future.
(Issue: Fall/Winter )
A: Properly designed and installed Ductile Iron Pipe systems could easily have a life expectancy of more than 100 years. Unlike other pipe materials, the physical properties of Ductile Iron Pipe do not change with age. As long as Ductile Iron Pipe is not subjected to loadings and pressures in excess of its original capabilities, the only factor that could shorten its life is corrosion. Internal corrosion has been effectively eliminated with the evolution of cement-mortar linings and other special linings. Also, not all soil environments are considered corrosive to iron pipe. This is evidenced by the fact that more than 600 utilities in the United States and Canada have had unprotected Cast Iron pipe that has provided service for 100 years or longer, and 23 utilities for more than 150 years. A common procedure used to determine if the soil is aggressive to iron pipe is the 10-point soil evaluation procedure outlined in Appendix A of the ANSI/AWWA C105/A21.5 Standard “Polyethylene Encasement for Ductile Iron Pipe Systems.” If the soil tests corrosive to Ductile Iron Pipe, then corrosion protection is warranted. Polyethylene encasement is the corrosion protection method normally recommended by the Ductile Iron Pipe Research Association and the manufacturers of Ductile Iron Pipe. If the soil is determined non-corrosive when tested in accordance with Appendix A of ANSI/AWWA C105/A21.5, or if it is determined corrosive and the pipe is encased with polyethylene in accordance with the standard, Ductile Iron Pipe could have a life expectancy of more than 100 years. If Ductile Iron Pipe is installed in aggressive environments without protection, its life expectancy would mainly be a function of that environment.
(Issue: Spring/Summer )
A: No. The majority of soils found in North America are not considered corrosive to Ductile or Cast Iron. Therefore, in these soils, corrosion protection of any nature is not required. In soils that are considered corrosive to Ductile or Cast Iron, it has long since been proven that the use of sacrificial metal (i.e., additional wall thickness) for corrosion protection is neither reliable nor cost-effective. Additional sacrificial metal, at best, might increase the service life of the product a few years, which is hardly any consolation to the user. Appendix A to ANSI/AWWA C105/A21.5 standard details an accepted procedure to determine whether the soil is considered potentially corrosive to Ductile or Cast Iron products. Effective, economical protection of Ductile and Cast Iron products in corrosive-soil environments can be achieved by simply encasing them in polyethylene at the trench in accordance with ANSI/AWWA C105/A21.5. This standard includes an approved method of encasing fittings.
(Issue: Fall/Winter )
A: In general, polyethylene encasement can remain effective at sustained temperatures up to around 180° F. Polyethylene encasement softens around 200° F and melts around 220° F to 230° F. Sustained temperatures above 180° F may eventually cause the polyethylene film to become brittle and crack. Stabilizing antioxidants can be added to the film during manufacturing to increase this temperature. As long as the polyethylene encasement continues to prevent direct contact of the pipe with the corrosive soil, it will remain an effective corrosion control system for Ductile Iron Pipelines.
(Issue: Fall/Winter )
A: Because buried Ductile Iron Pipelines are electrically discontinuous and are essentially grounded for their entire length, overhead AC power lines normally don’t impose corrosion or safety concerns.
A consequence of AC power lines and buried pipelines sharing rights-of-way is that AC voltages and currents can be induced by magnetic induction on the pipelines. The magnitude of the induced voltage and current on the pipeline is a function of a number of variables, including the length of pipeline paralleling the AC power line, the longitudinal resistance of the pipeline, and the resistance of the pipeline coating.
Ductile Iron Pipe is manufactured in nominal 18- and 20-foot lengths and employs a rubber-gasketed jointing system. These rubber-gasketed joints offer electrical resistance that may vary from a fraction of an ohm to several ohms, but nevertheless is sufficient for Ductile Iron Pipelines to be considered electrically discontinuous. In effect, the rubber-gasketed joints segment the pipe and prevent magnetic induction from being a problem. Also, in most cases, Ductile Iron Pipelines are installed bare and are therefore essentially grounded for their entire length, which further prevents magnetic induction on the pipeline.
During construction of Ductile Iron Pipelines in the vicinity of overhead AC power lines, certain safety precautions should be followed, e.g., “limit of approach” regulations governing construction equipment, grounding straps, or chains attached to rubber tired vehicles to provide a ground, etc.
(Issue: Fall/Winter )
A: DIPRA’s concern with controlled density backfill products is their potential to be considered corrosive to Ductile Iron Pipe. Some literature published by the manufacturers of the material states that it is not corrosive because of its high pH and is therefore similar to concrete. However, the material usually contains a high percentage of fly ash and, unlike concrete, it is very porous.
Our laboratory has conducted tests on controlled density backfill products in accordance with Appendix A of the ANSI/AWWA C105/A21.5 Standard. The backfill materials did not test corrosive to Ductile Iron Pipe after allowing them to dry out/hydrate; however, after allowing water to soak into the samples (not re-mixing) the backfill materials tested potentially corrosive. Therefore, if the porosity of the material allows the interface between the backfill material and the pipe to experience the addition of moisture in the field, it could cause the material to be considered corrosive.
Another concern is the possibility that the pipe is not completely encased by the material. This would occur if the pipe were placed in the trench, and controlled density backfill material was then poured around the pipe. In this case, the bottom of the pipe would not be encased in the controlled density backfill. This would also occur at the interface where the controlled density backfill material is terminated. In either event, accelerated pH differential corrosion cells may develop because of backfill material having a pH which would normally be much higher than the native soil.
Another point is the possible presence of cinders in the material. Without question, cinders are corrosive to iron pipe. Therefore, if cinders are encountered or suspected, DIPRA would not recommend use of the material for backfill.
As a result, DIPRA considers controlled density backfill material to be potentially corrosive to Ductile Iron Pipe and recommends polyethylene encasement of the pipe in accordance with ANSI/AWWA C105/A21.5 whenever it is used. Care also should be taken to prevent the pipe from floating.
(Issue: Fall/Winter )
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