FAQ: Can AC power lines cause corrosion of Ductile Iron pipe?
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.
FAQ: Can cement-mortar linings be repaired in the field?
Repair is achieved by first cutting out the defective or damaged lining to the metal so that the edges of the lining not removed are reasonably perpendicular to the pipe wall or slightly undercut. A stiff mortar is then prepared, containing not less than one part of cement to two parts of sand, by volume. This mortar is applied to the cutout area and troweled smooth with adjoining lining. To provide for proper curing of patches by preventing too rapid of a moisture loss from the mortar, the patched area is normally seal-coated immediately after any surface water evaporates, or alternatively the area is kept moist (e.g. with wet rags or burlap over the area or with the ends of the pipe or fitting taped over with plastic film, etc.). Of course, in potable water-related applications, no patch or curing components should be used in the repair that would negatively affect health or water quality.
FAQ: Can Ductile Iron products be glass lined?
Yes, Ductile Iron products can be successfully Glass lined. Glass lined pipe and fittings have been specified and utilized as a deterrent to interior build-up and clogging of problematic sludge and scum piping systems in wastewater and sewage treatment facilities for over 40 years. Not only is the excellent non-stick characteristic effective in combating the build-up of grease, sludge, and scum, but has been found to be the only deterrent to Struvite and Vivionite build-up as well.
FAQ: What types of joints can be buried?
USP recommends that only flexible joints be buried i.e. Mechanical Joint, TYTON JOINT® and TR FLEX®.
FAQ: Which way should the bells face in relation to the flow?
Some Ductile Iron users specify that pipe be installed with the bell end facing the direction of flow. This theory emanates from the pre-pressure joint era, when common joint sealing materials were cement mortar and jute, asphalt and jute, just asphalt, and various other materials. The theory is predicated on the liquid flowing into the next pipe length prior to leaving the existing length.
Since the introduction of the TYTON JOINT® Pipe in 1956, it has been subjected to various tests. From this testing it has been determined the properly assembled joint will withstand a 14 psi vacuum, a 1,000 psi internal pressure, and a 430 psi external pressure without leakage. Given these results, it is obvious flow direction within the pipeline is not an installation factor.
FAQ: Why is there a peen pattern on Ductile Iron pipe?
Ductile Iron pipe is centrifugally cast by pouring molten iron against the inside wall of an externally cooled rotating metal mold. The deLavaud casting process incorporates a metal mold which has a peen pattern on its inside diameter. This peen pattern is transferred to the pipe during the casting process. There are a number of reasons why the mold has this peen pattern. Before casting each piece of pipe, an inoculating dry spray is distributed on the inside of the mold. The peen pattern on the mold acts as an anchor pattern that holds and evenly distributes the inoculant. This inoculant allows the iron to solidify in a slower fashion that increases nodule count, helps refine the grain and nodular size, minimizes carbides, and makes the pipe more easily annealed. The inoculant also acts as a deoxygenizer which ties up the oxygen on the surface of the mold, thereby preventing the formation of pin holes. The peen pattern also helps dispense thermal shock and additionally helps the mold pick up the molten iron by increasing surface friction between the mold and the iron as the mold is rotated. The chill-free dual wet spray casting process involves first spraying a binder on the inside of the mold followed by the inoculating dry spray. Because of the binder, no peen pattern is required to hold and evenly distribute the inoculant.
FAQ: Is the standard 1-mil asphaltic coating applied to the outside of Ductile Iron pipe for corrosion protection?
No. The asphaltic coating is applied to the outside of Ductile Iron pipe in accordance with ANSI/AWWA C151/A21.51 to minimize atmospheric oxidation for aesthetic reasons. If soils are determined to be corrosive when tested in accordance with Appendix A of ANSI/AWWA C105/A21.5, DIPRA and its member companies recommend that polyethylene encasement in accordance with the AWWA C105 standard be installed for corrosion protection.
FAQ: ANSI/AWWA C150/A21.50, "Thickness Design of Ductile-Iron Pipe," Tables 50.1, 50.6, and 50.12 list depth of cover starting at 2.5 feet. Can Ductile Iron pipe subject to truck loading be installed with less than 2.5 feet of cover and, if so, what design approach should be used?
The ANSI/AWWA C150/A21.50 procedure used for calculating truck loads on buried Ductile Iron pipe, which is based on the teachings of Spangler and others, employs the same methods used in ANSI A21.1, the older design standard for Cast Iron pipe. The approach for calculating truck loading is adequate at any depth of cover. However, depths of cover less than 2.5 feet are generally not recommended under roads and highways due to the possibility of high dynamic loading. When 2.5 feet or more of cover cannot be provided, the procedure in ANSI/AWWA C150/A21.50 can still be applied. However, if impact factors higher than 1.5, which is incorporated in the standard, are anticipated, then such impact factors should be employed. Further, in those shallow covers, maintenance of the road surface over the pipe may be more of a concern than serviceability of the pipe.
FAQ: What is the correct procedure for installing gaskets? If it is so important to do correctly, why don’t the gaskets come installed in the pipe?
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.
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.
FAQ: ANSI/AWWA C150/A21.50 and ANSI/AWWA C600 standards recommend that for 14-inch and larger diameter Ductile Iron pipe, consideration should be given to the use of laying conditions other than Type 1. What is the reason for this?
The external trench load in ANSI/AWWA C150/A21.50 consists of earth load plus truck load. The earth load on pipe increases as the depth of cover increases; the truck load increases as the depth of cover decreases. Therefore, the maximum depth of cover normally is limited by the earth load and the minimum depth of cover is limited by the truck load. For lower pressure classes of pipe in sizes 14 inches and larger installed in a Type 1 trench, this band of allowable depth of cover is limited, or even non-existent. Also, for higher pressure classes of pipe in sizes 14 inches and greater, it would normally be more economical to specify a better trench and a lower pressure class of pipe than a higher pressure class of pipe and a Type 1 trench. Improved bedding is desirable, particularly in larger pipe sizes, to improve uniformity of axial support under the haunches.