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Green light given to new sealants for gas pipe joints.

The American gas industry operates approximately 62,000 miles of cast iron (CI) pipes of different diameters. Most of these were installed 50 or more years ago. With joints at 12-foot intervals, there are about 30 million pipe joints. Many gas utility companies spend a considerable portion of their maintenance budgets to maintain these CI pipes and joints.

Anaerobic sealants were developed over 10 years ago and have been used successfully in the United Kingdom for some time. However, a comprehensive understanding of how the sealants will perform under long-term service conditions has been absent. Also, some problems had occurred when the sealants were applied internally with robotic devices. A study, therefore, was sponsored by the Gas Research Institute (GRI) and Northeast utilities, including Consolidated Edison, Brooklyn Union, and Consumers Gas, at Cornell University, to learn exactly how these sealants work and, more importantly, how long will they last under North American conditions. An interdisciplinary study of these sealants, involving the School of Civil and Environmental Engineering and Fiber Science Program, was completed last year.

In the past, leaking CI pipe joints were sealed using conventional techniques that included metallic joint clamps, encapsulation, and shrink sleeves. An earlier Cornell University study (O'Rourke et al., 1985) had shown that, when properly installed, the encapsulants and shrink sleeves can seal leaking joints and also last for 50 or more years under normal service conditions. However, excavation around the joint and extensive cleaning of the gas main are required before the encapsulants and shrink sleeves can be installed. Because the cost of repairing leaking joints is related to the expense of excavation and restoration, it is important that sealing products be chosen for the maximum possible life span with minimum excavation. The recent Cornell study showed that anaerobic sealants used in combination with keyhole and vacuum excavation procedures are well-suited for these requirements.

Anaeorobic sealants are composed of methacrylates, and involve two chemical systems: 1) a monomeric system that polymerizes, in the absence of oxygen, into a rubberlike compound, and 2) an initiator system that starts the polymerization reaction. The presence of metal ions is essential for this reaction to proceed. Additionally, there are small amounts of accelerators, inhibitors, dyes, and stabilizers in the sealants. The basic monomeric sealant system, which is a mixture of several different methacrylates, typically makes up about 96 percent of the commercial product. A small percent of the monomers are multifunctional, and are responsible for making the cured polymer slightly crosslinked and less flowable.

The CI joints studied in this project were the most common bell and spigot type that were packed with jute and caulked with lead. During initial installation, the gap between the bell and spigot was packed first with jute yarn and then the lead was hammered into it. Most joints are over 30 years old, and the jute has deteriorated to differing extents depending upon the local environment. In addition, thermal expansion and contraction, repeated traffic loads, and chemical aging have been responsible for gas leakage at some joints.

The sealing of the joint is performed using a keyhole technique that requires only partial (top part) exposure of the joint, cutting down significantly on the excavation and hence the total repair cost. The sealant is injected through a hole drilled and tapped in the exposed bell part of the joint. For larger joints, two to three holes, strategically placed between 10 o'clock and 2 o'clock positions, may be necessary. The monomeric compositions, which have low viscosity, get absorbed and are transported within the porous jute packing in the joint.

The jute packing acts as a reservoir for the necessary metal ions, available in the form of iron, rust particles, and debris that have penetrated the fibers from the pipe surfaces. The gas pipeline environment provides the necessary anaerobic conditions for the polymerization reaction, turning the entire monomer mass into a rubber-like polymer. The key to success is to get the anaerobic sealant into leak paths within the jute packing and along the jute/joint interface. The Cornell study concluded that when properly injected, the sealant can reach these leak paths and can effectively stop the leaks.

The Cornell study consisted of field survey and discussions with utility personnel, evaluation of stresses and deformation at joints, mechanical aging tests on CI pipe joints, review of sealant properties and current testing procedures, performance evaluation of commercially available sealants, and establishing criteria for sealant evaluation and testing.

The detailed analytical modeling of buried CI joint deformation caused by traffic loads and thermal expansion/contraction performed at Cornell indicated that rotational and axial joint stiffnesses play key roles in the deformation sustained by the joint. However, thermally induced axial slip represented the most critical deformation for testing and qualifying joint sealants. For the study, several treated and untreated 300-mm (12-inch) diameter joints, were mechanically aged, simulating joint rotation due to traffic loading and thermal expansion and contraction. Probabilistic analyses and Monte Carlo simulations were performed to evaluate axial joint movements induced by temperature changes as large as 22 [degrees] C (40 [degrees] F) that are representative of conditions in the northern U.S. and Canada. The mechanical aging tests showed that anaerobic sealants not only stopped leakage, but maintained a gas tight seal. Also, the anaerobic compounds were found to increase the strength and stiffness of the joints, thereby further diminishing the likelihood of leakage under typical service loads.

A total of seven commercially available sealants, ranging from low viscosity to thixotropic, were studied. The study included extensive chemical/physical characterization and aging tests as well as related tests, such as jute swelling, porosity, and sealant evaporation. Chemical aging studies were performed in two parts: the first dealing with the shelf life of uncured sealants and the second dealing with the long-term behavior of cured (polymerized) sealants. The shelf life study indicated that sealants can be stored at room temperature up to six months without any noticeable changes in their curing behavior. Storing temperatures of 40 [degrees] C (104 [degrees] F) and higher are not recommended.

Long-term aging tests were conducted using modified pin and collar (ASTM #D4562-90) specimens developed for this project. The specimen configuration results in sealant deformation similar to that between the bell and spigot in a CI joint, and allows for monitoring changes in both the sealant properties and the sealant/metal adhesion The tests show that, under normal conditions, cured sealants maintain strength, elongation, and stiffness properties suitable for a durable seal over service lives of approximately 30 to 50 years. However, exposure to high temperatures (100 [degrees] C and above), such as those observed in the vicinity of steam pipes, change the properties within short time periods. As a result, these sealants are not recommended for such applications.

Because proper penetration of the jute packing and adequate hardening of the sealant within the jute matrix are necessary for an effective seal, the viscosity and gel times are important characteristics for successful anaerobic sealant performance. The jute specimens showed about 50 percent porosity, with pore radii ranging from 10 um to 400 um. This represents a relatively pervious medium and is equivalent in pore size to that of a medium grained sand and thus capable of allowing the sealant to permeate effectively through the jute. Two novel techniques, termed 'magnet method' and 'glass plate method', to characterize the gel (cure) times of these sealants were developed during this study. The glass plate technique was used extensively to obtain gel times under various environmental and temperature conditions. This technique does not require elaborate assembly, is easy to perform, and gives consistent results. The gel times indicate that an effective barrier against gas leakage will form within 24 to 48 hours.

Recommendations for chemical characterization as well as mechanical and chemical aging procedures were made for qualifying anaerobics in the future. These test procedures provide for the accelerated application of service loads to CI joint specimens treated by any type of externally or internally applied sealant.

Uniform criteria were developed to evaluate anaerobic sealant properties. The chemical and physical tests provide data to confirm effective spreading within the joint, as well as evaporation properties, cure times under normal joint environment, and durability of the seal. These emphasize the relevance of the tested properties with respect to intended applications as well as simplicity, reliability, and consistency.

The cost comparison between conventional and anaerobic sealing methods showed that externally injected anaerobic sealants, when used in combination with the keyhole excavation technique, have the potential for saving in the range of 30-50 percent with individual cases as high as 67 percent. In general, the savings are higher for larger diameter pipes.

The study recommends the use of anaerobic sealants for external injection, and as a promising candidate for internal injections by means of robotic devices.
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Author:Netravali, Anil M.; O'Rourke, T.D.; Hranicka, Tony
Publication:Pipeline & Gas Journal
Date:Jun 1, 1997
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