What advances have there been in sewer pipe linings?

By far, gravity flow domestic sewers create the most rigorous conditions for water/wastewater piping. There are two reasons for this, one physical and the other chemical. The physical challenge is that, since gravity sewers must be constructed to slope and grade, they are often installed under great depths of cover. Therefore, the piping system must be designed for very large external earth loads. Not every piping material in today’s market can be economically installed at these depths. However, the inherent toughness and strength of ductile iron pipe make it the ideal product for this service. The chemical challenge, of course, is that gravity flow sewers can be subjected to severe attack by sulfuric acid formed when slow moving, untreated sewage becomes septic. Hydrogen sulfide (H2S) is formed from the stagnant sewage by anaerobic bacterial action and is subsequently oxidized into sulfuric acid by aerobic bacteria in the space above the liquid surface. The sulfuric acid can be very concentrated, and pH levels below 2 can be developed. The iron pipe industry has wrestled with this problem for many years and has come up with various solutions, some good and some not so good. Until about 40 years ago, iron pipe for gravity sewer service was dipped in coal tar to coat it inside and outside. However, tar linings were brittle, could have pinholes, were not very abrasion resistant, and as we know today, coal tar is environmentally unfriendly. Since that time, various manufacturers and suppliers in the industry have developed and marketed several types of linings, including various polymers, special cements, and compounds of coal tar. American was one of the earliest in the field of special sewer linings with the development of Polybond, which was patented in 1967. The first Polybond linings were pure polyethylene with the only additive being carbon black for ultraviolet protection. These linings were only 20 mils (0.020") thick. The next development for Polybond linings was in the mid-1970s and consisted of an increase to 40 mils (0.04") thickness and the addition of an inert filler. This change vastly improved the physical characteristics of Polybond, and this material has been in service since that time. Several years ago, the technical staff at AMERICAN began to wonder if there were a material available that would make an even better lining for meeting our customers’ needs, or one that might replace AMERICAN’s Polybond. We looked at several types of material, and found drawbacks with them all. The following were considered: 1. Coal tar epoxy. As the name suggests, this material contains coal tar. We concluded that we did not want to expose our employees to this material. Also, we were not satisfied that this material or other solvent-borne epoxies could be made truly "holiday free." 2. Urethane. From a use standpoint, we could find no advantage in this material compared with polyethylene. Actually, it is susceptible to attack by organic acids and alcohols, where polyethylene is not. It also can cause allergic reactions in personnel applying the linings. 3. Calcium Aluminate Cement. This material is not acid resistant below a pH of 3.0, and acid levels on the walls of gravity flow sewers can develop a pH as low as 2. Also, we could find no application advantage of this material in our shops. At the same time this material search was going on, we were also in the process of working on a next generation Polybond, and we feel we now have the superior lining on the market, PolybondPlus™. This lining consists of a primer layer of fusion bonded epoxy and an interior layer of specially modified polyethylene. Let’s look at the characteristics of this PolybondPlus™ lining that make it superior. Chemical Resistance: Polyethylene at normal service temperatures up to 140° F is highly resistant to a wide variety of rather aggressive chemicals such as inorganic acids, alcohols, fatty oils, food chemicals, and detergents. Such corrosive liquids as concentrated sulfuric and hydrofluoric acids can be packaged in polyethylene bottles. Such chemical resistance makes polyethylene an outstanding material for sewer linings. Holiday Free: Holidays (voids) in a protective sewer lining allow the pipe surface to be exposed to and corroded by any sulfuric acid present. Each piece of PolybondPlus™-lined pipe or fitting is tested during the production cycle to ensure that it is holiday free. The holiday test used is ASTM G62-87 (Reapproved 1998) Standard Test Methods for Holiday Detection in Pipeline Coatings, Method B. The test voltage required by this method for 60 mils (0.06") PolybondPlus™ is 10,000 volts. This severe test ensures that PolybondPlus™ is truly holiday free. Adherence to Pipe Wall: Adherence of the lining to the pipe or fitting wall is dependent on several factors. Two of the most important are cleanliness of the pipe or fitting surface to be lined, and characteristics of the lining being applied. To provide the excellent bond needed for a sewer lining, the metal surfaces are grit blasted to remove all visible oxide or heat treating scale. The dual layer PolybondPlus™lining – consisting of a primer layer of fusion bonded epoxy and interior surface layer of specially modified polyethylene — is then heat fused to the prepared surface to produce a tightly bonded lining. All of the outstanding characteristics of PolybondPlus™ were developed and tested in AMERICAN’s headquarters laboratories. As a precaution and a double check, we also employed a third-party consultant to test this lining. His report confirmed the test results we had observed earlier. With the addition of new ID blast equipment, AMERICAN has committed to providing PolybondPlus™-lined pipe and fittings in the full size range, 4-inch through 64-inch, with technology and equipment to produce the best pipe and fittings for sewer service in today’s market.

Ben Helton, Technical Director, AMERICAN Cast Iron Pipe Company.