HDPE and the Art of Fusion

High-density polyethylene pipe (HDPE) has been around for more than 50 years, and today’s resins provide pipes and end users with higher performance qualities and life expectancy. While HDPE has been the product of choice in North America for gas distribution systems for many years, it is today being used in a wide variety of high-performance applications. Those applications range from municipal potable water to power plants and emergency fire protection systems.

The preferred joining method for HDPE — “butt heat fusion” of the pipe ends — is an integral part of the HDPE system and largely responsible for the success of this great piping system. HDPE is the only fusible piping system that has published guidelines for fusion procedures and training. A proper HDPE fusion joint is 100 percent leak proof and as strong as or stronger than the pipe itself — a claim that cannot be made by any other fused piping product.

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Although the term fusion sounds a little scary, it is actually a simple procedure that has been successfully used for more than 40 years in the gas distribution industry worldwide. Butt fusion involves utilizing a fusion machine and a simple, six-step procedure that can be easily taught. With relatively wide tolerances, it is a “far cry” from the extensive experience required of steel welders. Nonetheless, a fusion operator should be both conscientious and properly trained in order to ensure a high quality piping system.

Fusion Training

In 2007 the Municipal and Industrial (M & I) Division of the Plastics Pipe Institute (PPI) formed an Advisory Council of prominent representatives from the fields of contracting, municipal engineering, design consultants and academia to help the industry identify those projects and objectives most needed to increase user satisfaction with HDPE. Kevin Miller, president of Miller Pipeline, chaired a subcommittee that gave high priority to the need of a fusion training guide and this recommendation was adopted by the Advisory Council and reported back to the M & I Division.

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In fall 2008, the Division formed a task group to draft the guide based on PPI’s TR-33 (Generic Butt Fusion Joining Procedure for Field Joining of Polyethylene Pipe) and ASTM F 2620 (Standard Practice for Heat Fusion Joining of Polyethylene Pipe and Fittings). Representatives from distributors, pipe manufacturers and fusion equipment manufacturers all participated in the writing of this guide, which is currently being balloted and should be released later this year. The guide provides a “road map” for ensuring to the owner of a project that the fusion equipment operator was properly trained and tested.

Topics covered in the training requirement include job setup guidelines, fusion machine qualification and pipe handling. Attention is given to understanding the carriage pressures used when facing, heating and joining the pipe and special attention is given to the phenomenon of “drag pressure” and its effect on the fusion process. The “Six Steps” of making a joint are then outlined in a simple-to-follow training protocol.  These steps are: 1) Secure the pipe in the machine; 2) Face the pipe ends; 3) Align the pipe ends; 4) Melt the pipe ends; 5) Join the pipe ends; and 6) Hold and cool the pipe. The guide will include graphics illustrating both acceptable and deficient joints. Finally, the guide specifies a protocol for testing the operator’s fusion samples and issuing a written record of the training.

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The essential “Six Steps” to making a fused joint are broken down in simple language targeted to communicate well with the average construction worker, and PPI eventually hopes to provide the guide in Spanish as well. In writing the guide, the task group members used their collective experience in the industry to identify those areas of the process that, if not completed properly, are most likely to result in deficient joints. Contamination of the joint (from dirt, oil, or human contact), poor pipe alignment and excessive carriage pressure during the “heat soak” phase were considered areas where emphasis should be given to the proper procedure.

The guide gives simple but thorough treatment to “drag pressure,” which is sometimes not well understood by operators. There are two kinds of “drag” when using a butt fusion machine. The first kind is called “static drag” and is the frictional resistance of the carriage and hydraulic cylinders. Usually around 30 to 50 psi as indicated by the unit’s pressure gauge, it is the amount of force needed to move the carriage without any pipe in it. “Dynamic drag” is the amount of force required to move the carriage when pipe is loaded. The amount of dynamic drag must be added to the recommended final fusion pressure. This force can be very little or can potentially exceed the capability of the machine, in which case adequate final fusion pressure cannot be achieved. For this reason, the guide includes a section explaining drag phenomena.

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One of the most common causes of defective joints is applying too much pressure in the “heat soak” phase. Once a melt bead is formed, carriage pressure should be reduced to the minimal amount of pressure required to keep the pipe ends in contact with the heater (generally no more than drag pressure) and the carriage should be hydraulically locked into position. This prevents melt from being squeezed out and forming a cold joint. The training guide provides step-by-step guidance and an essential visual check for properly training an operator to avoid this pitfall.

The guide should be on PPI’s Web site, https://plasticpipe.org/publications/index.html, by July 1. At that point contractors, inspectors, pipe designers and owners will have an opportunity to avail themselves of this free resource that takes another step in improving the art — and the science — of heat fusion, making HDPE the most consistent, reliable, leak free, and sustainable piping system in the market today.

Ron Underwood is president of Connectra Fusion Technologies, based in Gainsville, Texas.

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