Heat Fusion and Joining

Introduction

An integral part of any pipe system is the method used to join the system components. Proper engineering design of a system will take into consideration the type and effectiveness of the techniques used to join the piping components and accessories, as well as the durability of the resulting joints. The integrity and versatility of the joining techniques used for polyethylene pipe allow the designer to take advantage of the performance benefits of polyethylene in a wide variety of applications.

There are three types of heat fusion joints currently used in the industry: Butt, Saddle and Socket Fusion. Additionally, there are two methods for producing the socket and saddle fusion joints. In addition to the fusion procedures that follow, electrofusion is recognized as an acceptable method of producing socket and saddle fusions but is not addressed here.

The fusion procedures that follow have been proven to consistently produce sound fusion joints when used correctly and are recommended for the joining of PolyPipe^® products. The recommended procedures for butt and saddle fusions are consistent with the Plastic Pipe Institute (PPI) TR-33, Generic Butt Fusion Procedures and TR-41, Generic Saddle Fusion Procedures.

Federal Regulations

Individuals who are involved in joining gas piping systems must note certain qualification requirements of the U.S. Department of Transportation Pipeline Safety Regulations. The U.S. Department of Transportation, D.O.T., requires that all persons who make fusion joints in polyethylene gas piping systems must be qualified under the operator’s written procedures (49 CFR, Part 192, §192.293(a)), and require that gas system operators ensure that all persons who make fusion joints are qualified (49 CFR, Part 192, §192.285(d)).

These fusion joining procedures, when used to join PolyPipe^® gas pipe and fittings, are qualified in accordance with U.S. Department of Transportation Regulations.

• D.O.T. regulations require that written procedures for butt fusion, saddle fusion and socket fusion joining of polyethylene gas piping must be qualified before use by subjecting specimen joints to required test procedures (CFR 49, Part 192, §192.283(a)).
• D.O.T. regulations require that each joint in a gas piping system must be made in accordance with written procedures that have been proved by testing to produce strong gastight joints (CFR 49, Part 192, §192.273(b)).
• D.O.T. regulations require that all persons who make joints in polyethylene gas piping must be qualified under the operator’s written procedures (CFR 49, Part 192, §192.285(a)).
• D.O.T. regulations require that the gas system operator must ensure that all persons who make or inspect joints are qualified (CFR 49, Part 192, §192.285(d) & §192.287).

Where an operator is already using qualified procedures that are in compliance with the D.O.T., these recommended procedures do not constitute a requirement for that operator to change to these fusion procedures.

Qualification Procedure Due to the requirements of the U.S. Department of Transportation, any person joining polyethylene gas pipe must receive training in each of the fusion procedures (49 CFR, Part 192). Each operator should make a sample joint for each procedure used. Each sample joint must pass the following inspections and tests:

1. Pressure and tensile testing as described in §192.283, CFR,
2. Ultrasonically inspected and found to contain no flaws, or
3. Cut into at least three (3) strips, each of which is:
1. Visually examined and found free of voids or discontinuity on the cut surface of the joint.
2. Deformed by bending, torque or impact, and if failure occurs, must not initiate in the joint area.

A person must be re-qualified under an applicable procedure during a 12-month period for the following conditions:

1. The individual does not make any joints under the procedure.
2. The individual has three (3) joints or 3% of the joints made, whichever is greater, that are found to be unacceptable by —192.513, CFR.

Each operator shall establish a method to determine that each person making a joint in plastic pipelines in his/her system is qualified in accordance with this section.

Heat Fusion

The principle behind heat fusion is to heat two surfaces to a designated temperature, and then fuse them together by application of a sufficient force. This applied force causes the melted materials to flow and mix, resulting in a permanent, monolithic fusion joint. When fused according to the recommended procedures, the fusion or joint becomes as strong as or stronger than the pipe itself in both tensile and pressure properties. PolyPipe^® fusion procedures require specific tools and equipment for the fusion type and for the sizes of pipe and fittings to be joined.

• Butt Fusion – This technique consists of heating the squared ends of two pipes, a pipe and fitting, or two fittings by holding them against a heated plate, removing the plate when the proper melt is obtained, promptly bringing the ends together and allowing the joint to cool while maintaining the appropriate applied force.
• Saddle Fusion – This technique involves melting the concave surface of the base of a saddle fitting, while simultaneously melting a matching pattern on the surface of the pipe, bringing the two melted surfaces together and allowing the joint to cool while maintaining the appropriate applied force.
• Socket Fusion – This technique involves simultaneously heating the outside surface of a pipe end and the inside of a fitting socket, which is sized to be smaller than the smallest outside diameter of the pipe. After the proper melt has been generated at each face to be mated, the two components are joined by inserting one component into the other. The fusion is formed at the interface resulting from the interference fit. The melts from the two components flow together and fuse as the joint cools.

Properly fused polyethylene joints do not leak. If a leak is detected during hydrostatic testing, it is possible for a system failure to occur. Caution should be exercised in approaching a pressurized pipeline and any attempts to correct the leak should not be made until the system has been depressurized.

Note: Polyethylene cannot be joined by solvent bonding or threading. Extrusion welding or hot air welding is not recommended for pressure applications.

Inclement Weather

Polyethylene has reduced impact resistance in sub-freezing conditions. Additional care should be exercised while handling in sub-freezing conditions. In addition, polyethylene pipe will be harder to bend or uncoil.

In inclement weather and especially in windy conditions, the fusion operation should be shielded to avoid precipitation or blowing snow from contracting pipe fusion areas and to prevent excessive heat loss from wind chill. The heating tool should also be stored in an insulated container to prevent excessive heat loss. Remove all frost, snow or ice from the OD and ID of the pipe; all surfaces must be clean and dry prior to fusing.

The time required to obtain the proper melt may increase when fusing in cold weather. The following recommendations should be followed:

1. Maintain the specified heating tool surface temperature. Do not increase the tool surface temperature.
2. Do not apply pressure during zero pressure butt fusion heating steps.
3. Do not increase the butt fusion joining pressure.

In butt fusion, melt bead size determines heating time; therefore, the procedure automatically compensates when cold pipe requires longer time to form the proper melt size.

The outside diameter of polyethylene pipe and fittings will contract in cold weather conditions. This can result in loose fit or slippage in the cold rings. For best results, clamp one cold ring in its normal position adjacent to the depth gage. Shim around the pipe behind the clamp with paper, tape, etc., and place a second cold ring over this area. This cold ring will prevent slippage while the inner cold ring will allow for the pipe to expand during the heating cycle of the fusion process.

The proper cycle time for any particular condition can be determined by making a melt pattern on a piece of scrap pipe using the recommended standard heating time. If the melt pattern is incomplete, increase the heating time by three (3) second intervals until a complete melt pattern is established. Each time the procedure is repeated, a new piece of scrap pipe should be used. For additional information concerning cold weather procedures, refer to ASTM D2657, Standard Practice for Heat Fusion Joining of Polyolefin Pipe and Fittings, Annex A1.

Fusion Confidence

Reliable fusion joints of polyethylene piping systems can be accomplished under reasonable latitude of conditions. The following is a listing of general notes to help ensure proper equipment and techniques are utilized:

1. The fusion operator must have adequate training and understanding of the equipment and tools and the fusion procedure. Improper understanding of the operation of the equipment and tools can produce a fusion of poor quality. The operator must understand thoroughly how to use the equipment and tools, their function and operation. The operator should adhere to the equipment manufacturer’s instructions.

Contact pressures and heating/cooling cycles may vary dramatically according to pipe size and wall thickness. Operators should not rely exclusively on automated fusion equipment for joint qualification. In addition, visual inspection and qualification should always be made. If necessary, test fusions should be made to determine correct pressures and heat/cool cycle times. Destructive test methods, such as bend back tests, may be necessary to formulate correct pressures and heat/cool cycle times (refer to Qualification Procedures).

2. Pipe and fitting surfaces must be clean and properly prepared. Any contaminants present on the surfaces or poor preparation of the surfaces cannot produce a quality fusion joint. Ensure that all pipe and fitting surfaces are clean. If surfaces are reintroduced to contaminants, they should be cleaned again.

3. Heater plates must be clean, undamaged and the correct surface temperature. Heater surfaces are usually coated with a non-stick material. Cleaning techniques should be used accordingly. If a solvent is deemed necessary, do not use gasoline or other petroleum products. Refer to the equipment manufacturer’s instructions for proper cleaning products.

Recommended heating tool temperatures are specified for each procedure. This temperature is indicative of the surface temperature, not the heating tool thermometer. The surface temperature should be verified daily by using a surface pyrometer. If a crayon indicator (melt stick) is used, it should not be used in an area that will be in contact with the pipe or fitting.

If the heater plate is not in use, it is recommended that it be stored in an insulated holder. This not only protects the heater surfaces from contaminants, but it can also prevent inadvertent contact, which can result in serious injuries.

4. Proper equipment and condition of tools and equipment for the job. Each type of fusion requires special tools and equipment. Fusions performed with the incorrect fusion equipment, materials or tools can result in a poor fusion.

Fusion Checklist

• Inspect pipe lengths and fittings for unacceptable cuts, gouges, deep scratches or other defects. Damaged products should not be used. Refer to PolyPipe® InfoBrief No. 17 for allowable surface damage according to the Plastics Pipe Institute (PPI) and the American Gas Association (AGA).
• Any surface damage at pipe ends that could compromise the joining surfaces or interfere with fusion tools and equipment should be removed.
• Be sure all required tools and equipment are on site and in proper working order.
• Pipe and fitting surfaces where tools and equipment are fitted must be clean and dry. Use clean, dry, non-synthetic (cotton) cloths or paper towels to remove dirt, snow, water and other contaminants.
• Shield heated fusion equipment and surfaces from inclement weather and winds. A temporary shelter over fusion equipment and the operation may be required.
• Relieve tension in the line before making connections. When joining coiled pipe, making an S-curve between pipe coils can relieve tension. In some cases, it may be necessary to allow pipe to equalize to the temperature of its surroundings. Allow pulled-in pipes to relax for several hours to recover from tensile stresses.
• Pipes must be correctly aligned before making connections.
• Trial fusions. A trial fusion, preferably at the beginning of the day, can verify the fusion procedure and equipment settings for the actual jobsite conditions. Refer to Qualification Procedures for detailed information on the bend back test procedure.

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