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Joining HDPE Pipe: Traditional & Modern Methods

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A pipeline is only as strong as its weakest connection. When it comes to joining HDPE pipe, there are several common methods that are used, each with its advantages and disadvantages. In this article, we’ll discuss four different methods: butt welding, electrofusion, flanging and mechanical coupling – focusing predominantly on the ease and speed of installation, the need for specialised equipment and experienced personnel, and the strength and reliability of the joint.

Butt Welding

Butt welding (or butt fusion) jointing is a thermofusion process which involves the simultaneous heating of the ends of two components which are to be joined until a melt state is attained on each contact surface, thereby producing a permanent, economical and flow-efficient connection.

Butt Welding

The two surfaces are then brought together under controlled pressure for a specific cooling time and homogeneous fusion is formed upon cooling. The resultant joint is resistant to end thrust and has comparable performance under pressure to the pipe.

This method of jointing requires an electrically heated plate to raise the temperature of the pipe ends to the required fusion temperature and is used for PE63, PE80 and PE100 grades of material for pipe of size 32mm and above of the same Standard Dimension Ratio (SDR). When joining pipes using butt welding techniques.

The important steps involved in making a butt welded joint are:

Pre-welding procedure

  1. Create an environment conducive to repeatability and reproducibility, thus a clean tidy and stable environment at the point of fusion
  2. Ensure the use of clean and appropriate equipment
  3. Only use trained personnel
  4. Draw up preliminary welding procedures
  5. Weld according to preliminary welding procedures
  6. Perform a visual inspection on the welded section
  7. Perform mechanical tests on welded sections
  8. Make process changes until all aspects achieve a pass status
  9. Continue with the welding processes and make sure to record all important parameters
  10. Make use of check sheets to ensure accurate tracking of all important steps and parameters

Welding Process

  1. Place the pipes in clamps with ends against the trimming tool and pipe markings aligned
  2. Align and level components using support rollers
  3. Tighten the pipe clamps to grip and re-round the pipes
  4. Use the trimming tool to ensure continuous shavings are cut from each surface
  5. Check that there is no visible gap between the trimmed faces
  6. Place the heater plate in the machine and close the clamps so that the surfaces to be joined are touching the plate
  7. Using the hydraulic system, apply the pressure previously determined
  8. Maintain the applied pressure until the pipe begins to melt and uniform bead of 2-3mm is formed on each end
  9. Check that the pipe does no slip in the clamps. The pipe ends must maintain contact with the heater plates
  10. Once the heat soak time is completed, remove the heater plate, ensuring that the plate does not touch the melted surfaces
  11. Immediately close the clamps and bring the melted surfaces together at the previously determined pressure
  12. Hold under pressure for the appropriate cooling time

Click here to find out more about Marley’s solution for butt welding

Electrofusion

Another technique using heat fusion joining technology is electrofusion. Electrofusion welding systems are considered the most highly developed, cost-effective and safe method for joining polyethylene pipes today.

This jointing process uses heat generated from electricity to melt both the pipe wall and the pipe surface, creating a completely homogenous joint that is stronger than the pipe itself. Thanks to its versatility, electrofusion can be used to weld together pipes or fittings from different PE materials and different thicknesses as long as there is compatibility of melt flow index and raw material density.

Electrofusion

Another key advantage of electrofusion is that it is particularly suited to installations where access is difficult, such as in trenches or around other pipes.

A typical electrofusion weld is carried out by a trained welder and includes the following steps:

Pre-welding procedure

  1. Create an environment conducive to repeatability and reproducibility, thus a clean tidy and stable environment at the point of fusion
  2. Ensure the use of clean and appropriate equipment
  3. Only use trained personnel
  4. Draw up preliminary welding procedures
  5. Weld according to preliminary welding procedures
  6. Perform a visual inspection on the welded section
  7. Perform mechanical tests on welded sections
  8. Make process changes until all aspects achieve a pass status
  9. Continue with the welding processes and make sure to record all important parameters
  10. Make use of check sheets to ensure accurate tracking of all important steps and parameters

Welding Process

  1. Check that pipe is cut at 90° to the pipe axis
  2. Mark the fusion zone on the pipe and scrape to remove oxide layer
  3. Clean the scraped area of the pipe, and fittings (avoid touching the fusion zone)
  4. Mark the insertion depth (half the length of the fitting) on both pipe ends to be joined, clamping the components in place if required
  5. Insert the pipe ends into the Electrofusion fitting. For larger diameters, assembly can be assisted by tapping around the face with a plastic hammer
  6. Start the fusion process, using a preheating phase if applicable
  7. Wait the appropriate cooling time and quality check the fusion
  8. Mark the fusion parameters on the pipe

Click here to find out more about Marley’s solution for electrofusion welding

Flanged Connections

Flanged joints are normally used to connect PE pressure pipelines to valves, pumping stations and pipelines of other pipe materials. The joint must be able to transfer long-term axial forces with maintained tightness, which requires use of suitable components and a correct assembly[2].

The components in a flanged joint for HDPE pipes are:

  • Stub-ends or flanged adaptors
  • Back-up flanges
  • Gaskets (pressure and chemical appropriate)
  • Bolts
  • Washers

Flanged Connections

The general procedure for making flanged connections is:

  1. Flanges and gasket should be centred, supporting one or both halves of the joint if necessary
  2. The end surfaces of the stub-ends must be close to each other before the bolts are tightened to avoid elongation of the pipe during joint assembly
  3. Bolts are tightened crosswise in at least six stages, starting with finger tightening, and consequently using a torque wrench
  4. Additional tightening to the recommended torque should be carried out several hours later and also on the next day

As a rule of thumb, the sealing pressure should be at least twice the maximum internal pressure in the pipe.

Mechanical Compression Coupling

Compression couplings are another form of non-permanent jointing for HDPE pipes. As indicated below, the general components of a compression fitting or coupling are a body, a threaded compression nut, an elastomer seal ring or O-ring, a stiffener and a grip ring.

Mechanical Compression Coupling

Philmac Compression Fitting

Compression fittings are popular because they do not require soldering, so they are comparatively quick and easy to use and require no special tools or skills to operate. Because compression fittings are not permanent, they are especially useful in installations that may require occasional disassembly or partial removal for maintenance.

The procedure for installing compression fittings is:

  • Cut the pipe at 90° to its axis
  • Inter over the pipe end in the following order: nut, clinch ring and O-ring on the mouth of the pipe
  • Insert the pipe end and the O-ring onto the body of the joint, up to the insertion depth tab
  • Push the clinch ring into the body of the joint
  • Engage the nut and fully tighten

Marley supplies a comprehensive range of compression fittings, including Marley Compression Fittings and Philmac Compression Fitting

PLEASE NOTE: This overview of HDPE jointing procedures is for informational purposes only and does not take into account safety considerations in installation instructions. Please consult a Marley representative for further advice or suppor

Reference:

  • TEPPFA Technical Guidance Document – AGU/2014/01: A Good Practice Guide for the Electrofusion Jointing of Larger Diameter Polyethylene Pressure Pipes
  • TEPPFA Technical Guidance Document – AGU/2014/02: A Good Practice Guide for Flange Jointing of Polyethylene Pressure Pipes

 

Contact Marley Pipe Systems

Marley Building Division
t. +27 11 739 8600
f. +27 11 739 8680

Marley Mining & Industrial Division
t. 0861 MARLEY (627539)
t. +27 12 045 0997

e. info@marleypipesystems.co.za

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