Corrugated Pipe Joints

The following ideas are posted for your review about existing technology and other interesting thoughts about corrugated pipe joints. These are our opinions, and no doubt other people may have ideas that are contrary to ours. Please review and read these and use the ideas to help you in your decision making process when buying technology to manufacture corrugated pipe.

Corrugated pipe bell joining system selection. (bell&spigot)

Since the mid 1980’s I have been actively involved in the polyethylene corrugated pipe industry, more specifically in the development of the double-wall (twin wall) product line. In the beginning we were happy to make pipe with a smooth liner, the improved hydraulics on the corrugated pipe with a smooth inner wall presented great opportunity in new applications such as culverts, storm sewers and even sanitary sewers. Our early coupling systems were simple, a coupling band, first made by roto-molding and then blow-molded as volumes increased.

Today, through factors both technical, competitive and market driven, the corrugated pipe industry have developed a plethora of joint designs; band couplings, double bell coupling, spin welded bells, in line bells (both single and double layer), traditional expansion socketing, extrusion welding, hotplate fusion welding, and even wire electro-fusion. I apologize if I’ve omitted any.

Coupling systems have become very complicated to discuss, largely due to the investment by machine producers and manufacturers into their preferred technology. When discussing joint design you will hear many claims, some substantiated but more often theorized about the relative performance of the many systems.

So if you are looking at buying a pipe corrugating line for the first time; and plan to make a pipe that will be used in an application requiring the water to stay in the pipe - or more often is the case out of the pipe, carefully consider the joint design you choose, it has significant long term implications on your business’ competitive position, and even more importantly the satisfaction of your customers.

To select the proper technology you should carefully answer the following questions:

With answers to the above questions you should now be in a good position to talk to the pipe machinery suppliers about the requirements of the joint design and the joint design they would recommend to meet your market need.

Here’s a quick review of the most common pipe joining technologies for corrugated pipe

Double bell coupling - a requirement for almost every manufacturer the double bell coupling is primarily offered as a repair coupling, although in the early stages of a product line development they are often the primary joining system. These products can easily be molded using the three methods of roto, blow and injection molding, however injection molding has become the principal method of manufacture for these parts offering low part cost and high volume production. A simple injection mold making a half coupler which is then welded together to make the double bell is a good way to develop a bell for spin welding, and supply a part for fittings production and double bell field joints. The double bell can be made with locking tabs, and is often used with a gasket. Equipment has been developed by Mecasoft to automatically push on a gasket and coupler in-line for many European manufacturers which sell the smaller diameter pipes with this type of coupler. The double bell coupler is prominent in Europe in pipe diameters below 200mm, but is still found in use in many areas on larger sizes. In North America it has been overcome by the success of the in-line single layer bell systems, and is typically available as a repair joint. Double-bell couplers are available almost everywhere, but must be matched to the diameter of the pipe. If you are in the design phase of your profiles, pay attention to what parts are commercially available in the US and Europe.

Spin welding / Friction welding – a robust well proven joining system, used in Europe and to a lesser extent in North America this joining method offers the flexibility of a secondary belling process, fast cycle times, infinite pipe length variations and low scrap rates. When you consider developing your corrugated pipe product line and require a bell to be molded for each pipe diameter then the ability to use this bell and friction weld it to the pipe becomes a very attractive and flexible option. The bells used for friction welding can also be used for making couplers and fittings allowing a complete product line for one mold investment. The molding methods are less flexible, and any manufacturer of spin welding equipment will demand am injection molded part offering the wall and dimensional consistency demanded by the process. The bell design has a significant impact on the injection mold cost, and if following North American design practice can result in a mold cost less than a blow mold for an equivalent sized bell. Adescor offer spin welders for sizes of 50mm (2”) ID to a size of 1200mm (48”) OD sized pipes in semi automatic to fully automatic machines. The process is most suitable for polyolefin’s such as PP and PE. The spin welding process is ideal for diameters less than 300mm ID, as this is the range of pipe diameters with highest line speeds and other processes result in slowed corrugator operation and increased control complexity. The friction welded bell is not the lowest initial capital investment, but results in a complete product line and excellent production speeds.

In line belling – seemingly the lowest production cost process for making a bell joint on a corrugated pipe the single layer IB process has become the dominant joining system on the North American market for corrugated polyethylene double wall pipe. It is also used on PVC in the manufacture of ribbed pipes and to a lesser extent on PP pipes in Europe. There is a significant debate over the benefits of single layer and double layer in-corrugator belling systems of which I have seen very little documented evidence supporting the benefits of one vs. the other. The market fact is the single layer bell has been widely accepted, albeit some designs are now reinforcing the larger diameter bells specifically where the gasket seats, and the double layer bell has had some adoption, but with the expected line speed slow downs the industry understands is the result of double layer bell production. The selection of an IB system and technology is complicated, there is no doubt it offers a great joint, with a low mold investment, and good throughputs, but make certain you understand the local standards and joint pressure specifications you must compete with. IB systems were first introduced in North America in the late 1980’s and there has been significant work in the area since. The most recent designs allow for a reduced corrugation with a gasket pocket fitting within a bell the same diameter of the pipe, offering many advantages such as decreased gasket cost, and ease of handling. When talking to your corrugator supplier about IB, ensure you ask for references, even better would be some joint test data, and don’t forget how critical the gasket designs are to make this joint design work well. Choose a reputable supplier.

Expansion Socketing – Developed over 40 years ago for PVC smooth wall pipes, traditional bell forming using a secondary heating process and expansion mandrel until recently has been limited to PVC double wall pipes. Work in the past few years has resulted in new technology that can bell successfully bell polyolefin’s more specifically PE corrugated pipes using the more traditional expansion forming technology. This system offers the flexibility to make various lengths without the purchase of additional molds for making a bell, albeit pipe bell tooling for each size must still be purchased and can add up to a significant investment. When reviewing this technology for PE pipes ensure you ask all the questions about production variability, memory and process stability. If the newer processes developed for polyolefin’s have the stability known of PVC belling this technology will be worthy of consideration.
Extrusion Welding – A method of adding a bell to larger pipes typically 450mm and greater is the manufacturing of a bell using roto molding and extrusion welding it to the pipe as it revolves on a rotary table. This can be automated and there are machines available on the market to do this with reasonable production rates. This method I consider an entry level technology and is quickly surpassed in the market by manufacturers who introduce one of the lower cost production methods of making and attaching a bell.

Field Welding – Traditional field welding of polyethylene pipe is widely accepted with tremendous machinery capacity available to welding pipes in the field. In the corrugated industry the lighter weight wall and difficulty in clamping the pipes has made field welding much less popular than in the PE pressure pipe industry. Systems using electro fusion wire and lower pressure field clamps have been developed, but also have resulted in high installation costs and poor acceptance by the contractor. Field welding of corrugated pipes is possible, proven and can offer special industrial applications a solid joint without the use of a gasket.

In Summary

Joining systems are a complicated and competitive area of the pipe industry. There are machinery and technology companies promoting their specific technologies; pipe companies competing on specifications designed around their proprietary design, and customer preference on the system to use locally.

The joining system you pick is a big and long term investment, make sure you only have to do it once, and if in doubt get good third party advice.