What Type of Ductile Iron Pipe Joint Is Right for You?

Which Ductile Iron Joint Type Is Right for You?  

There are several types of Ductile Iron (DI) pipe joints available, each offering unique benefits for different applications. Properly joining pipes is as crucial as the pipes themselves, and the conditions in which the pipe is assembled are key factors to consider.

- Some joints are designed to restrain the pipe.

- Certain joints are intended specifically for above-ground applications.

- Specialty joints allow for deflection, expansion, and contraction within the same coupling.

- Some joints enable longer spans between pipe sections.

- Other joints allow for assembly above water, with the capability to lower the pipe from a barge.

No matter your project, there’s likely a DI pipe joint suited for the job. Let’s explore the different types of DI pipe joints to find the best fit for your application.

First, a Little History of Ductile Iron Pipe

Technological advancements consistently create opportunities to improve products and enhance our quality of life, and waterline piping is no exception. It wasn't too long ago that people used hollowed-out trees for water distribution—wooden waterlines are still occasionally uncovered today.

Centuries ago, the first type of joint used in piping was the flanged joint, where leather was placed between the flanges, similar to today’s rubber gaskets. This was followed by the bell and spigot joint, assembled with jute packing and lead poured behind it. Over the decades, manufacturers have continued to innovate and develop new types of joints.

The assembly methods for these joints have also evolved, particularly since Ductile iron pipe became the standard in the 1950s. When repairing or replacing pipes, you may encounter different joint types, some of which are no longer in production but are still in use. For this reason, manufacturers continue to provide information on older joints to assist with ongoing maintenance and repairs.

Push-On Joints

The most common type of joint for underground applications is the Push-On Joint, known for its simplicity and ease of installation. This non-restraint joint creates a watertight seal through full-circumference radial compression of the gasket between the bell and spigot.  

If your line requires horizontal or vertical deflection, you can achieve it after the joint is assembled. The allowable deflection depends on the pipe’s size and manufacturer, with most push-on joints allowing for up to five degrees of deflection. A five-degree deflection equates to 19 inches of movement in any direction—after the joint is properly assembled.

Mechanical Joints

Mechanical Joint (MJ) Pipe is primarily used for fittings and valves. These joints can be configured as either non-restrained or restrained joints, with a wedge-action retained gland to provide restraint. Mechanical joints work by compressing the gland against the bell, which wedges the gasket to form a watertight seal.  

The development of MJ restraint glands allows design engineers to eliminate the need for thrust blocks, reducing project costs and increasing efficiency. Mechanical joints can also be deflected for alignment adjustments after tightening. A common mistake in assembly is neglecting to apply lubricant to the gasket. Without lubricant, the gasket can be forced into the gasket recess, leading to relaxation over time, loosening the bolts, and requiring re-tightening.

What type of joint is used for ductile iron piping?

Both push-on joints and mechanical joints are used with Ductile iron pipe installation. Push-on joints are comprised of a special bell with an internal groove, a plain end, and rubber gasket which is seated into the bell's groove.

Proprietary (Specially Designed) Restraint Push-On Joints

Proprietary Restraint Push-On Joints are tailored to each manufacturer, utilizing the basic push-on pipe design to create a watertight seal. Each manufacturer incorporates unique features, such as a deeper push-on bell and specially designed wedges or rings, to restrain the pipe against a welded bead on the spigot.

These restraint joints offer design engineers an efficient alternative to thrust blocks, reducing costs and improving project efficiency. They are especially suited for high-pressure applications, such as penstocks, or in situations where operating pressures exceed typical conditions.

Additionally, these joints allow for varying degrees of deflection based on the pipe's design and size. Deflection adjustments can be made after assembly, and the joints are quick and easy to install, requiring minimal labor. They are versatile and can be used in a wide range of installations, including:

- Buried  

- In-casing  

- Horizontal directional drilling/pipe bursting  

- Subaqueous applications  

- Aerial spans/bridges  

- Pipe on supports  

You can’t go wrong with choosing this type of restraint joint for your project.

Proprietary Mechanical Restraint Joints

Proprietary Mechanical Restraint Joints are distinct to each manufacturer. What sets these joints apart is the use of an additional gland positioned behind a welded bead, providing restraint capabilities. Instead of relying on wedges or set screws, the gland applies uniform compression across the entire spigot, ensuring secure and reliable restraint.

Locking Gaskets

Locking gaskets are used with push-on pipe to provide restraint and are unique to each manufacturer. While other companies are developing locking gaskets, it’s important to ensure they are approved for use with the pipe you're using. Some locking gaskets are created without the pipe manufacturer’s consent, and they may not perform correctly on the pipe.

The restraint mechanism engages after the joint is fully assembled, including any required deflection. Pulling back on the joint locks the teeth in place. Locking gaskets are an excellent option, particularly for smaller diameters (24 inches and below), and are primarily used in buried applications. However, they are not recommended for above-ground, bridge, or vertical applications.

One crucial consideration when using locking gaskets is that you have only one chance to assemble them correctly. They are extremely difficult to disassemble once put together, even with the use of shim kits designed to deactivate the wedges within the gasket.

Wedge Action Retainers

Wedge Action Retainers are used with mechanical joints to create a restrained joint that allows deflection. The wedge action retainer replaces the MJ gland, and after assembling the joint, you can achieve the necessary deflection before tightening the wedge bolts. The bolts are designed to break off once they reach the correct torque.

Wedge action retainers can be reused if needed, but it’s important to follow the manufacturer’s re-use guidelines to ensure the proper torque is applied for the restraint.

Bell Harness Restraints

A Bell Harness Restraint is an external restraint used on push-on joints. It consists of a split gland behind the bell and a wedge action retainer on the spigot in front of the bell. These are secured together with a threaded rod and two nuts to prevent disassembly under pressure. While effective, this type of restraint is labor-intensive and requires deeper excavation to assemble the harness around the push-on bell.

Flanged Pipe

The Flanged Joint is one of the oldest joint types, best suited for above-ground applications. It is rigid and doesn’t allow for deflection or expansion, making it unsuitable for underground use where ground shifts could damage the joint. The watertight seal is achieved by tightening the bolts and compressing a gasket between the flanges.

Grooved/Shoulder Joints

Grooved and Shoulder Joints use a u-shaped coupling to connect pipes with either a groove cut into the pipe or a shoulder threaded onto it. The coupling fits into the groove, securing and restraining the pipe. Inside the coupling is a gasket that compresses between the coupling and the pipe, forming a watertight seal. These joints are ideal for above-ground applications but should not be used underground.

Grooved joints come in either flex or rigid configurations, depending on whether deflection is required.

Ball and Socket/River Crossing Joint

The Ball and Socket Pipe offers the greatest degree of deflection, allowing up to 4.5 feet of offset for an 18-foot pipe section, with a deflection range of twelve to fifteen degrees per joint. The name of the joint describes its functionality—machined components allow for optimal deflection. Various pipe class options are available, allowing the pipe to either "float" or "sink" into place.

This joint deflection can be both horizontal and vertical, making it possible to assemble the pipe on the bank or a barge. The pipe adapts to the contours of the waterway bottom, ensuring seamless installation.

Seismic Couplings/Joint

Seismic Couplings/Joints allow for expansion, contraction, and deflection in specific areas. Each Ductile iron pipe manufacturer has a unique design to accommodate ground shifting.

While ground shifts may be commonly associated with earthquakes, they also occur due to liquefaction, such as when soil becomes saturated after heavy rains or during a drought. A fully restrained Ductile iron line with seismic couplings ensures stability against these shifting soil conditions.