The Road to the Rieber Gasket
PVC – THE PERFECT FIT
The thermoplastic, Polyvinyl Chloride, was first made in a laboratory in 1872. A patent was issued in 1913. It was not until 1926 when Waldo Semon, of BF Goodrich, perfected the process of production and various additives were used to process the plastic that it became a commercial product.
It was first made into pipe in the 1930s. From 1936 to 1939 over 400 residences were installed with
PVC drinking water and waste pipelines in central Germany. Various test pipelines of PVC were laid in Leipzig, Dresden, Magdeburg, Berlin, Hamburg, Cologne, Heidelberg and Wiesbaden during the period of 1936 to 1941.
While thermoset plastics, like Bakelite, can be heated in a mold to make something, it cannot be softened and re-hardened, or reprocessed to make the same article again. PVC is a thermoplastic. PVC turned out to be a perfect fit for making pipe as it could be formulated having an exceptional combination of tensile strength, flexural strength, and impact resistance. For the water works industry, drinking water quality and safety from PVC pipe proved superior to many existing products. PVC pipe has shown itself to be a superior solution environmentally. It can be recycled and has an excellent life cycle.
JOINING PVC PIPE
Being a thermoplastic PVC can be heated and softened, then cooled to re-harden and retain its original properties. Making a bell to join lengths of pipe proved easy. The end of a pipe can be heated and expanded, then cooled to make a “bell end”. The best part, using a combination of readily available solvents, the male and female surfaces could be wetted, such that when the two were joined together they weld together, creating a “solvent weld” joint.
Solvent weld joints are in use today and suitable for many applications. However, in larger diameter pipe, in wet, cold, or windy conditions, effective solvent cementing becomes difficult. Solvent cementing larger diameter pipe is labor intensive, expensive, and in many cases not practical. PVC pipe is now being made up to 60” diameter.
The Gebrüder Anger (brothers ahn- ger) are acknowledged pioneers of the PVC pipe industry. Willy (1926 – 2014) & Anton (1929 – 2013) Anger worked to perfect a better extrusion machine to manufacture PVC pipe, a machine having 2 processing screws. They also commercialized a revolutionary method to replace solvent cement to join pipe lengths together as described in their US patent 3,205,535.
To make a solvent weld joint, the end of the pipe length is heated, the PVC softens, a simple steel plug, or mandrel, is pushed inside the softened end, swedging and expanding this area. The PVC is cooled with water and the end of the pipe resembles a bell … or bell end.
The Anger brothers had a gasket solution in hand, a simple O-ring can seal a pipe. The problem to solve was how to get the O-ring gasket into the joint. They worked to perfect a forming mandrel with moving parts. The mandrel had segments which could be collapsed and expanded, such that the softened end could be pushed over the expanded segments, cooled, segments collapse, pipe bell pulled away. Now the bell end had a raceway to accept a gasket. As it would not be practical to make a rectangular raceway for an O-ring, the brothers designed a raceway having 30 & 60 degree angles and complementary radii. This raceway is known worldwide today as the Anger Raceway
BASIC GASKET DESIGN
Rubber is not compressible, like a sponge. When rubber is “compressed” it moves to fill in available space. Thus, as rubber “compresses”, it displaces. This simple act of physics is key to gasket design. A gasket, like an O-ring, and the original Anger B, relies 100% on displacement to create the seal. It is also possible to design a gasket that is not dependent on displacement of the rubber. A 100% lip seal gasket will pass water until the lip of the gasket creates a seal. Such a gasket relies on water pressure to enact the seal.
Compression gaskets are those that rely on displacement to affect the seal. Using the O-ring as the example, it is designed to fit a precisely calculated raceway groove. This groove will have available area to fill as the gasket is compressed and displaced. Approximately 10% to 15% of the gasket cross section is displaced after assembly. Reliability of the seal is greatly dependent on the precision of gasket raceway, O-ring, and spigot dimensions. Insufficient displacement, i.e. spigot too small / gasket raceway too large, can lead to low insertion and leakage. Given the case when the spigot is too large / gasket raceway too small, can lead to high insertion and gasket displacement.
Manufacturing of gasketed PVC pipe is not akin to machining steel. In the era of the Anger brothers process control yielded greater variability than today’s extrusion and belling technology. While gaskets relying on 100% compression / displacement worked, improvement was needed. Pipe joints were subject to leaking and high assembly force.
Lip seal gaskets are those that rely on water pressure to enact the seal. Unless a gasket is being designed for portable above ground irrigation pipe, it is not practical that a pipeline pass water at low pressures. Gasket designers soon combined the compression gasket with the lip seal gasket to create a “hybrid” gasket.
A hybrid gasket is a combination of compression and lip. Approximately 25% to 35% of the gasket cross section is displaced after assembly. As a result, the hybrid gasket allows for greater belling process variability when making the gasketed bell end.
The Road to Rieber
A belling mandrel having moving parts, one that was characterized having expanding and collapsing segments, became known as a “collapsing mandrel” or “expanding mandrel”. During the mid-1970s extruded and spliced hybrid seals were developed to fit in several shaped gasket raceways. PVC pipe market share grew and pipe diameters increased, as contractors installed this relatively new product. Many early gasket / raceway combinations resulted in poorly secured gasket inside the raceway. Some manufacturers used contact cement to glue gaskets in place.
To assemble a gasketed joint in the field the male pipe end is pushed into the female gasketed bell end. The end of the pipe comes in contact with the rubber gasket, the rubber is deformed and displaced, the pipe end (spigot) continues past the gasket and assembly is complete. A critical point in gasket design is the point when the spigot meets the rubber. The rubber must deform and displace without being dislodged from the raceway and pushed toward the back of the bell. In the US this occurrence is called “fish mouthing”. Gaskets were displaced, or “fish mouthed”, during field installation.
HYBRID SINGLE DUROMETER GASKET FOR PRESSURE PIPE
The gasket design focus moved to effective methods to retain the gasket in the raceway. One popular development was the addition of a separate polypropylene retaining ring. This plastic ring fit inside the ID of the rubber gasket and improved retention. However, the PP brought on disadvantages. It could become brittle in cold temperatures. It has different thermal expansion / contractions that the rubber and the PVC pipe. In the cases of contractor error, when joint assemblies were made in deflection, the gasket could be displaced. Contractor complaints were difficult to resolve. Was the “fish mouthing” due to the gasket or to improper installation?
HYBRID PP RING GASKET FOR PRESSURE PIPE
Gunnar Parmann
Bjorn Rieber, and the company Rieber and Son Plastic-Industri A/S of Bergen, Norway, were in many businesses in the 1970s including PVC pipe. A young engineer, Gunnar Parmann, analyzed the variables when making a gasket raceway using the known art of collapsing mandrels. He reasoned if the softened end of the pipe length could be pushed over an expanded steel mandrel it could be pushed over a forming ring.
Gunnar’s first US patent, 3,776,682, disclosed a method to push the heated and softened end of the PVC pipe over a metal forming ring. His next patent 4,030,872, having a priority date of 1974, discloses a method to push the heated and softened end of the PVC pipe over a composite metal / rubber gasket. Gunnar found that rubber alone could not withstand the forces involved when pushing the heated, and softened, pipe end over the gasket. Gunnar added a steel band to reinforce the gasket during belling.
While Gunnar was solving the belling process problem using a metal band component, this metal component turned out to be the key design element of success. Although forming the pipe bell over a 100% rubber gasket would have had manufacturing advantages, the gasket would be subject to similar “fish mouthing” issues as other gaskets on the market. The addition of the metal band added retaining properties never seen before. This “Rieber Gasket”, as it became known, jumped far ahead of all other designs in field installation reliability.
The second unintended consequence of the metal band shows up in PVC pipe belling and manufacturing. As mentioned previously, making a PVC pipe bell is not akin to machining a piece of steel. Heating, cooling, pressures, time parameters, all effect process capability as evidenced by dimensional variability. However, these variables are not associated with the Rieber gasket. The metal band component of the Rieber gasket, and dimensions of the gasket itself, define the critical dimensions of the pipe joint. The precision afforded by the Rieber gasket allows the gasket designer to use 100% compression seals as well as hybrid seals. Today Rieber designs are optimized for the purpose; water transmission, sewerage, cable protection, restrained joining systems.
The Parmann
Rieber style gaskets, sometimes misspelled as Reiber gaskets, can now be found throughout the world. In North America Rieber joints 1 ½”” – 60” for drinking water are acknowledged to be water tight, keeping potable water in and ground water out. Rieber seals are used predominately in gravity sewer systems, keeping sewerage in and water infiltration and root penetration out. Rieber gaskets can be made of various rubber elastomers as required by the engineer, marketplace, or application. Specialty elastomers resistant to contaminated soils, like Nitrile, are often specified. Rieber gaskets are available for electrical conduit and duct. No gasket resists displacement during contractor install better than the Rieber gasket.
May be the gasket could have been called The Parmann … Gunnar had a very good idea.