Although cast iron gas mains have been in use for over 100 years, they are still very viable. When iron ages it forms a
protective skin that prevents any further deterioration. Cut out a piece of cast iron pipe and look at the pipe wall
cross section. You will see that it is usually as good as new under this top surface. Studies have shown that the cast
iron pipes presently in use will last at least a further two hundred years. Engineers wish they could confidently say
the same about any other type of pipe. The cost of replacing the many thousands of miles of all sizes of cast iron pipes
throughout the country would be absolutely prohibitive and in most cases completely unnecessary.
The problem with cast iron pipes is that there is a potential leakage point at every joint. The changeover to dry natural gas
greatly increased the leakage of gas through these joints as it dried out all the joint packings. Some companies fogged with
swellants such as monoethylene glycol (MEG) to try to resurrect the elasticity of the joint packings but most companies have
done nothing to improve the quality of the packings.
Over the years many methods of joint sealing have been tried with varying degrees of success. Leak clamps, shrink sleeves,
epoxy wraps and internal patches have all been tried but proved to be temporary solutions at best. The only two practical,
economical solutions that have finally evolved are encapsulation and anaerobic sealing.
PLCS ENCAPSULATION is a very simple process whereby a few inches of pipe either side of the leaking joint are cleaned
down to bare metal using a small portable grit blaster working from the crew's air compressor. Although there is a perception
that this is difficult or hard work, it is actually very fast and easy, taking about 5 minutes for a 6" (150mm) bell joint. The
operator wears a helmet fed with cool air so that even on a hot day he is one of the cooler men in the crew. Primer is brushed
onto the joint area and a fabric mold wrapped around the joint and secured with Velcro. The mold is fastened to the pipe with
plastic ties or steel bands using hand tools. A two part polyurethane sealant is mixed for one minute and poured into the mold.
The sealant is then pressurized to above mains pressure by twisting down the mold neck. This action stops the leak, as gas cannot
leak from a low pressure to a higher pressure.
The whole process takes about 15 minutes for a small bell joint. Over the next hour the sealant cures to a solid, flexible
synthetic rubber, which permanently adheres to the pipe joint and completely encapsulates it. If care is taken to not damage
the fabric mold, backfill can begin immediately. Encapsulated joints are now sealed for the lifetime of the pipe, no matter
how the pipe joint subsequently flexes and moves, it will never leak again.
PLCS sealant easily passed the performance tests conducted by Cornell University for the New York Gas Group and earlier by
British Gas (LC8) in Europe. Since then the sealant has been approved and used by almost every gas company with cast iron pipes.
At present over sixty US gas distribution companies install PLCS encapsulation kits either with their own labor or using contractors.
Once operators have obtained experience with encapsulating low-pressure bell joints, they can move on to other types of joints
such as mechanical, coupling or split sleeves. Later they can move up to fabric molds for mains pressures up to 25 psi.
Eventually they can encapsulate almost any configuration of joint or fitting with a custom made special kit. Leaks at
pressures up to 60 psi can be overcome by using a custom made, re-usable metal mold to enclose the encapsulant. Training
is available at every stage.
The PLCS Flex-Kit is for use in keyhole or core drilled excavations. The joint is cleaned using a grit blast lance and primer is
applied remotely, with one wipe priming the pipe surface and the bell face. The mold is lowered into place using a fill tube.
The pre-installed bands are located with a hook tool and tightened by using a remote air ratchet. The sealant is poured down the
fill tube, which is then removed. A pressurizing piston cap is installed onto the neck of the mold to complete the encapsulation.
The whole operation is conducted from the road surface using long handled tools. Kits are available for 4", 6", 8", 10" and 12"
low-pressure bell joints.
PLCS encapsulations are sold with 10 years warranty. If any encapsulation fails, and the failure can be traced to faulty sealant,
PLCS will replace the kit free of charge.
ANABOND ANAEROBIC SEALANTS are also manufactured by PLCS. These are a special group of liquids, which cure to a solid when
placed in an environment where there is an absence of oxygen and a presence of metal particles to act as a catalyst. The inside
of a gas main joint provides just such an environment.
Anabond sealants are packed in half-liter tubes and are made in four viscosities. They penetrate around the internal circumference
of a joint through a combination of sealant pressure, capillary action, gravity and gas pressure. As they move through the interior
of the joint they pick up metal particles that act as the catalyst to cure them into a leak seal. The four viscosities are used under
the following circumstances:
Medium Viscosity - Blue
The most commonly used sealant for a typical joint leak that would cause soap solution to fuzz around a joint or cause soap
bubbles to continuously burst. If a significant volume of sealant is blowing out of the joint face when using Blue, it is
necessary to switch to Red sealant. Blue is also used for mechanical (rubber sealed) joints.
Low Viscosity - Yellow
If it is difficult to inject Blue sealant because of densely packed jute, switch to Yellow.
High Viscosity - Red
This is a thixotropic sealant for sealing joints where the jute has turned to dust or where other grades may blow out of the face
of the joint during injection. This situation is more prevalent on medium pressure joints.
Ultra-low Viscosity - Green
Green is used where the internal leak path is extremely small or long, for example if a joint has very tightly packed jute
or where the distance from the injection point to the leak point is significantly longer. It is invaluable with unusual joint
packings including cement packed jute. It is also used to wet out the jute on medium pressure joints, so that it will transport
thicker grades around the joint.
The Installation Procedure is simply explained as follows:
Long handled tools are also available to inject Anabond remotely through a vacuum excavated small hole.
- A hole is drilled and tapped 1/8" NPT through the bell about 1½" back from the bell face.
- The nipple on one end of the injection hose is screwed into this hole and a pinch clamp is closed to prevent gas from entering the hose.
- A tube containing the correct viscosity of sealant is placed into the barrel of the sealed cartridge gun and the injection hose is attached to the gun.
- As the injection hose is tightened, an integral piercer punctures the tube. The operator then squeezes the gun handle, injecting the sealant into the joint. Because of the patented design of the Anabond cartridge gun, there is no contact between the operator and the sealant throughout the process, even if the tube bursts.
- The sealant should be slowly pumped into the joint. Each tube taking 15 to 20 minutes. This ensures that the resin remains within the joint and is not pumped directly into the main.
- The hose assembly is unscrewed and a 1/8" NPT pipe plug is screwed into the hole.
Differing circumstances at each gas distribution company dictate which of these two methods of sealing to use. Some of these circumstances are:
In the cases of (d), (e), (f) we would recommend encapsulation. Almost all gas companies purchase both sealing systems, to be used as conditions dictate.
PLCS developed polyurethane encapsulation in 1968 and is the worldwide market leader. Over sixty US gas companies and virtually all gas contractors involved with leak sealing currently use our kits.
- The size and depth of the excavation.
- Is shoring required and under what circumstances? Shoring may be a requirement for encapsulation but not for anaerobic seals.
- Are the company's own crews installing the seals or is a contractor to be used? If a contractor is to warrant his workmanship, and be held responsible for a failure, he will usually want to encapsulate.
- Is the joint in a place where it will be subjected to high traffic vibration or movement? For example, is it near a bridge or railroad?
- Is the joint near a hazardous location, very close to a building, hospital or school?
- Is the joint somewhere where you want to fix the leak and never have to return, for example in the center of a busy junction or near traffic lights?
- Is the main working above low pressure?
Many US gas companies, and National Grid (British Gas), currently use Anabond anaerobic sealant.