September 2019, Vol. 246, No. 9
Tech Notes
Hand-Held Device Coats Pipeline Field Joints to OEM-Standards
By Sebastian Paus, Protective Coating, Sulzer Mixpac
Tape wrap contends with higher technology pipeline joint coating solutions, in many contexts with a degree of success. Wrapped with skill, tape can offer the requisite protection a pipeline joint needs.
However, that success depends to a great extent on the ability of the labor installing the low-adhesion, low-mechanical strength material. Tape represents an archaic approach to pipeline joint protection; while standard, it is hand applied, making it both labor intensive and subject to variable quality.
Air entrapment introduces unevenness and weakness to the covering, and unskilled personnel often find extreme difficulty keeping a constant tension around the pipe.
Historically, failures on pipelines in South Africa occur at the field applied joints which are done on site by the pipe laying contractor. One pipeline company opted to replace the old tape wrapping technique with a modern low-cost coating solution. The result, achieved by using Sulzer Mixpac MixCoat applicators, protects weld joints with the same quality repeatability as the pipeline’s OEM coating.
The pipeline segments would arrive in the field already coated with the fast-curing Polyclad 777 system manufactured by Stoncor, the biggest local coatings manufacturer in South Africa. The only parts left bare steel were the ends where they will be connected. The area 15 cm to each side of a spot to be welded was not coated in the factory because the temperature of the welding would simply burn it off.
To replace the tape wrapping method, these bare termini could be coated with a different substance that would extend over the coated portion of the pipe, but such overcoating invites adhesion problems. To ensure uniform coating, Stoncor sought to match the quality of the coating applied in the production plant, like for like.
Coating Practices
Standard application of coating for many pipelines depends on manual mixing. Two components are poured together, mixed manually, and applied with a brush. This process involves a number of risks. To properly mix the components, they must be re-poured into an entirely new can to avoid the unmixed residuum on the sides of the original cans from disturbing the ratio and weakening the mix.
In the field, this rarely happens since workers find skipping the step makes the process faster. Similarly, laborers may discard some limited amount of one or another component when mixing, neglecting to pour the full volume of a container. Typical pack sizes range from 1.5 to 1.8 liters, so if 100 ml is disposed of instead of mixed in, the resulting substance will be nearly 10% off ratio. Plus, this manual process takes time.
To address the issue of ratio inaccuracy, some contractors choose to use “squish packs.” These are plastic bags with either a welded separation sealing off component A on one side and component B on the other. The seal is broken, and the parts are mixed by manually squishing the bag. This process also has a downside for projects of large scale.
Squish bags mix a set amount of material, so if the area to be coated does not exactly correspond to a multiple of that set amount, waste is inevitable. If a joint requires one and a half squish bags worth of material, half a squish bag is wasted every time.
For either of these procedures, the material must be applied manually after mixing. Often brushes, rollers, or even soft gloves serve to apply the mixture to the surface of the pipe. This results in a less than ideal layer consistency.
Often when applying a two-component system to larger surfaces, appliers will use an airless gun. The machine mixes the components and applies a pressure reaching several thousand psi to atomize the material with a nozzle alone without requiring air, so no manual application is called for. However, these machines are heavy (about 500 kg), components must be pre-heated, and it takes between two and four hours to put an airless unit into service.
When laying a pipeline, equipment must traverse sometimes treacherous landscapes, very rarely with the aid of a paved road. Huge trucks must plow through the country to lay the pipeline. In such an environment, using two or five or 10 liters of paint for each joint, operating and moving an airless system across the terrain constitutes an unacceptably costly hassle.
Fast-Curing
Polyclad 777 cures within 15 seconds, a benefit in the factory when optimizing for efficiency and consistency. With such cure speeds in the production plant, pipe storage is simplified, since they do not require half a day to become touch dry. A segment can be moved just ten minutes after it is coated. When in the field, though, the quick cure time poses a challenge. In Stoncor’s case, manual mixing and squish bags were not options, and considering the limitations of an airless unit, another solution was required.
The Sulzer Mixpac MixCoat is a hand-held device that mixes the material and atomizes the blended coating with air, using less than one tenth of the force of an airless unit. Still, by spray applying the coating, the coat’s consistency is high compared to manual applications, free from runs or bubbles. In short, the solution removes the risk of a mixing failure and of a dosing failure while improving the application method significantly.
The advantages of applying the exact same coating system in the field as is applied over the rest of the pipeline are myriad, including turning the fast-curing nature of the fitting into a field benefit and giving the contractor the means to repair any weld damage to the factory coating while the joint is being sprayed.
Self-contained cartridges vastly limit the risk of contamination in the field and are easily disposable. While the system cost approximately twice the previous method upfront, it results in 15%-20% cost savings per joint compared to tape wrapping, Sulzer said. P&GJ
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