Anti-Friction Solid-Film Coatings for threaded connections of the casing and pump-compressor pipes

Introduction

The threaded paste and greases are the most widespread today at mounting pump-compressor pipes and casing. They are applied onto the thread surface and make dismounting easier, prevent jamming and scuff forming, provide required friction coefficient at unscrewing the pipes. Despite all simplicity and availability of this technology, it has some drawbacks such as:

• Cleaning and lubricating surfaces take extra time at mounting

• Pastes from different manufacturers have different frictional performances, and thus additional tests required to be held, and the corresponding correction of the used torque torsion at mounting

• Pastes should be applied repeatedly at every next assembly operation or after a long period of storage

• Dust and abrasive particles stick onto the surfaces lubricated with paste, what leads to irreparable damage of the surface in the course of mounting

• Pastes are not environment friendly

The solid lubrication technology based on anti-friction coatings has no drawbacks like these. Such lubricating materials can be applied both under field conditions and at a plant manufacturing pipes. 

Use of coatings to prepare the pipes for mounting excludes the labourous pipe lubrication operation at the mounting site. Besides, ASCs give a set of required operating performances including antiscuff and anti-corrosive properties, required friction coefficient, ‘dry lubrication’ of the surfaces.

Special features of Anti-Friction Solid-Film Coatings

Due to solid lubrication technology based on Anti-Friction Solid-Film Coatings (ASCs) it is possible not only to solve the task of preparing pipes for mounting right at the manufacturing plant but also to gain additional benefits. The coatings suitable for this application have been developed by the Modengy Russian company under their own MODENGY trademark. Many of them are already widely used in oil-and-gas industry.

Fig.1. Coating Structure

Anti-friction Sold-lubricant Coatings are materials alike paints whose base consists of the fine-dispersed powders of the solid lubricating materials which are spread evenly in the mixture solvents and binding substances. Polytetrafluorethylene, graphite, molybdenum disulfide, and some special compositions are used as solid lubricants. 

The solvent serves for more convenient and even applying the coating onto the surface, and evaporates completely after that. Polymerization and complete adhesion with the surface occur as a result of binding resin curing which can happen both at room temperature and by heating. In cured state coatings is a polymer matrix of the binding substance with the solid lubricating materials dispersed evenly in its cells.

Fig. 2. Molybdenum disulfide structure

Molybdenum disulfide (MoS₂) is a mineral of natural origin which is widely used as a solid lubricant. This substance has vividly expressed layered (sandwich) structure where the layers can slide easily relative to each other. 

Each of molybdenum atom is surrounded by two sulfur atoms forming in such a way a ‘sandwich’. Connection between the layers is weak enough, so the shear resistance along the interface plane is slight. On the same reason the friction coefficient is low and remains so at high loads and significant heating.

This mineral, due to its high adhesion, fills in the surface microroughness cavities and extends its supporting area. In the process of friction MoS₂ particles form a smooth and slippery protective layer able to withstand significant contact pressures, micro gaps are also getting tighter, and total tightness grows.

Molybdenum disulfide is especially effective at high contact pressures in mixed and boundary friction modes. Under such conditions, the mineral reduces friction, prevents scuffing and seizures, as well as makes the parts running-in easier. It differs by its unique anti-friction properties and chemical inertness. 

According to the experimental test results, it was defined that the mineral provides stable and low friction at contact pressures up to 3,000 MPa what favouredly distinguishes it from solid lubricants based on polymers.

MODENGY Anti-Friction Solid-Film Coatings

Fig. 3. MODENGY Anti-Friction Solid-Film Coatings for casing and pump-compressor pipes

MODENGY 1002 is a coating based on molybdenum disulfide and organic binder which has already proved itself in oil-and-gas industry as a product with good balance of anti-corrosive properties and wear resistance, as well as ability to cure fast at room temperature. It is available in 520 ml metal aerosol cans, as well as in 1 kg cans, and 4.5 kg pails.

You can apply it onto threaded parts of couplings and pipes by a usual method for painting, that is, by spraying out. Just 5 minutes later the coating layer gets dry, and the pipes can be transported to the storage place. In 120 minutes after it is applied, the product cures completely and is ready to assembly and operation loads. At heating up to +130 °C cure time makes 15 minutes.

The layer thickness can be adjusted by diluting the concentrate of the product with solvent which changes viscosity of the entire composition. MODENGY 1002 retains its operating characteristics providing the constant friction coefficient, whose scattering stays within ±0,01 in process of 5 to 8 screwing-unscrewing cycles. According to the results of the test in salt mist, this material provides the parts with corrosion protection for more than 160 hours.

If you have a need in the higher anti-corrosive properties, you should use MODENGY 1004 and MODENGY 1014 coatings. These materials cure by heating up to +200 °C within 40 minutes. According to the results of the test in salt mist, these materials provide corrosion protection for more than 500 hours and more than 672 hours, correspondently.

Fig. 4. MODENGY 1002 coating appearance on the thread surface

Table 1. Characteristics of MODENGY coatings for threaded connections

Advantages of MODENGY Anti-Friction Solid-Film Coatings:

• High load carrying capacity

• High antiscuff properties

• Easy way of applying by spraying with a standard paint gun

• Fast curing

• No need in further mechanical treatment of the surface

Conclusion

MODENGY coatings have the whole set of the required characteristics to manage friction in threaded connection efficiently, and to protect against corrosion. While preparing the pipes to installation, these materials enable to abandon labourous operations on lubrication under conditions of the mounting site. 

Besides, they provide tightness of the casing connections, significantly reduce time period for its assembly. The coating applying technology requires no significant cost for implement since you can use the same methods and equipment that are used for painting.