Keywords: Coating properties surface characteristics aerodynamic testing high pressure high velocity
Coating surface characteristics
The internal coatings for pipelines exhibit complex fluid friction. Firstly, the surface condition analyzed locally in terms of roughness parameters, Ra or Rz is different from that of shot blasted steel often taken as a reference. Secondly, although sometimes smooth locally, the surface of a coating may have long wavelength corrugations contributing to greater friction than that predicted from a measurement of local roughness. Finally a third parameter can intervene with certain types of coating which is the parietal (or wall) sliding. This phenomenon could be explained by a slight distancing on the molecular scale of the fluid from the wall promoting the flow of the gas (an acceleration). This phenomenon is particularly important when the boundary layer (viscous) presents a small thickness.
This last phenomenon is the opposite of that is sometimes observed with chemically modified coatings after contact with certain chemical agents leading to an attraction of molecules towards the wall, consequently, to a braking of the flow (a deceleration).
It is therefore important to know the aerodynamic performance of a pipeline internal coating, for new as well as for aged coatings.
Coating aerodynamic testing
An aerodynamic testing device has been designed for this purpose. The operating principle is based on the rotation of a gas, in a closed enclosure under pressure, using a rotating cylinder. The braking of the gas by the fixed external wall makes it possible to deduce the friction factor of that wall at the origin of this braking. The device allows measurement of gas friction under conditions of high pressure (up to 100 bar) and very high velocity (up to 40 m/s), i.e. with a very thin layer thickness (less than one micrometer). Transcribed in terms of Reynolds number, this gaseous friction can be translated in a condition of similitude based on a constant product of the density and the velocity of the flow.
The characterisation of the surface profile of various coating types has shown the large spectrum of amplitude and wave length deformation. The solvent based coatings present roughness amplitude smaller than steel but some deformations of a longer wave length. Water based coatings have shown significant roughness amplitude with relatively large undulation amplitude. To the contrary powder coatings present little roughness with some residual undulation amplitude. Like powder coating, polyamide coating has shown an extremely small roughness with undulation of a very large wavelength not detrimental to the aerodynamic performance.
A test apparatus has been developed for carrying aerodynamic test of internal coatings. It is suitable to an operation up to a gas velocity of 40 m/s and a pressure of 100 bar. Results found in these conditions may be transposed to any condition with a constant velocity * pressure product (i.e. 20 m/s and 200 bar or 10 m/s and 400 bar).
All new coatings have presented a beneficial material effect tending to reduce the fluid friction compared to a steel surface based on the same surface roughness. This may be explained by a certain degree of repulsion of the gas molecules from the wall providing less contact between these two and therefore less fluid friction.
Following the aerodynamic testing of a coating in the rotating cylinder unit, it is possible to transpose the friction factor established during testing to any industrial applications following a transposition law based on an equality of the hydraulic roughness and viscous layer thickness values. Aerodynamic performance of aged coatings will be reported in the Web Site Page “Pipelines” section “Aged coatings”.