Page 170 - Handbook Of Multiphase Flow Assurance

166 5. Flow restrictions and blockages in operations

Typical scraping velocities are between 1 and 3 m/s. The faster the scraper moves, the
more its cutting surfaces deform and the less efficient is the scraping run in removing wax.
Therefore an operational experience can show the optimum balance between the duration of
the scraping run and the amount of wax removed per run. In some cases only the soft layer
of wax gets scraped by the scraper while the aged harder underlying layer gets left behind.
If a pipeline carrying waxy crude has not been scraped for an extended time (over 6 months)
it is likely that the amount of accumulated wax is too great to be handled by a single full-
diameter scraper. In such cases the progressive scraping is implemented when softer material
(lower durometer) scrapers are used for initial scraping of wax, then progressively larger and
harder scrapers are used to remove the remaining wax deposit.
One way to avoid wax deposition in a pipeline and to reduce or eliminate the need for
scraping is cooling of the crude to ambient environment temperature before sending it
through the pipeline. No wax will deposit on the pipe wall by diffusion since there is no
temperature difference between the fluid and the pipe wall. However, this requires addi-
tional equipment or a length of pipeline and may be uneconomic. A pre-cooling system could
include a heat exchanger with non-stick cold surfaces which would pre-cool the crude and
send all precipitated solids with the flow. Prior laboratory tests indicated that wax sticks to
almost any material which can be withstand harsh production system conditions and erosion,
so such pre-cooling heat exchanger is not technically achievable yet. An oleophobic non-stick
surface could possibly be made of polyacrylonitrile polymer modified by oxyfluorination to
improve its antifouling, possibly with embedded SiO 2 nanoparticles, or periodically actively
heated to melt off the deposited wax.
Even if the hydrocarbon liquid is pre-cooled, precipitated wax may still accumulate in the
pipeline by the dispersion mechanism discussed in Burger et al. (1981). An example of wax
from a line where the main wax deposition mechanism is by dispersion is shown in the image
below in Fig. 5.35.

FIG. 5.35 A 48-in. diameter scraper used to scrape wax in an oil export pipeline.