Page 159 - Handbook Of Multiphase Flow Assurance
P. 159
Sand transport 155
Stokes equation (1851) for particle settling velocity in a fluid shows dependence on particle
diameter D:
−
2
Ws = D ×(densityparticledensity fluid )× g (18 viscosity )
×
_
/
_
which is valid where density_fluid × Ws × D/viscosity <1.
Measurements for particles transport in vertical gas-liquid flow are presented by Toda
et al. (1982).
Biot and Medlin (1985) discuss the applicability and methods for applying the settling
velocity calculations for solids transport in vertical flow with small and large particle
concentrations.
Wang et al. (2014) discuss drag on sand particles in liquid and multiphase flow.
Erosional velocity limits
Several guidelines are available to help determine the maximum fluid velocity at which it
is safe to operate a pipeline, without erosion of the pipe wall material.
The Norwegian NORSOK P-001 standard was developed before 1996 to replace individual
oil company specifications on erosion. The new P-002 standard (2014) replaces the P-001.
API RP 14E (2017) recommended practice has been historically widely used due to its ease
of application and relative conservatism, providing a solution which is highly unlikely to
result in an erosional rupture and accident.
Typical safe limit recommendation for C = 100 holds for solids-free fluid flow. However,
operating companies report (Mansoori et al., 2013; Salama, 2000) that the use of the API-14E
guideline is overly conservative and provide field observations that higher C-factors may be
acceptable. While the onshore erosion consequences may be more easily managed, the API-
14E is specifically for offshore platform systems where reliability is more important.
V _ maximumfts] = ( _ / 3 ) . 05
[
C/ fluiddensity lb ft
/
0.5
0.5
C = 100 lbm /ft s for continuous flow in carbon steel, 125 for intermittent flow excursion
to a higher rate, and 200 in corrosion resistant material.
The C = 100 factor corresponds to approximately 12–14 ft/s flow velocity.
Note that in metric units the C factor will differ from that in the field units.
0.5
0.5
C = 122 kg /m s for continuous flow in carbon steel.
A recent clarification by API (2017) states that pipes with a pressure rating over 10,000 psi
are outside the scope of this guideline.
A more recent 2015 DNV guideline is available.
A good discussion on erosion in elbows in hydrocarbon production systems is available
from the UK Safety office (2003).
While there are guidelines for erosional velocity limits in steel pipes, there is a shortage
in agreement on erosional guidelines in non-metallic pipes such as fiberglass pipes. The
NORSOK P-001 guideline provides maximum velocity recommendation for glassfiber-
reinforced pipe at 6 m/s, same as for steel pipe. DNV-O501 specifies that K-constant for a fiber
pipe be three to six times lower than for a steel pipe. Fiberglass pipe manufacturers provide
their own recommendations, some recommending higher velocities.