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  • Erosion Due To Flow

    Erosion can be defined as the mechanical loss of material by the impact of solid particles (e.g. sand, certain hard scales, catalysts) and / or liquid droplets.

    Erosion caused failures are not new. The oil and gas industry has suffered and continues to face many failures that can be attributed to erosion.

    Under aggressive operating conditions, flow velocity limits and thus the production limits are set to avoid erosion. If the velocity limits are overly conservative (lower values) then an oil company can suffer production losses and if they are too optimistic (high velocities) there is a serious risk of erosion damage and the loss of system integrity.

    Erosion mechanism can be a combination of several factors in flowing fluids. Some of the erosion mechanisms are described below:

    Liquid Droplet Erosion

    This form of ersoion happens due to the formation of very high pressures and stresses at the point where the liquid droplets contact the pipe wall. Wet gas with an annular mist flow regime can cause droplet impact but generally droplet erosion is a problem in very high velocity systems.

    Erosion-corrosion

    Erosion-corrosion occurs in environments that have the potential to be both erosive and corrosive. The erosion and corrosion can either be independent, in which case the total material loss is the material loss produced by each mechanism in isolation, or synergistic, in which case the total material loss is greater than the sum of the independent mechanisms of erosion and corrosion. It is important to note that erosion-corrosion is not a pure erosion phenomena.

    Cavitation-erosion

    Cavitation is caused by the high energy collapse of vapor bubbles (or cavities) in a liquid flow stream as a result of pressure recovery following pressure reduction to a value below the vapor pressure of the liquid at the flowing temperature. Cavitation-erosion is not caused by the impact of solid particles or liquid droplets, but by the impact of a very high velocity liquid pulse onto the solid boundary from a collapsing bubble.

    High pressure drops using single-stage control valves and sharp directional changes (e.g. short-radius elbows) can result in cavitation-erosion in piping systems. Multistage pressure reduction and long radius elbows provide better protection against cavitation-erosion.

    Effect of flow patterns on erosion in multiphase flow

    The flow patterns that are encountered in multiphase flow can be described as follows:
    a. Bubble Flow
    b. Plug flow
    c. Stratified flow
    d. Wave Flow
    e. Annular Flow
    f. Slug flow
    g. Churn Flow
    h. Mist Flow
    With the exception of stratified flow all the flow patterns that have been described above are commonly encountered in horizontal, vertical and inclined flow.

    The flow pattern is very important in determining the erosion cahracteristics of multiphase flow, i.e flow containing solid particles. The major factors that effect erosion in multiphase flow are:
    1. The phase that the soild particles travel in and
    2. How the phases are distributed in various patterns

    Due to the higher density and viscosity of the liquid compared with the gas phase, solid particles carried in the liquid phase follow the streamlines more closely and are less likely to impact the pipe wall if the fluid changes direction. Hence, for the same mixed phase velocities, flow regimes where the particles are carried in the gas exhibit higher erosion rates than flow regimes where the particles remain in the liquid.

    In annular flow the particles are carried in both the liquid film and the gas core. The phase distribution in annular flow usually consists of a slow moving liquid film on the pipe wall and more rapidly flowing gas core containing liquid droplets.

    In bubble flow the low relative velocity between tha gas and liquid phases means that there is very little entrainment of particles into the gas phase. This results in all the particles being carried in the liquid phase. The liquid flows as a continuous phase with the gas fairly evenly distributed in it. As both gas and liquid flow at similar velocities the effects of direction changes (bends) and other fittings on the phase distribution is much less pronounced than in annular flow.

    As with bubble flow, the relative velocities between the phases, in churn flow is low. However, the large amount of mixing and generally random distribution of the phases could lead to a small portion of the particles being carried in the gas phase.

    API 14E- "Recommended Practice for Design and Installation of Offshore Production Platform Piping Systems" as well as ISO 13703- "Petroleum and natural gas industries- Design and Installation of piping systems on offshore production platforms" provide calculation equations for erosional velocity in multiphase flow. The basic equation for calculating erosional velocity as per the above references is:

    Ve = C*(1/دپm)0.5

    where:

    Ve = fluid erosional velocity, ft/s (m/s)
    دپm = gas / liquid mixture density at flowing temperature and pressure, lb/ft3 (kg/m3)
    C = empirical constant

    The 'C' value as recommended by API 14E and ISO 13703 are as follows:

    English Units:
    C = 100 for continuous service; C=125 for intermiitent service; for solids-free fluids where corrosion is not anticipated or when corrosion is controlled by inhibition or by employing corrosion resistant alloys, values of C = 150 to 200 may be used for continuous service; values of 250 may be used for intermiitent service

    SI units:
    The values mentioned above should be multiplied by a factor of 1.22 (e.g. 'C' value of 100 becomes 122)

    The mixed phase density may be calculated as follows:

    Engllish Units:

    دپm = 12490SlP + 2.7RSgP / (198.7P + RTZ)

    where:

    Sl = liquid specific gravity at standard conditions (water =1)
    P = Operating Pressure, psia
    R = Gas/liquid ratio, ft3/barrel, at standard conditions
    Sg = gas specific gravity at standard conditions (air =1)
    T = Operating Temperature, R
    Z = Gas compressibility factor, dimensionless

    SI Units:

    دپm = 28833SlP + 37.22RSgP / (28.82P + 10.68RTZ)

    where:

    Sl = liquid specific gravity at standard conditions (water =1)
    P = Operating Pressure, kPaa
    R = Gas/liquid ratio, m3/m3, at standard conditions
    Sg = gas specific gravity at standard conditions (air =1)
    T = Operating Temperature, K
    Z = Gas compressibility factor, dimensionless

    Once Ve is known, the minimum cross-sectional area required to avoid fluid erosion is determined by the equation below:

    English Units:

    A = 9.35 + (ZRT/21.25P) / Ve

    where:
    A = minimum pipe cross-sectional flow area required, in2/1000 barrels liquid per day

    SI Units:

    A = 277.6 + (103ZRT/P) / Ve

    where:
    A = minimum pipe cross-sectional flow area required per unit volume flow rate, expressed in square millimeters per cubic metre per hour mm2/m3/h

    However, recent studies on erosional velocity limits that the 'C' values given as per API 14E are very conservative. I had made a post related to the 'C' values for erosional velocities based on recent studies and also had provided the link for a "Society of Petroleum Engineers" (SPE) paper which discusses in detail about 'C' factors that need to be considered based on the solid content and the various piping materials.

    Author ,
    Ankur.
    This article was originally published in forum thread: Erosion Due To Flow started by Mohamed View original post
    Comments 1 Comment
    1. elbastar85's Avatar
      elbastar85 -
      Thanks
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