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Chemical Process Technology

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Sunday, January 31, 2010

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In oil and gas production system, sand is carried and produced together with oil / gas production. Sand produced cause erosion, erosion-corrosion, vibration, blockage, reduce productivity, sand separation and handling, additional maintenance, etc. Downhole sand control is introduced to minimize sand production. With downhole sand control, it will not absolutely sand free. There is still possibility of sand produced with oil/gas production. The produced sand size may range from 50 to 100 micron. However, the sand production quantity is sufficiently low and the impact and consequence are mild. This post will discuss some facts related to erosion.

Erosion is a material  removal from a material surface with continuous particle, droplet and/or cavity impinging on the surface.Typical examples 
  • oil/gas production with sand particle
  • gas / vapor with sand
  • wet vapor with liquid droplet
  • two phase flow with liquid mist/slug
  • liquid cavitation with bubble collapse near/on the surface
  • scale  / corroded slag flowing in fluid impinging surface
  • flashing and/or cavitation downstream of control valve
Erosion corrosion is an acceleration in corrosion attack in material with the present of erosion phenomenon. Corrosion inhibitor (CI) is used to minimize / mitigate corrosion in Corrosion susceptible material i.e. acidic wet fluid flowing in carbon steel, CI form an "isolation" layer on the carbon steel surface and to isolate corrosive fluid in contacts with corrosion susceptible material and to minimize corrosion activity on the surface. Present of erosion phenomenon will remove this CI layer (and material surface) and accelerate corrosion attack. For corrosion resistance alloy/material (CRA), a strong passivated material is formed at the CRA surface to protect it from further corrosion. Present of erosion phenomenon will remove the passivated layer and promote erosion-corrosion.

Erosion phenomenon
There are several possible erosion type and its phenomenon :
  • Fluid shear stress erosion - This typically occurs in any flowing fluid where fluid is moving on surface and induce shearing stress on the surface. Higher shearing force induce higher shearing force and lead to higher material removal rate
  • Fluid impingement erosion (splashing & droplet impingement) - This typically occurs in multiphase flowing fluid where heavy phase is accelerated with light phase and induced high momentum and shearing stress on surface. Slugging flow in vapor-liquid system with large slug hammering surface induce high momentum with medium velocity and high mass flux. Mist flow with droplet accelerate at vapor velocity induce high momentum with high velocity and low mass flux. Both induce high shear stress on the surface and increase material removal rate
  • Particle impingement erosion - Similar to droplet impingement on material surface, solid particle (e.g. sand, welded slag, corroded slag, solid scale, etc) is accelerated with vapor/gas. High velocity solid impinging on the material surface and remove material from its surface. Solid with high material hardness increase further material removal rate
  • Fluid cavitation - Fluid with operating pressure above vapor pressure, flow through devices (e.g. control valve, restriction orifice, orifice plate, pump suction line, etc) results operating pressure drop below fluid vapor pressure where bubbles form and followed by pressure recovery (e.g. in control valve) and addition of external power (e.g. pump) lead to operating pressure again rise above fluid vapor pressure where bubbles collapse. This is commonly known as cavitation which results significant jet force acting on the surface and material removal from surface
  • Fluid flashing - Similar to fluid cavitation, fluid with operating pressure above vapor pressure, flow through devices (e.g. control valve, restriction orifice, orifice plate,  etc) results operating pressure drop below fluid vapor pressure where bubbles form and followed by pressure recovery (e.g. in control valve) lead to operating pressure again rise. In flashing case, recovered pressure is still below fluid vapor pressure, and bubbles permanently form downstream of these device. Increase bubbles formation lead to accelerate liquid, increase shearing force and material removal rate.
Common Erosion Locations
Erosion occurs in all particle/fluid reached components e.g.
  • Chokes valve
  • Elbow
  • Blind tee
  • Reducer & Constriction
  • Partially close valve
  • Check valve
  • Non Full bore valve
  • Branch
  • Straight pipe
Erosion rate severeness subject to the way erosion occur. Direct impingement (perpendicular to impacting surface) of solid/fluid on material surface induce higher erosion rate compare to parallel shearing. High erosion occurs at Tee where solid / fluid impinge perpendicularly to pipe, elbow where solid / fluid impinge in multiple angles and choke valves with change in flow direction. High erosion rate also occurs in straight pipe with annular flow. Swirling vapor flow in annular flow pattern forcing liquid phase flowing along the pipe surface increases liquid shearing rate  and  frequency on the pipe surface.

Factors Affecting Erosion
Erosion can be affected by many factors which subject to (1) Fluid characteristic, (2) characteristic of impacting solid / particle / droplet and (3) properties of material being impacted.

(1) Fluid characteristic

  • Fluid velocity - Fluid carrying impacting solid / particle / droplet / slug flowing at higher velocity, high momentum is generated and lead to high impacting force and increase erosivity.
  • Fluid viscosity - Fluid with high viscosity induce high dragging force on impacting solid / particle / droplet. Fluid with high viscosity has higher inertial in dragging impacting solid / particle / droplet to follow it flowing path and reduce tendencies and frequency impacting on the surface. Nevertheless, creation of eddies and flow path concentrated at particular location on the surface, will seriously promote erosion
  • Fluid density - Similar high fluid viscosity, high fluid density has high capability in carrying and affecting flow path of impacting solid / particle / droplet.
  • Flow pattern - Two phase flow with present of solid / particle, annular flow tends to push liquid and solid / particle concentrated at the pipe surface and increase erosivity. Slugging flow with severe slug impacting on the pipe surface would seriously increase erosivity and it is enhanced by present of solid / particle / sand
(2) Characteristic of impacting solid / particle / droplet

  • Particle production level - High solid / particle / droplet present in fluid leads to higher impacting frequency and higher erosivity. Minimizing solid / particle / sand production e.g. efficient downhole sand control, etc and improve fluid dryness e.g. dew point, well operated separator, filter coalescing, etc are the ways to minimize erosion
  • Particle velocity - Erosivity is highly affected by particle velocity. High particle velocity results high momentum on impacting surface and lead higher successive erosion. It is commonly understood that Erosion Rate (ER) is proportional to particle impacting Velocity raised to the power of n where n may range from 2 to 3 for ductile material (e.g. stainless steel) and possibly upto 6 for brittle material (e.g. some plastic material)
  • Particle density - There are two major contributions by high particle density. (1) high particle density results high impacting momentum and successive erosion. (2) high particle density increase particle flowing inertial and reduce the tendencies of fluid carrying capability and drive away from impacting the surface
  • Particle viscosity - Contrary to particle density, high particle viscosity increase fluid carrying and dragging effect and drive away from impacting surface
  • Impacting angle - Impacting  angle play a major rule in erosion. Direct impacting particle would results most severe erosion and reduce with impacting angle. Least erosion occurs when particle flowing parallel with impacting surface
  • Particle shape - Sharp particle compare to round particle tends to increase erosivity
  • Particle size - Very small particle tends to flow with flowing fluid and drag away from impacting surface. Very large particle tends to flow slower in the flowing fluid. Medium size particle results severe erosion as it flow at high velocity and momentum (less drive by flowing fluid) comparatively
  • Particle hardness - Hard particle (e.g. stone) results higher erosion than soft particle (e.g. mud)
 (3) Properties of material being impacted
  • Material hardness - Increase material hardness reduces erosivity
  • Material ductility - Some material with high ductility tends to reduce erosivity. Robber or polymer type material tends to absorb impacting energy and reduces erosivity. Stainless steel with work-hardening property tends to increase its hardness once it is impacted.
  • Material brittleness - Some material present high hardness but brittle. Erosion is so much affecting healthiness of the material but impacting momentum tends to increase material localise cracks (due to its brittleness property)
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