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Chemical Process Technology

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Tuesday, February 2, 2010

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Wet vapor potential condense and form liquid droplet (mist flow), vapor at high velocity will drag the droplet and flow approximately same speed as vapor. Whenever vapor with liquid droplet flow  change in direction at elbow, bend, tee, valve, reducer, etc, liquid droplet will high density tends to impinge on the pipe wall and results erosion. Droplet impingement on pipe wall results erosion is commonly occur in Mist flow.

As liquid condensation increase, liquid droplet coalesce and accumulate and slowdown due to increase in mass and shearing force near wall. Vapor flows in swirling pattern in pipe creates centrifugal force pushing  liquid stick to the pipe and moving forward. Swirling liquid moving forward at reasonable high velocity will results erosion on pipe wall and common occur in Annular flow.

Further increase in liquid flow will further slow down liquid movement in the pipe compare to vapor flow. Swirling flow and vapor dragging liquid surface tends to create liquid slug and restrict vapor in the pipe.  Vapor at high velocity behind slug will accelerate slug and potentially hammering on pipe wall, elbow, bend, reducer,etc. Severe vibration, noise level and erosion will occurs in Slugging flow.

Mist flow, annular flow and slugging flow erode pipe in different ways and results different level of erosion. Many researchers and experts have spend their time and effort in deriving the erosion rate for two phase flow phenomenon and derive criteria in designing a pipe in two phase flow.

Droplet Erosion velocity threshold
As discussed in "Erosion & Erosion - Corrosion", erosivity is highly affected by particle/droplet velocity. High particle/droplet velocity results high momentum on impacting surface and leads to 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). Many researchers have conducted experiments and derived the Droplet Erosion velocity threshold for solid free fluid.

Droplet Erosion velocity threshold (VE ) for solid-free fluid
  • DNV RP O501, VE = 70 ~ 80 m/s
  • Salama & Venkatesh, VE = 26 ~ 118 m/s
  • Shinogaya, VE = 80 m/s for Aluminum, 100 m/s for pure iron, 110 m/s for SS
  • Svedeman & Arnold, VE = 30 m/s
Looking at above results, there is no one common range for the Droplet Erosion velocity threshold (VE ) for solid-free fluid due to complexity of two phase gas liquid flow.

Erosion model & Erosion Velocity Criteria
There are many models have been studied and proposed :

  • API RP 14E Erosion model
  • Salama & Venkatesh model
  • Salama 2000 model
  • DNV ERBEND model
  • AEA Harwell model
  • Tulsa SPPS model
Among all, API 14E erosion model is one of the earliest model being used in designing two phase gas liquid flow. Many others models have evolved from this basic model.

API RP 14E recommends

VE = C / Sqrt (mixture density)

VE in ft/s
mixture density in lb/ft3

C =100 for solid free corrosive and continuous operation service
C =125 for solid free corrosive and intermittent operation service
C =150 to 200 for solid free non-corrosive or CI controlled and continuous operation service
C =250 for solid free non-corrosive or CI controlled and intermittent operation service

Today, general perception is that API RP 14E recommendation is highly conservative. Many experiments have demonstrated this perception and recommends higher C value to be used.

Salama & Venkatesh have similar model and recommends :
C = 300 for solid free flow.

Salama recommends :
C = 400 for solid free non-corrosive fluid
C = 300 for solid free corrosive fluid

NORSOK standard P-001 (Ed. 5) recommends :
Wellhead flow-lines, production manifolds, process headers and other lines made of steel and transporting two-phase or multiphase flow, have a velocity limitation. When determining the maximum allowable velocity, factors such as piping geometry, well-stream composition, sand particle (or proppant) contamination and the material choice for the line shall be considered.

As a guideline, the maximum allowable velocity can be calculated by:

VE = C / Sqrt (mixture density)

VE  in ft/s
mixture density in lb/ft3 
C =  150

  • Non corrosive service - For non corrosive well-stream and for corrosion resistant pipe materials the velocity should be limited to maximum 25 m/s if the well-stream includes only small amounts of sand or proppants (typical less than 30 mg sand/liter in the mixed flow).
  • Corrosive service - For carbon steel (CS) piping systems the corrosion rate often limits the life time. With increased flow velocity the corrosion rate tend to increase due to increased shear forces and increased mass transfer. The flow velocity should be restricted to maximum 10 m/s to limit the erosion of the protective layer of corrosion products and reduce the risk for a corrosion inhibitor film break down.
Again, above shows that there is no one common Erosion velocity limit for for solid-free fluid due to complexity of two phase gas liquid flow.

Solid / Sand Present in Fluid
With the present of sand in single and/or two phase gas liquid  flow further increase it complexity :

NORSOK standard P-001 (Ed. 5) recommends :
  • Particle erosion in non corrosive service - For well-stream contaminated with particles the maximum allowable velocity shall be calculated based on sand concentration, piping geometry (bend radius, restrictions) pipe size and added erosion allowance. For the calculation of maximum velocity and life time specialised computer programmes are available and should be employed.
  • Liquid flow with presents of sand, maximum allowable velocity (VMax ) are :
    • 5 m/s for CS
    • 7 m/s for SS/Titanium

NORSOK standard M-001, section 4.2.2.... recommends :
If sand production and/or particles from well cleaning and squeeze operations are expected, an erosion evaluation shall be carried out. The evaluation should be based on DNV RP-O-501

Salama recommends

VE = D * Sqrt (mix den) / [20 * Sqrt (W)] 

VE = Erosion velocity limit (m/s)
D = pipe internal diameter (mm)
W = sand production rate (kg/day)
Mix den = Mixture density in (kg/m3)

Author has worked projects for many well-known oil and gas companies e.g. SHELL, EXXONMOBIL, TOTAL, BP, etc. All companies philosophy in erosion and erosion-corrosion and criteria in designing two phase gas liquid and sand-laden fluid are different.

Some Facts from Literature / Studies
Following are facts related to erosion :
  • Material such as tungsten carbides, coating, ceramic, etc commonly formed part of valve internal component are vulnerable to erosion.
  • Particle impinging surface at varies angle results different erosion impact. Maximum impact is particle impacting perpendicular to surface
  • Corrosion inhibitor (CI) form layer at internal pipe isolating / minimizing corrosive fluid contacts with corrosion susceptible material. Erosion due to fluid and particle impingement on CI layer potentially remove this protective layer. Commonly maximum velocity to avoid erosion of CI layer is 20 m/s. Some special CI can tolerate upto 50 m/s
  • Sand production with downhole sand control, sand concentration at 1st receiver typically contains 1 to 50 ppmw of sand concentration. Past experience may reach 100 ppmw.
  • A well produce 5 to 10 lb/day of sand is typically regarded as "Sand-free production".
  • "Nominal solid-free" production is common defined as less than approx. 3 gram-per-m3 for liquid or less than 0.1 lb/mmscf for gas
  • Well with downhold sand control may contains sand sizes typically range from 50 to 100 micron. Those without downhole sand control may range from 50 to 500 micron
  • Erosion rate is commonly proportional to particle impact velocity into power of a factor range from 2 to 3 for steel.
  • Higher fluid viscosity and density increase drag effect and "holding" capacity. Viscous and dense fluid tends to reduce particle impacting on surface
  • Typical sand particle density is 2600 kg/m3
  • API RP14E recommendation is conservative for solid free liquid service from erosion aspect. However, it potentially under-estimate solid free gas/vapor service (subject to droplet erosion)
  • Rich amine potential expose erosion and cavitation effect when it is flashed from high pressure to lower pressure. Low threshold velocity should be used e.g. 1-2 m/s.
  • Elbow and tee are most vulnerable to erosion compare to others component
  • In gas and condensate production with present of solid / sand particle, API 14E has no clear recommendation to account for erosion rate.
Above is meant to provide some information for those engineers dealing in erosion. The complexity lead to many opinion and recommendation. What about yours in previous/present projects ???

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posted by Webworm, 3:19 PM


Anonymous Anonymous said...

very good article, but why the hell still not in SI units?

May 13, 2014 at 11:50 PM  
Blogger Webworm said...

As engineer, should be familiar on unit conversion...

December 13, 2014 at 9:45 PM  

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