Chemical & Process Technology

Tuesday, February 2, 2010

Facts about Erosion & Erosion-Corrosion

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 (V_E ) for solid-free fluid

DNV RP O501, V_E = 70 ~ 80 m/s
Salama & Venkatesh, V_E = 26 ~ 118 m/s
Shinogaya, V_E = 80 m/s for Aluminum, 100 m/s for pure iron, 110 m/s for SS
Svedeman & Arnold, V_E = 30 m/s

Looking at above results, there is no one common range for the Droplet Erosion velocity threshold (V_E ) 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

V_E = C / Sqrt (mixture density)

V_E 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:

V_E = C / Sqrt (mixture density)

where
V_E 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 (V_Max ) 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

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

where
V_E = 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 ???

posted by Webworm, 3:19 PM | link | 2 Comments |

Sunday, January 31, 2010

Erosion & Erosion - Corrosion

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)

Related Topics

Labels: Corrosion, Erosion