FLOW VELOCITY REQUIRED FOR SOLIDS MOVEMENT IN OIL AND GAS PIPELINES JOHN S SMART JOHN SMART CONSULTING ENGINEERS HOUSTON TX BLACK POWDER IN PIPELINES JOHN S. SMART JOHN SMART CONSULTING ENGINEERS HOUSTON, TX

What is Black Powder? A mixture of fine particle corrosion products and other solids found in both liquid and gas pipelines Can be all iron sulfide, all iron carbonate, or all iron oxide It is called “black powder” because the presence of iron salts makes it black.

What it is N0T !

SEM Photomicrographs of Black Powder

Sulfides Pyrrhotite – Fe 1-.85x S normal corrosion product from H2S in gas, strongly ferromagnetic, pyrophorric Mackinawite – semi- stable form of FeS that forms under low H 2 S activity (slightly sour) Greigite – formed by sulfate reducing bacteria Industry Amount H 2 S Allowed, Grains/100SCF

Oxygen Contamination Oxygen acts as cathodic depolarizer, accelerates other corrosion reactions Air contact converts FeS to Fe and elemental sulfur Inhibitors lose their effectiveness Oxygen Allowed, ppm or Vol%

Water Allowed in Natural Gas There must be liquid water to have corrosion 7 lbs/MMSCF: –Dew point = 12 o F at 600 psi –Dew Point = 32 o F at 1000 psi –Dew Point = 43 o F at 1500 psi 3 lbs/MMSCF specified in Europe – higher pressures and colder environments Industry Allowed Water in Natural Gas, lbs/MMSCF

GLYCOL IN BLACK POWDER Gas Transmission Lines carry some glycol as mist for a short distance, most carry as vapor (very small amount) from glycol dehydration Concentration of TEG is 97.66% at 7 lbs/MMSCF Will make a black sludge instead of a dry powder Bacteria can metabolize TEG at lower concentrations Can remove with a slug of methanol followed by pig Ref: Van Bodegom et al, CORR. 1987

SOURCES OF BLACK POWDER Producing wells Corrosion product Off-specification produced fluids (e.g.salt) Powder Movement in line Hydrotesting: sand and clays Plant turnarounds and upsets  water Construction debris, millscale, rouge

CALCULATION OF MINIMUM VELOCITY FOR SOLIDS MOVEMENT IN WATER, OIL, AND GAS PIPELINES Hydraulic Model used to calculate minimum velocity for solids movement in pipe by viscous drag Applied to sand and iron compounds in water, oil and gas Sets a minimum velocity for possible solids movement Erosion by solids occurs at much higher velocity

HYDRAULIC MODEL Model of Wicks (Shell, 1969) Particles must be “rolled” out of a bed to start movement For level flow, must add extra 10-15% for uphill movement Shape factor for particles required ( for PL solids) No interaction between particles (dry, non- magnetic) Round particles roll Long thin particles slide Rough particles bounce along, called “Saltation flow” Flake like particles move like leaves in the wind

FLUID VELOCITY TO MOVE IRON SULFIDE IN 40 o API OIL Velocity to move FeS in 40 o API Crude Oil much higher than sand in water Approximates FeCO 3, Fe 2 O 3, Fe 3 O 4 movement velocity

Velocity to Move Iron Sulfide in 20 o API Oil vs. Pipe Size Higher velocity to move solids due to thickness of laminar boundary layer over particle bed used in model High viscosity fluids will result in significant time for particle to settle after bouncing

Movement Velocity for FeS in No. 2 Diesel vs. Pipe Size Diesel often used for pigging as it can be recovered and sold When #2 Diesel used in pigging, FeS will settle out in front of a pig running at 5 ft/sec, possibly accumulating and sticking the pig.

Bed Height of Solids in No. 2 Diesel in a 24 Inch Pipeline When the pipeline fluid is moving less than the entrainment velocity, sediment can build up to a steady state level that can be calculated by Wicks Model

Accumulation of Solids in Pipelines CASE HISTORY –20” Crude Oil Pipeline –0.5” iron oxide = 22 ft 3 /mile = 2 tons/mile –1.0” iron oxide = 9 tons per mile –Must be removed for ILI Inspections

Sticking Pigs CASE HISTORY –Pipelines can plug with sediment during pigging –200 feet of pipe had to be cut out and replaced in this 12 inch line –Need to use pro- gressive pigging and jetting in front of pig in low velocity lines

Velocity to Move Black Powder in Natural Gas at 1000 psi and 60 o F Entrainment Velocity from 9-14 ft/sec depending on pipe size at 1000 psi Lines cleaned of black powder frequently show black powder again after a short time. Pigging upstream can cause black powder to move downstream Filter Seps needed ahead of centrifugal compressors Lines with “No Black Powder” problem may be just operating at low velocity

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IRON SULFIDE FRACTURES INTO FINE POWDER

FINE ROUGE IN NATURAL GAS FUELED DIESEL ENGINE

Movement of Wet Powder Dry powder

PROBLEMS WITH BLACK POWDER Pyrophoric Inaccurate ILI Inspections from ferromagnetic iron sulfide in internal pits Lowers Flow Efficiency Abrasive

Iron Sulfide is Pyrophoric

Flow Efficiency - I

Flow Efficiency - II Before Pigging 3 months after pigging 5 months after pigging

Abrasiveness Black Powder in gas compressor, 24 hours Compressor rotated at 28,000 rpm Cause was an increase in pipeline velocity resulting in 6 tons per day black powder into station

Abrasiveness

Ceramic Coated Gas Cooled Gas Turbine Blade Ceramic coating fluxed by black powder Caused by adding a second generator onto gas supply line

Solving Black Powder Problems Physical Stuff –Chemical cleaning –control contamination –Internal coating –Cyclone separator –Submicron filters –Analyze pipeline for black powder –Glycol Injection to control dew point –Corrosion Inhibitors Management Stuff –Operators, engineers and managers need to cooperate, no-one can do it alone –Managers need to recognize problem, direct effort, provide budget, and use analysis for forward planning –Operators, engineers need to learn to speak economics to managers.

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