FLOW VELOCITY
BLACK
REQUIRED FOR SOLIDS
MOVEMENT
IN OIL AND
POWDER
GAS PIPELINES
IN PIPELINES
JOHN S SMART
JOHN S. SMART
JOHN SMART CONSULTING ENGINEERS
JOHN SMART CONSULTING ENGINEERS
HOUSTON
TX
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 – Fe1-.85xS
normal corrosion product
from H2S in gas, strongly
ferromagnetic, pyrophorric
• Mackinawite – semistable form of FeS that forms
under low H2S activity
(slightly sour)
• Greigite – formed by sulfate
reducing bacteria
Industry Amount H2S Allowed,
Grains/100SCF
Oxygen Contamination
• Oxygen acts as cathodic
depolarizer, accelerates other
corrosion reactions
• Air contact converts FeS to
Fe304 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 = 12oF at 600 psi
– Dew Point = 32oF at 1000 psi
– Dew Point = 43oF 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 (1.5-2.0 for PL
solids)
• No interaction between
particles (dry, nonmagnetic)
• 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 40oAPI OIL
• Velocity to move FeS
in 40oAPI Crude Oil
much higher than
sand in water
• Approximates FeCO3,
Fe2O3, Fe3O4
movement velocity
Velocity to Move Iron Sulfide in
20oAPI 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
ft3/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 progressive pigging and
jetting in front of pig in
low velocity lines
Velocity to Move Black Powder in
Natural Gas at 1000 psi and 60oF
• 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
Gas Velocity to move Black Powder Constituents
in a 24 Inch Pipeline at 1000 psi
16
14
Velocity, ft/sec
12
10
8
6
4
2
0
salt
sand
Bentonite
Iron
Iron Sulfide magnetite hematite
Clay
Carbonate
FeS
(mill scale) (rouge)
FeCO3
Fe3O4
Fe2O3
Mineral
iron metal
Flow Rate (MMSCFD) of Natural Gas at 60oF
to Move 1 micron Iron Sulfide Black Powder
vs. Pressure and Nominal Pipe Diameter
1600
8" Pipe
1400
Flow Rate, MMSCFD
48"
1200
16" Pipe
1000
800
24" Pipe
36"
600
36" Pipe
400
24"
200
16”
0
8”
48" Pipe
0
200
400
600
800
Pipe Line Pressure, psi
1000
1200
1400
1600
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
Fuel cost (thousand dollars/ day)
PIPE LINE ROUGHNESS EFFECT ON FUEL COST STA BBB-1 TO STA BBB-2 (Fuel cost based on $ 3.19/MCF)
12
<---------------Ratio Out Of Station Limit------------------------------>
10
3 months
after pigging
8
5 months after
pigging
6
Before Pigging
4
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
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.
END