Skystone FRP Water Tank Explosion - NACE Calgary

NACE International
NACE Calgary International Section
Non-Metallics Mini Symposium
November 25, 2011

Examination of Explosion Failure of
Aboveground FRP Water Tank
Alex Tatarov, Ph.D., P. Eng., Frank Gareau, P. Eng..

Failure Site

Operating Conditions
• Pumpjack oil well.
• Oil emulsion consisting of sweet natural gas, oil
and water.
• A treater separates the three phases.
• The gas phase goes to a pipeline.
• The oil and water phases are stored in
atmospheric tanks and trucked away periodically.

Water And Oil Were Trucked Away
• Treater (100
bbl)
• Three oil
tanks (400 bbl
each)
• Water tank
(400 bbl)

100 bbl Treater

Automatic Discharge Valves

Conditions Prior To Failure
• Pumpjack was not operating.
• Well valves were not closed, so the well was
flowing.
• Two oil tanks were empty. Third tank
contained 6 feet of oil.
• Water was trucked away two days prior to
failure.
• Water tank was almost empty. 1.5 feet of
water from the bottom=30 bbl of water.

Conditions Prior To Failure
• Good weather, no storms, no precipitation within last 24 hours.
• Temperature from 9C to 19C (late April).
• Nobody was on site at time of failure.

Damage At Top Of Water Tank
• Fire
damage
• Mechanical
damage

Fire Damage At Top Of Failed Tank

Fire Triangle
Confined space additionally required for explosion

Fuel Source 1: Gas From Oil Tanks
• Upper parts of
tanks are
connected by 3
inch overflow
lines at the
upper part of
the shell, and
by a 3 inch vent
line at tank
tops .

Fuel Source 2: Oil Film On Water

Fuel Source 3: Gas/Hydrocarbons
Discharged From Treater With Water
• Pumping was shut down, but the well was still flowing.
• Gas-to-Oil Ratio (GOR) changed.
• More gas and light hydrocarbons came to a treater.
• They came in more volatile manner.
• Water discharge system could be activated to discharge liquid
high in hydrocarbons.

Ignition Source
1. Lightning Strike – Considered to be
unlikely as good weather was reported.
2. Heat – Discussed later; considered to be
unlikely.
3. Static electricity – Discussed later; most
probable source.

Air Source: Thief Hatches
• Water was
trucked away
prior to the
failure
• Thief-hatches
(pressure 8 oz
vacuum 4 oz)
allowed air to
get inside the
tank

Two Possible Scenarios
Water Dumping Scenario
• Automatic discharge
triggered water dumping.
• Water dumping resulted in
internal pressure increase.
• High internal pressure blew
the lid off.
• Metal parts of the lid hit the
railing, which resulted in
sparking.
• The gas-air mixture was
ignited.
Fire and Explosion Scenario
• Water tank contained
ignitable gas-air mixture
• Ignition somehow occurred.
• Gas burning resulted in
increasing pressure and
temperature.
• High internal pressure blew
the lid off.

Water Dumping Scenario:
Possible Metal To Metal Spark

Water Dumping Scenario - Unlikely
• The pressure increase due to water dump was calculated
using the worst-case scenario:
• All the thief-hatches are still closed (no gas escapes).
• The flare line is completely plugged.
• The entire volume of the treater filled with water.
• All this water is dumped into the water tank.
RESULTS:
• ΔP = 1psi or 7.2% of 14 psi
• Water flow 2 m 3 /min; 8 minutes to fill water tank
• The maximum speed of water level rising is 19 cm/min.

External Fire And Explosion Scenario
• Light hydrocarbons
evaporated from the
tanks during hot
days and then
condensed inside
the valve at night.
• Alternatively,
measuring depth of
oily water could
contaminate the
external surface.

Properties Of Common Hydrocarbons

Internal Spark and Explosion Scenario
API Standard RP 2003 Protection Against Ignitions Arising Out of Static,
Lightning, and Stray Currents in paragraph A.7 Ignition by Static
Electricity states:
• For an electrostatic charge to be a source of ignition, the following
four conditions must be met:
• a) a means of generating an electrostatic charge must be present;
• b) a means of accumulating an electrostatic charge capable of
producing an incendive spark;
• c) a means of discharging the accumulated electrostatic charge in
the form of an incendive spark (that is, a spark gap) must be
present;
• d) an ignitable vapor-air mixture must be present in the spark
gap.

Internal Spark and Explosion Scenario
• The electrostatic charge could be generated during flow of the
water-oil mixture through pipes from the treater to the water
tank.
• The electrostatic charge could be accumulated on the oil
surface, which is non-conductive.
• All the pipes connected to the water tank were grounded.
• The pipe openings to inside the tank contained plastic outlets.
• The manway lid was non-metallic.
• Bolts could not store energy necessary for ignition.
• The length of the grounding cable was adequate, so it did not
hang over the water surface.
• No foreign conductive objects inside the tank.
• There were no obvious spark gaps inside the water tank.

Static Electricity Spark Promoters
• Non-conductive liquid
• Metallic/conductive objects
• Absence of grounding

Splash Filling
NFPA 77 (Recommended Practice on Static Electricity)
• Flow velocity of incoming liquid should be less than 1 m/sec until the outlet
is submerged either two pipe diameters or 0.6 m, whichever is less.
• For tank capacity of more than 50 m 3 , the inlet flow velocity could be
increased to 7 m/sec after the fill pipe is submerged.

Fill Line Is Above Liquid Level
• NFPA max. velocity for the non-submerged fill line V max = 1m/s
• Inlet flow velocity V flow = 7.2 m/sec or ~ 7 times higher

Bottom Of Failed Tank

Failure Scenario
• Flammable gas or gas-air mixture were present in water and oil tanks.
• When the truck dumped the water, this mixture was sucked into the water
tank through the overflow lines and through vent lines.
• Air was sucked inside the water tank through thief hatches.
• Thus the ignitable gas-air mixture was created inside the water tank.
• A layer of oil was present on liquid surface in the water tank.
• The well was still producing and the treater was in unusual and volatile
conditions.
• An automatic discharge releases hydrocarbons from the treater through
water line.
• Fluid containing hydrocarbons was splashed over the layer of oil in water
tank with high velocity.
• This resulted in a static electricity spark and gas ignition.
• Ignition resulted in increase of gas pressure and temperature, and the lid
blew off.

Proposed Modification

RECOMMENDATIONS
• Minimize the chance of sparking due to static
electricity. Install a non-conductive riser on the end
of the drain outlet inside the water tank.
• Eliminate or significantly reduce the oxygen level
inside the tanks. Use fuel or sales gas to blanket the
contents of both the water tank and the oil tanks.
• Upgrade the automatic discharge system to a more
reliable system.

QUESTIONS

CONTACT INFORMATION
Alex Tatarov, Ph.D., P. Eng
403-510-4394
atatarov@skystone.ca
Frank Gareau, P. Eng.
403-516-4234
fgareau@skystone.ca