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1
2012 FAI Annual Seminar
The Safety and Integrity of
Ammonia Storage Tanks
Venkat Pattabathula
Global Ammonia Technology Manager
Incitec Pivot Ltd
Brisbane, Australia
Incitec Pivot
• Largest fertiliser manufacturing and distributing company in Australia
• Has largest explosive grade AN manufacturing base in North America, operates as Dyno Nobel
• One of the key values of the company
2
Presentation Outline
1. Introduction
2. Types of ammonia tanks
3. Tank design and safety aspects
4. Tank commissioning & decommissioning
5. Risk Based Inspection
6. Incidents
7. Summary
History of Ammonia
• Ammonia being produced for the last 100 years, first commercial production by BASF in Germany
in 1913.
• Currently about 165 million tonnes of ammonia
produced per annum worldwide.
• Storage of ammonia since industrial production,
initially in high pressure bullets and Horton Spheres.
• Low pressure (atmospheric) ammonia storage at
-33DegC widely accepted, safer and cost effective and storage capacity up to 50,000 tonnes.
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Types of Ammonia Tanks – Single Wall Tank
Insulation
Low Temp.
Carbon Steel
Piles
Double wall tank (double containment)
Insulation
Inner Tank
Piles
Outer Tank
4
Double wall double integrity (full containment) with insulation
in annular space
Insulation
Inner Tank
Piles
Outer Tank
Double wall double integrity (full containment) with insulation
on outer tank
Insulation
Inner Tank
Piles
Outer Tank
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Tank Design Features
- Tank shall be
• double wall double integrity (DWDI) with insulation on outer
tank
• designed for internal pressure of 14 kPag.
• designed, fabricated, erected and tested per API-620
Appendix R.
• elevated on piled concrete slab foundation of about 2
meters.
• hydrotested of both inner & outer tanks.
• designed for cyclonic wind and earthquake conditions per
country standards.
• provided with drain lines in both inner & outer tanks.
Tank Materials
• Both inner and outer tanks shall be of welded construction.
• Low temperature certified carbon-manganese
steel, impact tested at or below -40DegC.
• Welding and Charpy V-notch testing per QA
standards.
• Load bearing underneath the tanks with treated wood such as lignostone or equivalent.
• All nozzles/manway welds in the lower strakes
shall be post weld heat treated- stress relieved.
• No hard stamping of materials.
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Tank Relief
• A minimum of two pressure relief valves and two
vacuum breakers.
• Only one relief or vacuum breaker be taken off-line
during operation.
• Relief valves and safety devises per API-2000.
• A permanent N2 connection to maintain tank
pressure as one of the layers of protection in low-
low pressure scenario.
• Emergency shut-off valve in liquid supply line to
activate on high-high pressure.
Safety Aspects
• Remote shut-off valves in liquid ammonia main inlet and
outlet line to/from ammonia storage tank.
• The refrigeration system shall be based on recognized and
proven industrial compressors.
• There must be an auto compressor loading/unloading facility
for tank pressure control.
• Stand-by equipment for critical duties and utilities.
• The design shall take into account a closed vent and drain
system for ammonia.
• Redundancy in critical instrumentation & control
• Thermal relief valves in the ammonia lines where there is a
possibility of blockage or heat ingress.
• Fugitive ammonia emissions shall be minimised.
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Safety Aspects-cont’d
• A flare for controlled venting under extreme emergency
situations
• Any venting of ammonia to the flare shall be avoided or
minimised.
• Ammonia leak detection system
• Lightning protection and earthing protection
• Emergency power to one of tank boil-off refrigeration
compressor
• Wind direction indicator
• Emergency plant lighting
Electrical & Instrumentation
• Fitted with earthing bosses, no copper conductor stripes in direct contact with ammonia.
• Three independent level and pressure indications.
• High level shut-off valve to close the supply to the
tank.
• Adequate lighting in the storage area.
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Piping and Insulation
• All flanges shall be minimum of 150# rating.
• Bottom liquid nozzle connections shall be of 300#
rating.
• No screwed connections
• Insulation to be designed for Max +50C and Min
-40DegC.
• External insulation to be covered with a
continuous flat aluminium vapour barrier.
• No water ingress on the tank shell and base to
prevent ice formation.
• Spiral type stairway to the tank top.
Tank Non-Destructive Testing (NDT)
• Install waveguides to carry out acoustic emission (AE) testing.
• Conduct initial AE tests during hydrotest of the
tank to identify any construction defects.
• A second AE test during first filling of liquid
ammonia after cooling down operation.
• Hydrotest at maximum operating level of tank.
• Inspection Test Plan (ITP) to include witness
points during all stages of construction.
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Tank Commissioning
• Purge with nitrogen until the measured oxygen in
the discharge gas is less than 4%.
• Purge with ammonia vapour until the oxygen in
the tank is <0.5%.
• Cool the tank down to as low as possible with
injection of liquid ammonia at a cooling rate lower
than 2C/hour, preferably using a spray system.
• Measure the temperature in the tank away from the gas inlet.
• Take samples from the ammonia liquid in the tank and analyse them for water and oxygen.
Tank Decommissioning
• Empty the tank to the absolute minimum liquid level.
• Evaporate the remaining ammonia in a way that ensures
uniform and slow heating, not exceeding 2C/hour.
• Purge with warm ammonia or nitrogen until all liquid
ammonia is removed.
• Remove the ammonia gas in the tank by purging with
nitrogen and not with air, to eliminate the formation of an
explosive mixture. To prevent environmental issues, flare all
the ammonia vapour containing streams.
• Remove the nitrogen atmosphere by purging with air until
the oxygen content is >19%. If ammonia is still measured in
the gas phase due to residual oil, breathing equipment must
be used when entering the tank. Residual oil remnants may
require additional clean methods and additional personnel
safety requirements and equipment.
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Risk Based Inspection (RBI)
• Primary inspection method for the tank shall be
the acoustical emission [AE] test method. As a
global inspection technique it is capable of
detecting and locating defects in the area being
monitored.
• AE testing only detects active cracking conditions.
AE signals which indicate the potential for
cracking will require follow up examinations
utilizing UT for confirmation and sizing.
• Additionally inspection must include routine in-
service observation, periodic in-service inspection, and periodic inspection according to API 653.
Risk Based Inspection (RBI)- cont’d
• RBI assessments should be conducted by trained and qualified individuals knowledgeable in RBI
methodology and experienced in tank foundation design, construction, , properties of materials,
corrosion, stress-corrosion cracking, fatigue
cracking, fracture mechanics, and techniques for
storage tank inspection. The RBI assessment is performed following the procedures in API
580/581.
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Stress corrosion cracking (SCC)
• SCC is promoted by the presence of oxygen in combination
with ammonia.
• SCC is minimized when the following actions are taken.
- Special efforts are taken to quickly reduce the oxygen content of the tank when re-commissioning the tank following opening or ventilation.
- Tank commissioning is carried out under controlled conditions
such that the tank cooling rate does not exceed 2 degrees (Celsius) per hour.
- Stress relief or stress reduction techniques are used when making repairs to the tank.
- Use of lower strength weld materials that are compatible with the materials being welded.
- Water [0.2%] is intentionally added to the product in storage.
Inspection Frequency Risk Matrix
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Inspection Criteria
• Routine In-Service Observation
• Periodic In-Service Inspection
• Internal Inspection – Visual
• Internal Inspection – WFMT
• Vacuum Box Inspection
• Pressure- Relieving Devices
• Evaluation per API-579-1/ASME FFS-1
• Repairs to be approved by API-653 certified
inspector.
Ammonia Storage Safety Incidents
“Those who don't know history are destined to
repeat it.” Edmund Burke
• 23 incidents reported in the last 57 years of AIChE Ammonia
Symposium.
• All were given in Table 1 of the paper.
• Many incidents were on stress corrosion cracking (SCC).
• One of the SCC case was at BASF old plant in
Middlesborough, England
• Tank was inspected in 1989, found SCC. Repaired the
defects and tank was recommissioned.
• Tank integrity monitoring system was put in place.
13
Rupture of ammonia tank
• In 1990 Symposium, Bengt Orval Andersson reported on Lithuanian fertiliser plant accident happened in March 1989, destroyed a 10,000 tonne-ammonia storage tank.
• Whole ammonia tank slid from its foundation, smashed with great force through a surrounding wall of reinforced concrete, finally landed 40m from the foundation.
• Enormous devastation around the tank, 70cm deep liquid ammonia.
Large quantities of ammonia evaporated, vapour caught fire and whole plant engulfed in flames.
• About 32,000 people evacuated from a nearby town, rescue operation continued for three days. Seven people died, 57 were injured.
• Ammonia tank over-pressured when its contents rolled over.
• Warm ammonia supplied to tank bottom, caused the rollover while tank refrigeration compressors out of service.
Failure of inner shell of a double wall ammonia storage tank
• In 1999, a 5,000 tonne ammonia storage tank
decommissioned at Coromandel Fertilisers Ltd in India.
• Noticed ammonia liquid level in annulus area of the double
wall double integrity tank, ammonia collected from splashing
during ship unloading at high levels.
• Level transmitter indication wrong, inner cup failed when
draining from a hydrostatic head of ammonia in annulus.
• Bottom plates fractured and circumferential welds failed.
Tank repaired, bottom plates renewed.
• Many improvements made on tank instrumentation: new TI
on annulus with a low temp alarm; interlock to trip ammonia
pumps if annulus level reaches 400mm level; separate level
indications for inner tank and annulus and up-to-date
documentation on ammonia tank.
14
Ammonia Storage Fatality
• In 2005, NH3 tank emptied out for a repair, at Yara facility in
Rostock, Germany.
• One person killed while recommissioning the tank and
another injured.
• 100 tonnes of ammonia released.
• Tank rupture from high pressure
• Root Cause:
- Thin layer of oil prevented mixing of anhydrous ammonia
with aqueous ammonia which was used to cover tank
bottom. When a valve on tank outlet opened, caused violent
reaction and raised tank pressure causing rupture.
Ammonia Tank Rupture
15
Summary
• Different types of ammonia tanks were explained.
• Ammonia storage tank design and safety aspects were
highlighted.
• Guidelines for decommissioning and commissioning
ammonia storage tanks were provided.
• A detailed review of Risk Based Inspection (RBI) was given.
• Twenty three incidents from the last 50+ years of the AIChE
symposium were listed including the root causes and
mitigations put in place (refer Table 1).
Acknowledgement
• Incitec Pivot Management
• My co-authors – Raghu Nayak and Don Timbres.
• Walter Benson, Carl Jaske and Brian Shannon for the review of the paper.
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