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ERA Technology Limited
Technical Consultancy Services Forensic Engineering and Expert Witness
Claire Malpas Rotating Equipment Consultant [email protected]
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Agenda
• About ERA
• Introduction to Failure Investigations
• Case Study 1 – Reciprocating Compressor
• Case Study 2 – Gas Turbine
• How to make an investigation easier
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About ERA Technology
• Responsive, independent engineering consultancy
• Engaged by many of top 100 international organisations
• Impressive engineering knowledge drawn from origins in research and development
• World-class engineers and specialists
• Offices in UK, Europe and UAE – global network through sister companies
• Part of the
• Global providers of technical, quality and safety solutions to the Energy industries
www.era.co.uk
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Specialist asset experience
4
• Turbo-machinery – gas and steam turbines
• Engines
• Pumps
• Compressors
• Motors
We have particular knowledge of the following assets:
Rotating
Electrical
• Transformers
• Cables
• Switchgear
• Protection devices
Static
• Fired Heaters & Steam Reformers
• Coke Drums,
• Platformer reactors vessels & pipework
• Heat Exchangers, pressure equipment & boilers
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Through-life services
System Specification
Procurement
Construction
Commissioning
Detailed Design
Conceptual Design
Through-Life Management
Change of Use/Life Extension/End of
Life
Decommissioning
FEED, EPC Contractors
Construction Companies
Owner Operators
Specialist Contractors
• Safety Engineering
• Power Systems Design
• EMC
• Engineering Design Improvements
• 3rd Party Verification
• Power Systems Protection
• Remaining Life Assessments
• Fitness for Service
• Forensic Engineering and Root Cause Analysis
Engineering Subcontractors
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Why carry out a failure investigation?
• Insurers & Loss Adjusters:
• Understanding the root causes
• Separating out damage by event
• Identifying the limits of attributable damage
• Assisting during re-instatement
• Operators & Owners:
• Understanding the root causes
• Identifying corrective actions to avoid recurrence
• Assisting during re-instatement
• E.g. Liaising with suppliers, evaluating feasibility
• Confirming effectiveness of corrective actions
• Improve safety
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First steps in a failure investigation
• Collect the initial information
• Examine the damage
• As soon as possible after the event
• In-situ if possible
• Witness dismantling if possible
• Obtain evidence
• Damaged components in their as-found condition
• Interview witnesses
• Data and records
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Typical investigation techniques
• Physical examination of components
• Simulation e.g. FEA, calculations, metallography
• Brainstorming sessions
• Cause-effect mapping (Apollo approach)
• Elimination of unlikely causes
• Building hypothesis
• Testing hypothesis
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Case Study – Fool me once…
• Upstream Oil and Gas field in Africa
• Commissioning of a 2 stage reciprocating booster compressor system
• Catastrophic failure – hydrocarbon leak
• Following rebuild the unit failed again on start-up
• Parties Involved
• Owner/operator
• Package provider
• Compressor OEM
• Third party independent – ERA Technology
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Case Study
• Reciprocating compressors
• Process diagram/conditions
• Initial information from the failures
• Findings
• Recommendations
• Outcomes
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Reciprocating Compressors
• Positive displacement
• Fixed volume
• Does not like incompressible fluid
• Can be run by diesel engines, electric motors, or turbines
• Used where high pressure ratios are required, e.g. for export of dry natural gas or instrument air etc.
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Process Diagram
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Ultra low pressure gas in
Low pressure gas in
To main compressor
Aftercooler Compressor
Scrubber For liquid removal
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Process Conditions
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ULP gas in
LP gas in
0.3 bar
4 bar
12 bar To main compressor
10 MMSCFD
20 MMSCFD
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• During a commissioning test run the system tripped on hydrocarbon release in the area of the compressor.
• On inspection the cylinder head had come apart from the body.
• The unit was stripped and lots of damage found.
• Vibration monitoring had been faulty so did not trip the unit at the first sign of damage.
• The owners and contractors concluded it had been built incorrectly.
• Rebuilt the unit with a high level of precision and supervision.
1st failure
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2nd Failure
• During a test run the system tripped, on high vibration on the compressor cylinder no 5.
• The cylinder was stripped and damage was found to the piston.
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Areas of investigation
• Manufacture of the piston
• Assembly of the compressor
• Design of the package
• Operating conditions
• Start up procedure
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Piston plug manufacture
• Threads of the remaining plugs were sectioned and examined
• Identified a varied quality of plug engagement
• Force would still be required to remove the plug from the piston
• Identified the direction of force was inward to the cylinder
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Assembly of compressors
• The owners/operators suspected the assembly of the unit as the cause of the first failure
• Rebuild was conducted under strict supervision
• All stages of build signed off by qualified personnel
• No indications of poor build issues following 2nd failure
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Design of the package
• Package design from a reputable company
• All equipment conforms to industry standards
• One discrepancy from owners standards
• Layout appears similar to comparable packages
• However on discussion with the package provider there was some confusion over the specification relating to the operating conditions
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Operating conditions
• Specification called for 0 – 20MMscfd
• Originally designed for exactly 20MMscfd
• Recycle line added to the package design as an afterthought
• Actual flow ~5MMscfd through stage 1 and 10MMscfd through stage 2
• Recycle always used
• Leads to lower temperatures, higher liquid knock out – extra work for the scrubber
• Liquid carry over as a cause?
• Liquid in the compressor could cause the high pressure required to knock in the plug
• However, although it wasn’t designed for these conditions, the scrubber is able to cope with the calculated amount of additional liquid knock out.
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Process Conditions - design
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ULP gas in 0.3barg
LP gas in 4barg
0.3 bar
4 bar
12 bar To main compressor
10 MMSCFD
20 MMSCFD
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Process Conditions – during start up
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ULP gas in 0.3barg
LP gas in 4barg
4 bar
12 bar
36 bar To main compressor
10 MMSCFD
20 MMSCFD
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Outlet of 1st stage
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LP gas in
2nd stage recycle
1st stage recycle
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Knock on effects
• Is the scrubber able to cope?
• No, not with these conditions
• Liquid will build up in the scrubber and carry over to the cylinder
• Incompressible liquid will cause very high forces on the face of the piston
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Root cause of the failure
• High pressures at start up combined with recycling the gas at low temperatures increases the liquid to be separated in the scrubber.
• Scrubber cannot handle this volume of liquid so some carries over to the cylinder.
• The liquid carry over is likely to be the cause of the force on the plug causing the damage and release of the piston plugs.
• Poor plug thread engagement is a contributing factor.
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Recommendations
• Improved QA on the threads of the pistons and plugs would reduce the chance of a piston with a weak engagement from entering service.
• Reconfigure the cylinder from double acting to single acting reducing the volume flow from 20MMscfd to 10MMscfd.
• Review start up procedure to stop high pressures building in the system which is currently causing high liquid drop out in the second stage at start up.
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Outcomes
• Liability shift from owner/build contractor to OEM and package designer
• Financial settlement for the owner
• Rebuild according to the recommended changes
• Successful commissioning and operation
• Flexibility to reinstate the double acting compressor if flow rate increases
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Learning points
• Worthwhile carrying out a technical RCA
• Cause of failure different to first thought
• Could have avoided the second failure
• Justification for attributing liability
• Improvements to quality procedures reduce the risk of
reoccurrence
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Case study – Where’s the damage?
• Gas turbine is operated in a Mediterranean environment
• Used for power generation
• Condition monitoring contract with the OEM
• Surge protection tripped machine following control card failure
• Operator found that performance changed
• Waterwashing did not help
• Operator suspected mechanical damage
• Operator committed to investigation and repair costs, then called insurer
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Case Study
• Gas turbine power loss
• Causes of power loss
• Areas for investigation
• Physical damage
• Control system
• Environment
• Findings
• Learning points
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Power output
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40
41
42
43
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01-Jun 09-Sep 18-Dec 28-Mar 06-Jul 14-Oct 22-Jan 02-May 10-Aug
Po
wer (
MW
)
Date
Stall Event Lease unit
Expected loss within 8,000 hours up to 2%
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No sign of physical damage
• Unit was stripped at the OEM facility
• No evidence of significant damage to cause a power loss
• So where is the power loss coming from?
• Data from the turbine was provided, but very limited
• One data point per day
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What can affect power output?
• Common causes:
• Outside temperature, pressure, humidity
• Dirty filters
• Dirt build up on the compressor blades
• Increased clearances (wear and tear or damage)
• Other causes:
• Change of fuel calorific value
• Fogging / inlet cooling
• Steam/water injection
• Gas turbine tuning
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Maintenance reports do not suggest this
Waterwashed regularly, no sign of this at strip
Nothing significant found at strip
No fuel change
No change
No change
No control system changes
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An ISO day
• Temperatures onsite range from ~ 10˚C to 35˚C in the year
• Power output in previous year 35 – 45 MW,
• Need to correct for ambient conditions to make a comparison
• ISO standard day:
• 101.325 kPa
• 15˚C
• 60% humidity
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Corrected Turbine Performance
Linear performance
No signs of significant loss
Speed limited in later months
What is limiting the power?
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Gas Turbine limits
• Speed control
• Temperature control
• Turbine normally limiting on temperature
• Detailed data not available for analysis
• Only average temperature and spread
• Condition monitoring reports reviewed
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Spread data
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30
40
50
60
70
80
90
100
01-Jun 09-Sep 18-Dec 28-Mar 06-Jul 14-Oct 22-Jan 02-May 10-Aug
Sp
rea
d (
de
gC
)
Date
T48 Spread
Before Stall
After Stall
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Summary of findings
• No evidence of physical damage to the gas path
• Power reduction rather than power loss
• Speed limited by maximum limit on average exhaust temperature
• Probable failure of one probe
• Power reduction unrelated to stall event
• Cause of problem found in the condition monitoring report
• OEM did not highlight issue
• Repair work carried out was unnecessary
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Lessons learned
• Good understanding of gas turbine operation beneficial
• OEM reports should be read and not just filed
• Rushing to initiate repair work can lead to unnecessary work/cost
• Recording data can help to solve future investigations
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How to make an investigation easier
• Leave scene untouched as long as possible
• Retain or quarantine all components for analysis
• Record data for high value assets such as gas turbines
• Involve all parties as soon as possible
• Conduct witness interviews early
• Open and honest communication
• Have all parties attend the investigation together
• Do not rush to conclusions!
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