Selected recent global technical projects are summarized and presented in this section 

For more information on any of these projects or Tetra capabiltites contact your regional Tetra office here.

Root Cause Analysis of HP Drum Nozzle Cracking, 2018
FEA drumDuring the 2018 shutdown at a CCGT in Europe several of the HRSG HP drum riser nozzle and downcomer nozzle welds were checked and all were found to be cracked on the inside of the drum, at the weld toe or between weld beads, with the cracks following the fusion line back to the nozzle. The drum feedwater inlet and steam outlet nozzles (above drum waterline) were both undamaged. The drums were removed and all welds were repaired in the workshops.  Tetra performed Finite Element Analysis (FEA) and operational data analysis and could conclude that the operational stresses were high enough given the current cycling regime to cause the cracking observed. The quality and design of the original welds is also likely to have contributed to the cracking.

Client region: Europe

 CFD Analysis of GRP Piping Caustic Dosing, 2018 
CFD causticCaustic soda is injected into the product water in order to raise the pH as required by the specification. Inspection of GRP piping has often shown large amounts of deposition inside these pipes. Deposition is mostly observed downstream of the injection point of sodium hydroxide and can therefore be attributed to unsuitable chemical injection. Typical injection setups utilise four simple (no nozzle) injection ports at 90° from one another causing ineffective mixing of the flows, leading to the observed precipitation. Tetra performed a CFD analysis of the caustic injection process to investigate the degree of mixing of the two flows and simulate the effects of modifying the volumetric flow rate. 

Client region: Middle East. 

 Diverter Damper Design & Operation Review, 2018 
shaft cracksTetra was engaged by a client to investigate the recently observed cracking on the diverter damper actuator. The design documentation and failure mode was reviewed. From this initial review it could be concluded that the shaft cracking is most likely a result of the operating procedure, where  the damper is kept in an open position for an extended period of time. The exposure to hot flue gas in combination with the load acting on the toggle arms is the most likely source of the cracking

Client region: Middle East

 MSV Stellite Cracking Failure Analysis, 2018 
debondingStaff at a CCGT plant engaged Tetra to investigate the root cause of the cracking and damage appearing on the Main Steam Valve (MSV) stellite seat ring(s). The metallurgical analysis identified iron dilution from the F91 valve seat into the hard-facing alloy as the root cause of the debonding.  The extent of the iron dilution is a sign of excessive heat input, most likely caused during the welding process. This points towards the welding during fabrication as the actual root cause of the failure. The driving force for failure / debonding of the Stellite hardfacing could also include residual stresses from welding and/or transient heat transfer stresses and/or thermal shock during operation. 

Client region: Middle East

 Flow Accelerated Corrosion Risk Assessment, 2017 
 FAC1An assessment of the susceptibility of HRSG piping components was performed according to Tetra Engineering’s FACRisk™ methodology which includes the use of thermal modelling simulation software (PPSD). From each system, sub groups are ranked using both time to minimum wall thickness and wear rate. The highest risk components (such as elbows, tees and valves) are identified. Overall, due to relatively high operating pH level (>9.2), the overall risk of FAC failures at the CCGT plant in question was considered to be relatively low, with few specific areas showing increased risk. Three separate load cases were simulated and analysed to determine the potential FAC risk. The results of each simulation were incorporated into the final locations recommended for inspection. 

Client region: South East Asia

Boiler Tube Failure Root Cause Analysis, 2017 
temperatureAfter several tube failures in recent years in the 1st row of LTRH hanger tubes, Tetra was tasked with performing a Root Cause Analysis (RCA) to determine the underlying cause of the failures. A detailed review of operating data, failure reports and design information was carried out and potential causes investigated. The tube failure mechanism as determined by metallurgy was short-term overheating, with temperatures likely exceeding 700°C prior to failures. Two contributing causes were identified by elimination of all other possibilities: temporary loss of steam flow in affected leading row tubes coupled with high flue gas temperatures in certain tubes at the side of the boiler gas path.  Unfortunately, the underlying root cause of the loss of flow could not be confirmed, whereas the asymmetric flue gas temperature distribution is a known issue since commissioning.

Client region: Asia

 HP Evaporator Tube Failure Analysis, 2016 
 tensileoverloadDuring a recent condenser tube leak at a CCGT Power Station in the Middle East, two trips occurred on the boiler feedwater pumps leading to trips of one of the HRSGs.  During that time, three tubes failed on the HP Evaporator. Tetra Engineering performed an investigation into the failures with the aim of determining the failure mechanism, to estimate the root cause of failure, and to identify inspection priorities in case damage may have gone undetected. This work included metallurgical analysis and a review of key operating data. Failures were located directly under the risers at extreme ends of the HP Drum. The most probable source of tensile overload is tube quenching when flow restarted

Client region: Middle East

 Attemperator Sleeve Failure Study, 2016 
 sleeveIn 2015, it was discovered that the thermal sleeve in the attemperator in a client’s power station hot reheat line had failed, resulting in fracture around the circumferential weld in the sleeve’s mid section. For this reason, Tetra was engaged by the owner to investigate. The investigation showed failure from fatigue cracking. There were no signs of thermal fatigue damage, hence quenching from over-spraying was discounted as the root cause. The cracked area showed signs of high cyclic local strain, and given the moderate number of starts low-cycle fatigue was thought to be the root cause. Differential thermal expansion between the Grade 22 sleeve and the Grade 91 pipe, combined with lock-in of the sleeve on the pin locations, would raise sufficiently high stresses

Client region: Europe

 HP Bypass Finite Element Analysis, 2015 
bypassTetra was engaged by a client to perform a finite element analysis of the HP Bypass valve reducer, using measured displacements as input. Results from a strain gauge/displacement survey showed a large strain effect occurring during the start-up of the boiler. Initially, both the left and right side strain gauges on the reducer showed compressive strain. During the start something triggered the reducer to experience tensile strain that differed in its intensity between left and right. The purpose of the study was to try to simulate the observed strain fields and determine the root cause. 

Client region: Europe

 Site Condition Assessment, 2015 
site visit asiaA Tetra Senior Engineer visited a south east asian power plant site to make an assessment of the commissioning situation. The EPC engaged Tetra as they considered that their site work was being delayed due to lack of progress by the IPP to perform their scope of work.There was significant lack of communication between the two parties as well as discrepancies in the scope of work, which lead to schedule delays and cause of mutual blaming between parties. 

Client region: South East Asia

 High Energy Piping (HEP) Remaining Life Assessment (RLA), 2015 
jackheadTetra was engaged by a client to investgate the remaning life of the HEP on 3 units of a thermal power station. The plant intended to operate the units for another 100,000 hours. The scope of work entailed a piping stress analysis of the main steam and hot reheat lines, thermal stress analysis of the SH header outlet/main steam line and remaining life assessment (creep, fatigue, creep-fatigue) of critical areas based on inspection history and results from the pipe stress/finite element analysis. The figure shows time-dependent stresses and temperatures during start up. 

Client region: Europe

 Study of impact of cyclic loads on superheater tubes, 2015 
harp FEADuring the shutdown of the unit at night, the HP bypass was kept open to maintain a vacuum in the steam turbine condenser in order to guarantee a faster start the next morning. Therefore, a small quantity of high pressure steam from the HP drum was flowing through the SH on the way to the HP bypass valve. The small HP steam flow takes the path of least resistance and only goes through tubes on one side of the unit, affecting their temperature. The temperature of the tubes on the other side change differently and a temperature difference of around 80°C can be seen 2 hours after shutdown. The CCGT owner engaged Tetra Engineering to investigate the impact of these resultant cyclic loads on the fatigue life of the tube to header welds. The study concluded that the temperature difference of 80°C between different tubes does not cause high enough fatigue stresses alone to be considered critical

Client region: Europe

 Superheater Tube Root Cause Failure Analysis, 2015 
 SH metallurgyTetra Engineering completed a Tube Failure Analysis of failed SH tubes from a CCGT plant in South East Asia.

The results show the following damage mechanisms present

  • Short-term Creep and Fatigue damage (Creep-Fatigue interaction)
  • Short-term Creep and Stress Corrosion Cracking (SCC) or Hydrogen Embrittlement

The fact that short-term creep damage was observed on both of the failed tubes indicates a sudden change in temperature. It was possible that newly discovered damage to the turbine exhaust flow correction device could have allowed a larger mass flow to one side of the boiler (due to swirl influence), increasing the tube temperatures locally and/or increasing local stresses when expansion is prevented, thus accelerating creep damage. The plant had suffered from frequent fatigue failures, with several independent metallurgical analyses over the years confirming thermal fatigue as the previous root cause. No signs of any creep damage had previously been discovered.  

 Client region: South East Asia

 CFD Study of HRSG Gas Path, 2015 
 cfd hrsgTetra Engineering  performed a CFD analysis of the HRSG gas path at a CCGT in Asia in collaboration with CFD specialistR&R ConsultThe results showed a very uneven flow distribution at base load over the HPHTSH tube sheet, with a higher mass flow located towards the outlet header sections and towards the side walls. The swirling motion of the GT flow results in a different flow pattern for the low load case, compared to the base load case. The flow was more evenly distributed in the longitudinal direction at the SH, but a considerable variation is seen in the span-wise direction. The uneven flow distribution was thought to be a contributing factor to the repeated fatigue failures seen in the boiler over the last couple of years.  

Client region: South East Asia

 Field Assessment of Piping Water Hammer Event, 2014 
slugTetra Engineering was asked to assess the situation after a significant water slug/hammer event. Observations made during the site visit showed that the structural members were severely damaged with steel being heavily deformed and torn in some cases.  Piping supports were also severely damaged and would require repair.  

Client region: Middle East

 LP EVAP Tube Failure Analysis, 2014 
PPSD LP EVAPTetra Engineering was asked by a client to help to assess the cause of tube failures in the upper section of the LPEVAP on their HRSG units. The first tube failure was reported in 2010; other failures have followed since then at a rate of two to three per year. At the time of the first failure the plant had been operating for approximately three years. Tetra performed a root cause analysis which included a review of water chemistry logs, boiler simulations using PPSD as well as a FAC Risk Assessment. 

Client region: Middle East

 HRSG Boiler Preservation Study, 2014 
table preservationA CCGT power plant employed Tetra to perform a review of possible boiler preservation options within the context of the present and planned operating regime. The review relied on information supplied by the plant staff, Tetra field experience and latest industry trends and practices to identify the most promising strategy for preservation. The ideal solution for this client was Dry Layup that is systematically applied when it reaches the end of the standby period.  This would require that resulting startup timing meets requirements imposed by the governing network operator. 

Client region: Middle East

 Thermohydraulic Attemperator Study using PPS&D, 2014
PPSD attemperator The primary purpose of this study was to validate the effect of an interstage attemperator between two superheaters with respect to the primary superheater and primary reheater outlet steam temperatures. The CCGT in question had already had problems with creep failures of the P91 components, and needed to assure that exit temperatures did not exceed 570°C. Currently, the primary superheater outlet temperatures exceed 600°C at certain loads and the primary reheater temperatures exceed 590°C. Initial results were based on steady-state analysis, while final results included dynamic modeling of cold-, warm- and hot starts, as well as ramp up and ramp down. The study was performed using the software simulation tool PPS&D.

Client region: Europe

Stress Analysis Study of SH Section, 2014 
HPSH Assembly A CCGT Power Plant suffered repeated tube failures between 2003 and 2014 in the SH section of their boiler. Failures were exclusively located at the tube-header weld, at varying positions on the tube sheet. The client engaged Tetra Engineering to analyze the problem and simulate the existing stresses during various load conditions. The scope of work included a review of historical failures and metallurgical reports, operational data analysis, Finite Element Analysis (FEA) and Fatigue Analysis.

Client region: South East Asia

 HRSG Vibration Study, 2013 
ppsd vibrationA HRSG operated by a European client appeared to be subject to excessive vibration, based on high noise near the unit and vibration of the casing in certain sections of the roof.  The OEM and EPC had initially agreed that the vibration was a potential problem, so for this reason a study was commissioned by them. This included a field survey of noise using microphones, confirming that there were high noise levels in three different frequencies at some points outside the HRSG casing. Subsequently the OEM/EPC modified their position to hold that the observed noise was not a problem and that no further action was required. The client then requested that Tetra Engineering independently review the problem and perform additional analysis as required. The scope of work included field measurements, thermohydraulic boiler simulations and vibration risk calculations.

Client region: Europe

 CFD Analysis of failed 180° Bend, 2013 
cfd bendA CFD study on one of the 4th quadrant pipe bends at a thermal power plant steam boiler was performed. The purpose of the analysis was to analyse the change in flow velocity due to an increase in the bend nominal size from 3” to 4”. The connecting pipe run was kept at 3” and standard expander/reducers are used to join the pipe with the bend. Input process parameters were taken from a PPSD model.

Client region: Middle East

LP Economiser Header Failure Analysis, 2010
 rupture headerThe LP economiser lower header on a HRSG ruptured shortly after a diverter damper incident. Tetra was approached by the client to investigate the possible root causes for the rupture. The unit went into single element control at the time of the incident and the LCV cycled through fully open / fully closed positions to maintain the drum level after “shrink”. When the damper re-opened the LCV remained closed as the drum level increased (swell). The economizer temperature increased and there was an associated increase in pressure. The safety valve failed to operate as it was accidentally gagged, causing serious over pressurisation and a burst header.

Client region: Middle East

 

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Established in 1988, Tetra Engineering has more than 25 years experience providing solutions to the power industry. We specialize in solutions for HRSGs, conventional boilers & steam-cycle balance of plant.

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