Ventilation and Leak Dispersion in CCGT Enclosures

1. Ventilation and Leak Dispersion in CCGT Enclosures Patrick Phelps( Flowsolve ) and Douglas Wylie(GEC Energy Services) IPUC 7 - Luxembourg - May 2000

2. Ventilation and Leak Dispersion in CCGT Enclosures • Industrial Context • Health and Safety Issues • Application to an Existing Power Station • Application to New Enclosure Designs • Conclusions • Experimental Verification

3. Industrial Context - 1 Combined Cycle Gas Turbine (CCGT) Plants • Gas turbines drive an electricity generator • Engine exhaust waste heat recovered by a boiler to produce steam. • Steam turbine generates further output.

4. Industrial Context - 2 • Turbines are fuelled by gas at very high pressures • Liquid fuel system as back-up • Turbines are contained within acoustic enclosures. • Enclosures tend to be installation-specific designs

5. “ Frame 6” Turbine Generator

6. Industrial Context - 3 Each enclosure is divided into a number of compartments containing • Auxiliary equipment • Gas turbine and exhaust plenum • Reduction gearing and the generation equipment. Auxiliary/GT/exhaust compartment is usually self- contained, with a dedicated ventilation system

7. Ventilation and Leak Dispersion in CCGT Enclosures

8. Health and Safety Issues - 1 • Enclosure ventilation system removes some heat from the turbine casing • Enables operatives to carry out readings and routine maintenance under operating conditions. • However, enclosures remain a “thermally hostile and noisome” environment .

9. Health and Safety Issues - 2 • Gas from leakages can accumulate to flammable proportions in poorly ventilated regions of the enclosures - “dead zones” • OUTCOME - Big Bang • REMEDY - Use the ventilating air to safely dilute and disperse any gas leakage. • MOTIVATION - Legislation

10. UK Safety Assessment of CCGT Enclosures - 1 • Identify nature and potential sources of hazardous material releases • Determine leak frequency and inventory of releases • Investigate airflow characteristics • identify "dead zones" • Predict dispersion consequences safety-critical release scenarios • Implement remedial measures

11. UK Safety Assessment of CCGT Enclosures - 2 • Experimental investigation of airflow characteristics is difficult within the confines of a turbine generator enclosure, especially under operational conditions. • HSE promote computer simulation as the most appropriate technology in this case.

12. UK Safety Assessment of CCGT Enclosures - 3 CFD models can • simulate the dispersion consequences of releases under a variety of operating conditions • compare the efficiency of alternative ventilation strategies, to achieve the desired dilution / dispersion result

13. “Safe” Dispersion Criteria - 1 “Santon” Criterion “The ventilation arrangements within the turbine enclosure must be such as to ensure the safe dilution/dispersion of gas releases prior to activation of mitigation/shutdown systems by the gas detection system. The criteria to be applied are that the envelope of the 50% LEL concentration contour should not occupy more than 0.1% of the free volume of the enclosure, for a gas leak of sufficient magnitude to trigger the gas detection system.”

14. “Safe” Dispersion Criteria - 2 G = V * (0.01 * E) * (0.01 * S). • Gas concentration level (S) detected by sensors for activating emergency response systems (typically 10%) • Lower Explosive Limit (E) for the turbine fuel gas is around 5%. • Maximum undetected leak is thus of magnitude 10% of LEL • For compliance, ensuing flammable envelope (of the 50% LEL surface) must not exceed 0.1% of “the compartment free volume”.

15. “Safe” Dispersion Criteria - 3“Compartment free volume”

16. Reference Leak Scenario Zero momentum leak source • corresponds to jet release impinging immediately on an obstruction (casing, flange body} • No net directionality imparted to release. • A directional release would require additional assumptions…..

17. Application to an Existing Power Station - 1 A CFD-based simulation study commissioned by IVO Generation Systems and Regional Power Generators Ltd

18. Application to an Existing Power Station - 2

19. Application to an Existing Power Station - 3 Over 100 simulations performed Studies to determine • air flow distribution • worst case operating condition (hot,cold); • worst case leak location; • efficiency of alternative “retrofit” ventilation strategies, to achieve HSE compliance

20. Air flow Distribution at inlet toTurbine Compartment

21. Application to an Existing Power Station

22. Application to an Existing Power Station - Parameter Studies Over 100 simulations performed Studies to determine • air flow distribution • worst case operating condition (hot,cold); • worst case leak location; • efficiency of alternative “retrofit” ventilation strategies, to achieve HSE compliance

23. Hot Operating Conditions:Envelope volume - 0.69%

24. Cold Operating Conditions:Envelope volume - 2.28%

25. Application to an Existing Power Station - Parameter Studies Over 100 simulations performed Studies to determine • air flow distribution • worst case operating condition (hot,cold); • worst case leak location; • efficiency of alternative “retrofit” ventilation strategies, to achieve HSE compliance

26. “Worst Case” Leak Location • Under both hot and cold conditions, the worst case leak location was found to be in the “pit” region, in front of the lowest combustor flanges

27. Application to an Existing Power Station - Parameter Studies Over 100 simulations performed Studies to determine • air flow distribution • worst case operating condition (hot,cold); • worst case leak location; • efficiency of alternative “retrofit” ventilation strategies, to achieve HSE compliance

28. Alternative Ventilation Strategies :1 - Abject failures • Increasing ventilation rate • overhead pendant baffles • twin outlets • blowing air into the pit region • sucking air from the pit region • EGT “wavewall” idea

29. Alternative Ventilation Strategies :2 - Heroic failures • Reversed flow system • air supply through existing outlet • air extract to TG inlet plenum • Lateral side-gust system • air supply through side door • air extract through existing outlet • other inlets blocked off

30. Alternative Ventilation Strategies :3 - Final Success ! The “Corkscrew” Strategy • Close all existing inlets • plate over grated walkway tops • Single non-symmetric outlet • Two inlet slots , one high, one low, cut in connecting doors • 30-degree deflector plates create corkscrew effect

31. “Corkscrew” Ventilation Scheme

32. “Corkscrew” Ventilation Scheme

33. “Corkscrew” Ventilation Scheme

34. “Corkscrew” Ventilation Scheme

35. And so . . . . . . . . . . This led on to . . . . . . . .

36. Application to New Enclosure Designs A CFD-based simulation study commissioned by the Thermal Power Division of Kvaerner Energy Ltd

37. Application to New Enclosure Designs

38. Enclosure Geometry - Elevation

39. Enclosure Geometry - End View

40. Application to New Enclosure Designs

41. Turbine combustor flanges and associated pipework

42. Geometry Representation - 1

43. Geometry Representation - 2

44. Application to New Enclosure Designs - Workscope Over 25 different simulations performed Studies to determine sensitivity to: • nodalisation level & distribution; • leak location; • ventilating flowrate; • presence of internal geometric features; • inlet flow manipulation.

45. Findings - 1 • The worst case leakage scenario, under cold start-up conditions, was a zero-momentum leakage from the flanges in front of the lowest can combustor • The flammable gas cloud with the “reference” ventilation arrangement was twenty five times larger than the target value (11 times larger if the accessory compartment volume was included)

46. Reference Configuration{Flammable volume: 2.8% TC}

47. Reference Configuration{Flammable volume: 2.8% TC}

48. Reference Configuration{Flammable volume: 2.8% TC}

49. Reference Configuration{Flammable volume: 2.8% TC}

50. Reference Configuration{Flammable volume: 2.8% TC}