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

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

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

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

Frame 6 Turbine Generator Frame 6 Turbine Generator

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

Ventilation and Leak Dispersion in CCGT Enclosures

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

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

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

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

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

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.

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

Safe Dispersion Criteria - 3 Compartment free volume

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

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

Application to an Existing Power Station - 2

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

Air flow Distribution at inlet to Turbine Compartment

Application to an Existing Power Station

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

Hot Operating Conditions: Envelope volume %

Cold Operating Conditions: Envelope volume %

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

Worst Case Leak Location b 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

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

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

Alternative Ventilation Strategies : 2 - Heroic failures b Reversed flow system air supply through existing outletair supply through existing outlet air extract to TG inlet plenumair extract to TG inlet plenum b Lateral side-gust system air supply through side doorair supply through side door air extract through existing outletair extract through existing outlet other inlets blocked offother inlets blocked off

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

Corkscrew Ventilation Scheme

And so This led on to

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

Application to New Enclosure Designs

Enclosure Geometry - Elevation

Enclosure Geometry - End View

Application to New Enclosure Designs

Turbine combustor flanges and associated pipework

Geometry Representation - 1

Geometry Representation - 2

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

Findings - 1 b 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 b 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)

Reference Configuration {Flammable volume: 2.8% TC}

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

Findings - 2 b Increasing the ventilating air flow by 25% had little effect on ventilation efficiency ( gas leak size increased proportionately ).

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

Atomising Air System {Flammable volume: 1.9% TC}

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

Findings - 3 : Inlet Flow Manipulation Improved results (decrease in flammable cloud size) obtained by : b adding a scoop diverter at inlet ; b throttling - increasing the inlet velocity ; b concentrating the incoming flow towards the axial centreline; b blocking off the outermost inlet hole(s) on each side; and b biasing the massflow distribution.

Final Configuration {Flammable Volume: 0.2% TC}

Conclusions b Flammable cloud volumes of less than 0.1% of the free volume of the combined compartments are attainable (at least conceptually). b This would satisfy the current UK dispersion criterion. b Compliance achieved without major structural changes to enclosure or to air delivery system.

Future Efforts b Confirm high velocities do not provoke thermal shock problems in hot operation b Confirm required manipulation of inlet air supply is technically feasible (!) b Use model to predict likely over-pressures arising from deflagration of the confined gas plume.

Experimental Verification b No quantitative data available. b Qualitative comparison - video footage of smoke tests on installations with similar internal geometries. b Smoke trails confirm magnitude and direction of airflow in the region in front of the can flanges and around the front upper part of the turbine barrel

Ventilation and Leak Dispersion in CCGT Enclosures Thank you for your attention. When I count to three, When I count to three, you will awake and remember nothing …..

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