flowsolve phelps gas leaks

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Ventilation and Leak Dispersion

in CCGT Enclosures

Patrick Phelps ( Flowsolve )

andDouglas Wylie (GEC Energy Services)

I PUC 7 - Luxembourg - May 2000




in CCGT Enclosures

Industrial Context

Health and Safety Issues

Application to an Existing Power Station

Application to New Enclosure Designs

Conclusions Experimental Verification



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.




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



“ Frame 6” Turbine Generator




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




in CCGT Enclosures



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 .



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



UK Safety Assessment of

CCGT Enclosures - 1

Identify nature and potential sources of

hazardous material releases

Determine leak frequency and inventory ofreleases

Investigate airflow characteristics

identify "dead zones" Predict dispersion consequences safety-critical

release scenarios

Implement remedial measures




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 themost appropriate technology in this case.




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 thedesired dilution / dispersion result



“Safe” Dispersion Criteria - 1

“Santon” Criterion “The ventilation arrangements within the turbine

enclosure must be such as to ensure the safedilution/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% LELconcentration 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).

Gas concentration level (S) detected by sensors foractivating 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 compartmentfree volume”.



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



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 …..



Application to an Existing Power

Station - 1

A CFD-based simulation study

commissioned by

IVO Generation Systems

andRegional Power Generators Ltd




Station - 2




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



Air flow Distribution at inlet to

Turbine Compartment




Station




Station - Parameter Studies







compliance



Hot Operating Conditions:

Envelope volume - 0.69%



Cold Operating Conditions:

Envelope volume - 2.28%











compliance



“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











compliance



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




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




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 inconnecting doors

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

A li i N E l




Designs



Enclosure Geometry - Elevation



Enclosure Geometry - End View

A li ti t N E l




Designs

T bi b t fl d



Turbine combustor flanges and

associated pipework



Geometry Representation - 1



Geometry Representation - 2

A li ti t N E l




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.



Findings - 1

The worst case leakage scenario, under cold

start-up conditions, was a zero-momentum

leakage from the flanges in front of the lowestcan combustor

The flammable gas cloud with the “reference”

ventilation arrangement was twenty five timeslarger than the target value (11 times larger if the

accessory compartment volume was included)



Reference Configuration{Flammable volume: 2.8% TC}




Designs - Workscope




leak location;





Findings - 2

Increasing the ventilating air flowby 25% had little effect on

ventilation efficiency ( gas leak

size increased proportionately ).





Designs - Workscope




leak location;



Atomising Air System




{Flammable volume: 1.9% TC}





Designs - Workscope




leak location;



Findings 3 :



Findings - 3 :

Inlet Flow Manipulation

Improved results (decrease in flammable

cloud size) obtained by :

adding a “scoop” diverter at inlet ; “throttling” - increasing the inlet velocity ;

concentrating the incoming flow towards the

axial centreline; blocking off the outermost inlet hole(s) on each

side; and

biasing the massflow distribution.

Fi l C fi ti



Final Configuration{Flammable Volume: 0.2% TC}

Fi l C fi ti




Fi l C fi i




Fi l C fi ti




C l i



Conclusions

Flammable cloud volumes of less than 0.1% of the

free volume of the combined compartments are

attainable (at least conceptually) .

This would satisfy the current UK dispersion

criterion.

Compliance achieved without major structural

changes to enclosure or to air delivery system.

Future Efforts



Future Efforts

Confirm high velocities do not provoke thermal

shock problems in hot operation

Confirm required manipulation of inlet air supply

is technically feasible (!)

Use model to predict likely over-pressures

arising from deflagration of the confined gas

plume.



Experimental Verification

No quantitative data available.

Qualitative comparison - video footage of

smoke tests on installations with similarinternal geometries.

Smoke trails confirm magnitude and

direction of airflow in the region in front ofthe can flanges and around the front upper

part of the turbine barrel





in CCGT Enclosures

Thank you for your attention .

“ When I count to three,

you will awake

and

remember nothing …..”




in CCGT Enclosures

Patrick Phelps (Flowsolve Ltd)

and Douglas Wylie (GEC Energy Services)

flowsolve phelps gas leaks

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