review of helium venting analyses
DESCRIPTION
Review of Helium Venting Analyses. Chris Tutt AMS-02 Project Manager. Review of Hazard. Hazard to be addressed is release of asphyxiant gas into an occupied area. Three major helium reservoirs within AMS-02 payload and GSE. 2500-liter main helium dewar inside the payload. - PowerPoint PPT PresentationTRANSCRIPT
Chris Tutt AMS-02 Phase II Safety Review 1
Review of Helium Venting Analyses
Chris TuttAMS-02 Project Manager
Chris Tutt AMS-02 Phase II Safety Review 2
Review of Hazard Hazard to be addressed is release of asphyxiant
gas into an occupied area. Three major helium reservoirs within AMS-02
payload and GSE. 2500-liter main helium dewar inside the payload. 1000-liter master dewar used for filling main dewar. 1000-liter transfer dewar used for filling master dewar.
Venting analysis focused on main dewar as enveloping case, but results for all others are similar.
Chris Tutt AMS-02 Phase II Safety Review 3
Main Helium Dewar Main Helium Dewar has two major components
Helium Tank contains the cryogen itself Vacuum Case provides vacuum space around the
tank. 3 bar burst disk on helium tank defines the
Maximum Design Pressure for the system. Nominal operating pressure is ~16 mbar. All hardware will be extensively tested prior to
arrival at KSC. Structural analysis shows high margins for MDP. All welds will be inspected per MSFC-STD-504C. Both items will be proof pressure tested. Both items will be vacuum leak tested.
Chris Tutt AMS-02 Phase II Safety Review 4
Main Helium Tank
Chris Tutt AMS-02 Phase II Safety Review 5
Vacuum Case
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GSE Dewars
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Tank Overpressurization Two mechanisms for overpressurization of tank.
Failure to remove nominal boil-off from tank. Large external heat source increases boil-off beyond
pump’s capability to remove. First scenario requires weeks to reach burst
pressure, so does not present safety hazard. Only possible heat source for second scenario is
ambient atmosphere and requires air leak into the dewar vacuum space.
Chris Tutt AMS-02 Phase II Safety Review 8
Cause of Leak Two leak scenarios were considered in analysis
Loss of Vacuum (LOV) – Total loss of vacuum caused by large breach of Vacuum Case. Requires major accident:
• Forklift tine breaks through VC outer cylinder• Payload dropped during lifting operations• Large hardware falls on payload from significant height.
Maximum Credible Leak (MCL) – 3” leak through the double O-Ring seals in the VC upper and lower support rings.
• Leak size defined by Payload Safety Review Panel and used for all payload bay leak analyses.
LOV can be prevented operationally, so following discussion will focus on MCL.
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VC O-Ring Seals
Chris Tutt AMS-02 Phase II Safety Review 10
Each sealing surface has double O-ring seals.
In MCL, both seals are assumed to have failed in the same location.
CONICAL FLANGESUPPORT RING
O-RING TEST PORT
VACUUM S
PACE
INTERFACE PLATE
OUTER CYLINDER
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MCL Defined Leak Size
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Venting Analysis OverviewVenting Analysis consists of three basic
steps Calculation of time required for tank to reach
3 bar burst pressure after leak begins. Calculation of mass rate of flow of helium
leaving the main tank. Calculation of oxygen levels in surrounding
external space.Only the third step is location dependent.
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Time to BurstPre-burst pressure rise modeled as
isochoric heating. Heat flux from MCL calculated to be 655.5 W. Helium thermodynamic properties from NIST
handbook.Results driven by initial temperature and
fill level.Calculated times range from 54 min (1.9K,
95% full) to 86 min (1.7K, 80% full).
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Typical Pressure Rise Profile
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0 500 1000 1500 2000 2500 3000 3500 4000 4500
Time (sec)
Pres
sure
(MPa
)
Lambda Point
Tank Filled
Burst Pressure
Chris Tutt AMS-02 Phase II Safety Review 15
Time to Empty TankMass flow out of the tank modeled as
isentropic expansion through choked nozzle.
Results driven entirely by fill level. The more helium in the tank, the longer it
takes to empty. Below 90% full, helium becomes two-phase
prior to reaching 1 atm.Calculated times range from 131 min after
burst (80% full) to 239 min (95% full).
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Typical Mass Flow Profile
0.001
0.01
0.1
1
10
100
0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000
Time (sec)
Mas
s Fl
ow (k
g/s)
Burst Pressure
Pressure EqualizesHelium Goes Two-Phase
Helium Fully Vapor
Tank Empty
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Exterior VolumeAnalysis done for four KSC spaces
Space Shuttle Processing Facility Canister Rotation Facility Canister Payload Changeout Room
Results driven by three factors Volume of external space Air refreshment rate Gas diffusion model
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Original Gas Diffusion Assumption Original discussions assumed that helium vapor
would rise to the ceiling of external volume.
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Current Gas Diffusion AssumptionAt GOWG in November, GSRP requested
analysis be redone assuming gas spreads evenly throughout the room. Model used GSFC algorithm provided by
SHOOT team. Based on helium sensors measurements from
Tevatron accident at Fermilab.Venting memo describes results using
second assumption.
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GSFC Algorithm
Helium enters control volume at R m3/s.Ambient air removed by ventilation system
at Q m3/s.
Incoming Helium
Ventilation Exhaust
Incoming Air
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Oxygen Concentration Levels in PCR
18.50%
19.00%
19.50%
20.00%
20.50%
21.00%
4000 6000 8000 10000 12000 14000
Time (sec)
Oxy
gen
Leve
l (%
)
Pressure Equalizes
Helium Fully Vapor
Tank Empty
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Oxygen Concentration Levels in Canister
3.00%
6.00%
9.00%
12.00%
15.00%
18.00%
21.00%
4000 6000 8000 10000 12000 14000
Time (sec)
Oxy
gen
Leve
l (%
)
Pressure Equalizes
Helium Fully Vapor
Tank Empty
Chris Tutt AMS-02 Phase II Safety Review 23
Oxygen Concentration Levels in SSPF
20.80%
20.85%
20.90%
20.95%
21.00%
4000 6000 8000 10000 12000 14000
Time (sec)
Oxy
gen
Leve
l (%
)
Pressure Equalizes
Helium Fully Vapor
Tank Empty
Chris Tutt AMS-02 Phase II Safety Review 24
Oxygen Concentration Levels in CRF
20.50%
20.60%
20.70%
20.80%
20.90%
21.00%
4000 6000 8000 10000 12000 14000 16000
Time (sec)
Oxy
gen
Leve
l (%
)
Pressure Equalizes
Helium Fully Vapor
Tank Empty
Chris Tutt AMS-02 Phase II Safety Review 25
Overall TrendsPCR falls briefly below 19.5%, but high
ventilation rate allows rapid return to safe levels.
Canister’s small volume can be rapidly overwhelmed, but is not normally a manned volume when door is shut.
SSPF/CRF volumes are large enough and ventilation rates are fast enough that oxygen level never falls below 19.5%.
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Proposed Safety Controls GSE should be monitored for signs of
temperature or pressure rise while AMS-02 is in manned area.
If leak observed, personnel should be removed from vulnerable areas. Entire PCR Elevated structures within SSPF and CRF
Oxygen sensors should be used to determine safety of atmosphere prior to reentering any area after venting event or opening Canister in the PCR.
Additonal vent lines and building modifications should not be necessary.