Materials investigation of thermal triggers used in pressure relief devices on transit buses

34 Pages·2003·1.7 MB·English

by  Rolander, Nathan

DOT-FTA-MA- 26-7071-03-1
DOT-VNTSC-FTA-03-06
HE
4211
.U6
no.
03-06
Materials Investigation of Thermal Triggers Used in
irtment Pressure ReliefDevices on Transit Buses
or iransportation
FederalTransit
Administration
July2003
Final Report
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1.AGENCY USE ONLY (Leaveblank) 2. REPORTDATE 3. REPORTTYPEAND DATES COVERED
June2003 FinalReport-July2003
4. TITLEANDSUBTITLE 5. FUNDING NUMBERS
MaterialsInvestigationofThermalTriggersUsedinPressureReliefDevicesonTransitBuses
TT-383 U3077
6. AUTHOR(S)
NathanRolander*/**,DouglasMatson** ,WilliamP.Chemicoff*
7. PERFORMING ORGANIZATION NAME(S)ANDADDRESS(ES) 8. PERFORMING ORGANIZATION
*VolpeCenter **TuftsUniversityDept, ofMechanicalEngineering REPORTNUMBER
MA MA
Cambridge, 02142 Medford, 02155
DOT-VNTSC-FTA-03-06
9. SPONSORING/MONITORINGAGENCY NAME(S)ANDADDRESS(ES) 10. SPONSORING/MONITORING
U.S. Department ofTransportation AGENCYREPORTNUMBER
Federal Transit Administration
Office ofResearch, Demonstration and Innovation FTA-MA-26-7071-03-2
11. SUPPLEMENTARY NOTES U.S. DepartmentofTransportation
Thisworkwasperformedundercontractto: VolpeNationalTransportationSystemsCenter
MA
Cambridge, 02142
12a. DISTRIBUTION/AVAILABILITYSTATEMENT 12b. DISTRIBUTION CODE
This document is available to the public through the National Technical Information
Service, Springfield, VA 22161
13. ABSTFRACT(Maximum 200words)
This investigation pertains to the composition and general condition of the thermally activated trigger
mechanism of Pressure Relief Devices [PRD's], safety devices used on compressed natural gas cylinders
commonly used to store fuel on transit buses. These trigger mechanisms provide the essential function
of the PRD, and any problems with the trigger will result in poor device performance.
The report analyses the condition of the eutectic material used in the triggers, its composition,
condition, and any flaws that may occur. Following the experimental investigation, failure criterion
for the triggers were computed based on observed flaws in the trigger material. These computations
provide a measure of the reliability of the triggering devices, and thereby of their effectiveness.
14. SUBJECTTERMS 15. NUMBEROFPAGES
33
Pressure Relief Device, Natural Gas Safety, Alternative Fuel, Bus Safety
16. PRICECODE
17.SECURITYCLASSIFICATION 18.SECURITYCLASSIFICATION 19.SECURITYCLASSIFICATION 20. LIMITATION OFABSTRACT
OFREPORT OFTHISPAGE OFABSTRACT
Unclassified Unclassified Unclassified
NSN 7540-01-280-5500 Standard Form 298(Rev. 2-
89) Prescribed byANSI Std.
239 18298-102
tn
oojfe ( H£
vt//
* L)(o
NOTICE fU
This document is disseminated under the sponsorship ofthe U.S. Department of c 6
Transportation in the interest ofinformation exchange. The United States Government
assumes no liability for its contents or use thereof.
NOTICE
The United States Government does not endorse products or manufacturers. Trade or
manufacturers’ names appear herein solely because they are considered essential to the
objective ofthis report.
RECEIVED
JUN "
1 2094
US.DEPT.OFTRANSPORTATION
LIBRARY,NASSIFBRANCH
Metric/English Conversion Factors
English to Metric Metric to English
LENGTH (Approximate) LENGTH (Approximate)
1 inch(in) = 2.5 centimeters(cm) 1 millimeter(mm) = 0.04inch(in)
1 foot(ft) = 30centimeters(cm) 1 centimeter(cm) = 0.4inch(in)
1 yard(yd) = 0.9meter(m) 1 meter(m) = 3.3feet(ft)
1 mile(mi) = 1.6kilometers(km) 1 meter(m) = 1.1 yards(yd)
1 kilometer(km) = 0.6mile(mi)
AREA(Approximate) AREA(Approximate)
111ss1qqsuuqsaauqrraueeraermyeiailrnfedocoh((tss(q(qssqmyqidi.,nft,,myifidnt2222)))) ==== 2600....58609sssqqqsuuuqaaaurrraeeeremkcieemlnteotetmireemret(te(merm2rs)s2)((kcmm22)) 1101,ss0qq0uu0aar1sresqequckuaeianrlrteoeimmmmeeeettteteerrersr((((kcmmmm2222)))) ==== 0011...h421e6sscqqtsauuqraauerraeer(ehymeaii)rlndec=sh(2s((.qss5qqmayiicdn,r,,emisyind22)2))
1 acre=0.4hectare(he) = 4,000squaremeters(m2)
“1 1
MASS-WEIGHT(Approximate) MASS-WEIGHT(Approximate)
1 ounce(oz) = 28grams(gm) 1 gram(gm) = 0.036ounce(oz)
1 pound(lb) = 0.45kilograms(kg) 1 kilogram(kg) = 2.2pounds(lb)
1 shortton=2,000pounds(lb) = 0.9tonne(t) 1 tonne(t)= 1.000kilograms(kg) = 1.1 shorttons
VOLUME (Approximate) VOLUME (Approximate)
1 teaspoon(tsp) = 5 milliliters(ml) 1 milliliter(ml) = 0.03fluidounce(floz)
1 tablespoon(tbsp) = 15milliliters(ml) 1 liter(1) = 2.1 pints(pt)
1 fluidounce(floz) = 30milliliters(ml) 1 liter(1) = 1.06quarts(qt)
1 cup(c) = 0.24liter(1) 1 liter(1) = 0.26gallon(gal)
1 pint(pt) = 0.47liter(1)
1 quart(qt) = 0.96liter(1)
1 gallon(gal) = 3.8liters(1)
1 c1ubciucbiycarfdoo(tc(ucyud,ft,yfdt33)) == 00..0736ccuubbiiccmmeetteerr((mm33)) 11 ccuubbiiccmmeetteerr((mm33)) == 3163 ccuubbiiccfyeaertds(c(ucuft,ydft,3)yd3)
TEMPERATURE (Exact) TEMPERATURE (Exact)
[(x-3(2x)+(456/09))/]1°.F8 == yy °°CK [(9(/y5x)y1.+83B24]6°0C) == xx°°FF
PRESSURE (Exact) PRESSURE (Exact)
1 psi = 6.8948kPa 1 MPa = 145.04psi
ENERGY& ENERGYDENSITY (Exact) ENERGY& ENERGY DENSITY (Exact)
1 Btu = 1.05506kJ 1 MJ = 947.81 Btu
1 Btu/lb = 2.326kJ/kg 1 MJ/kg = 430Btu/lb
QUICKFAHRENHEIT-CELSIUSTEMPERATURE CONVERSION
-40° -22° 14° 32° 50° 58° 8 >° 104° 122° 140° 158° 17 5° 194° 212°
-40° -30° -20° -10° 0° 10° 20° 30° 40° 50° 60° 70° 80° 90° 100°
IV
8
7I.. Table ofContents -
Table ofContents - v
II. ListofFigures - vi
III. DocumentNomenclature - vii
Ill.a Acronyms - vii
IILb Definitions - vii
Abstract viii
1. Background 1
2. ThePressureReliefDevice 3
3. TriggeringDevice 5
4. Metallography 7
5. ThermalProfile 11
6. Geometry andForces 15
6.1. Constriction Angle Calculation 15
6.2. Ball Bearing Contact Angle 17
6.3. Spring Constant 18
6.4. ResultantVertical Force 1
6.5. Void ShearReduction Effect 18
6.6. Upper Bound Hodogram 20
7. Conclusions 21
8. Future 23
9. References - 24
v
II. List ofFigures -
Fig 2.1 - ExplodedMirada PRD Photograph 3
Fig 2.3 - AssembledMirada PRD 4
Fig 2.4 - Pressure Disk inplace (left) andpuncturedwith bayonet (right) 4
Fig 3.1 - Sectioned View ofMirada PRD, ball bearings removed 5
Fig 3.2 - Voidin eutectic material near top surface under microscope 6
Fig 4.1 - Electrolytic Polishing apparatus with Eutectic Trigger 8
Fig 4.2 - Eutectic Material Grain Structure 9
Fig 4.3 - Bismuth-LeadEutectic Imbalance 9
Fig 4.5 - Large Single Leadparticle under close inspection 10
Fig 5.1 - Thermocouple insertedinto eutectic trigger mechanism 11
Fig 5.2 - Temperature Profile Plots ofEutectic Triggers 12
Fig 5.4 - Phase Change Plateau ofTriggers 2 & 3 13
Fig 5.5 - MeltPlateau Measurements 14
Fig 5.6 - Phase Diagram ofBismuth-Lead 14
Fig 6.1 - Ball bearingstaticforce diagram 15
Fig 6.2 - PRD Trigger Geometry 16
Fig 6.3 - Surface Area Reduction Ratio Plot 19
III. Document Nomenclature -
IH.a Acronyms -
PRD Pressure ReliefDevice
CNG CompressedNatural Gas
DAQ
Data Acquisition
SCXI Signal Conditioning and Switching Interface
IH.b Definitions -
Eutectic Composition -
A specific alloy composition which solidifies at a lower temperature than
all other alloy combinations.
Metal Creep -
Progressive deformation ofa metal or alloy over a period oftime. The
process occurs via diffusion within the material. This time dependant strain is
caused by a constant load or stress, particularly at elevated temperatures.
Plastic Deformation -
Permanent deformation ofa material caused by continued applied stress
exceeding the materials yield strength.
Abstract
This investigation pertains to the composition and general condition ofthe thermally
activated trigger mechanism ofPressure ReliefDevices (PRD's). PRDs are safety
devices on compressed natural gas cylinders, commonly used to store fuel on transit
buses. These trigger mechanisms provide the essential function ofthe PRD, and any
problems with the trigger will result in poor device performance.
The report analyses the condition ofthe eutectic material used in the triggers, its
composition, condition, and any flaws that may occur. Following the experimental
investigation, failure criteria for the triggers were computed based on observed flaws
the trigger material. These computations provide a measure ofthe reliability ofthe
triggering devices, and thereby oftheir effectiveness.
Background
1.
Pressure ReliefDevices (PRDs) are standard equipment on all compressed natural
gas containers. The function ofa PRD is to vent the compressed natural gas in the case
ofa fire, preventing rupture and the subsequent high-pressure gas release with a possible
ignition and explosion. Compressed natural gas is stored at a maximum settled pressure
of3600 psi. Ifthe gas is released at high-pressure in certain environments, the result
could be catastrophic. Therefore, PRD design and manufacture must be ofvery high
quality. The PRD must offer a degree ofprotection and reliability that meets or exceeds
that ofthe cylinder in order to provide the proper degree ofsafety. The PRDs under this
investigation are used on CNG cylinders for transit busses. They came under scrutiny
beginning in the early 1990s, when many malfunctions occurred, where the natural gas
vented. The importance ofhaving reliable components is increasing with the shift
towards alternative fuels, and especially compressed hydrogen-powered vehicles.
Because hydrogen storage cells would see even higher pressures and temperature ranges,
the performance oftoday's compressed natural gas vehicles may serve as the performance
benchmark.
A primary goal ofthe study described in this report was to find evidence of
material creep or plastic deformation. Creep phenomena have been blamed for previous
PRD failures; however, to date no evidence supports this assumption, since trigger
material in the PRD is not exposed to high cylinder pressure. The analysis determined
the physical condition ofthe PRD by disassembly and dissection, including
metallography and thermal profiling. The results obtained provided insight into the
functionality ofthe PRDs and possible causes offailure.
Metallurgical analysis ofcontemporary PRDs was performed by the Tufts
University Mechanical Engineering Department for the U.S. DOT/RSPA/Volpe National
Transportation Systems Center. Previous PRD failure studies have not fully identified
the failure modes and mechanisms. Without proper failure mode identification a
satisfactory solution cannot be implemented. There are currently around 6,200 natural-
gas-powered transit buses in the United States. During a 1997 study of703 buses, a total
of 132 gas release events were recorded, resulting in one serious fire. In December 2000,
1
another unintentional release resulted in a deflagration. The calculated frequency of
release is between 0.0013 - 0.0358 per bus per year. Since that time, releases and failure
rates have decreased but still occur. This investigation was conducted in two phases;
engineering analysis ofthe physical and metallurgical aspects ofthe failed PRD's, and
statistical analysis to support the engineering results and detect failure patterns.
2