ADDENDUM #2
Fire Hazards Analysis Update
For the
BaBar Detector Project
February 15, 2005
(Final June 29, 2005)
Prepared for:
Mr. Frank O’Neill
Project Safety Officer, BaBar
Stanford Linear Accelerator Center
Menlo Park, CA 94025
Prepared by:
Hughes Associates, Inc.
3610 Commerce Drive, Suite 817
Baltimore, MD 21227
CONTENTSFIRE HAZARDS ANALYSIS UPDATEFOR
THEBABAR DETECTOR PROJECT
1.0
BACKGROUND
The BaBar Project is located in the PEP-II Research Hall at Interaction Region 2 (IR-2). IR-2
and all PEP-II facilities are located inside the SLAC Radiological Control Area.
A comprehensive fire hazards analysis (FHA) was completed for the BaBar Project at the
Stanford Linear Accelerator Center (SLAC) in December, 1996 [HAI 1996]. Then, in 2001 an
addendum to the FHA was completed. The addendum included evaluation of several design
features that were not finalized at the time of the original FHA, or represented modifications
from the original design(s).
2.0
SCOPE AND OBJECTIVES
Several proposed changes to the BaBar Detector design are under consideration. In most cases
these changes are a result of desired improvements in the experimental performance. This
addendum (i.e., Addendum #2) includes evaluation of these proposed changes or modifications
to the experimental facilities. The evaluation included the following factors:




Potential impact on original estimates of fire development documented in the initial
FHA;
Compliance with DOE 420.1;
Any effects on the original MPFL and MCFL estimates, and
Any life safety effects.
This addendum provides a summary of detailed evaluations of several design changes.
Reference should be made to the original FHA [HAI 1996] regarding descriptions of BaBar and
its associated equipment and facilities. That information is not repeated here.
3.0
UPDATES/CANDIDATE CHANGES
The following changes or additions to the BaBar Detector Project were evaluated as part of this
addendum:

Replacement of the resistive plate chambers in the gaps of the BaBar Instrumented Flux
Return(IFR) with Limited Streamer Tubes (LST);
 Adjustments to the gas mixtures for BaBar Barrel IFR upgrade;
 Addition of high voltage system electronics, cables, and racks for the LST installation;
 Inclusion of a high sensitivity smoke detection system (VESDA) for the IFR racks; and
 Installation of a dry air gas system in the Detector gaps for the LSTs to improve control
of the HV currents.
The changes/additions were evaluated and recommendations were provided to the BaBar Safety
Officer at the time the individual change/addition was being considered. The following is a
summary of the results of these evaluations.
2
3.1
Replacement of the RPCs with LSTs
Description:
Replacement of the Resistive Plate Chambers (RPC) in the gaps of the BaBar Instrumented Flux
Return (IFR) with Limited Streamer Tubes (LST) is ongoing. The original RPCs consist of
layers of phenolic resin strips (bakelite) and polystyrene. The LSTs are fabricated from
polyvinyl chloride (PVC), a vinyl resin. The top and bottom barrel sextants LST installation was
completed over the summer of 2004. The remaining four barrel sextants are planned for LST
installation in late 2006. A VESDA high sensitivity smoke detection (HSSD) system installed in
2004 provides very early warning smoke/fire detection coverage for all of the electronics
associated with the LST installation.
Of concern was the effect of this change on the fire hazards associated with the BaBar
experiment in the context of the existing (DOE approved) FHA. A detailed fire hazard
evaluation of the use of LST technology was performed. This effort included small scale fire
testing of sample materials and the use of specific materials flammability correlations available
in the literature to assess the relative flammability. A letter report detailing this work is attached
as Appendix A [HAI 2003].
Potential Fire Hazards:
The concern associated with the use of the LST technology is the potential for increasing the fire
hazard in BaBar. The primary ignition source is from the high voltage electronics. A secondary
source is the gas mixture which is circulated through the IFR. Normally the gas mixture is
nonflammable. And, the gas mixture system includes detection and fail safe shutdown if the gas
mixture fails to maintain predetermined individual gas concentrations.
Analysis:
Small scale laboratory fire tests (ASTM E 1354 Standard Test Method for Heat and Visible
Smoke Release Rates for Materials and Products Using an Oxygen Consumption Calorimeter)
and flammability correlations were used to perform a limited fire hazard analysis of the LST
materials. Of particular interest were measurements of ignition time, duration of visible flaming,
and heat release rate. The test results were used with available flammability correlations to
compare the flammability characteristics of the LST materials and the RPC materials.
Results/Findings:
The results of this analysis indicated that the use of the LST materials will reduce the intrinsic
fire hazards associated with BaBar. The LST material (PVC) exhibited better ignitability and
heat release rate characteristics than the existing RPC materials. In addition, the PVC was
determined not to exhibit significant flame spread under the tests conducted. The change out to
LSTs will eliminate both the polystyrene foam and the linseed oil from the IFR. And, the PVC
exhibited a strong tendency to intumesce when exposed to an external heat source, resulting in an
insulating char layer that would inhibit flame spread.
3
A concern with the use of the PVC material is the production of HCL in the IFR gaps if exposed
to an ignition source. No attempt was made to quantify the HCL production rate and the effects
on the exposed electronics of exposure of the LSTs to an ignition source or small, incipient fire.
Under such an event, acid gas damage to the exposed electronics will occur.
Compliance:
The change out to the LST technology does not affect compliance with DOE 420.1 or related
NFPA standards.
MPFL/MCFL Impact:
The original MPFL and MCFL calculations presented in the initial FHA are not affected by
replacement of the RPCs with LST technology. In the original calculations, it was assumed that
a fire in BaBar would result in extensive damage to the Detector and its electronics. While the
use of the LST technology will result in less damage due to direct fire exposure, corrosive effects
on exposed electronics due to burning PVC will occur.
Life Safety:
No changes in the life safety analysis presented in the initial FHA occur as a result of this
modification.
Recommendations:
None; the change out to LST technology is not expected to increase the fire hazard in BaBar.
3.2
Adjustments to the gas mixture for BaBar Barrel IFR upgrade
Description:
An analysis of the proposed gas mixture for the IFR was performed as part of the initial FHA.
The objective was to determine if the proposed mixture of gases resulted in a flammable gas
mixture. The introduction of a flammable gas mixture into the BaBar RPC system was
determined to be an unacceptable fire hazard. As a result of the original analysis, a specific nonflammable gas mixture consisting of argon, isobutane and Halon 134a was identified for use in
BaBar [HAI 1996]. This mixture was determined by the initial thermodynamic analysis that
included a 50% safety factor, and resulted in a limit of 4% isobutane. Further analysis, including
review of the operational accuracy of flow measurement and shutdown equipment and
procedures for the gas mixing and delivery systems resulted in increasing the isobutane limit to
5%.
The introduction of the LST technology required flexibility in the selection of the gas mixture.
As a result, a detailed review of flammability test methods was conducted [HAI 2003b; reference
Appendix B] and a procedure for determination of allowable gas mixtures [SLAC 2004;
4
reference Appendix C] was developed. The procedure was designed to ensure that the mixture
remain outside the flammable range, but permits some flexibility in the make up of the mixture
in order to improve the performance of the LSTs.
The optimum proportions of the gas constituents have not been determined in terms of the LST
performance but they are expected to be in the range of 8-9.7% isobutane, 2.5-3% argon, and the
remainder carbon dioxide. Such mixtures remain nonflammable. The operational accuracy of
the gas mixing and control equipment will maintain the selected gas mixture below the LFL.
Any mixture excursions outside the predetermined boundaries that insure a nonflammable
mixture will result in gas shut off and an alarm.
Potential Fire Hazards:
The introduction of a flammable gas resulting from an upset condition in the gas mixing process
in and of itself would not result in a fire. However, if air is inadvertently introduced into the
detector, the gas mixture could be ignited due to an electrical short or other transient ignition
source associated with detector operation.
Analysis:
A procedure was developed to determine the boundary limits for mixtures of isobutane, argon
and carbon dioxide. Once the boundary limits were identified, it was necessary to determine the
required safety factor for normal detector operating conditions between the flammable and nonflammable regions. The dominant factors associated with this part of the analysis were the
accuracy and repeatability of the gas mixing system itself.
The gas mixing system will use mass flow controllers with a specified accuracy of 0.5% of the
flow controller’s full scale, with a repeatability of 0.2%. Input flows of the three gases will be
metered by the mass flow controllers to make a precise gas mixture when the three flows are
combined.
The isobutane concentration will be cross checked independently. This has been done for the
PRC gas system with an error range of 0.25-0.5% of the relative isobutane concentration.
The performance of the actual mixing control system will be continually monitored by a second,
independent set of in-line mass flow meters, one of which will meter the flow of a particular gas
component. A controller will be set to open a relay if the particular flow it is monitoring drifts
outside of a small range around a nominal rate. An open relay from any of the three controllers
will cause solenoid shutoff valves in all three of the supply gas lines to close, automatically
shutting down the gas supply system and sounding an alarm. The process controllers have an
accuracy of 0.2%.
The stability of the isobutane fraction of the gas mixture used in the RPC chambers is much
better than 1% of nominal value. A window of +0.5% is sufficient to contain the variations. The
standard deviation of the isobutane fraction is 0.25% of the nominal value over periods longer
than a month. Given this information, an operating region for the gas mixture was determined,
5
based on reducing the net allowable isobutane concentration in the mixture by a fixed percentage
from the boundary of the flammable and non-flammable regions, allowing for the stability of the
mixing system.
The gas system is capable of operating within a range of +0.5% of nominal. A reliable
separation from the flammable region can be achieved by reducing the set points for the amount
of isobutane in the mixture by 3% from the concentration that is just outside the flammable
region. Therefore, a 3% reduction in the isobutane concentration below the lower flammable
limit was selected as an upper limit for any potential gas mixture to be produced in the mixing
system. The process controllers will be set to stop all gas flow if the mixture fails either of two
tests: a) if a particular gas component drifts enough to bring the isobutane fraction of the gas
mixture to within 2% of the flammable boundary; or b) if a particular gas component drifts
enough to move the gas mixture away from a suitable operating region.
Results/Findings:
Based on the review and analysis of the gas mixing system a protocol was developed for
selection of the individual gas concentrations for the gas mixture to the LSTs. The resulting
flexibility will allow experimentalists to adjust the gas mixture within relatively tight limits to
improve detector performance. Controls associated with the mixing and delivery of the gas
mixture were designed to provide adequate safety margins. Automatic shutdown of the gas
mixing and delivery system was incorporated into the design. The alarm threshold associated
with shutdown was based on maintaining the flammable gas constituent at a concentration that
was 3% below the edge of the flammable limit.
Compliance:
The primary safety systems associated with the gas mixing and delivery system are in
compliance with applicable standards and DOE 420.1, allowing limited flexibility in the make up
of the gas mixture. The resulting mixtures will remain non-flammable. In the event of an
excursion beyond the safe limits established for the system, the gas mixing and delivery system
will be automatically shut down. This approach does not represent a significant compromise in
fire safety.
MPFL/MCFL Impact:
No changes in the MPFL or the MCFL analyses presented in the initial FHA occur as a result of
this modification.
Life Safety:
No changes in the life safety analysis presented in the initial FHA occur as a result of this
modification.
6
Recommendations:
None.
3.3
High Voltage System, Electronics, Cables and Racks for LST Support
Descriptions:
Support systems for the LST modules will include a high voltage system, electronics and cables.
The high voltage system distribution boxes will be located outside the shield wall on top of the
electronics hut. Cables will be connected from the distribution boxes to the LST modules inside
BaBar. External high voltage current supply and control lines will be located inside the
electronics hut. There will be 94 high voltage cables for each sextant.
New LST FEC crates will be positioned adjacent to BaBar as close as possible to the LST
modules. The crates will be located close to the Detector in order to minimize the cable runs.
Output from the FEC crates will be transferred to IFB boards which have DAZ and control lines
connected to the data acquisition system. The new LST high voltage cables will be routed
through the shield wall. New LST crates and racks will be located on top of a platform installed
on the East side of the Detector near the shield wall. The high voltage cable trays will run from
the electronics hut to the BaBar central cable trays. The vertical section will be attached to the
new platform. High voltage cable wireways will be installed for each sextant. New racks for the
LST readout crates will be located adjacent to the Detector.
The high voltage cables are manufactured by Kerpenwerk GmbH & Co., and will be installed
from the electronics hut to the LST modules inside the Detector. The cables will penetrate the
shield wall. The outer jacket of the power cables is fire retardant polyethylene which is halogen
free. The cable product meets CERN Safety Instruction 23, which requires the use of cable
materials that exhibit fire retardancy as well as low toxic or corrosive products of combustion.
The signal cables from the LST modules are Amphenol 34 channel twist/flat ribbon cables
encased in halogen free polyethylene sheaths. This cable also meets CERN Safety Instruction
23. The cables from the FEC crates to the IFB crates as well as the cables that connect IFB
crates to the BaBar data acquisition system will not be changed.
The tests used to evaluate the cable materials are IEC 60332-1 and IEC 60332-3/Category C.
These are small scale tests that evaluate either single wire or bundled cable configurations. IEC
60322-1 is somewhat analogous to UL 1581 VW-1, but is generally considered to be less severe
with regard to flammability (i.e., ignitability, heat release rate, and flame spread). IEC 60332-3
is similar to IEEE 383, a vertical tray cable fire test used in the U.S. nuclear power and other
industries. All of these tests are prescriptive, characterized by “go/no go” criteria. Quantitative
measurements regarding the flammability or combustion product production rates are not taken.
Therefore, the results of these tests can not be used to predict the fire behavior of these cables
under different fire scenarios.
7
Potential Fire Hazards:
The primary ignition sources associated with the LST installation involve overheated electronics
or cable connections, both inside the Detector in the vicinity of the LSTs and in the crates and
equipment racks (directly outside the Detector). The primary fuel sources are the cable materials
and electronics.
Analysis:
A review of the descriptions of the electronics to be installed as part of the LST upgrade was
performed. In addition, candidate cable materials and configurations were evaluated, along with
the test methods used to evaluate and assess their suitability for use in BaBar. In situ
considerations included fire hazards under normal operating conditions as well as while the
Detector is opened for service/maintenance operations.
Results/Findings:
The installation of the LST electronics and associated power and signal cables in BaBar is not
expected to increase the potential fire hazard level in the Detector. Electronics fires are rare, and
usually occur during initial system testing. During this period, BaBar will be open and
accessible. This minimizes the likelihood that an electronics or connector overheat/burnout
condition will evolve to ignition of adjacent combustible materials and spread of a fire without
intervention by the experimental and/or safety staff. In addition, although the cable sheaths may
ignite under electrical shorting or arc conditions, the fire propagation rates and production rates
of smoke, corrosives and toxic gases are lower than those associated with untreated cable sheath
and insulation materials.
Under conditions when BaBar is open, the possibility does exist when experimental staff could
be exposed to an incipient overheat condition or fire in the Detector, or in the adjacent equipment
rack areas. However, due to the materials selected for the LST upgrade, rapidly developing fires
are not expected, and it is more likely that the presence of staff in the vicinity of BaBar will
result in more rapid intervention.
Under conditions when BaBar is closed for operation, a high sensitivity smoke detection (HSSD)
system located outside the magnet coil at the forward end of BaBar will provide very early
warning of an overheat or pre-burning condition in the LST region. The alarm associated with
the HSSD system is transmitted to the electronics hut which is occupied continuously while the
detector is operating. In the event of an alarm, the Detector operation will be terminated and the
gas mixture flow to the detector interrupted.
A HSSD system has been installed in the IFR equipment racks adjacent to BaBar. This system is
intended to detect any overheat condition and terminate power and electrical signals. This will
limit the extent of damage to the electronic equipment and cabling in the racks due to an
overheat condition, electrical short or similar incident. Review of the installation drawings and
specifications indicate that the number of sampling points and their locations, as well as the
8
detection system flow rates will provide adequate coverage for the equipment racks adjacent to
BaBar.
Compliance:
The installation of support electronics, high voltage power, and associated cables and equipment
racks will not effect compliance with DOE 420.1 or related NFPA standards. The use of fire
retardant polyethylene sheathed wire and cable materials should not increase the potential for
ignition and fire propagation. And, the production rates of smoke and corrosive/toxic
combustion products under expected fire conditions are considerably lower than for typically
used cable materials.
MPFL/MCFL Impact:
The installation of the specified electronics and cables will not qualitatively change the
MPFL/MCFL estimates provided in the initial FHA.
Life Safety:
No measurable changes in the life safety analysis presented in the initial FHA will occur as a
result of these electronics and cable upgrades.
Recommendations:
The changes in electronics and cables in support of the LST upgrade are not expected to increase
the fire hazard in the BaBar Detector, provided that HSSD systems are in place to detect an
overheat/incipient fire condition in the LST areas and in the outside equipment racks.
The HSSD system in the electronics racks adjacent to BaBar should be extended to include the
LST crates. Reliance on HSSD detection sensitivity (in lieu of automatic fire suppression) is
crucial to detection of overheat or incipient fire conditions in the equipment racks or LST crates
before the onset of significant damage to the Detector or support systems.
3.4
Dry Air Gas System
Description:
Studies of HV currents associated with the operation of the LST’s indicated that the currents
were very sensitive to increases in the ambient Dew Point. These studies also demonstrated that
flowing dry air past the HV boxes reduced the currents substantially. Therefore, a dry air gas
distribution system has been installed to maintain relatively dry air in proximity to the HV boxes,
thus reducing the HV currents.
The dry air source is located on the forward west side of the Detector. The dried air is
introduced at 100 psi to a distribution panel through a 0.25 in. poly-flo tube. At the distribution
panel the air first goes through a solenoid valve, which is plugged into an outlet on the magnet.
9
After the solenoid valve, the air goes through a regulator to reduce the pressure to below 40 psi.
The flow rate of the dry air is maintained at 200 cu ft/hr, the desired flow rate. The flow is
divided at this point through a manifold and subsequently directed to each sextant. At this flow
rate the gaps will be provided with five air changes per day.
Potential Fire Hazards:
Limited ignition sources and additional fuel loading are associated with the dry air gas system
installation.
Electrical shorts due to equipment failures can occur, as with any other
electrical/electronic components installed in or adjacent to the Detector. The amount of
additional fuel loading associated with the tubing is minor. The air flow through the Detector
could potentially increase burning, depending on the proximity of combustible fuels and the
magnitude of an ignition or overheat condition. However, the air flow rate is relatively low, and
the power to the dry air gas system is designed to be interrupted if the HSSD system goes into
alarm.
Analysis:
A review of the description of the dry air gas system, including electrical components and power
supply, was performed. The fire hazards associated with minor amounts of Poly-flo tubing and
the introduction of dry air at 200 cu ft/hr were examined.
Results/Findings:
Provided the air flow is terminated, given detection of an over-heat or burning condition, the
installation of the dry air gas system is not expected to qualitatively increase the potential fire
hazard level in BaBar. As previously indicated, electronics fires are infrequent and usually occur
during initial system testing and operation. The effects on burning of increasing the air flow for
a limited time at the relatively low flow rates associated with this system are negligible if the air
flow is interrupted in response to an HSSD alarm. Also, since the MPFL/MCFL analysis
assumed extensive damage and operational interruptions if a fire occurred in the Detector, the
relative impact of the installed dry air system on the MPFL/MCFL estimates are minor.
Compliance:
The installation of the dry air gas system will not adversely effect compliance with DOE 420.1
or related engineering standards.
MPFL/MCFL impact:
The installation of the dry air gas system, with fail-safe interruption, will not qualitatively change
the MPFL/MCFL estimates provided in the initial FHA.
10
Life Safety:
No measurable changes to the life safety analysis provided in the initial FHA will occur as a
result of installation of the dry air gas system.
Recommendations:
Installation of the dry air gas system in BaBar is not expected to increase the overall fire hazard
in BaBar, provided that the air flow is terminated in the event of detection of an overheat
condition or a fire. This feature should be integrated into the system controls to ensure that
power interruption is fail-safe.
4.0
AUTOMATIC FIRE SPRINKLER EXEMPTION
An exemption was provided by DOE for automatic fire sprinklers in IR-2 above BaBar based on
analyses documented in the initial FHA. This exemption was based on analyses of the potential
effectiveness of automatic sprinklers under rational fire scenarios as well as damage potential to
BaBar due to exposure of the detector and its associated external electronics to water. As a result
of these analyses alternative fire protection features were adopted; they include restriction of the
flammability of materials used in the detector, implementation of a wire and cable specification
that limits the potential ignition and fire propagation inside the detector and outside the detector
in the electronics racks and cable trays, and high sensitivity smoke detection (HSSD).
The modifications to BaBar reviewed in this addendum do not qualitatively increase the fire
hazards associated with operation of the detector. In fact, the LST materials and the related
electronics and cabling are expected to improve the fire hazard conditions for the BaBar
experiment. Therefore, continuation of the sprinkler exemption is appropriate.
5.0
REFERENCES
[HAI 1996]
Fire Hazards Analysis for the BaBar Detector Project, prepared for Stanford
Linear Accelerator Center, Hughes Associates, Inc., December 19, 1996.
[HAI 2001]
Addendum, Fire Hazards Analysis for the BaBar Detector Project, prepared for
Stanford Linear Accelerator Center, Hughes Associates, Inc., December 19, 2001.
[HAI 2003a] Fire Hazard Impact Assessment of LST Materials, prepared for Stanford Linear
Accelerator Center, Hughes Associates, Inc., August 11, 2003 (Attached as
Appendix A).
[HAI 2003b] Memorandum, Analysis of Candidate Gas Mixtures for BaBar Barrel IFR
Upgrade, prepared for Stanford Linear Accelerator Center, Hughes Associates,
Inc., October 8, 2003 (Attached as Appendix B).
[SLAC 2004] Messner, R. and O’Neill, F., Internal Memorandum, Development of Procedure
for Selecting Gas Mixtures for the BaBar Barrel IFR Upgrade, Stanford Linear
Accelerator Center, March 22, 2004 (Attached as Appendix C).
11
APPENDIX A
A-1
A-2
A-3
A-4
A-5
A-6
A-7
A-8
A-9
A-10
A-11
A-12
A-13
A-14
A-15
A-16
A-17
A-18
A-19
A-20
A-21
A-22
A-23
A-24
A-25
A-26
A-27
A-28
A-29
A-30
A-31
A-32
A-33
A-34
APPENDIX B
B-1
October 8, 2003
HAI #1020-003
ANALYSIS OF CANDIDATE GAS MIXTURES
FOR
BABAR BARREL IFR UPGRADE
1.0 Background
Hughes Associates, Inc. (HAI) has been tasked to determine if certain isobutane, argon, carbon
dioxide mixtures under consideration for the BaBar barrel IFR upgrade are flammable. A
mixture is considered flammable when it is capable of propagating a flame in a range of oxidant
(typically air) concentrations. The limits are known as flammability limits or explosive limits.
Several tests have been developed to determine these flammability limits.
2.0 Flammability Apparatus
The most widely used test is the US Bureau of Mines Test Apparatus2 (see figure 1). The
apparatus consists of a 5 cm (2 inch) diameter glass tube 150 cm (~5 foot) long. Before the start
of the test, the lower end of the tube is closed by a glass plate (b) and the contents of the tube
evacuated with a vacuum pump. The mixture is then allowed to enter the tube. The test starts
once the plate at the bottom is removed and a spark (at y) or a small flame is introduced at the
bottom of the tube. The mixture is considered flammable if the flame propagates half the length
of the tube (75 cm/2.5 ft). This distance was chosen to ensure that a flame would self propagate
and would not be influenced by the spark or flame at the bottom.
B-2
Figure 1. US Bureau of Mines Flammability Test Apparatus
Another test method that is under approval in Europe is known as the CEN Test Apparatus3. It is
similar to the US Bureau of Mine Apparatus, but uses a slightly different tube and it has different
passing criteria (see figure 2). The CEN Test Apparatus uses a 6 cm (2.5 inch) glass tube 30 cm
(1 foot) long. The mixture is injected into the tube and ignited at point 2. The mixture is
considered flammable if the flame detaches from the electrode as shown in figure 3.
B-3
Figure 2 CEN Test Apparatus
B-4
Figure 3 Flame Detachment Criteria
3.0 Differences Between Test Methods
The major difference between the two tests is the criteria used for determining flame
propagation. One of the factors that affects flame propagation and the determination of the
flammability limits is temperature. This effect is illustrated in Figure 4, which shows the lower
flammable limit of 10 different hydrocarbons at various temperatures7. The Bureau of Mines test
criteria were chosen to limit the effects that the ignition source has on the flammability limits.
The CEN test procedure does not attempt to limit the ignition effects. This influences the
temperature of the mixture; i.e., results in a somewhat higher sample temperature than that
associated with the Bureau of Mines test. Based on these differences, the CEN Test will typically
predict lower flammable limits as compared to the Bureau of Mines Test.
Other factors that influence the flammability limits are pressure and gravity. Figure 5 shows the
changes in the flammability limits for propane, carbon dioxide mixtures and propane, nitrogen
mixtures in air at various pressures.7 Increases in pressures have a small effect on the lower
limit, but can increase the upper limit by a factor of four. Additionally, pressure increases the
amount of inerting necessary to render a mixture nonflammable.
Gravity has an effect on the flammability limits of a mixture. The upward propagation will
always have the lowest limits due to the influence of the buoyant combustion products. Both the
Bureau of Mines Test and the CEN test measure upward flame spread. Table 1 shows the
flammable limits of Butane in air.2
B-5
Table 1 Flammability Limits of Butane in Air2
Direction
Upward
Horizontal
Downward
Globe
Lower Limit
1.8
1.9
1.9-2.2
1.6
Upper Limit
8.4
6.5
5.5-7.4
5.7
Figure 4 Lower Flammable Limit for 10 Hydrocarbons
as a function of Temperature7
B-6
Figure 5 Effects of Pressure on the Flammability Limits7
B-7
4.0 Flammability of Isobutane
The flammability of isobutane has been determined using the US Bureau of Mines Test and for
the CEN Test. Typically these tests are performed in air (see Table 2). They have also been
performed on other isobutane mixtures in air such as nitrogen and carbon dioxide. A
flammability diagram showing the flammability limits in various concentrations of nitrogen and
carbon dioxide using the US Bureau of Mines Tests can be seen in Figure 62 . From these
diagrams, the maximum concentration of isobutane in an inerting atmosphere to prevent the
mixture from becoming flammable can be determined. These concentrations are provided in
Table 3.
Table 2 Flammability Limits for Isobutane in Air
Test
US Bureau of Mines2
CEN Test4
Lower Limit
1.8
1.55
Upper Limit
8.4
8.4
9.5% C4H10
5.4% C4H10
Figure 6 Flammability of Isobutane with Nitrogen or Carbon Dioxide2
B-8
Table 3 Maximum Isobutane Concentrations to Prevent Flammability
Gas
Carbon Dioxide
Nitrogen
Argon
US Bureau of Mines2
9.5
5.4
Not Available
CEN4
7.95
4.1
2.4
5.0 Flammability Limits of Isobutane, Argon, Carbon Dioxide
Mixtures
HAI evaluated the flammability of various isobutane, argon, carbon dioxide mixtures. The
compositions of these mixtures are shown in Table 4. Flammability diagrams are not available
for these mixtures.
Table 4 Isobutane, Argon, Carbon Dioxide Mixtures
Mixture
Mixture 1
Mixture 2
Mixture 3
Argon
2.5
4
12
Isobutane
9.5
8
8
Carbon Dioxide
88
88
80
The flammability of a hydrocarbon mixture, such as isobutane can be determined based on
adiabatic flame temperatures. A mixture is considered flammable if the calculated adiabatic
flame temperature of the mixture is at or above the calculated adiabatic flame temperature of the
experimentally determined flammable limit mixture. The adiabatic flame temperatures were
calculated using the STANJAN chemical equilibrium program.5
For typical hydrocarbon mixtures the lower limit adiabatic flame temperature is approximately
1600 K (150 K).1 The adiabatic flame temperature of the mixture increases as the mixture
approaches the stoichiometric point and decreases from that point to the lower limit. Zabetakus6
has determined that the upper limit adiabatic flame temperature using carbon dioxide as a diluent
with propane and methane is 60-110 K higher than flame temperatures using nitrogen as a
diluent, while argon is approximately 10 K lower than nitrogen when used as a diluent for these
hydrocarbon gases.
The adiabatic flame temperatures for the isobutane, argon, carbon dioxide mixtures are between
that of nitrogen and carbon dioxide, based on the work of Zabetakus. The adiabatic flame
temperatures were calculated for an isobutane-carbon dioxide limit mixture, an isobutanenitrogen limit mixture, and the argon, isobutane, carbon dioxide mixtures in Table 4, using the
US Bureau of Mines Flammability Data.2 These results can be seen in figure 7. Due to
experimental error in calculating flammability limits, such as in making precise mixture
concentrations, and impurities in the gases, the calculated adiabatic flame temperatures of the
limit mixtures could vary by up to 90 K from the tested value. These errors are denoted as error
bars in figure 7.
B-9
2500
Adiabatic Flame Temperature (K)
2000
1500
C4H10-CO2
C4H10-N2
8% C4H10 4% Ar
88% CO2
1000
8% C4H10 12%Ar
80% CO2
9.5% C4H10 2.5% Ar
88% CO2
500
0
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
Equivalence Ratio
Figure 7 Adiabatic Flame Temperatures for Isobutane-Gas Mixtures
B-10
1.5
6.0 Results/Conclusions
The results of these calculations show that Mixture 1 (2.5% argon, 9.5% isobutane and 88%
carbon dioxide) has a flame temperature above that for pure isobutane-carbon dioxide. This
indicates that the proposed mixture could become flammable when mixed with the correct
proportions of air. Mixture 2’s flame temperature is below that calculated for the limit
temperature with nitrogen, although it is still within the error bar for the carbon dioxide limit
flame temperature. Mixture 3 has adiabatic flame temperatures just above that for nitrogen (and
within the error bar for carbon dioxide). This would indicate that both of these mixtures are at the
edge of the flammability limits, but could be tested as flammable when tested at room
temperature conditions using the US Bureau of Mines test criteria.
The flame temperatures calculated in figure 7 were based on ambient conditions (1 atm, 20 C).
Given that Mixtures 2 and 3 are so close to the adiabatic flame temperature to propagate flames
in the US Bureau of Mines test, it is very likely that small increases in temperature and/or
pressure would cause these mixtures to have flame temperatures above that for carbon dioxide at
ambient conditions. Additionally, all of the mixtures would be considered flammable when
tested in accordance with the CEN test criteria. This is due to the concentration of isobutane
being above the CEN maximum isobutane concentration to prevent flammable concentrations.
When dealing with flammable mixtures or potentially flammable mixtures, the mixture is either
kept outside its flammability limits or devices designed to deal with the explosion (suppression
and/or venting). If the mixture is designed to be kept outside the flammability limits a safety
factor is typically employed. These safety factors range between 2 and 4 depending on how well
the mixture can be controlled.
The three mixtures are at the edge of the flammable limits for isobutane. Therefore, HAI
recommends that all three gas mixtures be considered flammable gases.
B-11
7.0 REFERENCES
1. Beyler, Craig, Flammability Limits of Premixed and Diffusion Flames, Section Two, Chapter
7, The SFPE Handbook of Fire Protection Engineering Third Edition, National Fire
Protection Association, Quincy, MA 2002.
2. Coward, H. F., Jones, G. W., Limits of Flammability of Gases and Vapors, Bulletin 503,
Bureau of Mines AD-701 575, 1952.
3. Determination of Explosion Limits of Gases Vapours and Their Mixtures, prEN 1839,
European Committee for Standardization, February 1995.
4. Flammability Gas Safety Code Rev G, CERN, November 1996
5. Reynolds, W. C., The Element Potential Method for Chemical Equilibrium Analysis;
Implementation in the Interactive Program STANJAN. Department of Mechanical
Engineering, Stanford University, Palo Alto, CA, 1986.
6. Zabetakis, M. G., Lambiris, S, Scott, G. S. Flame Temperatures of Limit Mixtures, Seventh
Symposium (International) on Combustion At London and Oxford 28 Agust-3September,
1958, The Combustion Insitute, Butterworths Scientific Publications, 1959 pg 484-487.
7. Zabetakis, Michael G, Flammability Characteristics of Combustible Gases and Vapors,
Bulletin 627, Bureau of Mines 1965.
Jc/ekb 100803
B-12
APPENDIX C
C-1
SLAC Memorandum
Date:
March 22, 2004
From: R. Messner, Frank O’Neill
To: Ed Budnick, Hughes Associates, Inc.
We are writing this note to outline a procedure that determines allowable gas mixtures for the
Limited Streamer Tube (LST) system. The gas mixture we are considering is a three component
mixture, consisting of argon, isobutane, and carbon dioxide. The optimum proportions of gas
have not been determined from the point of view of chamber performance, but they are expected
to be in the range of 8-10% isobutane, 2.5-3% argon, with the remainder carbon dioxide.
The ultimate goal is to avoid restricting the physicists to a specific combination of gases, and
instead to outline all the mixtures of argon, isobutane, and carbon dioxide that would be
considered non-flammable and hence safe to use in the BaBar detector. We can do this by
providing a contour of the boundaries of the flammable range of the various gas mixtures in air;
in tandem with this we also want to devise a mixing system with sufficient safety factors built in
to maintain the gas mixture within the safe regions prescribed by those boundaries.
This note will go over a number of issues that we have found important to consider. We require a
standard to determine the flammability of gas mixtures as well as the relevant measurements for
the gases we intend to use. It turns out that the flammability measurements available in the
literature are only for mixtures of two gases, one being the inert gas and the other isobutane (or
butane, which is equivalent.) Consequently we need to extrapolate from these measurements to a
three component mixture of isobutane with two inert gases which is then mixed with air.
The numbers available in the literature that define the flammability limits of gas mixtures are not
all consistent with one another. There are what appear to be contradictory measurements of the
flammability limits for the same mixtures; however, upon closer examination, the source of the
discrepancy is seen to be due to differences in the measurement methods and, as a consequence,
the empirical definitions of flammability.
Two papers published by the Bureau of Mines deal with the issue of the definition of the
flammability of gases; moreover, they have become world standards. The first is “Flammability
Characteristics of Combustible Gases and Vapors”, by Michael G. Zabetakis, Bureau of Mines
Bulletin 627, 1965, reproduced by the U.S. Department of Commerce, National Technical
Information Service (Paper 1). The second is “Limits of Flammability of Gases and Vapors”, by
H.F. Coward and G.W. Jones, Bureau of Mines Bulletin 503, 1952, reproduced by the U.S.
Department of Commerce, National Technical Information Service (Paper 2).
We have found that it is important to make a distinction between the concept of ignitibility and
the concept of flammability if one is to understand the differences among definitions. The
introduction of Paper 2 contains the following observations which discuss exactly this point.
“The experimental determination of limits of flammability is more difficult than may be
expected, as is shown by the contradictory figures reported from time to time. This
C-2
bulletin presents the results of a critical review of all figures published on the limits of
flammability of combustible gases and vapors when admixed with air, oxygen, or other
“atmosphere.”
When a source of heat of sufficient size and intensity is introduced into a weak mixture,
some combustion occurs, even when the mixture is incapable of self-propagation of
flame. This is often visible as a “cap” of flame, which may be large if the source of heat
is ample. The flame cap may be fixed in relative position to the source of ignition, as in a
miner’s flame lamp burning in a gassy atmosphere, or may become detached from the
source and float for a limited distance in a moving atmosphere, or may travel away 2 or 3
feet from an initiating spark or flame in a still atmosphere. Such flames are not selfpropagating, as they are extinguished when the influence of the source of ignition is lost.
When a weak source of ignition is employed, some flammable mixtures, especially those
near the limits, may not inflame. The source of ignition is not strong enough to be
satisfactory for the test.
As the test concerns the capability of the mixture to propagate flame, not the capacity of
the source of energy to initiate flame, it is axiomatic that the limits are unaffected by
variations in the nature and strength of the source of ignition. When statements are made
that limits vary according to the means of ignition, it is clear that the observers used
either such strong sources of ignition that the caps of flame gave the appearance of
general inflammation or such weak sources that flame was not started in mixtures which
were, in fact, flammable. Under these conditions they were determining the limits of
ignitibility by the particular sources of ignition they used, not the limits of flammability
of the mixture itself.”
Note that whether a mixture is truly flammable as distinguished from merely ignitable is
determined by the capacity of the gas mixture to self-propagate a flame. The Bureau of Mines
bulletins present flammability limits which require such a self propagation of the flame.
At least two other standards for determining flammability limits exist. One is the ISO
10156:1996 standard which, as far as I understand it, is a close replication of the Bureau of
Mines standard. The second is the CEN standard, which is frequently used by CERN (see, for
example, the tables in the CERN Flammable Gas Safety Code.) However, upon closer
examination, the CEN standard can be seen to be a measurement of the ignitibility of a gas
mixture as it requires only a “cap” of flame removed a little from the ignition source. As such, it
entails a built in safety factor over the flammability standards involved in the Bureau of Mines
limits. We prefer to build a safety factor into the design of our mixing system rather than into the
definition of the boundary of the allowed gas mixtures.
We feel that the measurements presented in the Bureau of Mines bulletins represent the most
reliable determination of the limits of gas mixtures which would present a fire hazard to the
BaBar detector. The Bureau of Mines measurements were made in response to the needs of a
number of industries, including the mining, petroleum and gas-manufacturing, and those
involving the use of anesthetics, insecticides, and solvents. The design of the experimental
C-3
conditions and apparatus employed for the measurements included the consideration of a number
of practical factors that influence flame propagation. These include, for example, a discussion
about the height and width of the vessel used to test for a flame; that is to say, the criteria are
grounded in practical experience.
We will use the Bureau of Mines limits as our primary reference; if needed, we will reference the
ISO limits; and finally, as a last resort, we will use the CERN measurements (which, of course,
also include a safety factor.)
The Bureau of Mines report by Coward and Jones (Paper 2) contains graphical representations of
the flammable regions for isobutane and various inert gases in air. An example is shown in
Figure 1, taken from Figure 34 in Paper 2, which shows the flammable regions for isobutane and
carbon dioxide in air, and for isobutane and nitrogen in air. We have added red hatching to
indicate the range of mixtures of isobutane and carbon dioxide that are flammable. Before we
can use this graph for our purpose, however, we need to take note of the unusual (to us)
description of the amount of inert gas in the mixture, namely “Added Inert Gas in Original
Atmosphere, Percent” (my underlining.) The most useful specification of the amount of inert gas
for this type of plot is its percentage in the final gas mixture because we can use this to determine
what mixtures of isobutane and an inert gas that will never be flammable when mixed with air.
This will be explained below.
C-4
Figure 1. Figure 34 of Paper 2.
In Paper 1, Zabetakis also takes note of Coward and Jones’ representation of the inert gas
concentration before changing the coordinates of the plot to produce his own set of graphs (page
27 of Paper 1):
Coward and Jones have (Paper 2) have presented graphically the limits of flammability of
the first six members of the paraffin series in air containing various inerts, based on a
representation found useful in some mining applications and treating inert gas or vapor as
part of the atmosphere with which the combustible is mixed. The composition of a point
on such a diagram, except one that represents only combustible and air, cannot be read
directly. Instead, one must determine the composition of the atmosphere, add the
combustible content, and then compute the total mixture composition. This has been done
for methane through hexane (figs 28-33). (Italics mine.)
We have made the equivalent transformation of coordinates to produce the plot in Figure 2
below. This uses the butane volume percent on the vertical axis and the carbon dioxide volume
percent on the horizontal axis. We carefully read off the numbers from the graph by Coward and
C-5
Jones; we checked the faithfulness of our numbers by reproducing our version of the original
graph on a transparency which could be overlaid on the original plot. Then we performed the
transformation on the numbers for the amount of carbon dioxide to produce Figure 2. This way
we have made use of modern computing techniques which we feel give us a better representation
of the original data of Coward and Jones than we would have by using Zabetakis’ version.
(Zabetakis’ plot does not retain the original data points and probably was made with French
curves to produce the outlines of the flammable boundaries.) The boundary of the flammable
region in Figure 2 was produced via a smoothing technique, but this is only to guide the eye and
is not used in the analysis. We can use the data points from the plot by Coward and Jones
directly, since they thoroughly map out the nose of the flammable region.
The advantage of specifying the volume percent of both the butane and the carbon dioxide on the
axes of the graph is the following: any constant mixture of carbon dioxide and isobutane can be
represented as a line starting at the origin and drawn upwards and to the right. Different
combinations of air and the specific mixture of butane and carbon dioxide will fall somewhere
upon the appropriate line. Moreover, the line that is just tangent to the boundary of the
flammable region represents the maximum concentration of isobutane in carbon dioxide that will
never become flammable when mixed with air, first because it never enters into the flammable
region and second because it has the largest slope. (Note also that the intersection of the
boundary of the flammable region with the vertical axis, corresponding to pure isobutane with no
added inert gas, gives the upper and lower flammable limits of isobutane in air.)
C-6
Percent CO2 in final mix
Tangent Line
Data_from_Cowards_Flammability_Plot
10
8
6
4
2
0
0
10
20
30
40
50
Percent CO2 in final mix
Figure 2. Graphical representation of the region of flammability of isobutane and
carbon dioxide in air, copied from the data of Figure 34 of Paper 2, but with
the amount of CO2 computed to be that in the final gas mixture.
For isobutane in carbon dioxide, Figure 2 gives a maximum isobutane concentration of 9.7% in
carbon dioxide (obtained from reading the point with the values of 4.3% isobutane and 40%
carbon dioxide) for the mixture to be non-flammable in air. This number is called the Tci value in
the CERN gas manual; the Tci value is defined to be the concentration of a flammable gas in a
mixture with an inert gas for which the mixture is just not flammable in air.
C-7
Carbon Dioxide
Nitrogen
10
8
Tangent Lines for Mixtures of N2 and CO2
6
4
Point 1,
pure CO2
2
Point 2, pure N2
0
0
10
20
30
40
50
Percent Inert in final mix
Figure 3. A copy of Figure 34 of Paper 2, but with the amount of inert gas computed
to be that in the final gas mixture. We also want to illustrate graphically
the idea of ‘averaging’ the flammable regions of two inert gases.
We are really interested in determining the flammable region of isobutane in a mixture of two
inert gases. We have not found any empirical measurements for such mixtures at this time.
Consequently the question is how to combine the knowledge of the properties of isobutane in the
two individual inert gases and determine the Tci of isobutane in an arbitrary combination of the
two inert gases. Common sense tells us that the Tci values for combinations of gases should be an
average of the properties of the two gases so long as no new chemical potential is introduced.
Since argon is a noble gas, this should be applicable to us.
One prescription we found in the ISO 10156:1996 standard converts each inert gas component to
an equivalent amount of nitrogen, where the quantity of nitrogen is determined by a coefficient
of equivalency for each inert gas relative to nitrogen. The ISO 10156:1996 standard gives a table
of approximate coefficients, but more accurate measurements can be used instead. The resultant
mixture is then tested to see if it satisfies the flammability criterion for nitrogen. This amounts to
taking a weighted average of the Tci of isobutane in each of the inert gases with the weighting to
be done by the size of the Tci value for each inert gas relative to the Tci of nitrogen. This is an
approximation, and is not always appropriate for all gases; again we note that we are considering
argon, so its participation in combustion is minimal. Its effect can be considered a dilution of the
heat carrying capacity of the carbon dioxide and hence we expect that this kind of approximation
C-8
should be valid. We note that the ISO expression is again a linear function of the two inert gases;
it averages the heat transfer properties of the two gases.
Figure 3 will illustrate the approach we have decided to take: the flammable regions for mixes of
isobutane in an inert made by combining carbon dioxide and nitrogen must lie between the
extremes of isobutane in pure carbon dioxide and isobutane in pure nitrogen. We have decided to
use the two boundary conditions given by the Tci of isobutane in pure carbon dioxide and in pure
argon and use a linear extrapolation between the two extremes.
First we need to make a special graph to represent the transition from isobutane in one inert to
isobutane in a mixture of the two inerts and finally to isobutane in the second inert. In Figure 4a
we make a plot showing the concentration of isobutane on the vertical axis versus the
concentration of argon on the horizontal. The amount of carbon dioxide is always to be taken as
100% minus the percentage of argon and minus the percentage of isobutane. The plot can be
seen to represent the entire region of possible three component gas mixes so long as one stays
within the ‘physical’ boundary.
We use the two points obtained from the boundary conditions and draw a line between them. The
line thus drawn represents the Tci limits of isobutane in the complete range of inert gas mixtures.
Note that when the argon content is zero, the gas mixture is isobutane in pure carbon dioxide,
and the Tci is 9.7%. When the carbon dioxide content is zero, the Tci is 2.4%, the Tci of isobutane
in pure argon. We use a CERN value for argon because we have no number for the Tci of
isobutane in argon from the Bureau of Mines. In Figure 4b we concentrate on the region of the
plot containing the line between the two boundary conditions; we also show the line obtained if
we use the CERN Tci values for isobutane in both argon and carbon dioxide to illustrate the
differences between determining the allowable region using the Bureau of Mines flammability
numbers on one hand and using the CERN ignitibility numbers on the other.
The gas mixture is flammable above the line and non-flammable below the line. We are most
interested in regions with very little argon, which is close to the pure carbon dioxide
measurement; this is where we expect the linear approximation we have used to be quite good.
An expanded view of the region around zero argon concentration is given in Figure 5.
C-9
Representation of a three component gas
100
%CO2=100-%Ar-%Isobutane
80
Not 'Physical'
60
Possible Mixes
40
CERN T
Bureau of Mines T
20
ci
ci
0
0
20
40
60
80
100
Per Cent Argon
Figure 4a. The representation of a three component gas. Physical mixtures are below
the blue line. Flammable and non-flammable regions of CO2-Isobutane-Argon
Mixtures in air are separated by the red line.
This graph gives the experiment the kind of information it needs to know in order to decide if an
arbitrary gas mixture will be considered non-flammable and suitable for use in the detector. Once
a particular mixture is chosen, it can also be verified by an empirical test to be non-flammable
and small adjustments made if required.
A graphical interpretation of the nature of the boundary plotted in Figures 4 and 5 can be seen by
going back to Figure 3. Consider the flammability contours for isobutane in CO2 and again in
nitrogen for illustrative purposes. The flammability regions for different mixtures of CO2 and
nitrogen will be contours that lie between the contours for either of the two gases alone. We are
taking 1) the point of intersection between the isobutane and CO2 contour and the line
representing the maximum isobutane making a non-flammable mixture of isobutane and CO2,
illustrated as Point 1; and 2) the point of intersection between the isobutane and nitrogen contour
and the line representing the maximum isobutane making a non-flammable mixture of isobutane
and nitrogen, illustrated as Point 2. A third line drawn between these points represents a “path”
of the point of intersection between the contours of the nitrogen/CO2 mixtures and the tangent
line corresponding to the maximum non-flammable mixture of isobutane and the nitrogen/CO2
C-10
mix. The displacement along the line is determined by the Tci value for the particular
nitrogen/CO2 mix. Figures 4 and 5 represent the equivalent information for mixtures of carbon
dioxide and argon.
Per Cent Argon (using CERN LEL numbers)
Per Cent Argon (using BOM for CO2; CERN LEL for argon)
10
Bureau of Mines T
ci
CERN T
8
ci
6
CERN T
ci
4
2
%CO2=100-%Ar-%Isobutane
0
0
20
40
60
80
100
Per Cent Argon
Figure 4b. Comparison of the allowable regions determined by the use of the
CERN and Bureau of Mines (BOM) determinations of the
Tci point of isobutane in carbon dioxide.
C-11
Per Cent Argon (using BOM for CO2; CERN LEL for argon)
12
Flammable
10
8
Non-flammable
6
4
%CO2=100-%Ar-%Isobutane
2
0
0
5
10
15
20
Per Cent Argon
Figure 5. An expanded region of Figure 4b.
In fact, we can show that the gas mixtures represented by the lines in Figures 4 and 5 represent
EXACTLY a combination of two nonflammable mixtures (both of which are right at the Tci
limit), with the first mixture being isobutane and argon and the second mixture being isobutane
and carbon dioxide; the two portions of isobutane add up to the quantity of isobutane derived
from the linear extrapolation. Imagine that one has 100 liters of gas. Pick the amount of argon to
be 20 liters (20% argon point.) The maximum isobutane in 20 liters of argon via the Tci
limitation is 0.4918 liters; that leaves 100-0.4918-20 liters for the isobutane/CO2 mix. The
amount of isobutane one can have in that is 0.097*(100-0.4918-20) = 7.7123 liters. Add the two
amounts of isobutane together and one finds a net 8.204 liters of isobutane, in 20 liters of argon
and 71.796 liters of CO2. This point (and any similarly generated point) falls exactly on the line
in Figure 4b. Coward’s review in Paper 2 mentions Le Chatelier’s principle; this has the
conclusion that lower limit mixtures of gases, if mixed in any proportions, give rise to mixtures
that are also at their lower limits (page 5.) This is exactly equivalent to what we are doing.
In summary, we feel we have a well defined procedure to establish a boundary below which any
given gas mixture is non-flammable.
C-12
The final step is to determine how far from the boundary between the flammable and nonflammable regions to run the apparatus. The driving considerations include the accuracy and
repeatability of the gas mixing system. The proposed mixing system will use mass flow
controllers; these have a quoted accuracy of 0.5% of the flow controller’s full scale, with a
repeatability of 0.2%. Input flows of the three gases will be metered by the mass flow controllers
to make a precise gas mixture when the three flows are combined; the controllers for two of the
gases can be slaved to the output reading of the third gas, so that the gas mixture remains
constant independent of the instantaneous flow rate.
The isobutane concentration will be cross checked independently of the mass flow meters
themselves on a regular basis; this has been done for the RPC gas system using LLNL gas
analysis resources. The errors for this kind of measurement have been quoted as 0.25-0.5% of the
relative isobutane concentration.
The performance of the actual mixing control system will be continuously monitored by a
second, independent set of inline mass flow meters, one of which will meter the flow of a
particular gas component; the output level from each of the meters will go into a DC Process
Controller. Each controller in turn will then be set to open a relay if the particular flow it is
monitoring drifts outside of a small window around the nominal rate. An open relay from any of
the three controllers will cause solenoid shutoff valves in all three of the supply gas lines to
close, shutting off the system. We have named the unit containing the Process Controllers and
the solenoid control hardware the Gas Interrupt Box. The Process Controllers have a quoted
accuracy of 0.2%. Note that the mixing system and the monitor system will be independent of
one another. It should be emphasized that the monitor system will shut off all the gas flows if
any one gas flow is detected to be drifting outside of preset limits; this is something that will be
done in hardware, independent of operator intervention. Readings will be also monitored by the
online monitoring system and recorded for later analysis, and alarms passed to the experimental
operators in real time.
We have studied the stability of a similar gas mixing system in use for the RPC chambers
currently installed in BaBar. We find that the stability of the isobutane fraction of the gas
mixture is much better than 1% of the nominal value; a window of +- 0.5% is sufficient to
contain the variations. As indicated by the frequency plot given in Figure 7 the standard
deviation of the isobutane fraction is 0.25% of the nominal value over periods longer than a
month. This observed performance suggests that we can operate quite close to the boundaries of
the non-flammable regions without expecting excursions into the flammable regime.
We propose to define a rule that sets the RUNNABLE region by the use of a second line to
define an operational boundary; this is to be constructed by reducing the net allowable isobutane
concentration in the mix by a fixed percentage from the line separating the flammable from the
non-flammable regions. This can be done if we take the Tci point for isobutane in carbon dioxide
and rescale the amount of isobutane by a fixed amount that reflects, as one consideration, the
stability of the mixing system; likewise we would produce a second number constructed by
scaling the Tci point for isobutane in argon. Then a line would be drawn between these points.
The scale factor by which the Tci points are reduced can reflect the uncertainty in the
C-13
determination of the value of the Tci point itself as well as the precision and accuracy of the gas
mixing system.
Figure 6. Snapshots of the isobutane concentration for the RPC system; these are
five twelve hour periods taken a week apart over a span of five weeks.
Since experience demonstrates that a gas system of the type we are intending to build can run
within a window +-0.5% of nominal, we can achieve a reliable separation from the flammable
region if we reduce the set points for the amount of isobutane in the mix by 3% from the Tci
points. Thus 9.7% isobutane in carbon dioxide becomes a point at 9.41%, for a 3% reduction in
the amount of isobutane. The 2.4% point for isobutane in argon would become 2.32% isobutane.
A line drawn between these points represents the boundary between allowed and ‘not-allowed’
gas mixtures for actual operation. Meanwhile the Gas Interrupt Box will have the Process
Controllers set to stop all gas flow if a particular gas component drifts 2% from nominal. This
value has been chosen to avoid nuisance trips while maintaining a suitable separation from the
flammable region.
A visual summary of our scheme is illustrated in Figure 8. The red line represents the boundary
between the flammable and non-flammable regions of gas mixtures. The blue line represents the
operational boundary below which all gas mixtures must lie in order to be acceptable for use in
the LST detector. The example point is set well below both boundaries to emphasize that the gas
mixture can be anywhere below the blue line. Finally, the dashed green line represents a third
type of boundary. We expect any actual delivered gas mixture to remain below the dotted green
line; the location of the line includes the effects of drifts in the mixing system. This results from
a combination of a limit on the set point of the gas mixing system, the blue line, in conjunction
with the observed stability of the operation of the RPC mixing system.
C-14
Percentage Isobutane
2000
1500
-1 % of nominal
+1 % of nominal
1000
500
0
Range
Figure 7. The frequency distribution of the RPC isobutane concentration sampled over a period
of five weeks as shown in Figure 6. The arrows show a window of +- 1% of the nominal value.
The standard deviation is approximately 0.2 % of the nominal value.
C-15
Per Cent Argon (proposed limit, 9.7% isobutane at 0% argon)
Runnable Region (9.41 % isobutane at 0% argon)
Runnable Region + 1.0% drift
Example Point (8.65% isobutane, 3% argon)
Compare_errors_vs_flammable_region_II
10
%CO2=100-%Ar-%Isobutane
9.5
9
8.5
8
All Operational Settings Are To Be Below the Blue Line
7.5
0
2
4
6
8
10
Per Cent Argon
Figure 8. The range of potential LST gas mixtures, showing the boundary between flammable
and non-flammable mixtures in red, and the boundary limiting
the range of the allowed operational settings in blue.
A visual presentation of the functionality of the gas mixing system and the gas monitoring
system is given in Figures 9 and 10. The commissioning of the system will include verification
of the accuracy and stability of the new system to the standards demonstrated above and will also
include verification of the performance characteristics of the Gas Interrupt Box and its
monitoring system.
C-16
Mass Flow Meters
Pressure Sensors
CO2
Mass Flow Controllers
CO2
Ar
Ar
Iso
Iso
Flow Control Box
Shut-off
solenoids
will close
if the gas
mix drifts
out of the
allowed
region
Vent
Gas Interrupt Box
(homemade)
Monitors gas flows;
will shut off inputs if
flows or ratios are
not correct
Mass Flow Meter Readings
One master flow;
slaves set by a
ratio to actual flow
of master
Status
Sample
Port
A fourth mass
flow meter can
provide a tiebreaking vote
in case of
MFM/MFC
disagreement
Can also pick
up small drifts
in mixed gas
flows
Figure 9. Functionality of the gas mixing system, with the gas interrupt box included.
The gas flow is controlled by mass flow controllers. The inline mass flow meters
confirm the operation of the mass flow controllers and shut off the system
if any gas flow deviates a set amount from the nominal value.
C-17
Reading In
From Flowmeters
Simpson
0-5 Volts
CO2
Simpson
2
S
%
S
Mix
2
%
Reading In Range?
Simpson
Iso
2
%
S
2
S
%
Reading Out
To Monitor
Trip?
Reading In Range?
To Monitor
Solenoid
Power
To Solenoids
Argon
Simpson
Shut Off Power
Reading In Range?
To Monitor
DC Process Controllers
With on/off control
Will trip when any value
gets out of range
Figure 10. Functionality of the gas interrupt box. The readings from the mass flow meters are
used as input to DC Process Controllers. If any flow deviates sufficiently from the nominal
value, the gas interrupt box will shut off all gas flow by allowing the solenoids
on the gas input lines to close. Note that there is a fourth mass flow reading
made of the total mixed gas flow which will also be monitored.
C-18