Transcript A presentation at the MICE Video Conference on Wednesday the 5th of May 2004 Response to the Safety review comments by Wing Lau --

A presentation at the MICE Video
Conference
on Wednesday the 5th of May 2004
Response to the Safety review comments
by Wing Lau -- Oxford
Safety Review Panel – Main Points – status review
• Hydrogen Gas Handling & Venting system
• Remove buffer tank and vent the hydrogen out directly - implemented
• Remove relief valves in the hydrogen vent lines and have burst disks only –
retained
•
Completely separate vent system for the absorber and vacuum spaces -
implemented
•
Detail specification of the Relief valve – work in progress
• Is hydrogen detector appropriate in the vacuum line – still under consideration
• Hydrogen detectors are needed in the ventilation system and in the personnel space
around the experiment – will be implemented
•
Examine the level to which piping should be Argon jacketed – will be addressed
•
Replacing the flame arrestor with a vent pipe with an inert atmosphere -
implemented
• Adopt Fermilab requirement vacuum system volume 52x H2 liquid volume – not
implemented
Safety Review Panel – Main Points
• R & D on the Metal Hydride system
• The use of hydride system requires active control.
• The panel suggested a scaled model test.
•
It also asked the group to examine the safety issues
associated with this system
• R&D proposal defined and submitted
Safety Review panel – Additional Points
• Practicality of using intrinsically safe electrical
equipment – response already drafted
• Pipe joints – will be as requested
• Detection of Hydrogen in Personnel areas – agreed
• Attention to Interlocks, alarms and control system ongoing.
• Continuation of HAZOP assessment – agreed
• Response to Absorber system leak scenario ongoing
• Potential of liquid hydrogen sloshing in warmer part
of the feed pipe – to be addressed in level control.
• Leak between the helium and hydrogen
compartment in Absorber unit - ongoing
Here is the final version of the report – printed in Powerpoint
format for the purpose of this presentation, but will be in MS WORD
format in the actual report.
3 main comments made by the Review Panel
1
The issue
The Safety Review panel’s comments
Hydrogen Gas
Handling &
Venting
system
Can we do without the buffer tank and just
vent the hydrogen out directly?
This is now implemented. See Appendix 1.
Can we get rid of the relief valves in the
hydrogen vent line and have burst disk
only?
The issue of having relief valves as well as burst discs is that the relief
valves will close again after activation and prevent back streaming into
cold spaces. For this reason, we do not recommend getting rid of the
relief valves. See Appendix 2 for details
Completely separate vent system for the
absorber and vacuum spaces
There is no reason why we can not have separate vent lines for the
Absorber and Vacuum space. In particular if the bilge space at the base of
the Absorber is implemented then the hydrogen release in the event of a
catastrophic event will be more controlled. See Appendix 2
Detail specification of the Relief valve
Is hydrogen detector appropriate in the
vacuum line?
Work is in progress to have detail specification of the Relief valve. The
issue of how effective the hydrogen detectors are and where they are
placed needs to be considered further and we are aware of the problem.
Work is in progress to address this issue.
The panel suggested that hydrogen
detectors are needed in the ventilation
system and in the personnel space around
the experiment
We are aware of this need. Hydrogen detectors will be placed in the
ventilation system and around the equipment. It is just that they were not
shown on the diagram. Refer to Appendix 2 for diagram.
Our Reply
The issue
2
R & D of the
Metal
Hydride
system
The Safety Review panel’s comments
Our Reply
Examine the level to which piping should
be Argon jacketed.
Argon jacket will be placed around any portion of the absorber vacuum
space that may come into direct contact with the air. The pipes that
connect from the absorber vacuum vessel to the relief devices or the
vacuum pump should also be argon jacketed. The vacuum pump should
pump from the absorber vacuum to a space with inert gas (argon or
nitrogen) so that no back streaming of oxygen occurs back through the
vacuum pump. The exit pipe from the pump should have an inert gas
between the pump and a check valve in the line at eventually leads to
air. As long as there is a relief valve or a burst disc between the absorber
vacuum and the buffer vacuum, one does not have to argon jacket the
buffer vacuum. If there is no relief device between the buffer vacuum
and the absorber vacuum the buffer vacuum has to be argon jacketed.
Piping outside the ventilation hood, i.e. on the lines between the hood
and the experiment will be argon jacketed
Replacing the flame arrestor with a vent
pipe with an inert atmosphere
This will be implemented in our design
Possibility of adapting the Fermilab
requirement concerning the vacuum system
volume relative to the liquid volume
We have looked at the adapting the Fermilab requirement and concluded
that this will not apply to the RAL requirement.
Please also see additional comments in Appendix 2 below
The use of hydride system requires active
control. The penal suggested an scaled
model test. It also asked this group to
examine the safety issues associated with
this system.
We have developed a plan for testing the hydride system. The plan
involves the building of the hydrogen system for the first MICE Absorber
cell. As part of this process we have been looking at how the system will
be controlled which has led to some small changes in the
instrumentation. We have received better information from the supplier
on the behaviour of the hydride in the beds. A plan has been submitted
for inclusion in the development programme for the coming financial
year – see appendix 3.
The issue
3
Window
development
The Safety Review panel’s comments
Our Reply
Continue development of the welded seal.
Demonstrate performance of Indium seal is
reliable and repeatable under thermal
cycling conditions.
What actions to take if pressure is detected
in the inter-space between the double seal.
Monitoring and interlock triggers to be
more specific.
The R&D programme on the welded Window option has already
addressed this point. Thermal couples will be attached to the window to
flange junction and on various locations at the window to see what
temperature they can get to during welding.
The selection of a bolted window will be conditioned to the satisfactory
demonstration of the reliable and repeatable.
If the leak rate is larger than 10-2 torr /s, we stop the cold-He cooling and
warm up immediately.
If the H2 leak is smaller than 10-2 torr /s, we continue the monitoring,
and repair the seal at next chance.
This means that we can pump the H2 gas at the vacuum better than 10-4
torr using 100 litre/s pump.
11 further safety issues recommendations by the Review panel
The issue
The Safety Review panel’s comments
A
Practicality of
using
intrinsically safe
electrical
equipment
This may not always be possible. We
should consider turning off all the
electric power without causing additional
hazard whenever hydrogen gas is
detected
Most of the standard cryogenic probes are well within the “intrinsically safe” power
limits set by the NEC. In addition, pressure valves and other equipment can be made
“intrinsically safe” by straightforward modifications (usually, a sealed cover or
container). We have to carefully consider what the thresholds for some minor action
(increase or decrease the metal hydride bed) as opposed to major action (system purge,
power shut-off). A similar set of questions are being reviewed for the “forced-flow”
LH2 absorber test at FNAL.
B
Effect of stray
magnetic field
Hazard with stray magnetic field which
causes tools and other equipment to
become projectiles.
Access into the experimental hall will be restricted and the area around
the experimental hall will be fenced.
The Operating Procedure will request that a scientist / engineer on shift
before switching on power supply for magnet to go into the fenced area
and to check and remove all the tools or subjects which might become
projectile.
C
Exposure of
thin window
Equipment shall not be operated with
thin absorber / vacuum windows exposed
The Operating Procedures will contain this requirement.
D
Pipe joints
All hydrogen gas system should be
welded pipes, VCR, conflate and other
flanged connections. No Swagelocks and
plastic tubing allowed.
All hydrogen Gas system are welded pipes, VCR, conflate and other
flanged connections. Please see Table 1 below for pipe joint details.
We have agreed to the Panel’s suggestion and will make sure that no
Swagelocks and plastic tubing be applied to the hydrogen piping and
pipe joints.
E
Detection of
Hydrogen in
Personnel areas
Hydrogen Gas detector should be
installed in the personnel areas around
the experiment. Actions taken if
hydrogen is detected should be thought
out.
Hydrogen detectors will be installed in various places inside the
experimental hall.. In case hydrogen is detected these sensors will trigger
the personnel evacuation alarm and a high rate mode of the ventilation
system.
Our Reply
F
Attention to
Interlocks,
alarms and
control system.
Interlocks, alarms and controls should be
carefully thought out and specified.
Response to alarm must not increase the
hazard and unnecessarily affect the
operation.
The design of MICE safety system will take this recommendation into
the account.
G
Continuation of
HAZOP
assessment
HAZOP analysis to be continued and
expanded to include a scenario of the
absorber and vacuum window failing at
the same time.
The HAZOP presented at the Review was only preliminary. We need the
operating modes to be more fully defined before we can go into the next
stages of HAZOP. We will also do the FMECA type analysis for all the
failure modes we can think of.
H
Response to
Absorber
system leak
scenario
To plan in advance how we respond to
leaks or problems of various level with
the absorber system.
A number of possible levels and scenarios of absorber leaks will be
analysed and the appropriate response procedures will be specified in the
MICE Operation Procedures.
I
Potential of
liquid hydrogen
sloshing in
warmer part of
the feed pipe.
Consequence of liquid hydrogen sloshing
as a result of it being pushed to the warm
pipe needs further looking in.
This is being addressed together with our design philosophy on the Level
Control. See our Appendix 4 for our further comments
J
Leak between
the helium and
hydrogen
compartment in
Absorber unit
An understanding of what happens if
there is a leakage developed between the
helium and the hydrogen compartment of
the Absorber unit needed.
This is an on-going activity. We need to assess this problem and how to
deal with it and how to detect the leak etc. The helium system will be
design for 18 bar, leak tested to a high standard after the necessary
thermal cycling as specified in our leak test requirement table.
K
QC on window
thickness
QC standards for window thickness
should be developed.
This is now prepared and will be implemented once approved by the
MICE Project manager.
Appendix 1 ----- Buffer Volumes
• Original Design
• One evacuated buffer volume for both absorber and
vacuum space venting
• Separated from volumes by relief valves
•Assessment from the review
• Buffer volume is more effective if directly connected
• Vacuum space
• RAL safety does not require 12 x volume for
vacuum space around absorber
• Current design gives ~ 8 –10 x volume
• Absorber volume
• Design includes buffer volume in the absorber line
•Window protection – response time
•Simplification of control
Appendix 2 --- Changes in MICE hydrogen system
In summary the AFC Safety Review Panel recommendations are implemented:
• Original buffer vessel is removed
• Vent manifold is added. The manifold is filled with nitrogen.
• Venting lines are separated.
Other changes:
• Buffer vessel is added in between absorber vessel and hydride bed.
• Ventilation system is removed. Most of the equipment is now sits
under hydrogen extraction hood.
Appendix 2 --
Version: 21/11/2003
Hydrogen system baseline layout
VP
P
He Purge system
Vent outside
flame arrester
Metal Hydride storage
unit
(20m3 capacity)
P
P
1 bar
18 K He
14 K He
from Cold box
Chiller/Heater
Unit
Fill valve
P
P
X2
X2
1.6 bar
P
2.0 bar
Purge valve
Liquid level gauge
H2 Detector
Vent outside
flame arrester
P
Vacuum
Ventilation
system
H2 Detector
Internal Window
LH2 Absorber
H2 Detector
Safety window
P
Purge valve
LHe Heat
exchanger
Vent outside
flame arrester
Evacuated vent
buffer tank
2.0 bar
Vacuum vessel
1.6 bar
1.4 bar
VP
VP
P
Pressure
gauge
P
Pressure
regulator
Valve
Pressure
relief valve
Non-return
valve
Bursting disk
VP
Vacuum pump
H2 Gas bottle
Appendix 2 --
Hydrogen system revised baseline layout
High level vent
High level vent
Non return
valve
0.1 bar
Vent manifold
Vent outside
flame arrester
Vent manifold
H2 Detector
Hydrogen zone 2
Extract hood
VP2
PV8
P
P1
Metal Hydride storage unit
(20m3 capacity)
PV7 P
P
PV2
PV1
PV3
Buffer 1 m3
vessel
P
Tbed
1
bar
PV4
Chiller/H
eater
Unit
Tchill
Fill valve
P
HV1
14 K He
in
18 K He
out
Zone 2:
An area within which any
flammable or explosive substance
whether gas, vapour or volatile
liquid, although processed or
stored, is so well under conditions
of control that the production (or
release) of an explosive or
ignitable concentration in
sufficient quantity to constitute a
hazard is only likely under
abnormal conditions.
0.5 bar
P2
P
Hydrogen supply
P
P
P
0.9 bar
Design pressure 1.6 bar abs
Test pressure
2.0 bar abs
Burst pressure 6.4 bar diff
Absorber window
Nitrogen
supply
H2 Detector
Purge valve
HV3
Tabs
Safety window
Helium
supply
HV2
Purge valve
P3
P
Windows:
P
PV6
0.9 bar
0.5 bar
VP1
P
Pressure
gauge
P
Pressure
regulator
Valve
Pressure
relief valve
Non-return
valve
Bursting disk
VP
Vacuum pump
Appendix 3 --- R&D programme on metal hydride storage system
Conceptual question: a small-scale rig vs. a full-scale prototype ?
Decision: go for a full-scale system which later will be used in MICE.
R&D goals:
• Establish the working parameters of a hydride bed in the regimes of storage,
absorption and desorption of hydrogen.
• Absorption and desorption rates and their dependence on various parameters
such as pressure, temperature etc.
• Purity of hydrogen and effects of impurities.
• Hydride bed heating/cooling power requirements.
• What set of instrumentation is required for the operation of the system?
• Safety aspects including what is the necessary set of safety relief valves, sensors
and interlocks.
•Status
•Programme on hold pending funding approval for 2004/05
Appendix 4 -- Hydrogen level control – design considerations
•Level Control – what variations do we need to respond to:
•Level will vary due to temperature changes in the absorber
•Variation in density of LH2 could give ~ 1 – 2 liters volume change
•Such changes cannot be accommodated in small pipes
•25mm dia = 2.2m/liter
•Such level changes will be relatively slow under normal
operating conditions
•Energy to go from 14 – 18K ~ 50kJ for 20 liters
•Nominal heat load /absorber is few W
•Time 14 – 18K is ~ 5 – 10 hrs
•Most significant effect will be intermittent gas boil off due to
changes in level – especially so for the horizontal pipe
Appendix 4 --- Hydrogen level control – design considerations
•Level Control – Where is best place to monitor/control level
•Absorber neck tube
•Insufficient volume
•Horizontal pipe
•Not practical
•Vertical pipe
•Need to thermalise the horizontal pipe
•Small volume available
•Main absorber volume
•Ullage - 2 liters is 10%
•Temperature of absorber body will be uniform
•Increase in volume will cause very little boil off
•Less active role for control system – hydride bed
•External buffer volume 1m^3 could absorb ~ 0.5 –1 litre
before activating the relief system – assuming no return to
the hydride bed - need further work