1. Mechanical Safety Systems and DSEAR Compliance
M Hills

2. Outline Safety Philosophy Design approach and codes followed
Mechanical design overview
Passive safety systems
DSEAR compliance
Lightening protection
Crane use
Summary of Risk Assessment

3. Safety Philosophy Have a safe and usable system
System should be safe against two simultaneous failures
As far as reasonably practicable:
Maintain separation of hydrogen and oxygen atmospheres
Avoid ignition sources in areas where explosible atmospheres may form

4. Approach (from ‘05 review)
All vacuum vessels designed as “pressure vessels” per BS or ASME code
implies testing to 1.25x “design pressure” (pressure where relief valve is set, 1.6 bar)
Done (ASME code)
Absorber and vacuum safety windows designed for 4x design pressure (internal) and 1.7 bar (external) without buckling
Done (Absorber safety windows tested to >8bara)
Two barriers between LH2and possible contact with oxygen
barrier is either window or Ar jacket
Done (transfer line jacketing (with N2) and Test Cryostat safety/insulating vacuum)
Hydrogen evacuation paths for absorber (vent pipe) and storage system (vent hood)
Done (dedicated relief lines for both Absorber Pot and Hydride Bed + ventilated enclosure)
R&D program, including rigorous testing procedures, will serve to validate design

5. Codes and Regulations
DSEAR (Dangerous Substances and Explosive Atmospheres Regulations)
IEC61508 (Functional Safety of Electrical/Electronic/Programmable Electronic Safety-related Systems)
Pressure Equipment Regulations 1999 (“Pressure Equipment Directive”, BS5500 and ASME)
Local codes:
SC33 - Safety of pressure and vacuum systems
SC20 - Controlling explosive and flammable gases and dusts

6. Hydrogen System P&ID Gas Panel Bottle Store Venting Vac. Cryostat
Pumps
Cryostat
Heater/
Chiller
Unit
Hydrogen
Charging Station

7. Passive Safety
Relief valves and burst discs are designed into the system to deal with rapid boil-off safely without the need of the control system
Order of preference is to return gas to the hydride bed first and only relieve it outside the hall if the pressure continues to rise
All relief valves and burst discs are set to ≤1.9bara (burst pressure of absorber windows in AFC > 8bara)

8. Relief Scenario 1
Boil-off in absorber

9. Relief Scenario 2
Rapid Boil-off in absorber

10. Relief Scenario 3
Leak from Absorber Pot into vacuum space

11. DSEAR
Workplace Directive designed to protect employees from the hazards associated with potentially explosive atmospheres
Ultimate aim is to protect people (not necessarily equipment)
Aim is to avoid bringing the three elements of the ‘ignition triangle’ (hydrogen, oxygen and an ignition source) into contact
Approach taken:
Identify ‘Hydrogen Zones’
Specify and procure the correct equipment
Calculate adequate levels of ventilation and ensure this is available

12. DSEAR Zoning Test Cryostat (or AFC absorber vacuum space)
Gas Panel Enclosure
Pump Enclosure
Connecting ductwork
...are Zone 2.
(“A place in which an explosive atmosphere....is not likely to occur in normal operation, but, if it does occur, will persist for a short period only.”)
The MICE Hall is not

13. Equipment Selection
All equipment used in the Zone 2 is rated to at least ATEX category 3 (safe under normal operation).
Vacuum Pumps
Gas Panel Valves
Flow Meters
Pressure Sensors
Ventilation Fans
Spark proof heaters and lighting (inside Vacuum Pump Enclosure)
Gas Panel valves are pneumatically operated from solenoids outside the Hydrogen Zone (i.e. The Gas Panel Enclosure)
All instrumentation and valve read-back inside the Gas Panel Enclosure and Test Cryostat are intrinsically safe (except the heaters – see Phil’s IEC61508 talk)

14. Other Considerations
All pipework that would normally contain hydrogen is either jacketed or within the ventilated area
Cryogenic pipework inside safety/insulating vacuum jacket
N2 jacketed transfer line
Gas panel, buffer tank, hydride bed and relief valves all inside ventilated enclosure
Dedicated ventilated enclosure for vacuum pumps on hall roof
High integrity pipework used throughout
Metal to metal seals
X-ray inspection used to check weld quality
All joints on hydrogen pipes inside the hall are either jacketed or in a ventilated area
Relief lines are routed through the ventilation ducts to contain leaks

15. Ventilation Rates From BS EN 60079-10-1:2009…
Given system design we only consider a “secondary release”
Therefore we need to achieve at least a “medium” degree of ventilation to claim a Zone 2
For the purposes of calculation, medium ventilation is taken to be when:
The hypothetical vapour cloud produced by the release is less than the volume under consideration
The cloud persists for less than 30 minutes
Secondary release: a release which is not expected to occur in normal operation and, if it does occur, is likely to do so only infrequently and for short periods

16. GP Enclosure Ventilation
Full details of the calculation are in the documentation (procedure followed is that outlined in BS EN :2009)
Calculate rate of release (in kg/s) based on properties of the gas and geometry of release
Use this together with the air changes/hour to estimate the hypothetical volume of the vapour cloud and its time of persistence
Assumptions
Ventilation rates are 100 air changes per hour and 450 air changes per hour (upon detection of hydrogen)
NPT and VCR connections are assumed to fail in such a way that would produce a leak cross sectional area of 0.25mm2. [Not necessary to consider a catastrophic failure – i.e. a pipe rupture – under DSEAR.]
Worst case scenario pressure developed inside the pipework is 1.9 bar (burst disc set point)
The availability of ventilation is considered to be fair. [Two fans installed and each one each capable of providing the ventilation rates above.]
Results at 100 air changes/hour
Hypothetical volume = 1.92m3 (Enclosure volume ~ 5m3)
Time of persistence = 7 minutes
Results for 450 air changes/hour
Hypothetical volume = 0.43m3 (Enclosure volume ~ 5m3)
Time of persistence < 2 minutes

17. Vacuum Enclosure Ventilation
Assumptions
The pump enclosure is 3.5m long x 3m wide x 2.3m high (= 24.2m3)
Ventilation rate is 75 air changes per hour
[Note: >150 air changes/hour is achievable with one fan]
Leak assumed to occur through pump shaft seal. A typical pump inside the enclosure will be a Leybold two-stage rotary vane vacuum pump (e.g. TRIVAC D65B) with a shaft diameter of 25mm and a seal clearance of 50 microns
The maximum pressure inside the pump has been taken as 1.3bara
Results for 75 air changes/hour
Hypothetical volume = 23.4m3
Time of persistence = <10 minutes
Additional considerations for the Vacuum Pump Enclosure
Pumps must be maintained between 12°C and 40°C
Ventilation rate is used in combination with the heaters to achieve this
This is controlled by a dedicated control unit, but the ventilation rate is never allowed to fall below 75 air changes/hour
For both ventilation systems the fans are ramped up to full speed if a hydrogen leak is detected.

18. Lightening protection
Interception rods to be fitted to all high level equipment on the roof
Will be earthed back to locations on the ISIS mound outside the hall
All frameworks and ducting grounded

19. Crane Use
Crane operations are forbidden during any period when hydrogen is present as a gas or liquid in any part of the MICE Hydrogen Delivery System, Test Cryostat or AFC module. This includes the following situations:
When the Hydride Bed is being charged.
When either the Test Cryostat or any Absorber is being filled or emptied with hydrogen.
When either the Test Cryostat or any Absorber is filled with liquid hydrogen and in a stable state.
Implication: to perform any lifting operations with the hydrogen system running will require all hydrogen to be returned to the metal hydride bed, a process which will take several hours and will incur the further penalty of having to re-liquefy the hydrogen to recommence running (4-7 days TBC).

20. Risk Assessment Summary
Highlighted the importance of controlling occupancy
During R&D Testing the Hall Personal Protection System (PPS) will be in operation
Only “controlled access” allowed
Personnel will be constantly present in the adjacent control rooms (ISIS Main Control Room (MCR), MICE Local Control Room (MLCR) and HLCR). However, these are separated from the system by a 50mm thick magnetic shield wall and 1m+ concrete wall.
Access to area outside south wall needs to be controlled
Hydrogen Detection System is vital early warning signal
Signal from this to be shared with the ISIS staff in the MCR
Use of the Charging Station and handling bottles will comparatively high risk compared to normal operations
Follow procedures and local rules