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Mechanical Safety Systems and DSEAR Compliance

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

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Mechanical Safety Systems and DSEAR Compliance

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  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 Bottle Store Cryostat Hydrogen Charging Station Gas Panel Venting Vac. Pumps Heater/ Chiller Unit

  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 60079-10-1:2009) • 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

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