Hydrogen Pre-Operation Safety Review 4 th October 2011 Mechanical Safety Systems and DSEAR...

Hydrogen Pre-Operation Safety Review 4th October 2011

Mechanical Safety Systems and DSEAR

ComplianceM Hills


Outline

1. Safety Philosophy2. Design approach and codes followed3. Mechanical design overview4. Passive safety systems5. DSEAR compliance6. Lightening protection7. Crane use8. Summary of Risk Assessment

2


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

3


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

4


Codes and Regulationsi. DSEAR (Dangerous Substances and Explosive

Atmospheres Regulations)

ii. IEC61508 (Functional Safety of Electrical/Electronic/Programmable Electronic Safety-related Systems)

iii. Pressure Equipment Regulations 1999 (“Pressure Equipment Directive”, BS5500 and ASME)

iv. Local codes:i. SC33 - Safety of pressure and vacuum systemsii. SC20 - Controlling explosive and flammable gases and dusts

5


H

H

H

H

Hydrogen System P&ID

Cryostat

BottleStore

Gas Panel

Vac. Pumps

Heater/ChillerUnit

HydrogenCharging Station

Venting


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)

7


Relief Scenario 18

H

H

H

H

Boil-off in absorber


Relief Scenario 29

H

H

H

H

Rapid Boil-off in absorber


H

H

H

H

Relief Scenario 310

Leak from Absorber Pot into vacuum space


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:i. Identify ‘Hydrogen Zones’ii. Specify and procure the correct equipmentiii. Calculate adequate levels of ventilation and ensure this is available

11


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

12


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)

13


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

14


Ventilation Rates– 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:

i. The hypothetical vapour cloud produced by the release is less than the volume under consideration

ii. The cloud persists for less than 30 minutes

15

From BS EN 60079-10-1:2009…

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


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

16

C

dtdVfVZ

min/

0

lnX

kLEL

C

ft m

293

// max

min

T

LELk

dtdGdtdV

m

0

0 /

V

dtdVC


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.

17


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

18


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).

19


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

20

Hydrogen Pre-Operation Safety Review 4 th October 2011 Mechanical Safety Systems and DSEAR...

Documents

Transcript of Hydrogen Pre-Operation Safety Review 4 th October 2011 Mechanical Safety Systems and DSEAR...