Mechanical Safety Systems
and DSEAR Compliance
M Hills
Hydrogen Pre-Operation Safety Review
4th October 2011
Outline
1.
2.
3.
4.
5.
6.
7.
8.
Safety Philosophy
Design approach and codes followed
Mechanical design overview
Passive safety systems
DSEAR compliance
Lightening protection
Crane use
Summary of Risk Assessment
Hydrogen Pre-Operation Safety Review
4th October 2011
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
Hydrogen Pre-Operation Safety Review
4th October 2011
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
Hydrogen Pre-Operation Safety Review
4th October 2011
4
5
Codes and Regulations
i.
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.
ii.
SC33 - Safety of pressure and vacuum systems
SC20 - Controlling explosive and flammable gases and dusts
Hydrogen Pre-Operation Safety Review
4th October 2011
Hydrogen System P&ID
Gas Panel
Venting
H
H
Bottle
Store
Vac.
Pumps
H
Cryostat
H
Heater/
Chiller
Unit
Hydrogen Pre-Operation Safety Review
4th October 2011
Hydrogen
Charging
Station
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)
Hydrogen Pre-Operation Safety Review
4th October 2011
7
Relief Scenario 1
H
H
H
H
8
Boil-off in
absorber
Hydrogen Pre-Operation Safety Review
4th October 2011
Relief Scenario 2
H
H
H
H
9
Rapid
Boil-off in
absorber
Hydrogen Pre-Operation Safety Review
4th October 2011
Relief Scenario 3
H
H
H
H
10
Leak from
Absorber Pot into
vacuum space
Hydrogen Pre-Operation Safety Review
4th October 2011
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.
ii.
iii.
Identify ‘Hydrogen Zones’
Specify and procure the correct equipment
Calculate adequate levels of ventilation and ensure this is available
Hydrogen Pre-Operation Safety Review
4th October 2011
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
Hydrogen Pre-Operation Safety Review
4th October 2011
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)
Hydrogen Pre-Operation Safety Review
4th October 2011
13
Other Considerations
–
All pipework that would normally contain hydrogen is either
jacketed or within the ventilated area
–
–
–
–
–
High integrity pipework used throughout
–
–
–
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
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
Hydrogen Pre-Operation Safety Review
4th October 2011
14
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:
i.
ii.
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
Hydrogen Pre-Operation Safety Review
4th October 2011
The hypothetical vapour
cloud produced by the
release is less than the
volume under consideration
The cloud persists for less
than 30 minutes
16
GP Enclosure Ventilation
–
Full details of the calculation are in the documentation
(procedure followed is that outlined in BS EN 60079-101:2009)
–
–
–
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
–
–
–
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
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
Hydrogen Pre-Operation Safety Review
4th October 2011
dV / dt min  dG / dt max 
k  LELm
T
293
C
dV0 / dt
V0
VZ 
f  dV / dt min
C
t
 f LELm  k
ln
C
X0
17
Vacuum Enclosure Ventilation
–
Assumptions
–
–
–
–
–
–
Results for 75 air changes/hour
–
–
–
Hypothetical volume = 23.4m3
Time of persistence = <10 minutes
Additional considerations for the Vacuum Pump Enclosure
–
–
–

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 twostage 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
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.
Hydrogen Pre-Operation Safety Review
4th October 2011
Lightening protection
Hydrogen Pre-Operation Safety Review
4th October 2011
18
–
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
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).
Hydrogen Pre-Operation Safety Review
4th October 2011
19
Risk Assessment Summary
–
Highlighted the importance of controlling occupancy
–
–
–
–
–
Hydrogen Detection System is vital early warning signal
–
–
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
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
Hydrogen Pre-Operation Safety Review
4th October 2011
20