prEN 17527

Helium cryostats - Protection against excessive pressure

This document specifies the minimum requirements for the protection of helium cryostats against excessive pressure, including the specific risks associated with cryostats for superconducting magnets and cryostats for superconducting radio-frequency cavities, coldboxes of helium refrigerators and liquefiers as well as helium distribution systems including valve boxes. It includes risk assessment, protection concepts, dimensioning of pressure relief devices, types of pressure relief devices, substance release and operation of helium cryostats.
In order to fulfil the aim of this document, the characteristics of pressure relief devices are taken into account.

Helium Kryostate - Schutz gegen Drucküberschreitung

Dieses Dokument legt die Mindestanforderungen für den Schutz von Helium-Kryostaten gegen Drucküberschreitung fest, einschließlich der spezifischen Risiken im Zusammenhang mit Kryostaten für supraleitende Magnete und Kryostaten für supraleitende Hochfrequenz-Kavitäten, Coldboxen von Helium-Kühlgeräten und -verflüssigern sowie Helium-Verteilersystemen einschließlich Ventilboxen. Es umfasst die Risikobeurteilung, Schutzkonzepte, Dimensionierung von Druckentlastungseinrichtungen, Bauarten von Druckentlastungseinrichtungen, Stofffreisetzung und den Betrieb von Helium-Kryostaten.
Um den Zweck dieses Dokuments zu erfüllen, werden die Charakteristiken von Druckentlastungs-einrichtungen berücksichtigt.

Cryostats pour hélium - Protections contre les surpressions

Le présent document spécifie les exigences minimales applicables à la protection des cryostats hélium contre les surpressions, ceci inclut les risques spécifiques associés aux cryostats comportant :
   des aimants supraconducteurs ;
   des cavités radiofréquence supraconductrices.
Il s’applique aux boîtes froides de réfrigérateurs et liquéfacteurs d'hélium, ainsi qu'aux systèmes de distribution d'hélium, notamment les boîtes à vannes et plus généralement aux cryostats. Il inclut l'évaluation du risque, les concepts de protection, le dimensionnement des organes de sécurité contre les surpressions, les différents types d’organes de sécurité, la décharge de l’hélium en situation accidentelle et le fonctionnement des cryostats hélium.
En conformité avec son objet, le présent document traite des caractéristiques des organes de sécurité contre les surpressions.

Kriostati za helij - Zaščita pred prekoračitvijo tlaka

General Information

• Status: Not Published
• ICS: 13.240 - Protection against excessive pressure
• ICS: 23.020.40 - Cryogenic vessels
• Technical Committee: CEN/TC 268 - Cryogenic vessels
• Drafting Committee: CEN/TC 268/WG 6 - Specific helium technology applications
• Current Stage: 4599 - Dispatch of FV draft to CMC - Finalization for Vote
• Due Date: 07-Jun-2021
• Completion Date: 07-Jun-2021
• Ref Project: oSIST prEN 17527:2020 - Helium cryostats - Protection against excessive pressure

Standards Content (sample)

SLOVENSKI STANDARD
oSIST prEN 17527:2020
01-julij-2020
Kriostati za helij - Zaščita pred prekoračitvijo tlaka
Helium cryostats - Protection against excessive pressure
Helium Kryostate - Schutz gegen Drucküberschreitung
Cryostats pour hélium - Protections contre les surpressions
Ta slovenski standard je istoveten z: prEN 17527
ICS:
13.240 Varstvo pred previsokim Protection against excessive
tlakom pressure
23.020.40 Proti mrazu odporne posode Cryogenic vessels
(kriogenske posode)
oSIST prEN 17527:2020 en,fr,de

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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oSIST prEN 17527:2020
DRAFT
EUROPEAN STANDARD
prEN 17527
NORME EUROPÉENNE
EUROPÄISCHE NORM
July 2020
ICS 13.240; 23.020.40
English Version
Helium cryostats - Protection against excessive pressure

Cryostats pour hélium - Protections contre les Helium Kryostate - Schutz gegen Drucküberschreitung

surpressions

This draft European Standard is submitted to CEN members for enquiry. It has been drawn up by the Technical Committee

CEN/TC 268.

If this draft becomes a European Standard, CEN members are bound to comply with the CEN/CENELEC Internal Regulations

which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.

This draft European Standard was established by CEN in three official versions (English, French, German). A version in any other

language made by translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC

Management Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,

Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,

Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and

United Kingdom.

Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of which they are

aware and to provide supporting documentation.

Warning : This document is not a European Standard. It is distributed for review and comments. It is subject to change without

notice and shall not be referred to as a European Standard.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels

© 2020 CEN All rights of exploitation in any form and by any means reserved Ref. No. prEN 17527:2020 E

worldwide for CEN national Members.
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Contents Page

European foreword ............................................................................................................................................ 4

Introduction .......................................................................................................................................................... 5

1 Scope .......................................................................................................................................................... 6

2 Normative references .......................................................................................................................... 6

3 Terms and definitions ......................................................................................................................... 6

4 Symbols .................................................................................................................................................. 12

5 Process flow-charts ............................................................................................................................ 17

5.1 Process flow-chart concerning risk assessment and protection concepts, see

Figure 1 ................................................................................................................................................... 17

5.2 Process flow-chart concerning scenario-specific dimensioning of helium circuit PRDs,

see Figure 2 ........................................................................................................................................... 18

6 Risk assessment .................................................................................................................................. 19

6.1 General information on risk assessment ................................................................................... 19

6.2 Sources of excessive pressure relevant for dimensioning ................................................... 19

6.3 Sources of excessive pressure to be mitigated ......................................................................... 20

6.4 Combined scenarios ........................................................................................................................... 22

6.5 Risk assessment before ordering .................................................................................................. 22

6.6 Risk assessment in the design phase ........................................................................................... 23

6.7 Evaluation of risks by the end-user .............................................................................................. 23

7 Protection concepts ........................................................................................................................... 23

7.1 General.................................................................................................................................................... 23

7.2 Single-stage protection concept .................................................................................................... 23

7.3 Multi-stage protection concepts .................................................................................................... 24

8 Dimensioning of pressure relief devices .................................................................................... 27

8.1 Method for the dimensioning of pressure relief devices ...................................................... 27

8.2 Calculation of the minimum discharge area ............................................................................. 27

8.3 Calculation of the fluid state properties at relieving conditions ....................................... 28

8.4 Calculation of the relieving mass flow rate ............................................................................... 30

8.5 Calculation of the mass flux ............................................................................................................. 39

8.6 Calculation of the discharge coefficient ...................................................................................... 42

8.7 Transfer line systems ........................................................................................................................ 43

8.8 Dimensioning of vacuum vessel PRD ........................................................................................... 43

9 Pressure relief devices ...................................................................................................................... 44

9.1 General.................................................................................................................................................... 44

9.2 Pressure relief valves ........................................................................................................................ 44

9.3 Bursting discs ....................................................................................................................................... 44

9.4 Combinations of pressure relief valves and bursting discs ................................................. 45

9.5 Magnetic pressure relief devices................................................................................................... 45

9.6 Pressure relief devices for insulating vacuum vessels .......................................................... 45

9.7 Mechanical supports for pressure relief devices .................................................................... 45

9.8 Materials for pressure relief devices ........................................................................................... 46

10 Substance release ............................................................................................................................... 46

10.1 General.................................................................................................................................................... 46

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10.2 Discharge lines and discharge systems ...................................................................................... 46

10.3 Direct release in to the environment .......................................................................................... 46

11 Operation of helium cryostats ....................................................................................................... 47

11.1 Inspection before commissioning ................................................................................................ 47

11.2 Periodic inspections of pressure relief devices ....................................................................... 47

Annex A (informative) Thermodynamic characteristics of helium ................................................. 49

Annex B (informative) Risk assessment .................................................................................................... 53

Annex C (informative) Protection concepts ............................................................................................. 68

Annex D (informative) Dimensioning of pressure relief devices ..................................................... 76

Annex E (informative) Types of pressure relief devices ................................................................... 108

Bibliography .................................................................................................................................................... 119

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European foreword

This document (prEN 17527:2020) has been prepared by Technical Committee CEN/TC 268 “Cryogenic

vessels and specific hydrogen technologies applications”, the secretariat of which is held by AFNOR.

This document is currently submitted to the CEN Enquiry.
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Introduction

Helium cryostats, other than cryogenic vessels used for storage of cryogenic liquids covered by

EN ISO 21009-2 and EN 13458, include additional specific components such as superconducting magnets

and cavities, electrical heaters, heat exchangers, bellows, circulation pumps and internal control valves.

These components imply additional risks such as sudden excessive pressure, which strongly influence

the design of pressure relief systems and are not covered by existing standards. Helium cryostats are

characterized by a variety of complex and individual design solutions, often exploiting small design

margins for cutting — edge performance. Therefore, a common and specific technical solution for the

protection against excessive pressure cannot be standardized. Rather, the approach on how to obtain the

state of the art protection can be standardized and therefore is covered by this document, specifying the

procedure and minimum requirements for the various aspects in the main part of the document.

Additional information, example solutions and exemplary measures are provided in the extensive Annex,

which mirrors the structure of the main part.

This document covers the typical sources that may lead to excessive pressure in helium cryostats and the

conditions, which are relevant for the protection against excessive pressure during system failures, in

order to harmonize risk assessments and design best practices. The document uses common SI-based

units.

The user of this document may refer to CEN/CENELEC Internal Regulations Part 3, which deals with the

use of verbal forms for the formulation of provisions.
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1 Scope

This document specifies the minimum requirements for the protection of helium cryostats against

excessive pressure, including the specific risks associated with cryostats for superconducting magnets

and cryostats for superconducting radio-frequency cavities, coldboxes of helium refrigerators and

liquefiers as well as helium distribution systems including valve boxes. It includes risk assessment,

protection concepts, dimensioning of pressure relief devices, types of pressure relief devices, substance

release and operation of helium cryostats.

In order to fulfil the aim of this document, the characteristics of pressure relief devices are taken into

account.
2 Normative references

The following documents are referred to in the text in such a way that some or all of their content

constitutes requirements of this document. For dated references, only the edition cited applies. For

undated references, the latest edition of the referenced document (including any amendments) applies.

EN 13445-2, Unfired pressure vessels — Part 2: Materials
EN 13445-3, Unfired pressure vessels — Part 3: Design

EN ISO 4126-1:2013, Safety devices for protection against excessive pressure — Part 1: Safety valves (ISO

4126-1:2013)

EN ISO 4126-3:2006, Safety devices for protection against excessive pressure — Part 3: Safety valves and

bursting disc safety devices in combination (ISO 4126-3:2006)

EN ISO 4126-6, Safety devices for protection against excessive pressure — Part 6: Application, selection and

installation of bursting disc safety devices (ISO 4126-6)

EN ISO 21013-3, Cryogenic vessels — Pressure-relief accessories for cryogenic service — Part 3: Sizing and

capacity determination (ISO 21013-3)

ISO 4126-9, Safety devices for protection against excessive pressure — Part 9: Application and installation

of safety devices excluding stand-alone bursting disc safety devices
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https://www.iso.org/obp
— IEC Electropedia: available at https://www.electropedia.org/
3.1
relevant national third party
inspection body authorized by national regulations
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3.2
back pressure

pressure existing at the outlet of a pressure relief device as a result of the pressure in the downstream

system
[SOURCE: EN ISO 4126-1:2013, 3.11, modified]
3.3
bath cooling

cooling method in which the object to be cooled is submerged in a vessel filled with a liquid cooling

medium

Note 1 to entry: The enthalpy of evaporation is used for cooling, with the phase change providing a nearly constant

cooling temperature.
3.4
blowdown
reseat
difference between set and reseating pressure
[SOURCE: EN ISO 4126-1:2013, 3.15]
3.5
build up back pressure

pressure existing at the outlet of a pressure relief device caused by flow through the device and the

downstream system
[SOURCE: EN ISO 4126-1:2013, 3.13]
3.6
bursting pressure
burst

value of the differential pressure between the upstream side and the downstream side of the bursting

disc when it bursts
[SOURCE: EN ISO 4126-2:2019, 3.10]
3.7
chattering

unstable discharge of a pressure relief valve characterised by high frequency opening and closing

3.8
coincident temperature
temperature of the bursting disc associated with a bursting pressure
[SOURCE: EN ISO 4126-2:2019, 3.14, modified]
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3.9
cryogenic fluid
fluid with a normal boiling point below 120 K respectively −153 °C
3.10
cryogenics

study of technologies, procedures and equipment at temperatures below 120 K respectively −153 °C

3.11
cryostat

vacuum-insulated device for the operation of components at cryogenic temperatures using cryogenic

fluids
Note 1 to entry: A cryostat is considered as an assembly.
3.12
current lead

electrical connection between the power supply unit and the superconducting magnet for charging,

operating or discharging the magnet

Note 1 to entry: In larger systems, current leads are actively cooled due to the high thermal conduction and the

large temperature gradient. The current lead consists of e.g. a copper part as electrical conductor and a heat

exchanger for cooling.
3.13
dewar
vacuum-insulated storage and transport container for storing cryogenic fluids
3.14
driven mode

operation of a magnet by a power supply unit, where the magnet is always

connected to the power supply unit or, in the event of quench, to an external protective circuit or a

discharge resistor
3.1.5
full-lift Pressure Relief Valve
PRV

PRV that opens instantaneously within 5 % of the pressure increase up to the design limited lift. The

proportion of the lift up to the instantaneous opening (proportional range) may not exceed 20 % of the

total lift
3.16
leak rate

pV throughput of a specific fluid which flows through a leak under specific conditions

Note 1 to entry: For vacuum and/or cryogenic technologies, the leak rate is often expressed in the unit mbar l/s.

[SOURCE: EN ISO 20484:2017, 4.3.5]
3.17
maximum allowable pressure

maximum gauge pressure for which the equipment is designed, as specified by the manufacturer

[SOURCE: EN ISO 4126-1:2013, 3.6, modified]
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3.18
maximum credible incident

worst incident within the realm of possibility that has a propensity to cause significant damage

Note 1 to entry: The MCI is the design bases for the dimensioning of the primary PRD.

3.19
multi-layer insulation

very effective thermal insulation method used in cryogenics, which significantly reduces the heat

transport caused by thermal radiation

Note 1 to entry: Multi-layer insulation (MLI) consists of multiple layers of highly reflective films enclosing the

cryogenic components. MLI is used along with vacuum insulation.
3.20
nominal operating pressure
operate
pressure expected during normal operation
3.21
overpressure
over
pressure increase over the set pressure
[SOURCE: EN ISO 4126-1:2013, 3.7]
3.22
performance tolerance
burst

range of pressure between the specified minimum bursting pressure and the specified maximum bursting

pressure, or the range of pressure in positive and negative percentages or quantities which is related to

the specified bursting pressure
[SOURCE: EN ISO 4126-2:2019, 3.15]
3.23
persistent mode

magnet operation without connection to the power supply unit, where the

power supply unit is disconnected after charging the magnet and the superconducting magnet operates

in short-circuit mode
3.24
pV throughput

rate at which a volume of gas at specified pressure passes a given cross-section of the system

Note 1 to entry: The pV throughput is expressed in mbar l/s.
[SOURCE: EN ISO 20484:2017, 4.2.3, modified – Note 1 to entry has been changed]
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3.25
quench

spontaneous transition of a superconductor or a superconducting

component from the superconducting state to the normal conducting state
3.26
relieving pressure

pressure used for the sizing of pressure relief devices; for a pressure relief valve the relieving pressure is

greater than or equal to the set pressure plus overpressure
[SOURCE: EN ISO 4126-1:2013, 3.10, modified]
3.27
reseating pressure
reseat

inlet static pressure at which the disc re-establishes contact with the seat or until zero lift

Exact wording is: value of the inlet static pressure at which the disc re-establishes contact with the seat

or at which the lift becomes zero
[SOURCE: EN ISO 4126-1:2013, 3.8, modified]
3.28
set pressure
set

predetermined gauge pressure at which the pressure relief valve commences to open

Note 1 to entry: It is the gauge pressure measured at the valve inlet at which the pressure forces tending to open

the valve for the specific service conditions are in equilibrium with the forces retaining the valve disc on its seat.

[SOURCE: EN ISO 4126-1:2013, 3.5, modified]
3.29
specified bursting pressure
sp,burst

bursting pressure quoted with a coincident temperature when defining the bursting disc requirements

(used in conjunction with a performance tolerance)
[SOURCE: EN ISO 4126-2:2019, 3.11, modified]
3.30
specified maximum bursting pressure
burst,max

maximum pressure quoted with a coincident temperature when defining the bursting disc requirements

(used in conjunction with minimum bursting pressure)
[SOURCE: EN ISO 4126-2:2019, 3.12, modified]
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3.31
specified minimum bursting pressure
burst,min

minimum pressure quoted with a coincident temperature when defining the bursting disc requirements

(used in conjunction with maximum bursting pressure)
[SOURCE: EN ISO 4126-2:2019, 3.13, modified]
3.32
superconducting magnet

magnet whose coils are cooled, usually with helium, to a temperature at which the conductor becomes

superconducting, effectively removing all electrical resistance

Note 1 to entry: At sufficiently low temperatures and within certain operating parameters, superconducting

materials do not have an ohmic resistance, which allows an electrical current to flow without energy loss.

[SOURCE: IEC 60050-815]
3.33
superimposed back pressure

pressure existing at the outlet of a pressure relief device at the time the device is required to operate

[SOURCE: EN ISO 4126-1:2013, 3.13, modified]
3.34
test pressure
test
pressure to which the equipment is subjected for test purposes

Note 1 to entry: In helium cryostats, hydrostatic tests are generally impractical and even disadvantageous, so that

only pneumatic tests are permissible.

Note 2 to entry: National regulations referring to risks associated with the use of compressible media during proof

tests shall be observed.
[SOURCE: EN 764-1:2004]
3.35
thermal acoustic oscillation

resonant gas oscillation built-up spontaneously within a connecting tube between high and low

temperature levels
3.36
vacuum jacket

evacuated outer shell of a cryostat surrounding the cryogenic components. The insulating vacuum

inhibits the heat conduction by residual gas

Note 1 to entry: The vacuum jacket itself normally remains at ambient temperature.

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3.37
valve box

assembly for distributing coolant to several cryostats and for controlling the flow in supply and return

lines

Note 1 to entry: A valve box consists of a vacuum vessel with an arrangement of cryogenic valves and pipework.

4 Symbols
Affected cryogenic surface area cm
Initial filling level –
Cross section of leak m
leak
Affected surface area of the superconducting device cm
scd
Minimum discharge area of the PRD m
C Discharge function –
c Specific heat capacity of the vessel/pipework material J/(kgK)
Specific heat capacity at constant pressure J/(kgK)
Velocity at the throat of PRD m/s
Electromagnetic energy stored in the magnet J
mag
Maximum thermal energy of the electrical arc J
max
Thermal energy transferred directly into the vessel/pipework wall in case of J
wall
dielectric breakdown
Specific enthalpy J/kg
Specific enthalpy of saturated liquid J/kg
Specific enthalpy of saturated vapour J/kg
h''
Specific heat input J/kg
adjusted specific enthalpy J/kg
0,x
Enthalpy difference of air J/kg
air
Specific enthalpy at the throat J/kg
I Current A
Average heat transfer coefficient to the helium flow W/(m K)
Discharge coefficient –
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Certified discharge coefficient –
Length of the pipe section m
L Inductance H
Characteristic length of heat transfer problem m
Latent heat of fusion of vessel/pipework material J/kg
fusion
Relieving mass flow rate kg/s
Total mass of the cryogenic fluid in the system kg
Helium mass stored in the cryostat kg
 Mass flow rate to vacuum space in case of LBV (loss of beamline vacuum) kg/s
LBV
 Mass flow rate to vacuum space in case of leak of cryogenic fluid kg/s
lcf
Mass flux at the throat of PRD kg/(m s)
P Perimeter of the pipe section m
Pressure drop bar(a)
Relieving pressure bar(a)

* Relieving pressure from which on critical (sonic) flow occurs in the throat bar(a)

area
Adjusted relieving pressure bar(a)
0,x
Ambient pressure bar(a)
amb
Back pressure bar(a)
Built-up back pressure along the downstream pipework bar(a)
Superimposed back pressure bar(a)
Bursting pressure bar(g)
burst
Maximum bursting pressure bar(g)
burst,max
Minimum bursting pressure bar(g)
burst,min
Initial pressure inside the cryogenic circuit bar(a)
Thermodynamic critical pressure bar(a)
crit
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Pressure Drop along the transfer line bar(a)
line
Overpressure bar(a)
over
Perimeter of the projected heat transfer in the direction of the flow cm
proj
Maximum allowable pressure bar(a)
Set pressure of PRV bar(g)
set
Test pressure bar(g)
test
Pressure at the throat of PRD bar(a)
Initial pressure inside the vacuum vessel or the beamline bar(a)
Sub-critical flash pressure bar(a)
Pressure and gap distance product in Paschen curve Pa m
Heat load W
Heat load caused by LIV W
LIV
Heat flux caused by LIV W/cm
LIV
Heat load caused by a quench W
quench
 Heat flux caused by a quench W/cm
quench
Heat load caused by a LBV W
LBV
Leak rate mbar l/s
R Ideal gas constant J/(mol K)
Fluid specific gas constant J/(kg K)
Adjusted specific entropy J/(kg K)
0,x
T Temperature K
Temperature at ambient conditions K
amb
Melting temperature of vessel/pipework material K
fusion
Nominal temperature of He K
i,He
Relieving temperature K
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Inlet helium temperature of pipe section i K
0,,in i
Adjusted relieving temperature K
0,x
Adjusted relieving temperature of pipe section i K
0,,xi
Initial temperature inside the cryogenic circuit K
Saturation temperature K
sat
Surface temperature of the superconducting device following a quench K
scd
Temperature of the cryogenic vessel/pipework wall K
wall
Surface temperature of pipe section i K
w,i
Breakdown voltage Paschen curve V
breakdown
Ionization voltage of gas V
ion
Specific volume m /kg
v ' specific volume of saturated liquid m /kg
v '' specific volume of saturated vapour m /kg
Specific volume upstream of PRD m /kg
0,x
Specific volume at ambient conditions of the of ambient air at m /kg
amb
T and p
amb amb
Specific volume inside the cryogenic circuit m /kg
Helium volume of the cryostat m
Initial specific volume m /kg
Specific volume at relieving conditions m /kg
Specific volume at the throat of the PRD m /kg
Volume of the vacuum space m
Dimensionless vapour mass fraction (quality) –
z Thickness of vessel/pipework wall m
α Temperature ratio –
ε Dimensionless void fraction –
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Critical pressure ratio for two-phase flow –
Tp and
Isentropic expansion coefficient of ambient air at
amb amb
Length ratio –
ρ 3
Density of the vessel/pipework material kg/m
Relative humidity %
Omega parameter –
Discharge function –
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5 Process flow-charts

5.1 Process flow-chart concerning risk assessment and protection concepts, see Figure 1

Figure 1
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5.2 Process flow-chart concerning scenario-specific dimensioning of helium circuit PRDs, see Figure 2

Figure 2
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6 Risk assessment
6.1 General information on risk assessment
As a bas
...