SIST EN 17527:2022

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

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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
p
b

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
∆p
reseat

difference between set and reseating pressure
[SOURCE: EN ISO 4126-1:2013, 3.15]
3.5
build up back pressure
∆p
bb

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
p
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
p
s

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
p

operate
pressure expected during normal operation
3.21
overpressure
∆p

over
pressure increase over the set pressure
[SOURCE: EN ISO 4126-1:2013, 3.7]
3.22
performance tolerance
∆p

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
p

0
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
p
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
p
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
p
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
p
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
p
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
p
bs

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
p
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
2
Affected cryogenic surface area cm
A

c
Initial filling level –
a

i
2
Cross section of leak m
A

leak
2
Affected surface area of the superconducting device cm
A

scd
2
Minimum discharge area of the PRD m
A

th
C Discharge function –
c Specific heat capacity of the vessel/pipework material J/(kgK)
Specific heat capacity at constant pressure J/(kgK)
c
p
Velocity at the throat of PRD m/s
c

th
Electromagnetic energy stored in the magnet J
E

mag
Maximum thermal energy of the electrical arc J
E

max
Thermal energy transferred directly into the vessel/pipework wall in case of J
E

wall
dielectric breakdown
Specific enthalpy J/kg
h

Specific enthalpy of saturated liquid J/kg
h'
Specific enthalpy of saturated vapour J/kg
h''
Specific heat input J/kg
∆h
adjusted specific enthalpy J/kg
h
0,x
Enthalpy difference of air J/kg
∆h

air
Specific enthalpy at the throat J/kg
h

th
I Current A
2
Average heat transfer coefficient to the helium flow W/(m K)
k
Discharge coefficient –
K

d
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Certified discharge coefficient –
K

dr
Length of the pipe section m
l

L Inductance H
Characteristic length of heat transfer problem m
L

c
Latent heat of fusion of vessel/pipework material J/kg
L

fusion

Relieving mass flow rate kg/s

M
Total mass of the cryogenic fluid in the system kg
M

c
Helium mass stored in the cryostat kg
M

He
 Mass flow rate to vacuum space in case of LBV (loss of beamline vacuum) kg/s
M

LBV
 Mass flow rate to vacuum space in case of leak of cryogenic fluid kg/s
M

lcf
2
Mass flux at the throat of PRD kg/(m s)

m

th
P Perimeter of the pipe section m
∆p
Pressure drop bar(a)
Relieving pressure bar(a)
p

0
* Relieving pressure from which on critical (sonic) flow occurs in the throat bar(a)
p

0
A
area
th
Adjusted relieving pressure bar(a)
p

0,x
Ambient pressure bar(a)
p

amb
Back pressure bar(a)
p

b
Built-up back pressure along the downstream pipework bar(a)
∆p

bb
Superimposed back pressure bar(a)
p

bs
Bursting pressure bar(g)
p

burst
Maximum bursting pressure bar(g)
p

burst,max
Minimum bursting pressure bar(g)
p

burst,min
Initial pressure inside the cryogenic circuit bar(a)
p

c
Thermodynamic critical pressure bar(a)
p

crit
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Pressure Drop along the transfer line bar(a)
∆p

line
Overpressure bar(a)
∆p

over
Perimeter of the projected heat transfer in the direction of the flow cm
P
proj
Maximum allowable pressure bar(a)
p

s
Set pressure of PRV bar(g)
p

set
Test pressure bar(g)
p

test
Pressure at the throat of PRD bar(a)
p

th
Initial pressure inside the vacuum vessel or the beamline bar(a)
p

v
Sub-critical flash pressure bar(a)
p

x
pd
Pressure and gap distance product in Paschen curve Pa m


Heat load W
Q

Heat load caused by LIV W
Q

LIV
2
Heat flux caused by LIV W/cm

q

LIV

Heat load caused by a quench W
Q
quench
2
 Heat flux caused by a quench W/cm
q

quench

Heat load caused by a LBV W
Q

LBV
Leak rate mbar l/s
Q

l
R Ideal gas constant J/(mol K)
Fluid specific gas constant J/(kg K)
R

c
Adjusted specific entropy J/(kg K)
s

0,x
T Temperature K
Temperature at ambient conditions K
T

amb
Melting temperature of vessel/pipework material K
T

fusion
Nominal temperature of He K
T
i,He
Relieving temperature K
T

0
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Inlet helium temperature of pipe section i K
T

0,,in i
Adjusted relieving temperature K
T
0,x
Adjusted relieving temperature of pipe section i K
T
0,,xi
Initial temperature inside the cryogenic circuit K
T

c
Saturation temperature K
T

sat
Surface temperature of the superconducting device following a quench K
T

scd
Temperature of the cryogenic vessel/pipework wall K
T

wall
Surface temperature of pipe section i K
T

w,i
Breakdown voltage Paschen curve V
U

breakdown
Ionization voltage of gas V
U

ion
3
v
Specific volume m /kg
3
v ' specific volume of saturated liquid m /kg
3
v '' specific volume of saturated vapour m /kg

3
Specific volume upstream of PRD m /kg
v

0,x
3
Specific volume at ambient conditions of the of ambient air at m /kg
v

amb
T and p

amb amb
3
Specific volume inside the cryogenic circuit m /kg
v

c
3
Helium volume of the cryostat m
V

He
3
Initial specific volume m /kg
v

i
3
Specific volume at relieving conditions m /kg
v

0
3
Specific volume at the throat of the PRD m /kg
v

th
3
Volume of the vacuum space m
V

v

x
Dimensionless vapour mass fraction (quality) –
z Thickness of vessel/pipework wall m
α Temperature ratio –
ε Dimensionless void fraction –
γ

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Critical pressure ratio for two-phase flow –
η

c
κ
–
Tp and
Isentropic expansion coefficient of ambient air at
amb amb
ξ
Length ratio –
ρ 3

Density of the vessel/pipework material kg/m
ϕ

Relative humidity %
ω
Omega parameter –
ψ

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
...