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TITLE: AURAL PRESSURE COMFORT LIMITS IN TUNNELS
1.
Reasons for this note
There are no formal rules in GB standards setting out limit criteria for aural pressure
comfort for staff and passengers travelling on trains in tunnels. This is because
Railway Group Standards are in place to control safety and compatibility whilst
issues such as pressure comfort are considered the responsibility and choice of train
operators.
This note sets out the limits applied to pressure changes in tunnels that British Rail
derived before 1996 and which have been used to set aerodynamic speed limits in
some GB railway tunnels. These criteria were also applied during the most recent
West Coast Mainline upgrade when line speeds were increased, and modified
criteria were also used to set speeds in the tunnels of HS1. On both routes, some
tunnel modifications were introduced to permit higher speeds while still ensuring
compliance with the limits
2.
Need for aural comfort limits
When trains enter and leave tunnels, they generate pressure changes, also called
pressure waves, a few percent above or below ambient atmospheric pressure.
These pressure waves move along the tunnel at the speed of sound, reflecting at
any interface with the open air, such as at portals or at the top of airshafts. On
reflection, the pressure waves are modified with waves above ambient pressure
returning back into the tunnel as waves below ambient pressure and vice versa. This
means that a train moves through a complex superposition of pressure changes as it
travels through the tunnel. These waves can superimpose to give relatively large
positive or negative pressure transients. For unsealed trains, the pressure variations
in the tunnels are propagated directly into the vehicles of the trains, whereas for
partially sealed or sealed trains the pressure changes are attenuated by the sealing,
leading to slower, modified pressure variations inside the train.
Figure 1 shows how pressures are generated by a single train travelling in a plain
tunnel and travel back and forth. The upper part of the figure shows a wave
diagram, which is a representation of the path of the train nose and tail and the
pressure waves generated. The horizontal axis is time and the vertical axis is
distance in the tunnel. The waves generated by the train nose are shown as solid
lines, whilst those generated by the tail are dotted. Waves coloured red are above
ambient pressure and waves coloured green are below ambient. The path of the
train nose is shown as a solid black line and that of the tail is a dotted black line.
The red dotted line parallel to the path of the train nose shows a fixed location on
the train for which the train pressure time history is shown in the lower part of the
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Figure. It is possible to see how pressure waves shown on the wave diagram and
passing this location contribute to the build-up of the pressure time history.
Nose
wave
Tail
wave
Nose path
Tail path
Figure 1: Wave diagram and pressure time history at a point on a train
Some people are quite sensitive to these pressure changes, particularly when they
are suffering with head colds, whilst others may be completely unaware of the
effects. The problem is therefore subjective and response can be further affected by
any activities being undertaken at the time and also the state of health of the
particular individual.
A very small percentage of the travelling public with existing aural health issues may
suffer permanent hearing damage if pressure changes exceed a certain value. For
this reason, the LOC & PAS TSI has requirements limiting the pressure changes
generated by TSI compliant trains travelling in tunnels, and requirements are placed
on Infrastructure Managers in the INS TSI to ensure that pressure changes do not
exceed limit values in tunnels when compliant trains travel on their infrastructure.
Thus, to ensure aural medical health and pressure comfort for train travellers, some
limits have to be applied to the generated pressures. In the tunnel design process,
the tunnel area should be large enough for the intended speed of operation to
satisfy both the intended passenger aural comfort criteria and the medical health
criterion.
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3.
Determination of pressure limits
Having established that the aural response to tunnel pressure variations is
subjective, it is necessary to undertake subjective testing to establish comfort limits.
(The medical health limit used in the TSIs was obtained on the other hand through
technical judgement, discussion and agreement by medical experts, see Section 5).
British Rail Research undertook a range of subjective tests in the 1980s and 1990s
using a pressure chamber. This had a diaphragm in its ceiling which could be
adjusted in real time to produce variations in the internal pressure by varying the
chamber’s air volume. Either pressures corresponding to simple pressure time
histories, such as ramps, or realistic train pressures measured on trains in tunnels or
predicted from theoretical models could be simulated. Volunteers sat in the
chamber as the pressures were varied and were then asked to score individual
pressure events, such a ramp change or a single tunnel, on a ranking sheet.
The British Rail Research results showed that the subjects varied greatly in their
responses to the same pressure changes, that response is partly physiological and
partly psychological and, critically, that the change of pressure, both in terms of
range and the time over which it occurs, is most important.
The same pressure chamber was also used by DB to undertake subjective tests, in
connection with the introduction of sealed ICE trains on the Neubaustrecke in
Germany. Due to the difference in the way pressures change within sealed trains (ie
slower changes, generally reduced amplitudes compared with the pressures outside
the train), DB pressure comfort limits focused much more on rates of change of
pressures than did British Rail’s limits.
4.
Pressure comfort limits
The following extracts taken from UIC 771-11, 2nd edition, February 2005,
adequately describe aural pressure comfort limits applied in different countries.
Appendix A - Pressure comfort design criteria for railway tunnels
General discussion and examples
A.1 - Existing pressure comfort criteria adopted by railway administrations
Very few operators have published criteria for their operations and it is believed that there
is still a genuine uncertainty in what is the correct choice for their own operating
conditions. The following is a list in approximately chronological order of their adoption of
those criteria which are well known together with comments on any particular operating
conditions relevant to that operator.
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A.1.1 - Japanese Shinkansen operations
Max. change of pressure = 1 000 Pa, (no time period given but suspected to be a few
seconds)
Max. rate of change of pressure = 200 Pa/s
The latter rate of pressure change criterion is thought to have recently been relaxed to 300
- 400 Pa/s.
Operating conditions:
-
high-speed operations: 210, 240, 270 km/h,
-
sealed rolling stock,
-
tight bore double-track tunnels.
There is no published corresponding criterion for conventional unsealed Japanese stock.
A.1.2 - British Railway Operations
A.1.2.1 - Inter-City Route Operations
Pre-1986:
Max. change of pressure = 3 000 Pa within a time period of 3 seconds
After 1986, this was revised to:
Max. change of pressure = 4 000 Pa within a time period of 4 seconds
Operating conditions:
-
moderate to high-speed operations: 160, 200 km/h,
-
unsealed rolling stock,
-
tight bore double-track tunnels.
These criteria are based on the very worst case. For example, they cover the worst case of
two trains passing in double-track tunnels.
A.1.2.2 - Rail Link (London to Channel Tunnel)
Recommended criterion:
Max. change of pressure = 2 000 Pa within a time period of 4 seconds in single-track tunnels
Max. change of pressure = 3 500 Pa within a time period of 4 seconds in double-track
tunnels
Operating conditions:
-
high-speed operations: 225-300 km/h,
-
unsealed rolling stock,
-
large bore single/double-track tunnels.
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A.1.3 - U.S. UMTA - Underground rapid transit systems
Max. change of pressure = 700 Pa within a 1.7 second period
Max. rate of change of pressure = 410 Pa/s (as an average rate over longer periods than 1.7
second)
Operating conditions:
-
low-speed operations: 80-100 km/h,
-
unsealed rolling stock,
-
tight bore tunnels,
-
regular commuter customers.
A.1.4 - German Railways "Neubaustrecken" (new lines)
Until recently, criteria similar to those of the Japanese Shinkansen services have been used.
However, German Railways (DB) now use criteria which are defined by the following three
components, all of which have to be complied with:
Max. change of pressure = 500 Pa within a time period of 1 second
and = 800 Pa within a time period of 3 seconds
and = 1 000 Pa within a time period of 10 seconds
However, these maximum changes refer to single-train operation only, not to the two train
passing situations that can arise in double-track tunnels.
Operating conditions:
-
high-speed operations: 240, 280 km/h,
-
sealed rolling stock,
-
large bore tunnels.
A.1.5 - Other Railways
Italian Railways (FS) have recently stated their criterion for new lines will be:
Max. change of pressure = 1 500 Pa (no time period stated)
Max. rate of change of pressure = 500 Pa/s
Operating conditions:
-
high-speed operations,
-
sealed rolling stock,
-
moderate-size bore tunnels.
SNCF: for the Atlantique high-speed line, which has only few tunnels, SNCF dimensioned
the tunnel cross- sections to limit pressure changes (including when two trains cross) to
approx. 5 kPa/3 seconds. Experience has shown that this criterion is not sufficient,
particularly for high speeds where the dP/dt gradient becomes preponderant.
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SNCF is currently studying new criteria including a gradient restriction to approx. 500 Pa/s;
this target does require a minimum isolation of the trainsets.
Eurotunnel adopted the early BR criterion for general pressure changes and a special
criterion to cater for the small regular pulses generated when passing the Pressure Relief
Ducts.
Max. change of pressure = 3 000 Pa within a time period of 3 seconds (individual pressure
pulses)
Max. change of pressure = 450 Pa (or frequently repeated pressure pulses)
A.2 - Example choice of criteria - BR philosophy for unsealed trains
(Abstracted from “Pressure comfort criteria in rail tunnel operations” by R G Gawthorpe in: Proc. 7th
Int Symp on the Aerodynamics and Ventilation of Vehicle Tunnels (Brighton, UK, 27-29 November
1991). BHR Group Ltd, Elsevier 1991, pp 173-188).
No single definition of comfort criterion can be considered universally appropriate at all
times by all people for all situations. Different people on different days under different
railway operating circumstances appear to demand differing standards of comfort.
Nevertheless, it is important that a consensus standard can be assumed for a particular
type of service.
Previous studies reported in the above-mentioned publication suggest that there are four
different train operating circumstances each requiring a separately considered comfort
standard. The four categories attempt to differentiate between three key operational
situations:
1.
different types of service (e.g. long-distance Inter-City/short distance commuter);
2. different numbers of pressure pulses experienced on the journey (e.g. up to, say, 4
tunnels per hour/over 4 per hour);
3. different standards of comfort for the service (e.g. conventional/enhanced). It is
assumed that, in the context of pressure comfort, these two standards are achieved with
"unsealed" and "sealed" rolling stock.
Rolling stock of the "sealed” variety prevent the full pressure transients external to the
train reaching the passenger environment. Examples of such operations are the Shinkansen
services in Japan and the ICE train operations on the DB Neubaustrecke routes. Standards
relating to such "sealed" stock will not be considered further here in this leaflet.
The four comfort standards suggested in the BR study are:
A -
Conventional Inter-City (I-C) and regional services with up to, say, 4 tunnels per hour
(less than 10% of route length). Normal "unsealed" rolling stock.
B -
Inter-City services with large numbers of tunnels (for example, 30% of route length)
and "unsealed" rolling stock.
C -
Sealed train operations - not considered further here.
D -
Rapid transit, Metro or Urban Underground services with large numbers of station
stops/intermediate tunnel sections. Normal "unsealed" rolling stock.
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The difference between Type A and Type B is simply the number of tunnels or alternatively
the relative length of route within tunnel. The consequent large number of pressure
transient events in Type B would generate more discomfort unless action is taken and
therefore it demands a correspondingly tighter pressure criterion.
Examples of Type A operations are those with BR Mk 3 or Mk 4 coaches on the British InterCity routes, SNCF TGV-Atlantique and Corail services, Eurofima coach operations.
Examples of Type B operation are the proposed BR/SNCF Channel Tunnel TMST Services on
the future BR Rail Link to London, and existing Eurofima services on the Italian Direttissima.
Type D and Type B operations are to some extent similar. Both use unsealed rolling stock on
routes causing large numbers of pressure transients. However, a more restrictive pressure
limit is justified for Type D operations due to the fact that daily commuters may be using
the service and could expect a less severe environment than the less regular travellers using
the services of Type B.
The criteria have evolved as a result of subjective assessments during either on-board train
tests on the route concerned, or train tests simulating a future train service on a new route,
or in laboratory simulations in a pressure chamber of a particular train type and route.
A.2.1 - Extreme case criteria
"Pressure transients in short tunnels" suggests the following pressure criteria for the three
types of operation:
Type A
Maximum pressure change 4.0 kPa in any 4 s period
Type B
Maximum pressure change 3.0 kPa in any 4 s period
Type D Maximum pressure change 1.0 kPa in any 4 s period
These criteria are based on the consideration of the rare worst case pressures that can
occur as distinct from lower pressures experienced "normally" in these tunnels.
A.2.2 - Normal case criteria
In the definition of the extreme case pressure criteria in point A.2.1, the worst case
situation only has been considered. Such a worst case situation may be extremely rare (as,
for example, for a double-track tunnel where two trains have to pass within the tunnel and
at a particular location in the tunnel). Owing to this rarity, it is then reasonable that the
defined limiting pressure change might be severe. It would certainly be a value that was not
acceptable as the normal case pressure change which may occur on most journeys through
the tunnel.
Thus, criteria can be chosen corresponding to those in point A.2.1 but this time in terms of
the normal operating case:
Type A
Maximum pressure change 2.5 kPa in any 4 s period
Type B
Maximum pressure change 2.0 kPa in any 4 s period
Type D Maximum pressure change 0.7 kPa in any 4 s period
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Pressure criteria based on the maximum peak-to-peak change of pressure within a set
period are recommended for tunnels covered by this leaflet. A period of 3 or 4 seconds has
been found to be a suitable value based on experience of unsealed train operations and
from test work described in "High-speed tests with ICE/V passing through tunnels, and the
effects of sealed coaches on passenger comfort", "Approximating unsteady friction at high
Reynolds Numbers" and "Pressure transients in short tunnels"1.
It is suggested that the criteria of Table 1 could be used for single-track tunnels. These
criteria are appropriate for single-track tunnel operations where passengers are exposed to
the same pressure change every time the train passes through the tunnel. For this reason,
the values are noticeably lower than those given above in point A.2.1 — page 15, where for
double-track tunnels the worst case is a rare event. Journeys with 10 to 30% in tunnels
should adopt values which take account of other special features of the service, e.g.
duration of journey, prestige status of service.
Table 1 : Pressure comfort criteria for single-track tunnels (BR proposal)
Type of train operation
5.
Pressure criterion
Conventional Inter-City (I-C) and regional services with up to
4 tunnels per journey hour or tunnels less than 10% of route
length
2 500 Pa in any 4 s period
Inter-City services with over 30% of route in tunnel
2 000 Pa in any 4 s period
Medical health criterion
The following extract taken from UIC 771-11 gives some background to the medical
health criterion limiting pressure changes in railway tunnels, and used in the TSIs.
F.2 - Medical health criterion
A medical working group undertook an extensive study (ERRI Report C 218/RP5) to evaluate
the allowable limit, from a medical health point of view, of pressure changes experienced
on board trains. The limit applies to both sealed and unsealed stock. For sealed stock, the
limit needs to be satisfied to account for safety of personnel in the event of sealing failure
(e.g. a broken window). The limiting pressure condition for medical health was evaluated
with a view to stipulating that future operations within the "Interoperable" category must
not generate pressures outside the train (as well as inside the train) which infringe the limit.
The recommended medical health criterion to adequately safeguard the health of the
railway travelling public from air pressure changes in tunnels is stated below.
1
See UIC 779-11 Bibliography - page 85
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The maximum pressure change (peak-to-peak), to which train passengers and crew are
subjected, must not exceed 10 kPa within any part of the time taken by the train to pass
through any particular tunnel.
The following proviso was added:
A small minority of people with certain conditions of the ear should follow medical
advice, in the same way as is normal practice with air travel, before undertaking highspeed train travel.
6.
Pressure sealing
Pressure sealing trains is a way of either ensuring a higher standard of aural
pressure comfort or permitting trains to travel at higher speeds in tunnels. The
following extract taken from UIC 771-11 gives information about pressure tightness
and base line pressure comfort for trains with different levels of sealing.
F.3 - Pressure tightness
Pressures experienced inside the train during transit through a tunnel, as a result of the
generated train borne pressure histories on the outside of the train, depend on the
characteristics of the external train borne pressure history and the structural construction
of the train. The internal pressure tends to follow the external pressure fluctuation, and for
trains with some degree of sealing the internal pressure will be attenuated and have a time
delay when compared with the external pressure. The pressure tightness coefficient, ,
gives a measure of how the internal pressure varies in response to the external pressure
fluctuations, i.e. it defines the degree of pressure tightness (or sealing) of a train.
There are two different pressure tightness coefficients that are commonly used, the
dynamic pressure tightness coefficient, dyn, and the quasi-static pressure tightness
coefficient, tstat. dyn is the true pressure tightness coefficient experienced by the moving
train-tunnel system and thus describes the true dynamics of external and internal pressure
fluctuations. The dynamic pressure tightness coefficient, dyn, is used to describe the degree
of sealing for trains in all three new tunnel design tools. To carry out accurate
measurements of the dynamic pressure tightness coefficient, dyn, of a train, a full-scale test
is required. Hence, sometimes the quasi-static pressure tightness coefficient, stat, which is
usually measured in a constant external pressure environment for one stationary coach, is
used to estimate the dynamic pressure tightness coefficient, dyn. stat is often 2-3 times
higher than dyn. An equivalent leakage area, Seq, is sometimes used as an alternative
specification of pressure tightness.
The dynamic pressure tightness coefficient, dyn, is defined as:
 dyn 
p( t )
dpint / dt
in which:
p(t)
= differential pressure, (pext - pint), at time t
pext
= pressure external to the train, varies with time (= pext(t))
pint
= train internal pressure, varies with time (= pint(t))
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dyn is always positive and varies from 0 to ∞ such that, for a completely unsealed train,
dyn = 0, and for a train which is perfectly sealed, dyn = ∞. It should be noted that the
extreme value due to complete tightness is difficult to achieve. Furthermore, dyn usually
has two values: dyn (pext > pint) and dyn (pext< pint) as leakage paths will, in general, have
different characteristics depending on whether the internal pressure is greater or less than
the external pressure.
F.4 - Base-line pressure comfort criteria
The following base-line pressure comfort criteria have been recommended for unsealed
and sealed train operation in tunnels (ERRI Report C 278/RP1):
Unsealed trains (generally dyn < 0.5 s)
The pressure experienced by a passenger on board a train should not exceed a change of:
- 4.5 kPa within a period of4 s for the worst case involving two trains passing in a doubletrack tunnel in a critical crossing situation
- 3.0 kPa within a period of4 s for a single-track tunnel
Sealed trains (generally dyn > 0.5 s)
The pressure experienced by a passenger on board a train should not exceed a change of:
- 1 000 Pa within a period of 1 s
- 1 600 Pa within a period of 4 s
- 2 000 Pa within a period of 10 s
The criteria in this case are both for single-track tunnels and for the case involving two
trains passing in a double-track tunnel in a critical crossing situation.
A useful discussion of the pressure tightness coefficient and the equivalent leakage
area is given in:
Chiu, TW & Johnson, T. ‘The Need for a Standardised Definition of a Sealing
Parameter’. Conference on ‘Cost-Effectiveness of Pressure Sealed Coaches’, Utrecht,
The Netherlands 12-13 October 1999.
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