THE IMPACT OF PRESSURE AND SUCTION WAVES ON INSTALLED COMPONENTS, WITH PARTICULAR REFERENCE TO ENCLOSURES IN RAILWAY TUNNELS

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1 THE IMPACT OF PRESSURE AND SUCTION WAVES ON INSTALLED COMPONENTS, WITH PARTICULAR REFERENCE TO ENCLOSURES IN RAILWAY TUNNELS ABSTRACT Heiko Holighaus Rittal Communication Systems, Haiger, Germany Environmental conditions in tunnels, which under certain circumstances can be extreme, place heavy demands on installed components. Enclosures, in particular, which are intended to protect other components from these environmental influences, have to be designed to be capable of coping with these conditions. This is the only way that the safety of the installed components and ultimately the safety of the tunnel can be improved. Considering each of the requirements in isolation is inadequate, however. Optimized holistic concepts are required. The effects of the environmental conditions to enclosures are considered as well as methods of resolution. Keywords: Enclosure design, tunnel application, environmental conditions 1. INTRODUCTION Tunnels are currently being planned and built in many different countries, both for road and rail traffic. They are being built, firstly, because of modern-day requirements for fast connections, and, secondly, because the space is simply not available to provide ideal routes using conventional means of construction. Hence, the biggest tunnel projects are currently to be found in the Alpine countries. At the same time, however, tunnels are also being built in countries which would not normally be associated with them at first thought, such as in the Netherlands. Longer tunnels, smaller tunnel cross-sections and higher traveling speeds place higher requirements on both the materials and components involved. To be able to work safely and reliably, the active components in particular have to be protected from all harmful environmental influences. The more effectively these components are protected from such influences, the greater safety of the tunnel. Standard enclosures are often unable to meet these requirements. At first consideration it might be assumed that the very fact that the components are installed in a tunnel would mean that they are well protected from all major weather influences. However, this view is deceptive, as closer scrutiny of the conditions in tunnels makes clear. Given that environmental conditions in railway tunnels are far more extreme than in any other types of tunnel, it is this type of tunnel which is the focus of this examination. 2. ENVIRONMENTAL CONDITIONS IN TUNNELS 2.1. Climatic conditions Climatic conditions in tunnels vary considerably depending on the length and nature of the tunnel. At the portals and the entry and exit areas it can be assumed that the climatic conditions are the same as in the open air. Deep inside tunnels through rock, on the other hand, conditions can be expected to be totally different. Here the air temperature will be between C and the level of air humidity will be high. In addition, it can also be expected that mountain water and/or condensation water will be dripping from overhead. The presence of salts brought into the tunnel from outside can also lead to the formation of aggressive media, and the concentration of these salts increases over time because there is no rain to wash them away.

2 The following table (Table 1) provides a summary of some of the typical climatic conditions which have to be accounted for: Temperature Temperature change Air humidity Salts Condensation water/mountain water Table 1: Typical climate conditions EN Railway application ETS Nonweatherprotected locations e.g. Customer specification C C C 0.5 C/min up to 20 C in total 0.5 C/min ± 30 K every 30 min % 8-100% % These climatic conditions provide a good basis for corrosion of all kinds. It is precisely these environmental conditions which often make higher quality, non-corrosive materials essential. In addition to treated sheet steel and aluminium, use is also made of stainless steels such as (V2A) or (V4A) Mechanical conditions In addition to the climatic conditions, a whole host of mechanical influences also have to be taken into consideration (see also Table 2). Falling rocks Dust Iron dust Concrete dust Rail rust Brake abrasion Vibrations Impacts Table 2: Typical mechanical conditions Pressure pulse Sand Dust Windpressure Vibration Shock EN Railway application ± kpa/s ETS Nonweatherprotected locations e.g. Customer specification --- ± 5 kpa mg/m³ 300 mg/m³ mg/m³ mg/(m²h) m/s 2,3 m/s² eff Hz 5 mg/m³ 20 mg/(m²h) m/s 1500 N/m² Hz Hz 1 m/s² Hz ms ms ---

3 Particular attention should be given to the dynamic fluctuations in air pressure. These should never be underestimated. Rise in pressure When a train travels through a tunnel it has a piston effect which generates a pressure wave ahead of it and a suction wave behind it (see Figure 1). Direction of travel Fall in pressure Figure 1: Pressure wave The level of the pressure wave depends on the speed of the train v, the cross-section of the tunnel A and the type of tunnel construction (single rail or multi-rail). What is important for the design of the enclosures and the dynamic response are the times during which the pressure is at its most extreme. For speeds of 300 km/s = 83 m/s and an enclosure width of 1m, the time for the train to pass by the enclosure is 12 ms. In reality, however, the actual change in pressure does not happen so fast. Measurements taken in a number of tunnels have demonstrated that the change in pressure from a maximum to minimum takes a number of seconds (8 seconds in Figure 2) Δp 6.5 kpa (A=82m², v=250 km/h = 70 m/s) Figure 2: Measurements recorded in an ICE tunnel in 1989

4 Measurements recorded in the Simplon tunnel (train speed v=140 km/h, pressure drop within 12 sec.) indicated that the typical pattern of air pressure development is as shown in Figure 3. Figure 3: Measurements recorded in the Simplon tunnel (Switzerland) in July 2005 Hence, the dynamic load on the enclosures is not a momentary surge, therefore, but a rapid change in pressure. 3. REQUIREMENTS OF ENCLOSURES 3.1. Requirements in relation to environmental conditions The environmental conditions described in section 2 lead to a range of requirements to be met by the enclosures if they are to provide adequate protection for the installed components. Protection from dust and water: Protection category IP65 or higher (in accordance with EN 60529) Climate control: Compliance with minimum or maximum temperatures for inside for the components in the enclosure (e.g C) Resistance to pressure: ±6.5 kpa (new requirement is ±10 kpa see section 3.2) Service life: years for passive components with a maintenance plan years for active components with a maintenance plan The special challenges here do not lie in trying to meet each of the requirements in isolation, but meeting them in combination. This means, for example, that the enclosure has to remain dust-tight even while the pressure is changing New requirements in terms of resistance to pressure Smaller tunnel cross-sections and high train speeds result in more severe conditions in respect of resistance to pressure. Whereas just a few years the maximum alternating load was still considered to be no more than ±5 kpa, it has now been raised to ±6.5 kpa, with some of the latest invitations for tenders even demanding ±10 kpa.

5 To illustrate what this means for an enclosure, where the typical size is 800 x 2000 mm² (width x height) and the alternating pressure is 10 kpa = N/m², the door has to be able to withstand a pressure of ±16000 N (which equates to a weight of 1.6 tons!). Are these new requirements justified or exaggerated? It is possible to check these new requirements on an approximate basis using empirical evidence in the railway tunnels where pressure differences of ±5kPa occur. Assumptions: Influence of a 10% reduction in the tunnel cross-section. Assuming an isentropic change of state, the laws of gases (equation (1)) can be used as the basis for estimating the increase in pressure. This produces a factor for the increase in pressure caused by the reduction in the crosssection as shown in equation (2): χ p V = const. (1) χ Air p V 1 0 1, 4 = = 1,11 = 1,157 p 0 V (2) 1 where p is the pressure, V the Volume and χ the adiabatic exponent. An increase in travelling speed from 220 km/h to 300 km/h (83 m/s). If the dynamic pressure (equation (3)) is taken into account for the increase in the travelling speed, the resultant factor is as follows (equation (4)): 1 2 p = ρ v (3) dyn 2 2 p km h = = 1,86 km p (4) h where p is the dynamic pressure, ρ is the density of air and v is the speed. The new pressure load is therefore calculated as follows: kn kn p1 = ( 1, ,86) p0 = 2, ,75 (5) 2 2 m m Hence, the new requirements of ±10 kpa are realistic for high speed trains in tunnels with reduced cross sections. 4. ALTERNATIVE SOLUTIONS 4.1. Design approaches A number of alternative design approaches come into consideration for ensuring that the enclosures meet the specified requirements. A) Reinforcements One alternative is to reinforce the enclosure in such a way that it would withstand the pressure loads under all circumstances while meeting all the other requirements such as protection from dust at the same time (see Figure 4). However, it has to be appreciated that the enclosure will certainly be more expensive if this approach is taken, because of the costs of the additional reinforcement.

- 265 - Figure 4: Reinforcement of enclosures B) Pressure compensation The effort and expense of reinforcements can be eliminated if the enclosure can be provided with a means of pressure

However, a major disadvantage of these is that they allow air humidity (water vapour) to penetrate the enclosure unhindered.

6 Figure 4: Reinforcement of enclosures B) Pressure compensation The effort and expense of reinforcements can be eliminated if the enclosure can be provided with a means of pressure compensation. These days it is even possible to obtain pressure compensation solutions which are also dust-proof and waterproof at the same time. However, a major disadvantage of these is that they allow air humidity (water vapour) to penetrate the enclosure unhindered. The installed components must be suitable for relative humidities of up to 100%. Humidity penetrating the enclosure then becomes critical if a cooling unit is used for the air-conditioning in the enclosure. This is because the humid air can condense in the cooling unit, leading to more humid air coming into the enclosure (even reinforcing the process because of the pressure difference) and thus even more condensation water inside the enclosure. Good solutions are necessary to get the condensated water out of the enclosure due to the requirements of section 2! C) Seals Where seals are used for the doors and walls there are also a number of different options available. Rubber strip seals provide a very tight seal, such that the enclosures are very tightly sealed and the differences in pressure are maintained for a long time. However, the disadvantage of these is the sealing joint, because it has to be glued once the seal has been fitted. The alternative is to use PU foam seals which can be foamed-in without a seam. These seals have properties which permit faster pressure compensation without any loss of sealing effectiveness against dust. Figure 5: Sealing preventing dust coming into the enclosure (after 15 years tunnel application)

- 266 - D) Lock systems What is important in lock systems is that the locking points are not too far apart.

Under pressure the door can deform between the locking points, such that it is then no longer effectively sealed.

7 D) Lock systems What is important in lock systems is that the locking points are not too far apart. For an enclosure of 2 m in height, for example, three locking points will be insufficient under pressure. Under pressure the door can deform between the locking points, such that it is then no longer effectively sealed. In addition, a further problem with insufficient locking points is that the mechanical load on the individual locks may become too strong, meaning Figure 6: Locking system for three sides that failure is inevitable. The latest locking systems now also permit the top and bottom edges of the doors to be locked as well so that the door can now be secured all the way around (on the fourth edge by the hinges). E) Accessories The aforementioned requirements also apply to all other accessories such as cooling units, cable entry elements, indicator instruments, etc., with contacts to the outside Verification Once the concept has been developed the product should then be verified, ideally before completion of the design engineering phase. This is possible at an early stage with the aid of computations using finite element methods, for example. These can be used to look into both individual aspects and complete solutions. The following four illustrations (Figures 7 10) provide examples of calculation results. A pressure load of -10 kpa is put to the enclosure in Figure 7. The non-reinforced door will bend to the outside, displacements and forces will be to high for that door and the hinges (Figure 8). Figure 7: Pressure load Figure 8: Displacement of a non-reinforced door

- 267 - After installing a reinforcement frame onto the door (Figure 9) the result of a new calculation show smaller displacements and forces to the door (Figure 10).

on prototypes. One reason for this is that in many cases not all of the components of the enclosure are depicted in the theoretical models and the models themselves involve necessary simplifications.

The alternative is to commission a suitable institute to carry out the development work and supervision at a cost.

8 After installing a reinforcement frame onto the door (Figure 9) the result of a new calculation show smaller displacements and forces to the door (Figure 10). Figure 9: Reinforcement frame Figure 10: Deformation in a reinforced door However, in addition to theoretical considerations and computations it is also imperative that suitable tests are performed on prototypes. One reason for this is that in many cases not all of the components of the enclosure are depicted in the theoretical models and the models themselves involve necessary simplifications. Since special tests of this kind, such as pressure tests, are not covered by any standards, manufacturers have to rely on their own creativity to develop and carry out their own tests. The alternative is to commission a suitable institute to carry out the development work and supervision at a cost. Figure 11 shows a measuring arrangement for carrying out alternating pressure tests involving 200,000 pressure changes of Pa. This arrangement has a corresponding upstream control unit and data logger for performing and analysing the tests. Figure 11 also shows several pressure load cycles from this test Pressure [mbar] :28:02 11:28:12 11:28:23 11:28:33 Time Figure 11: Pressure test set-up and measured data for +5 kpa According to the new pressure requirements a new test has been developed to simulate pressure cycles of the enclosure with pressure differences of ±10 kpa (see Figure 12). For 200,000 cycles this test takes 25 days (24 hours).

- 268-110 90 70 50 30 10-10 -30-50 -70-90 -110 10:17:11 10:17:20 10:17:28 10:17:37 10:17:46 Time Figure 12: Pressure test and measured data for ±10 kpa The test also permits theoretical computations

Figure 13 shows such a check during a pressure test on a non-reinforced door. The door displacement at 20mbar underpressure is 30 mm (800 mm wide door in this instance)!

9 :17:11 10:17:20 10:17:28 10:17:37 10:17:46 Time Figure 12: Pressure test and measured data for ±10 kpa The test also permits theoretical computations to be checked, which guarantees that the computation parameters and boundary values specified in the finite element calculations are more reliable. Figure 13 shows such a check during a pressure test on a non-reinforced door. The door displacement at 20mbar underpressure is 30 mm (800 mm wide door in this instance)! Pressure [mbar] Figure 13: Displacement of a non-reinforced door at 20mbar underpressure 5. SUMMARY It has been demonstrated that the diverse high requirements made of enclosures cannot be treated in isolation but have to be brought together in a single concept in order to achieve maximum safety and reliability for all components involved. From a rough calculation it is apparent that the latest new requirements regarding pressure differences are realistic. In addition to theoretical computations of loads, tests are absolutely imperative because not all details can be accounted for in models. Overall there is a requirement for optimized, suitable climate control solutions and simple and efficient maintenance solutions which are specifically designed for an integrated concept. Given the diversity of components that customers want installing and the wide variations in power losses involved it will always be impossible to develop a standardized enclosure solution. Nevertheless, service-proven solutions can always be used as the initial point of reference, at least, for the development of something new. 6. REFERENCES Physik für Ingenieure, Hering, Martin, Stohrer; 5.Auflage 1995, VDI-Verlag Kraftfahrtechnisches Taschenbuch Bosch, 21. Auflage 1991, Düsseldorf, VDI-Verlag EN EN EN ETS EN

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