3.The influence of railway infrastructure stress on its quality
The type of the draft, connection and buffing gears has a significant influence on the dynamic and power behaviour of a train. Transversal forces are influenced by the rigidity or the mobility of the used coupling systems.
The concepts presented above are relevant especially for the exploitation of the infrastructure dedicated to mixed traffic operation (passengers and freight). The elements of the running track are subject to a level of stress which varies on a large scale and may suffer displacements in time. The situation is applicable to the Romanian conventional railway system which doesn’t provide lines exclusively dedicated to passenger trains. In the states with a strongly developed railway system, high – speed infrastructure is subject to special stresses, easy to quantify in the stage of conception and design. Thus, a maximum efficiency is obtained regarding the long term performance of the running track.
Otherwise, the emergence of eventual restrictions or limitations on the high-speed lines causes significant commercial and energy effects. For this reason, the wear – out process of infrastructures dedicated to high – speed traffic requires thorough monitoring, while repair works need to be dealt with in time.
When the maximum admitted tonnage is systematically exceeded during operation, the risk of running track faults, remaining deformations of the superstructure or structure occurs, finally leading to infrastructure (efficient) lifecycle shortening between two repairs or adoption of speed restrictions. The accentuation of running track degradation is inversely related to the level of the exceeded maximum admitted tonnage – illustrated in figure 6.
In the left side of the figure the running track quality variation graphics are presented, corresponding to three particular cases of tonnage adopted in operation (highlighted in the right side):
Case I – operation in rigorous compliance with the designed tonnage. The degradation process has a normal variation identical to that set by the designers. In such a situation, the lifecycle (the variation time of the running track quality value from Ci to Cf ) marked in figure by Pt1 , reaches the maximum value – the most favourable case related to infrastructure endurance.
Pt1 – lifecycle (between two repairs) corresponding to operation while observing the maximum admitted tonnage – case I;
Tf1 – the moment when the quality of the running track reaches the minimum value (Cf ) for the situation mentioned in case I;
Case II – operation with exceeding the maximum admitted tonnage by a small value. The degradation process presents an accentuated variation and shortening of the lifecycle is observed, marked with Pt2 ;
Pt2 – lifecycle (between two repairs) corresponding to operation with exceeding the maximum admitted carried gross tonnage by a small value – case II;
Tf2 – the moment when the quality of the running track reaches the minimum value
(Cf) for the situation mentioned in case II;
Case III – operation with exceeding the normal (maximum admitted) tonnage by a high value. The degradation process has a strong decreasing variation. The lifecycle marked with Pt3 has a minimum value. It is the most unfavourable situation considering infrastructure endurance.
Pt3 – lifecycle (between two repairs) corresponding to operation according to case III;
Tf3 – the moment when the quality of the running track reaches the minimum value (Cf) for the situation mentioned in case III;
Considering the three adopted cases, the following relation between the lifecycles corresponding to each case can be noted:
Pt1 > Pt2> Pt3
1.1. Influence of other types of stress
Carried gross tonnage represents only one type of railway infrastructure stress. Experience has shown that any overload, regardless of its nature, shortens the lifecycle between two repairs. The influence of stress on infrastructure quality in relation to the lifecycle (Pt) is presented in figure 7. Four cases are emphasized. The first two are in compliance with the projected value, and the last two exceed this value.
The notes specific to figure 7 are:
• I – stress value inferior to the projected value;
• II – stress value equal to the projected value;
• III – stress value exceeding the projected value;
• IV – stress value significantly exceeding the projected value;
• PI , PII , PIII , PIV – lifecycles (the intervals during which the quality of the running track reaches from Ci to Cf), corresponding to stress cases I, II, III and IV;
• TfI, TfII , TfIII , TfIV – the moments when the quality of the running track reaches the value Cf , corresponding to stress cases I, II, III and IV.
Considering the analysis of the problem similar to figure 6, it can be noted:
PI> PII> PIII> PIV
A stress value under the projected limit spares infrastructure elements by reducing the wear-out process. On the other hand, such a situation is not favourable because the capacity of the infrastructure could be inefficiently used, a fact that would determine excessive increase of the pay-off period.
2. Investments and repairs in railway infrastructure
The process of maintaining the railway lines in the projected technical and operation parameters demands strict compliance with specific works and repair cycles. Their aim is to maintain the technical condition above particular values imposed by the specific regulations in force.
The specific recurrence of the running track maintenance process through periodical repairs is schematically shown in figure 8. As the process is continuous and practically uninterrupted, a number of n repairs at equal periods of time between them were suggestively presented in the figure.
The notes in figure 8 have the following meaning:
• Pr0-1 – the first lifecycle, starting with the moment of putting in operation or the first moment considered in the analysis (T0 ), until the initiation of the first repair;
• Pr1-2 – the lifecycle between the first and the second repair;
• Tr1 , Tr2 , Trn – the nominal duration of a repair process. Usually, for the same type or category of repair, the durations have equal values;
• ΔC – the range of values comprised by the quality of the running track.
In all cases, the lifecycles (Pr ) are delimited by the moments when the quality of the running track varies from a maximum to a minimum level, imposed to infrastructure manager through specific regulations.
2.1. The influence of stress on the running track wear-out and repair cycle
The wear-out phenomenon registered to railway infrastructure elements that have direct contact with railway vehicles depends on the stress level – the values of vertical load and traffic intensity.
Figure 9 presents the situation of two types of running track stress with different values and the effects on the periodicity of the repair processes.
It can be observed that when the stress value is low, the lifecycle between two repairs is longer – an advantageous situation considering the financial effort for maintenance. In the cases when the stress exceeds the projected value, we can observe that the same lifecycle P considered starting with the initial moment T0 requires more repairs. This is caused by the fact that the overload of infrastructure elements accentuates the wear-out process. Thus, the lifecycles (Pr ) are shortened.
The notes from figure 9 are the following:
• Tr1 , Tr2 , Trn – the nominal duration of repair processes for a normal stress;
• T’r1 , T’r2 , T’r3 , T’rn – the nominal duration of repair processes in case of running track overload;
• Pr0-1 , Pr1-2 – the lifecycles between two repairs corresponding to normal running track stress;
• P’r0-1 , P’r1-2 , P’r2-3 – the lifecycles between two repairs corresponding to running track overload;
The comparison between the two situations presented in figure 9 shows that
Pr0-1> P’r0-1 .
2.2. Introducing restrictions and limitations in order to reduce the wear-out and degradation phenomena of railway infrastructure elements
In order to maintain the wear-out between the projected limits, it would be necessary to resize the entire repair cycle corresponding to each stress regime. For the majority of infrastructure managers, such an adaptation procedure of the repair cycles depending on stress regime cannot be completed. The financial efforts for such a procedure are considerable or even impossible to complete.
The solution available from practical perspective in order to compensate for the wear-out phenomenon for preserving the cyclic period (Pt) is introducing speed or tonnage restrictions or limitations at a certain moment Tres , determined as a result of technical studies (figure 10). In this point, the variation indicator of the technical condition decreases. Such a solution cannot eliminate the indirect negative effects generated by the restrictions: additional energy consumption, travel time prolongation, numerous variable regimes etc.
During the period when the restriction or limitation is introduced (marked in figure 10 with Pres ) a series of inevitable devastating effects appear, even though infrastructure elements are spared. Of these, the most important are: additional energy consumption, travel time prolongation, additional wear-out.
Introducing restrictions or limitations reduces the wear-out process (degradation), prolonging the lifecycles during which the value of wear-out reaches the maximum level. Such a process is presented in figure 11 and is comparable with the situation in figure 10.
By applying this solution, the modification of the number of repairs necessary for a particular period of time is avoided; the Pr intervals for both situations remain at the same value:
Pr0-1=P’r0-1 , Pr1-2=P’r1-2
2.3. The effectiveness of repairs depending on the amount of allocated funds
Any repair or remediation process in railway infrastructure requires the allocation of a specific amount of funds. Its value depends on factors such as: the superior level of running track quality, which is desired to be provided, the duration of lifecycle between two repairs (Pr), the quality and characteristics of the available materials, the available working technology, the execution time (Tr), the geographic and environmental conditions, the amount of available funds etc.
Figure 12 schematically shows a repair process (Tr) for which funds that have an equal value to the necessary one are allocated. Such a scenario is favourable from the perspective of operation and maintenance of the running track, due to the fact that the superior (Ci) and the inferior (Cf) limits of running track quality are maintained at the same value through the repair process. The occurrence of restrictions or decrease of values of infrastructure operation parameters is avoided.
A much more advantageous situation regarding railway infrastructure maintenance occurs when the funds allocated for the repair processes exceed the necessary value (figure 14). Thus, after the repair is completed, the initial superior value of running track quality (Ci1) can be improved, a greater value marked in the figure Ci2 being obtained. The difference between Ci1 and Ci2 depends on the value of the additional investment (marked in figure 13 in the right side through the crosshatched area). In comparison with the previous period Pr1, by keeping the same inferior quality level (Cf), a prolongation of the lifecycle (Pr2) is also registered after the repair with additional investment.
The case presented in figure 13 may be considered a railway infrastructure modernization process, respectively improvement of general performances of the conventional railway system.
An increase of the superior (Ci) and inferior levels (Ci) of running track quality is obtained by transforming classical railway lines in high speed lines. Such a situation is presented in figure 14.
The figure shows that the lifecycle after the repair process Pr2 is approximately equal to the previous Pr1.
Due to multiple financial problems of certain railway infrastructure managers, as the national infrastructure manager (CNCF „CFR” SA), the amounts allocated to the various repair and maintenance processes are often smaller than the necessary values. Thus, a series of technical problems appear over time, such as: the decrease of the maximum traffic speeds, tonnage limitations, traffic restrictions and even closure of lines due to lack of funds. Figure 15 schematically shows a repair process with investment lower than the necessary value. As a result, the process registers a deficit of financing.
Besides the decrease of the superior level of running track quality (from Ci1 to Ci2), a shortening of the lifecycle of infrastructure after the repair (Pr2) is generated as indirect effect, compared with the previous (Pr1). To compensate for these disadvantages, the necessity of intensifying the number of repairs executed per time unit or introducing speed and tonnage restrictions occurs (figure 9, respectively 11).
Thus, in order to compensate for the disadvantage related to the shortening of the lifecycle, the inferior level of running track quality is decreased (figure 16). By this means, the two values are equal:
Pr2= Pr1 , and Ci2< Ci1 and Cf2< Cf1 .
Figure 17 shows a railway infrastructure maintenance process with successively repeated deficit of financing. A cumulative decrease of the superior value of running track quality (Ci) and lifecycles between repairs (Pr) is generated.
Through the scenario presented in the figure, there are multiple indirect negative and cumulative over time effects.
The relations of inequality between the superior values of running track quality and between the lifecycles are:
Ci3< Ci2< Ci1 and Pr3< Pr2< Pr1
Similar to the solution in figure 16, the lifecycles between the repairs can be maintained equal through the successive decrease of the inferior value of running track quality as well.
Figures
[ by Viorel LUCACI – Expert, Romanian Railway Safety Authority – ASFR
Marian CIOFALCĂ – Head of Service, Romanian Railway Safety Authority – ASFR ]
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