The requirement of twin pipe system in coaching stock:
Charging time of Auxiliary reservoirs:
As the free delivery of compressed air from the loco is 1500 liters to 2000 liters per minute at normal speed, to charge 6000 liters of air in the auxiliary reservoirs, it takes about 180 to 240 seconds.
To charge AR by 5 Kg/Cm2 = 240 Seconds (Maximum)
To charge AR by 1 Kg/Cm2 = 48 Seconds (Maximum)
The application time is the time taken by the Distributor valve to admit a pressure of 3.6 Kg/Cm2 into Brake cylinder from the Auxiliary reservoir during Full-service application or Emergency application.
The application time for the coaching stock is 3 to 5 Seconds.The application time for the Goods stock is 18 to 30 seconds.
After the Full service or Emergency brake application, the brake cylinder gets a maximum pressure of 3.8 Kg/Cm2 from Auxiliary reservoir.
The releasing time is the time taken by the Distributor valve to release the air from Brake cylinder from 3.8 Kg/Cm2 to 0.4 Kg/Cm2.
Note: The releasing time does not depend upon the piston displacement but it depends upon the BC pressure only.
- The releasing time for the coaching stock is 15 to 20 Seconds.
- The releasing time for the Goods stock is 45 to 60 seconds.
The difference in the application time and release time between the coaching and goods stock DVs is achieved by varying size of the choke in DV between the AR and BC for the application, and by varying size of the choke in DV between Brake cylinder and Exhaust for the release.
Hence the DVs of coaching & freight stocks are not interchangeable, since the application and release timings are different for coaching and freight stock.During brake application, as the air from Auxiliary reservoir is sent into brake cylinder, there is always a reduction in the AR pressure and it is likely to drop to 4.2 Kg/Cm2.
Hence during release, it must be ensured that before the air from brake cylinder is released completely, the AR should be recharged to 5 Kg/Cm2, so that the system can be kept ready for next brake application.To charge AR from 4.2 to 5 Kg/Cm2 the DV takes approximately 36 Seconds, and to release the air from BC from a pressure of 3.8 Kg/Cm2 to O.4 Kg/Cm2 the DV takes 45 to 60 Seconds in the case of goods stock. From the above facts, as it is possible to recharge the AR from 4.2 to 5 Kg/Cm2 within the release time of 45 to 60 seconds, a Single pipe system itself is sufficient for the Goods stock.
In the case of coaching stock, to release the air from BC from the pressure of 3.8 Kg/Cm2 to 0.4 Kg/Cm2 the DV takes 15 to 20 Seconds. And to re-charge AR from 4.2 to 5 Kg/Cm2 after application, DV takes approximately 36 Seconds From the above fact, it is clear that it is not possible through DV to recharge the AR from 4.2 to 5 Kg/Cm2 within the releasing time of 15 to 20 seconds for the Coaching stock.
Hence it has become necessary to introduce one more pipe called feed pipe to recharge the AR always to 6 Kg/Cm2, from the other end of the AR within the release time of 15 to 20 seconds and thereby auxiliary reservoir pressure is maintained to an optimum level for repeated brake applications.