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August 1997 - Page 56


E qualizing Reservoir
I s tated earlier that the engineer makes a service appli cation of the brakes by moving his brake valve handle to the application position, opening a small hole, which reduces brake pipe pressure slowly. He watches the brake pipe pressure fall on the air brake gauge. When he gets the amount of reduction he desires, he moves the brake handle to the Lap (blocked-off) position. Actual ly, t h i s is only true with the very early air brake system s . As trains got longer, and had more brake pipe volume, it took too much time for the air to travel through all the cars to vent at the engineer's brake valve. Compromising safety, his attention was fixed on the air brake gauge for an inordinate time. So another small reservoir, known as an equalizing reservoil was installed; its size allows pressure to be reduced almost instantaneously. The engineer's brake valve now reduces the air in the equalizing reservoir instead of the brake pipe. He can get the desired re duction ( say 1 0 psi ) very quickly and then can take his eyes off the equalizing reservoir gauge to look out ahead. An equalizing valve is connected between the equalizing reservoir and the brake pipe; it is this valve that vents the brake pipe air to the atmosphere until it matches the equalizing reservoir pressure. Also, since the late 1 950s or early 1 960s, the en gineer's brake valve has been self-lapping. He or she no longer has to move the brake valve back to the Lap position after making a reduction, because the position of the brake valve handle determines the amount of reduction. One more consideration: The engineer can change the maximum pressure of the brake pipe by adjusting the Feed Valve at his con trol stand. I have used 90 psi as the standard pressure to which the brake pipe is initially charged and subse quently recharged. On the railroad I work fOl; 90 psi is the standard. Some railroads use 80. Some mountain grade lines use 1 00 psi on loaded coal and grain trains. What is the significance of the different pressures? During normal service braking operations, there i s none. A 1 0 psi reduction from a 1 00 psi brake pipe, a 90 psi brake pipe, or an 80 psi brake pipe all result in 25 psi in the brake cylinder and thus equal braking ef fort. But what happens if you make a 26 psi reduction from a 90 psi brake pipe, since 90 minus 26 equals 64 psi in the brake p i pe? As service reservoir pressure flows into the brake cylindel; the brake cylinder pres sure rises. Because of the 2 . 5 to 1 ratio of volumes, when enough air has flowed into the brake cylinder to lower the service reservoir 26 psi, the brake cylinder pressure is 64 psi ( 2 . 5 times 26=64 ) . This air came from the service reservoir which is also now at 64 psi . Since the reservoir pressure and the brake cylinder pres sure are equal , no more air will flow into the brake cylinder. This condition is called a Full Service brake ap plication, because reducing the brake pipe further will not increase the amount of brake cylinder pressure. ( Even if you reduce the brake pipe pressure to zero psi , the reservoir and brake cylinder pressure will stiLl be 64 psi-the same as with only a 26 psi reduction . ) This Full Service, o r equalization of pressures, oc curs at 64 psi for a 90 psi charged system and 7 1 psi for a 1 00 psi charged system (resulting in higher full service brake effort ) . A n engineer w h o makes a reduction greater than these values is just wasting time-no higher braking ef fort results.

Brian Solomon

All of this is academic, however, since normal train operations seldom require a brake application greater than 1 5 psi. I ndeed, any reduction greater than 1 2 psi is considered heavy braking. So why would mountain grade rai l roads use 1 00 psi in the brake pipe? Two reasons. As we just saw, the Full Service braking effort is h igher if it is needed. Suppose a 1 0 psi reduction is made from a 1 00 psi system, resulting in 25 psi ( 1 0 psi times 2 . 5 ) in the brake cylinders. Part way down the mountain, the grade lessens and train speed drops. The engineer releases the brakes, and the brake pipe returns to 1 00 psi. The train immediately begins to ac celerate down the grade. The engineer immediately re sets the air brakes by making another reduction. But the car reservoirs have only just begun to recharge, so t h ey c o n t a i n o n l y 90 p s i . [ f he m a ke s a 1 0 p s i reduction o f the brake pipe, h e will get n o brakes, be cause the brake pipe will be at 90, as are the reser voirs. But if he makes an additional 1 0 psi reduction (a total of 20 psi ) , the engineer will get the same brak ing effort as the original set, 25 psi in the cylinders. This means the 1 00 psi system gives the engineer one additional 1 0 psi set and relea e before he begins to run out of air. So why not always use 1 00 psi? There are penal t i es that go a l on g w i t h t h a t extra pressure . Weak hoses or v a l v e gaskets may fai l . A n d , if the train

56 August 1 997

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