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August 1997 - Page 52


D ean Sauvola

ABOVE: Since air brakes are fail-safe with the reservoirs charged, this Canadian Pacific train came to a halt soon aHer these hoses separated and emergency valves were tripped in both directions, causing a full brake application through out the train. LEFT: When braking eHort is needed, the engineer reduces the pressure in the brake pipe through his brake valve; the control valve, sensing the pressure diHerence, then admits air to the brake cylinder and the brakes apply. RIGHT: Once the reservoir pressure decreases to equal the brake pipe pressure, the control valve returns to Lap position, and the engineer's valve self-laps so that a constant pressure is maintained in the brake cylinder.
in a short time. He or she soon will have no brakes be cause there will be very little air left in the reservoirs. Railroaders call this "pissing away your air." (Now be fore you go and tell the press at the scene of a runaway train wreck that a dumb engineer must have been at the controls, please understand that runaways can also occur in ways that are not the engineer's fault. ) Another complication o f this simple brake system is that a long train has a long brake pipe, containing a lot of air. When I want to make a brake application by re ducing the brake pipe pressure, it takes time to vent enough air to do so. This is not a problem under normal braking conditions, but what happens in an emergency? Solution: an emergency vent valve has been added to each car. This valve monitors the brake pipe air pressure. I f the pressure drops slowly, the emergency valve does not react, no matter how low the pressure goes . B u t if it d rops q u i c k l y, the emergency valve opens the car's brake pipe to the atmosphere. This quickly dumps the brake pipe air to the atmosphere at the car. In other words, all the air does not have to go t h rough the entire brake pipe, up to the engineer's valve, and out to the atmosphere. All I have to do is start the emergency application by quickly venting brake pipe air at the head end. The first car's emergency valve senses the fast drop and vents all brake pipe air at that car quickly; the next car senses a fast drop and also goes to emergency, then the next, and so on. Within seconds, the entire train is in emergency, dumping all the brake pipe air at each car. I get a fast and full application of the brakes throughout the train. If you are standing near a train when the lo comotive uncouples, you can hear these emergency valves vent the brake pipe pressure locally as the car you are next to makes the tell-tale "psssssht." If you are standing some distance off to the side, you can hear each car trigger in succession, rapidly down the train. These emergency vent valves stay open for about two minutes, ensuring that the train will be stopped before the engineer can release the brakes. Anything-the train coming uncoupled, a bursting h o s e , t h e engineer, t h e c o n d u ctor-that causes a quick drop in brake pipe pressure at any car will trig ger that car, which in turn triggers adjoining cars and puts the whole train in emergency. All well and good-in theory. But what about that doofus engineer who used up all his brakes, leaving little pressure in the reservoirs? If he puts his train into emergency, he will still get very little braking-in effect, he will just get what is available more quickly. To ensure that there is always air pressure on each car for an emergency application, t h e b a s i c s y s t e m w a s m o d i fi e d by a d d i n g a second-or emergency-reservoir to each car. The emergency reservoir is charged with compressed air from the brake pipe, just like the service reservoir. After the initial charging time in the yard, it contains 9 0 p si . This air is never used during normal braking. How ever, if the engineer initiates an emergency application by m a king a quick reduction, each car's emergency valve h'iggers, just as described above, but now it also connects the emergency reservoir air to the brake cylin der in combination with the service reservoir air. When I make a service application, the brake pipe air vents through a small hole in my brake valve, lower ing brake pipe air pressure slowly. When I want an emergency application, r move my brake valve over to the emergency position-a big hole in the valve that al lows air to escape quickly, hence the terms for an emer gency application, "Big hole 'em . " Just like freight cars, locomotives have air brakes that apply when brake pipe air pressure is reduced. This is not always desirable, especially when "stretch braking" with the throttle open and car brakes set to control slack action . At these times, an engineer can prevent the locomotive brakes from applying by de-

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