Western Railroad Discussion > Maintenance question

Do the air reservoirs that keep air pressure at operational levels have relief valves? When those valves fail, do they fail in the 'open' position so that air pressure is lost and the train stops? It would seem to me that that would be a nightmare to find for the conductor. Thanks.

The air systems on all of today's locomotives actually have two pressure relief valves that are set at 150-psi. All the air systems also have air dryers, some of which are automatic and turn on-off to vent water and moisture that has condensed within the air system when the air is compressed. These valves are the ones commonly heard making the popping sounds that you hear when passing by a locomotive, (also called "spitter valves"). If an air line were to come apart or break in two and cause a leak that would discharge the main reservoir air pressure, the air compressor usually kicks in and starts pumping to overcome the leak. (The air compressor magnet valve which controls the start up of the air compressor is generally set to start pumping air at 120-psi, and shuts off at 130-140 psi, or what ever pressure the railroad chooses to set it at, usually 10-20 psi higher than startup air pressure). If the main reservoir air pressure drops below 100-psi, some model locomotives, (like all of the GE models), will stop loading and display a "Low main reservoir" warning and alarm. The train can physically continue rolling as this function does not stop the train. In instances where a DPU unit is on the rear of the train and has no other working locomotive mu'd to it and if it were to develop a significant air leak that caused the main reservoir pressure to drop below the train line brake pipe pressure, then the brakes on the DPU remote locomotive would apply to stop the locomotive and the crew on the head end would be able to see this by their air brake and brake cylinder air pressure gauges on the DPU controls. This still would not stop the train and the crew, if they chose to ignore these readings could continue on which would burn off the brake shoes on the DPU locomotive.

But to answer your question about making it hard for the crew to find the leak, the answer would be no. If the leak is on the locomotive, it is usually still blowing a lot of air and is easy to hear, (due to the air compressor is still pumping until it reaches shut-off pressure). But a train-line air leak in the train or on a freight car is harder to find sometimes, because once the train goes into emergency, (due to a sudden loss or drop in train-line air pressure), all the air in the train-line and in each car's air reservoir is lost. So there is no air blowing, (or very little if the crew is trying to recharge the train-line before finding the leak), and then, Yes it is hard to find the leak in the train. And, Yes the safety relief valves generally do fail in the open position, but with the air compressor still pumping air into the system the leak is easy to hear and see, and the valve, although stuck open, doesn't vent as much air as fast as the air compressor can pump it up, so most locomotives can maintain at least 50-psi or so of air even though the relief valve is stuck wide open.

What a clear, complete, terrific answer. Thanks to poster and postee for both!

What needs to be done to recover from an emergency application? Is the automatic brake just recycled?

Approximately how much time is needed for a short train, 3 or 6 cars, as opposed to a long train?

TIA!

And when they fail open Mechanical tells you to hit the valve with a rock to reset it.

The "spitters" are actually main reservoir drain valves that are designed to remove water from the air system. The moisture in the air distills out when the air is compressed and settles in the reservoirs.

Locomotives that are equipped with air dryers do not need the spitters, although some locomotives have both an air dryer and spitters. Some units with air dryers have had the spitters replaced with manual drain valves.

I only know of one main reservoir safety valve per locomotive, although the air compressor intercooler has a safety valve mounted on top which is set to pop about 45 PSI. The main reservoir safety valve, on the other hand, operates between 147 and 153 psi and must be replaced if it operates outside of these limits.

Tom Dial, Machinist, UPRR (Ret.)

>Do the air reservoirs that keep air pressure at operational levels have relief valves? When those valves fail, do they fail in the 'open' position so that air pressure is lost and the train stops?

There are no releif valves on a car's air reservoirs.

There is one safety valve on a locomotive's main reservoirs, and it must exceed the air compressor's pumping capacity. It has no manual test lever, and if stuck open must be replaced and sent in for repair. The maximum operating pressure varies from railroad to railroad.

There are automatic drain valves, electrically operated, on either the main reservoirs, the air dryers, or both. "Spitters" are sometimes used in place of automatic drain valves because they are self contained, using no electrical control circuits. Their drawback is that the least spec of dirt makes them spit continuously.

>What needs to be done to recover from an emergency application? Is the automatic brake just recycled?

Wait approximately 90 seconds and release the brakes.

>Approximately how much time is needed for a short train, 3 or 6 cars, as opposed to a long train?

Recovery time is the same regardless of train length; it's the time the brakes actually release on the last car which varies.

InsideObserver Wrote:
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> Recovery time is the same regardless of train
> length; it's the time the brakes actually release
> on the last car which varies.

In terms of minutes, are we talking about a few (2 to 3), or quite awhile (5 to 10)?

After you wait the 90 seconds and reset the automatic brake valve, the time it takes to get your air back up to at least 75 lbs. at the rear varies by train length and temperature, as well as how many units are in the consist.

3-5 cars builds up in a matter of about 20 seconds, I'd say.

A 100 car train in the cold might take 30 mins. or more. Sometimes a lot more.

Then there is occasionally a vent valve that won't reset....ugh. Beat on it, big hole the train again, or just wait until it resets on its own.