What are those spitting sounds that locomotives make?

[cefinkjr]

After reading all of the preceding responses, I'd say the proper answer to the question, "What are those spitting sounds that locomotives make?" would be, "Spitting."

[oltmannd]

They are technically thermionic drain valves.  They replace the time-based solenoid valves for expelling condensation from the compressed air system.

http://www.dayton-phoenix.com/productCategoryDetail.php?categoryId=112

 

[zugmann]

Deggesty

Or, "Ripe Peaches," "Black Maria," Bohannon's Favorite," or "Apple."

 

Believe me, that is very prevalent noth of the M-D line, too.  Just wish those that partake in such a habit would be kind enough to remove all spit vessels from the cab when they get off the engine.  Sorry, but that is plain disgusting to leave behind.

[power58]

Saw a neat sticker on a CSX Air tank " I May Spit a lot but you're getting clean air"  Leave Valve in AUTOTMATIC Position at all times. Valve responds automatically to water formation by sputtering frequently.

[Deggesty]

Firelock76

If those locomotives are located south of the Mason-Dixon line they could be indulging in some "Red Man", "Beech-Nut", or "Days Work."

 

Or, "Ripe Peaches," "Black Maria," Bohannon's Favorite," or "Apple."

[aegrotatio]

How often would they spit?  When I see locomotives here in Virginia they spit at least once per second, usually more.

 

[YoHo1975]

beaulieu

It comes from the Air Dryer that removes moisture from the air compressor system recharging the air brake reservoirs. The spitting sound is a small amount of air being used to expel the accumulated water from the bottom of the dryer. How frequently the spitting occurs will depend on how much humidity there is in the air and how much the air compressor is running.

 

 

This explains why I heard it with a regularity when I was younger during long humid Chicago Summers, but now out here in comparatively dry California, not so much.

[ndbprr]

A funny related story. Forty years ago I worked for Armco Steel in Middletown Ohio. We had just rebuilt a galvanize line with pneumatic instrumentation. Now the standard line for products designed for use on air is,"customer will supply clean dry plant air". Every plant air system ever designed at that time had gallons of oil and water in it. One of our guys had a brilliant idea. On the other side of the control room was an anneal facility that used 15# nitrogen as a shield gas in annealing the coils that had a -60 degree dew point and no oil since it was made cryogenically. We used it to control the instrumentation. Several months later the L&N service guy was pulling his hair out trying to understand why the instrumentation had no water or oil problems. When we finally told him what we had done his reaction was, "you can't do that. In 20-30 years the seals will dry out".

[rfpjohn]

They're actually called "spitter valves". There is one on each main reservior and they are used to expell moisture from the air brake system. It seems like they tend to spit more in high humidity, cold weather conditions. In very cold weather, they will sometimes freeze open, blowing continuously until manually closed. This condition occurs far less frequently since the introduction of the Salem twin tower air dryer.

This is a clear, moist discharge, as opposed to the tebacky drool!

[Firelock76]

If those locomotives are located south of the Mason-Dixon line they could be indulging in some "Red Man", "Beech-Nut", or "Days Work."

[ndbprr]

Think sump Pump float switch. Water reachs a certain level and the valve opens. Air pushs water out. Water gone valve shuts. Only takes a couple of seconds on an engine.

[Wizlish]

aegrotatio

Is it the air brake system trimming itself by outgassing?

What does this sentence mean?  It appears to be saying that the little valves are adjusting air pressure by releasing small amounts to atmosphere -- I'd agree that if they were safety or pressure-trimming valves, something would be (massively!) wrong with the design or maintenance of the pressure regulation in the air compressor or the air system as a whole.  But that is not what the little valves are doing, as beaulieu has pointed out.

The sound comes from traps that clear condensed water from the air system, as noted.  When air is compressed some of the water vapor (humidity) it contains condenses out, and it's desirable for a number of reasons to get rid of that condensate promptly.  Since the condensate is liquid it runs down to low points in the air system, where special valves can be provided which open briefly when liquid is present (and then close again).  Air pressure is used to blow out the accumulated water quickly, and to provide a positive action for the valve mechanism; only a very small amount of compressed air is used for each 'shot'..  I believe the traps that remove condensate from steam-heating trainlines work on similar principles but of course lower pressure.

[beaulieu]

It comes from the Air Dryer that removes moisture from the air compressor system recharging the air brake reservoirs. The spitting sound is a small amount of air being used to expel the accumulated water from the bottom of the dryer. How frequently the spitting occurs will depend on how much humidity there is in the air and how much the air compressor is running.

[aegrotatio]

What are those spitting sounds that locomotives make?  Is it the air brake system trimming itself by outgassing?  Should it be doing this on a properly maintained locomotive?