Why Are Marine Boilers Water-Tube Instead of Fire-Tube?

That is correct. Water tube boilers can safely bring pressures to about 350 psi. Also water droplets in a turbine environment will cause a corrosion issue with the blades. Thus the supersteam heater.

The availability of water, unlike a tender that can bring a limited supply, makes Water-Tube boiler ideal for ships.

For what it's worth, a 350 psi water tube boiler is pretty much a "low pressure" and low volume boiler. Most large water tube industrial and ship's boiler are far in excess of 600 psi.

Plus, the other advantage of water tube boilers, vs. fire tube boilers is the ability to provide extremely high and STEADY demands. The fire tube boiler, like a steam locomotive, has the ability to cope with VERY RAPID load/demand level changes, i.e. full throttle operations for up-hill grades, then no throttle operation for down-hill operation.

I've read that turbine-powered luxury liner United States (the fastest ship in the world) makes steam in its 8 boilers as hot as 1000 degrees F with pressures as high as 975 PSI. 

BAD ORDER

That's right...no surface-condensing locomotives!  

Bad Order Harry

Technically any steam boiler 15 psig or less is low pressure. Everything above is considered high pressure.

You mean everything above one atmosphere gauge (15 psig) is considered high pressure?

Bad Order

Since condensors were used to reuse and recirulate water in various makes of steam

automobiles, such as later Stanleys, Doble, etc., (I don't know if President Taft's car,  the first White House auto, a White Steamer, was a condensor model)  I have wondered why that wasn't used and practical for locomotives vs. troughs between the track and constant water stops.  Here I am reading that ships, with an apparent water supply surrounding them, use condensors??

C'mon, Colorado!

Surely you know that ships can't take in salt water as feed for their boilers!

They take in salt water to cool their condensers externally, of course, and use the condensate as feedwater.

HUT HUT HUT! 

bad order hal

I'll stick with the 15 PSIG. And yes, anything above that is considered high pressure.

one atmosphere equals 14.7 psia equals 0 psig

HEY PENNSY K4  4-6-2,

I've known that atmospheric pressure was 14.7 PSI since about the 5th grade (1947), when the Hughes Hercules (Spruce Goose) took its one and only flight.  

Historic Hal

BAD ORDER

And as you probably know, water-tube boilers in stationary applications such as electric generating plants operate on steam pressures significantly higher than 975 psi.  Inlet steam pressures of 2,400 psi or 3,500 psi (supercritical) are commonly used in large generating units.  Ultra-supercritical boilers with pressure as high as 5,000 psi have been employed.  That was the specs on the boiler at coal-fired unit #1 at the Eddystone Plant of Exelon (formerly Philadelphia Electric).  That unit started up in 1960 and was retired in 2011. 

And to keep this message train related, “younger” units still operate at that power plant which is located near the old Baldwin Locomotive Works.

Bill

I spent a career in the Navy on steam powered ships.  Water tube boilers are more efficient than fire tube boilers which is very important for fuel consumption.

My first ship, a guided missile destroyer, had four boilers operating at 2,000 psi.  Most Navy boilers operated at about 600 psi.  The higher the pressure the more efficient the boiler.  

In a typical navy boiler the tubes are connected to a steam drum at the top of the boiler.  The saturated steam from the tubes is sent to the superheater.  (Saturated steam is steam that still has water droplets in it.  Saturated steam can destroy turbine blades.)  The superheater adds more energy to the steam and drys it.  The superheated steam is sent to a high pressure turbine and then a low pressure turbine.  The turbines are connected to the propeller shafts through reduction gears.  A typical ship will develop 30,000 or more shaft horsepower.  

The steam exhausts from the turbines to the condenser.  The condenser condenses the steam back to water.  This is done by passing the steam through many tubes surrounded by seawater in the condenser.  The flow of sea water around the condenser tubes is maintained by the movement of the ship through the sea.  The seawater flow is maintained by a pump when the ship is in port.

The condensed water is then pumped back into the bottom of the boiler through a device called the feed water drum.  It is then reheated and the cycle is repeated.

The navy uses recycled water for several reasons.  First, fresh water is precious at sea.  No ship can carry enough water for even a few days and the water has to made from seawater.  All navy ships have water making condenser systems.  This includes submarines.      

Second, the water in navy boiler has to be pure water to operate at high pressures.  Even 1 part per million of salt or other contamination can cause corrosion and a water tube or high pressure pipe to fail.  The water in navy boilers is constantly tested and treated.  New fresh water and chemicals are added to dilute any contaminates.  No navy boiler would last more than a few hours with water picked up from a water tank or a trough between the rails. (The navy adds salt back into the drinking water system to improve the taste.)   

Unlike train steam boilers, navy boilers are operated constantly for months without shutting down for maintenance.  I have been on a 9 month voyage where the boilers were only shut down a couple of times for a few hours in port.  Most of the steaming is at constant speeds but the boilers are able to provide emergency power instantly when needed.  When enemy shells start falling around a ship off Vietnam you can go from cruise to full ahead emergency in seconds.  The acceleration is awesome and needed.  I have experienced it.  

Although boilers were highly efficient and powerful, the navy has switched to gas turbines and diesels for most new ships.  These items are simply less costly to operate and easier to maintain.  When a turbine is bad, for example, it is hauled out through the stack and new one is installed.  The ships still have to make fresh water, however for the crew and some auxiliary systems.

I think that the only steam powered ships that are left are nuclear carriers and submarines.  These ships uses the reactor as the heat source and a relatively low pressure boiler.  I have forgotten the pressure but I believe it is in the 200 to 300 psi range.

Nuclear ships are unable to make superheated steam.  These ships have special turbines that are designed to handle saturated steam.  Otherwise, the steam cycle is identical to that described above.

I hope that this provides some insight to the difference in boiler systems between trains and ships.  

Joe

There is the Baldwin built water tube boiler locomotive at the Franklin Institute in Philadelphia. I believe that ran at at 600 to 1000 psi. Maybe because of the tough economy, WWII, or the coming of the diesel locomotive the design was not pursued. 

There were quite a few condensing steam locomotives in Europe and Africa. (Among other places.)

Probably one of the most impressive is the South African Railways class 25. 

And don't forget the famous Project X-12 Nuclear locomotive.  Even though it was never built, it would have been quite something to see.