Seagoing Steam Engines

More old-time navy questions.

How did steam-powered seagoing ships get enough fresh water to boil? Of course the could distill it, but that seems to take a lot of power. They could just bring some with them, and then recycle the same stuff forever.

One clever idea an officemate had was just to make the engine out of nonferrous metals that would not react with the sea water, boil the sea water and just clean the boilers regularly.

Do you mean before the introduction of multiple expansion engines?

I believe that in a emergency seawater could and was used, but caused problems, not just from rust but it foamed.

Also most oceangoing warships were sail driven until the screw (propeller) became in wide use as a paddle wheel was too easyally disabled and took away from gunports. By that time ways to reclaim the water were invented.

I think that’s pretty much it.

I’m not an expert on maritime steam, but railway steam was reluctant to use this technology, because reintroducing the vapour to the boiler presented huge problems in terms of traces of lubrication oils and other impurities which would do Bad Things™ to the boiler and mechanicals. It was used to some degree on desert railways because they had limited water available lineside. I assume the maritime types had no choice but to use this technology, and I’m sure they became good at minimising the problems. Short answer: with steam engines, it’s something you don’t do unless you really have to.

I don’t believe ocean-going steam engines were at all practical until they included condensors - the high consumption of fresh water would have seriously limited their range (high consumption of coal was enough of a problem).

On a ship, a condensor is not a huge problem - you have unlimited quantities of cooling water readily available (the sea). For a train, this isn’t so and a steam-to-air heat exchanger would be impractical. So steam trains essentially always used an open cycle, and depended on supplies of feed water at their stops.

Of course, even in a closed-cycle steam engine some water is lost and must be replaced. The normal scheme is to use an evaporator that boils salt water and condenses fresh water from it. If you keep the temperature low (by running the process at a partial vacuum) and don’t overly concentrate the salt, you can avoid scaling up your evaps.

Ferrous metals are not the only ones subject to corrosion. They do tend to be both cheaper and stronger than non-ferrous atternatives.

And this scheme would require redundant boilers, so the ship would not be rendered immobile by the need to cool & clean the boiler (a nasty job of which I have some experience).

I don’t think there’s any practical way to reintroduce low-pressure steam to a high-pressure boiler. You have to condense it to water so you can use a pump to overcome the pressure difference.

In one of his books, Daniel Gallery described the engineering plant on the Casablanca class escort carriers. They had something called a “Skinner” engine, which used high-pressure steam in all cylinders, and required that lubricant be mixed with the steam before injection. After it was exhausted the steam was supposed to be passed through a filter of fuller’s earth to remove the lubricant, but Gallery said that the filters were constantly breaking down — so, according to him, “our boilers sometimes had to put up with a little bit of oily water.”

You’re right. I worded that poorly.

Steamships make their own feedwater.

Its done using condensors to reduce boiling point of seawater to produce a vacuum, the condensate is then pumped away and beome feedwater.

Not all steamships can do this, but its a common enough method.

This system can also dearaerate the feedwater too.

Modern nuclear-powered submarines and aircraft carriers are steam driven. The steam is condensed and reused as feedwater in a closed-loop cycle. Makeup water is produced by distillation and ion exchangers to produce ultrapure water. At the high temperatures and pressures of the steam cycle, chlorides in excess of the part-per-million range can cause corrosion and rupture in a matter of minutes, so impurities are closely monitored. Chemicals such as phosphates are added to maintain pH in a narrow band.

Some of the first old-time low pressure steam systems actually did use seawater for feedwater, IIRC.

Corrosion aside, there is a more immediate problem faced with impurities in the boiler. In a steam locomotive, oil and other impurities can cause the water to foam, and this can introduce water in liquid form direct to the cylinders, a condition known as “priming”. As water doesn’t compress readily, priming can result in immediate failure of the cylinder, which is more of a concern to the engineer than the fact that boiler replacement may be brought forward at some distant future time.

In a naval steam plant, water in liquid form impinging on the turbine blades can indeed cause damage to the turbines. This is generally only an issue during plant warmup and startup, and the operators blow the lines dry as necessary.

However, the more common problem is contamination with chlorides. At the temperatures and pressures present for a naval steam plant, chloride pitting and ruptures of the steam tubes from corrosion can occur in a matter of minutes, not some distant time in the future.

I don’t know of any modern marine steam plant that uses cylinders and pistons. The steam produced is used to drive turbines.

Yeah, I was talking from a railway perspective, but your post is interesting. Is this catastrophic failure which can occur in a generally “as new” unit, or does it assume extant damage?

Corrosion is not the only problem, a buildup of solids, on waterside walls, will reduce conduction of heat flow through the metal, ferrous or not. This will result in overheating and failure of the metal. While pH is important, to decrease corrosion, impurities in the water are probably more critical. As the boiler water changes from solid to vapor it leaves those solids behind, increasing the buildup in the water. There are various methods of dealing w/ this.

Compared to the energy needed to move several hundred tons of ship through the water? Distilling plants are very low energy systems. Modern (WWII era and up) shipboard distilling plants run at a vacuum, which reduces the temperature that the water has to be raised to, to make it boil. By discharging brine, instead of trying to boil off all the water in a given amount of salt water, too, there are a number of advantages. There are all sorts of places where regenerative processes can be used to improve efficiency, too: using discharging brine to pre-heat incoming water is just one way.

For the most part, a distilling plant on a ship was going to be designed to meet two requirements: to make up the water losses from the enginerooms, and then to provide some fresh water for the crew. No steam plant is going to be tight enough enough to be able to operate without make up water - even if it’s nominally a closed cycle. Valves, fittings and other connections will leak.

Now, for really old steam plants, it’s not always going to have been necessary for the water going into the boilers to be pure water, instead of salt water. While temperature does affect how quickly corrosion occurs, the rapid chloride corrosions that scare modern boiler operators are happening at temperature and pressure regiemes that are completely foreign to early steam engines. For example - I don’t think that our distilling plants were made out of anything but normal stainless steel, though I’ll admit I might be wrong.

As long as the peak pressure for the boiler is no more than about 30 PSIG, I’d say that it’s probably possible to operate a steam plant using salt water. It would last longer, of course, if they used fresh water of course, but that’s not the same thing as saying that it was unsafe to have low pressure steam ships running with salt water in their boilers.

As temperatures and pressures in steam plants went up, of course, the concerns about rapid corrosion from salt water become much more important. But, for example, all the technology needed to design and operate a modern steam powered distilling plant is inherent in the design of the triple expansion steam engine. (The biggest hurdle I’m thinking of is the need for steam jet ejectors to reliably create a vacuum in the distilling chamber.)

I suppose every shift you could send some guy down to the evaporator room to renew the vacuum with a hand pump.

Nope, its a continuous process, you pump in water, and you have a vacuum pump.

The vacuum pumps tend to be high maintenance as the seals tend to be hard carbon brush rings.

De-aerators tend to be quite cold, even though its tepid water going this is due to the low due to the low pressures, you are sucking energy out of the water to evaporate it.