Condensing steam containing piston lubricant

[Tony MartyrParticipant @tonymartyr14488]

[Tony MartyrParticipant @tonymartyr14488]

Having worked on steam turbine ships I know a bit about boiler water chemistry therefore I'm a little puzzled about the condensate system supporting the triple expansion engine I've built. If I add oil injection into the steam the condensate is going to be a water/oil emulsion that will coat tubes etc and I'm not sure what will happen in the boiler. This dilemma has occurred to me a little late in the day and I would be interested to hear of experience of others

Tony

[John OlsenParticipant @johnolsen79199]

The boiler treatment is usually alkaline, so what happens is that the oil gets converted into soap and the boiler primes, which is of course highly undesirable. Some steam launch plants do use cylinder oil. Usually absorbent material is used in the hot well to absorb the oil. There are a number of things that people have tried, including paper towels. Also the hot well can be constructed as a labyrinth so that the water has to go under a number of barriers to get from the inlet to the outlet. Since the oil will float this helps to prevent it getting to the boiler feed.

It is also possible to put an oil separator ahead of the condenser. I have experimented with a centrifugal type…it seems to separate oil in bench testing, but hasn't been tried under proper working conditions since the boat is not finished yet. The principle is that the exhaust steam pipe (about 1 inch diameter) enters tangentially into the side of a larger tube, about four inches diameter. The large tube is vertical and the tangential tube enters near the top. The bottom of the large tube is formed into a funnel shape with an oil drain at the bottom. The steam exit is out the top, via another 1 inch tube that protrudes coaxially partway down the large tube. The idea is that the steam entering tangentially will whirl around inside the large tube and the heavier oil droplets will be flung against the outside wall and run down to the bottom. The steam, being lighter, can make it around the turn to get into the coaxial exit tube. Some water will also be collected in the bottom, this can be minimised by insulating the whole affair.

Most steam launches that use lubrication don't seem to worry too much about oil in the condenser. I presume that it would not hurt to flush the oil out fairly often, maybe by blowing a bit of steam through. They are usually keel condensers. So long as there is plenty of area a little oil will not hurt too much.

The other approach to consider is to run the plant on wet steam with no oil. The water has reasonable lubricating properties. With a triple, you may find that the last cylinder is not doing a lot of good because the steam has mostly condensed before it gets that far. Small compound engines don't gain much efficiency because the heat loss in small cylinders is higher.

You could find it useful asking about this on the Steam Boat Association web forum.

John

[Tony MartyrParticipant @tonymartyr14488]

Thanks for this John,

It is nice to hear that a group of experienced folk like the SBA have to deal with this problem and will direct future queries in that direction even if I can't afford to build a hull!

I intend to use dry superheated steam so the HP and IP pistons would seem to need a lubrication, the LP should be exhausting at lower than atmospheric (if the condenser manages to pull a partial vacuum) so should be OK anyway. I had considered a centrifuge like the Alfa Laval units we used to get water out of bunker C fuel oil but that is a considerable development task!

Your hot-well modifications sound a sensible (cheap!) option – I was just about to start its construction and as I haven't yet designed the cooling water system I can provide for a condenser flush branch.

Tony

[John OlsenParticipant @johnolsen79199]

If you can manage it then reheating between each stage would be interesting, but don't get the steam too hot or you will have lubrication problems. With steam launch sized plants it is typically possible to get around 20 inches or so of vacuum. The limit tends to be either air in the steam, eg dissolved in the feed water, or leaking in through the LP glands. Super heating tends to help with getting a better vacuum since it means there will be less steam to condense.

Don't let the cost of a hull put you off, it tends to cost more in time than in money. One tip to bear in mind is that restoring an old hull is likely to take more time and money than building a new one.

John

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