DSK wrote:

One thing I'm curious about is how well they could maintain vacuum with
the turbine bypassed. IIRC the air pumps were driven off the wing engine
shafts. I don't think the Olympic class plants were designed to do
crashback drills.

One could reasonably assume that the plant was designed to accomodate
the loss of the turbine and the condenser could operate at the vacuum
required to allow full power from the recips.

Recips don't require as great a vacuum as a turbine to operate at their
peak efficiency. Plant efficiency without the turbine would drop
considerably as the energy in the recip exhaust would be lost to heating
the sea.

Recips could and would crash stop quite well, all ships are designed
with emergency backing in mind. A recip will generally stop quickly due
to the internal friction but steam can be admitted in the reverse
direction without harm ... it is a compressible fluid and acts as a
cushion in normal operation. Reversing is a simple matter of changing
the operative eccentric, all tghe parts move in the same plane as
before, only shaft rotation, thrust, and crosshead guide thrust changes,
and the engines are designed with that in mind.

Rick

Rick wrote:
One could reasonably assume that the plant was designed to accomodate
the loss of the turbine and the condenser could operate at the vacuum
required to allow full power from the recips.

Sure, but for how long? It's a liner, designed to maneuver with the aid
of tugs. Warships have far greater ability to steam in maneuvers, but at
the cost of efficiency & space. A liner has to make money.


Recips don't require as great a vacuum as a turbine to operate at their
peak efficiency.

I guess you could run them with the exhaust vented up the stack, like a
railway locomotive. But you'd run out of feedwater after a while!

... Plant efficiency without the turbine would drop
considerably as the energy in the recip exhaust would be lost to heating
the sea.


Right... and this is why I think it would be difficult to maintain
vacuum if you ran without the turbine for any length of time.

Recips could and would crash stop quite well, all ships are designed
with emergency backing in mind.

Sure, that's their "brakes."

Thinking a bit more on the issue, it doesn't seem that an Olympic class
ship would ever need to keep up vacuum with the turbine bypassed, any
longer than the minimum needed to maneuver in and out of port, and to do
*one* crashback.

With regard to the Titanic specifically, it's not certain what bells
were rung or if they were answered before the collision. If Murdoch
really did ring up full astern, it's barely possible that could have
been enacted before the crash. IIRC the surviving stokers said that EOT
bells weren't rung up until after the collision, and some said it was a
"Stop" not a "Reverse" bell.

... A recip will generally stop quickly due
to the internal friction but steam can be admitted in the reverse
direction without harm ... it is a compressible fluid and acts as a
cushion in normal operation. Reversing is a simple matter of changing
the operative eccentric, all tghe parts move in the same plane as
before, only shaft rotation, thrust, and crosshead guide thrust changes,
and the engines are designed with that in mind.

Don't you think that if the throttleman overdid the reverse steam, it
could damage the crosshead bearings, or the shaft couplings and/or line
bearings, or maybe fold up the prop? Other ships lost prop blades and so
forth at times. The Olympic class props had the blades bolted to the hub
so that the pitch would be adjustable (the ship had to be in drydock for
them to adjust it).

Anyway, my experience with recprocating steam engines has all been on
much much smaller machinery, some of it from that era and some even
earlier

Regards
Doug King

DSK wrote:

Rick wrote:

One could reasonably assume that the plant was designed to accomodate

Sure, but for how long? It's a liner, designed to maneuver with the aid
of tugs.

As long as it takes to fix the problem. The condenser is large enough to
handle the steam flow required to produce full rated power. The
circulator pump moves more water than is required to remove the heat
leaving the turbine. Reduced power operation might have been required,
just as modern plants - even military plants - but there is no time
limit associated with that condition.

Warships have far greater ability to steam in maneuvers, but at
the cost of efficiency & space. A liner has to make money.

I don't believe wasrships have any exclusive claim to maneuverability
.... that statement applies equally to a warship or a merchant plant. Our
plants are optimized to operate at a continuous maximum power and steam
flow. Anything less is taking life easy on plant at the cost of a slight
increase in brake specific fuel consumption. There are many factors
involved in how fast we go for how long that have nothing to do with the
engine.

... Plant efficiency without the turbine would drop considerably as
the energy in the recip exhaust would be lost to heating the sea.

Right... and this is why I think it would be difficult to maintain
vacuum if you ran without the turbine for any length of time.

The relationship you imply assumes the condenser was sized to require
the use of the turbine ... that simply is not the case. The turbine
allowed the recovery of waste heat at full power. Their is a subtle
relationship between the efficiency of the recips and the cutoff
settings used when operating with and without the turbine online. If
cutoff is extended, as it might have been when turbine was online, the
recips produced greater power but at less efficiency than when cutoff
was shortened so as to allow maximum expansion in the engine.

Maximum expansion of minimum steam (short cutoff) comes at the cost of
wide temperature variations and condensation and re-evaporation withing
the cylinders which is avoided at high speeds and extended cutoff which
leaves more energy in the steam for the turbine to extract.


Don't you think that if the throttleman overdid the reverse steam, it
could damage the crosshead bearings, or the shaft couplings and/or line
bearings, or maybe fold up the prop? Other ships lost prop blades and so
forth at times. The Olympic class props had the blades bolted to the hub
so that the pitch would be adjustable (the ship had to be in drydock for
them to adjust it).

Don't know why it should. The crosshead bearing only "feel" the side
thrust imposed by piston rod movement relative to the connecting rod and
that is a function of load.

Line bearings don't know or care what direction the shaft rotates.

The torque on the shaft instantly changes from driving the prop to being
driven by the prop the instant the throttle is closed. The inertia of
the rotating shaft is absorbed by impeding the inlet of steam from the
throttle to the cylinder. When the force of the steam exceeds the force
of the piston being driven by the shaft inertia the engine will reverse.
The throttle is not opened wide at the same instant that reverse is
selected ... give the operator a little credit for knowing how to
operate the engine. The time involved is very short though, you do not
have to wait until the shaft stops rotating all by itself.

Ships still shed blades and even break shafts ... even though the props
and shafts are designed to function under torgues and stresses that the
engine would never be able to deliver under any and all concievable
conditions of normal or emergency operation.


Rick

Sure, but for how long? It's a liner, designed to maneuver with the
aid of tugs.


Rick wrote:
As long as it takes to fix the problem. The condenser is large enough to
handle the steam flow required to produce full rated power.

I don't think so. There's a volume/temp relationship involved. If you
put steam at a higher pressure & temp into the condenser, at some point
you are going to exceed it's cooling capacity.




... The
circulator pump moves more water than is required to remove the heat
leaving the turbine. Reduced power operation might have been required,
just as modern plants - even military plants - but there is no time
limit associated with that condition.

???

AFAIK there's a time limit, either you regain vacuum and resume steaming
normally, or you don't... and you restrict steam into the engines until
you either have to shut down or you regain vacuum.


Warships have far greater ability to steam in maneuvers, but at the
cost of efficiency & space. A liner has to make money.


I don't believe wasrships have any exclusive claim to maneuverability
... that statement applies equally to a warship or a merchant plant.

No, but warships are designed & built with far less regard to cost and
far greater regard to increased capacity. No merchant ship is ever going
to give up the tonnage & space for such a power plant. For example, a
Fletcher class destroyer's hull was about half filled by it's boiler &
engine rooms.


... Our
plants are optimized to operate at a continuous maximum power and steam
flow. Anything less is taking life easy on plant at the cost of a slight
increase in brake specific fuel consumption. There are many factors
involved in how fast we go for how long that have nothing to do with the
engine.

Agreed.


... Plant efficiency without the turbine would drop considerably as
the energy in the recip exhaust would be lost to heating the sea.

Right... and this is why I think it would be difficult to maintain
vacuum if you ran without the turbine for any length of time.


The relationship you imply assumes the condenser was sized to require
the use of the turbine ... that simply is not the case. The turbine
allowed the recovery of waste heat at full power. Their is a subtle
relationship between the efficiency of the recips and the cutoff
settings used when operating with and without the turbine online. If
cutoff is extended, as it might have been when turbine was online, the
recips produced greater power but at less efficiency than when cutoff
was shortened so as to allow maximum expansion in the engine.

Maximum expansion of minimum steam (short cutoff) comes at the cost of
wide temperature variations and condensation and re-evaporation withing
the cylinders which is avoided at high speeds and extended cutoff which
leaves more energy in the steam for the turbine to extract.

Right. But you're assuming that the condenser was enough oversized to
accept all the additional energy from the steam normally extracted by
the turbine. That's my point.

Suppose you took a somewhat more modern plant and installed a pipe from
the crossover into the LP exhaust trunk, shunting exhaust from the HP or
IP turbine directly into the condenser. That would be a similar
situation... and I'm thinking it would be difficult or impossible to
keep vacuum. But after reflection (as I said in my last post), I'm also
thinking it wouldn't matter so much with the Olympics because they would
only have to do this for a short time.




Don't you think that if the throttleman overdid the reverse steam, it
could damage the crosshead bearings, or the shaft couplings and/or
line bearings, or maybe fold up the prop? Other ships lost prop blades
and so forth at times. The Olympic class props had the blades bolted
to the hub so that the pitch would be adjustable (the ship had to be
in drydock for them to adjust it).


Don't know why it should. The crosshead bearing only "feel" the side
thrust imposed by piston rod movement relative to the connecting rod and
that is a function of load.

Wouldn't stopping the engine against the force from the prop potentially
create greater loads than normal forward operation?


Line bearings don't know or care what direction the shaft rotates.

True, but they care a *lot* about compression on the shaft, which is
going to produce axial forces. If you're trying to stop the prop against
the force of water going by, it seems to me you could generate at least
as much compression as if you are trying to accelerate the ship.


The torque on the shaft instantly changes from driving the prop to being
driven by the prop the instant the throttle is closed. The inertia of
the rotating shaft is absorbed by impeding the inlet of steam from the
throttle to the cylinder. When the force of the steam exceeds the force
of the piston being driven by the shaft inertia the engine will reverse.

But there's more than just shaft inertia involved here... there's the
inertia of the whole ship driving the prop.

The throttle is not opened wide at the same instant that reverse is
selected ... give the operator a little credit for knowing how to
operate the engine.

Right. That's why I said (quite a few posts ago) that it would make a
big difference having a skillful throttleman on the valve gear lever...

BTW there's a name for this lever, I forget what it is... probably a
Scottish name...

... The time involved is very short though, you do not
have to wait until the shaft stops rotating all by itself.

Right. On the ships I was on, the throttleman who did the best
stop-and-lock drills had bragging rights.


Ships still shed blades and even break shafts ... even though the props
and shafts are designed to function under torgues and stresses that the
engine would never be able to deliver under any and all concievable
conditions of normal or emergency operation.

I dunno about "all concievable conditions"... that sounds really
expensive! And remember, back in 1912 engineering metallurgy had not
advanced as far.

Fresh Steam-
Doug King

DSK wrote:

I don't think so. There's a volume/temp relationship involved. If you
put steam at a higher pressure & temp into the condenser, at some point
you are going to exceed it's cooling capacity.

Well, in the mode and condition which I described, I KNOW that the
engines can run indefinitely without exceeding the condenser capacity.

... The circulator pump moves more water than is required to remove
the heat leaving the turbine. Reduced power operation might have been
required, just as modern plants - even military plants - but there is
no time limit associated with that condition.


???

What part of steam plant operation and construction elicited the "???" ?
Most condensers and circpumps have a larger capacity than required for
normal operation up to a sea temperature around 85* F. Under some
conditions of sea temperature and load, reduced power operation may be
required to match condenser capacity. Once the balanced is achieved
there is no time limit to operation.

AFAIK there's a time limit, either you regain vacuum and resume steaming
normally, or you don't... and you restrict steam into the engines until
you either have to shut down or you regain vacuum.


Warships have far greater ability to steam in maneuvers, but at the
cost of efficiency & space. A liner has to make money.



I don't believe wasrships have any exclusive claim to maneuverability
... that statement applies equally to a warship or a merchant plant.


No, but warships are designed & built with far less regard to cost and
far greater regard to increased capacity. No merchant ship is ever going
to give up the tonnage & space for such a power plant. For example, a
Fletcher class destroyer's hull was about half filled by it's boiler &
engine rooms.

The Fletcher is smaller than most merchant steamships so of course the
plant takes up a larger proportion of the hull volume ... hardly a
reflection on plant capacity or maneuvering traits.

Right. But you're assuming that the condenser was enough oversized to
accept all the additional energy from the steam normally extracted by
the turbine.

Not at all, read my posts.

Suppose you took a somewhat more modern plant and installed a pipe from
the crossover into the LP exhaust trunk, shunting exhaust from the HP or
IP turbine directly into the condenser. That would be a similar
situation... and I'm thinking it would be difficult or impossible to
keep vacuum. But after reflection (as I said in my last post), I'm also
thinking it wouldn't matter so much with the Olympics because they would
only have to do this for a short time.

We carry just such a pipe. It is made precisely for use in case a LP
turbine casualty occurs and allows HP turbine exhaust to go directly to
the condenser. No big deal, just reduce power to maintain the vacuum
required for the auxilliaries and carry on for as long as it takes.

Just exactly the same way Titanic would operate if the turbine was
unavailable.


Wouldn't stopping the engine against the force from the prop potentially
create greater loads than normal forward operation?

Don't know why it should, steam is compressible.

True, but they care a *lot* about compression on the shaft, which is
going to produce axial forces. If you're trying to stop the prop against
the force of water going by, it seems to me you could generate at least
as much compression as if you are trying to accelerate the ship.

Run that by again ... "compression" of the shaft? I would love to see
the figures on that one ...

But there's more than just shaft inertia involved here... there's the
inertia of the whole ship driving the prop.

Not to seem too cynical but I have to ask if that is the source of the
shaft compression that is going to effect the line bearings ...

I dunno about "all concievable conditions"... that sounds really
expensive! And remember, back in 1912 engineering metallurgy had not
advanced as far.

Yes, it is and always has been expensive. All conceivable conditions are
often met and/or exceeded on ships over their life of steaming around
the world essentially non stop for a quarter century or longer.

Rick

Rick wrote:
Well, in the mode and condition which I described, I KNOW that the
engines can run indefinitely without exceeding the condenser capacity.

OK. I'm a bit surprised to hear that, since it's definitely not true of
most plants I know about. Otherwise loss of condenser vacuum wouldn't be
given such a prominent place in the drill book.


... The circulator pump moves more water than is required to remove
the heat leaving the turbine. Reduced power operation might have been
required, just as modern plants - even military plants - but there is
no time limit associated with that condition.



???


What part of steam plant operation and construction elicited the "???" ?
Most condensers and circpumps have a larger capacity than required for
normal operation up to a sea temperature around 85* F. Under some
conditions of sea temperature and load, reduced power operation may be
required to match condenser capacity. Once the balanced is achieved
there is no time limit to operation.

Well, that's what I meant. You're taking that balance very much for
granted. "Under some conditions, reduced power operation may be
required" is definitely true, but an understatement IMHO. Sometimes you
have to shut down and start over, to restore vacuum!

Run that by again ... "compression" of the shaft? I would love to see
the figures on that one ...

Sorry about that.... got it backwards... in stopping the shaft against
the momentum of the ship, the shaft would be in tension... not going to
produce much axial load that way! Make that strain on the couplings, not
the line bearings

BTW I wonder why our brilliant Navspritjax didn't catch that mistake?

Interesting discussion, thanks for joining in.

Fresh Buckets O' Steam
Doug King

DSK wrote:

Rick wrote:

Well, in the mode and condition which I described, I KNOW that the
engines can run indefinitely without exceeding the condenser capacity.


OK. I'm a bit surprised to hear that, since it's definitely not true of
most plants I know about. Otherwise loss of condenser vacuum wouldn't be
given such a prominent place in the drill book.

Drill book?

What drill book?

Cheers

DSK wrote:

OK. I'm a bit surprised to hear that, since it's definitely not true of
most plants I know about. Otherwise loss of condenser vacuum wouldn't be
given such a prominent place in the drill book.

Loss of vacuum is a show stopper ... loss can occur for any number of
reasons unrelated to engine operating conditions ... and most often has
nothing to do with what the engine is or was doing. There is a reason
the vacuum guage is the largest and placed squarely in view of the engineer.

Well, that's what I meant. You're taking that balance very much for
granted. "Under some conditions, reduced power operation may be
required" is definitely true, but an understatement IMHO. Sometimes you
have to shut down and start over, to restore vacuum!

The balance is taken for granted, at a given vacuum I can remove a given
amount of heat ... removing a given amount of heat will produce a given
vacuum for a given steam flow, it is very simple, don't put any more in
than you can remove. Loss of vacuum equates to a loss of power in a
steam plant, it's like putting a potato in an exhaust pipe, if you can't
get the potato out quick enough the engine stops ... no magic there.
There is no need to "shut down" if you completely lose vacuum,
everything shuts down all by itself ... especially if you are relying on
SSTG's for electrical power. Have been faced with losing vacuum many
times but have never seen or heard of having to shut a plant down in
order to restore it ... most vacuum comes from the condensation of steam
in the condenser so stopping all that steam from entering is the worst
thing you can do.

Rick

Rick wrote:

DSK wrote:

Rick wrote:

One could reasonably assume that the plant was designed to accomodate


Sure, but for how long? It's a liner, designed to maneuver with the
aid of tugs.


As long as it takes to fix the problem. The condenser is large enough to
handle the steam flow required to produce full rated power. The
circulator pump moves more water than is required to remove the heat
leaving the turbine. Reduced power operation might have been required,
just as modern plants - even military plants - but there is no time
limit associated with that condition.

Warships have far greater ability to steam in maneuvers, but at the
cost of efficiency & space. A liner has to make money.


I don't believe wasrships have any exclusive claim to maneuverability
... that statement applies equally to a warship or a merchant plant. Our
plants are optimized to operate at a continuous maximum power and steam
flow. Anything less is taking life easy on plant at the cost of a slight
increase in brake specific fuel consumption. There are many factors
involved in how fast we go for how long that have nothing to do with the
engine.

... Plant efficiency without the turbine would drop considerably as
the energy in the recip exhaust would be lost to heating the sea.

Right... and this is why I think it would be difficult to maintain
vacuum if you ran without the turbine for any length of time.


The relationship you imply assumes the condenser was sized to require
the use of the turbine ... that simply is not the case. The turbine
allowed the recovery of waste heat at full power. Their is a subtle
relationship between the efficiency of the recips and the cutoff
settings used when operating with and without the turbine online. If
cutoff is extended, as it might have been when turbine was online, the
recips produced greater power but at less efficiency than when cutoff
was shortened so as to allow maximum expansion in the engine.

Maximum expansion of minimum steam (short cutoff) comes at the cost of
wide temperature variations and condensation and re-evaporation withing
the cylinders which is avoided at high speeds and extended cutoff which
leaves more energy in the steam for the turbine to extract.


Don't you think that if the throttleman overdid the reverse steam, it
could damage the crosshead bearings, or the shaft couplings and/or
line bearings, or maybe fold up the prop? Other ships lost prop blades
and so forth at times. The Olympic class props had the blades bolted
to the hub so that the pitch would be adjustable (the ship had to be
in drydock for them to adjust it).


Don't know why it should. The crosshead bearing only "feel" the side
thrust imposed by piston rod movement relative to the connecting rod and
that is a function of load.

Line bearings don't know or care what direction the shaft rotates.

The torque on the shaft instantly changes from driving the prop to being
driven by the prop the instant the throttle is closed. The inertia of
the rotating shaft is absorbed by impeding the inlet of steam from the
throttle to the cylinder. When the force of the steam exceeds the force
of the piston being driven by the shaft inertia the engine will reverse.
The throttle is not opened wide at the same instant that reverse is
selected ... give the operator a little credit for knowing how to
operate the engine. The time involved is very short though, you do not
have to wait until the shaft stops rotating all by itself.

Ships still shed blades and even break shafts ... even though the props
and shafts are designed to function under torgues and stresses that the
engine would never be able to deliver under any and all concievable
conditions of normal or emergency operation.



Please take it easy on Doug. He's our resident propulsion engineer. His
knowlege is second to none. But seriously, a good accurate post IMO.
Often HP steam was bled to the LP cylinders to start big marine steam
engines? Do you do you do that on your tug?

Cheers

Nav wrote:

Often HP steam was bled to the LP cylinders to start big marine steam
engines? Do you do you do that on your tug?

Steam was admitted directly to the condenser via a "snifter valve" in
order to raise vacuum enough to allow smooth starting, never heard of
admitting drum pressure steam to the LP, it would sure jerk the poor
engine around.

My tug has a very large for its power and quite elderly diesel. I only
operate other people's steamboats for a living.

Rick