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

otnmbrd wrote:

Interesting .... thanks. I would have thought, considering the mass
involved that you would have had to reduce "ahead" steam first, prior to
"pulling one lever".

Recip engines have to stop before reversing simply because they rotate
in the opposite direction, so do steam turbines. In either case "ahead"
steam is secured since it is pointless and counterproductive to have
steam working on both "sides" of the engine. In the case of a recip, it
is mechanically impossible due to the valve gear geometry.

Admitting steam in the astern direction on a recip changes the side of
the piston which sees greater pressure, this first slows then stops
piston travel ... the forces are the same, the only thing that changes
is the direction of crankshaft rotation.

The turbine is locked to the reduction gear and shaft so it has to stop
before rotation can occur in the opposite direction. There is no problem
admitting steam to the astern turbine while the engine is rotating in
the ahead direction, we do it quite regularly to assist in slowing the
engine rapidly during maneuvering.

Rick

otnmbrd wrote:

Interesting .... thanks. I would have thought, considering the mass
involved that you would have had to reduce "ahead" steam first, prior
to "pulling one lever".

Sorry, I neglected one point of your post because it is so much a part
of normal or emergency operation as to require no thought ... of course
the throttle is closed prior to changing direction, no marine steam
engine is routinely started at full throttle. The shaft speed may not
change much when and as the throttle is closed rapidly but closing the
throttle is part of the sequence of operation.

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:
Titanic's turbine specs

Turbine Statistics
Type: Parsons direct coupled LP
Turbine Weight 420 tons
Rotor Diameter 12 feet
Rotor Length 13 ft 8 in
Input blade lengths 18 inches
Output blade lengths 25.5 inches
Operating Pressure in 9 p.s.i.a
Operating Pressure out 1 p.s.i.a
Operating Speed 165 r.p.m.
Operating Output 16,000 h.p.

An apology would be in order, but isn't really expected.


Yes it is -from you! I said the inlet pressure it was 9 psi to correct
your statement that the inlet was at vacuum. You then said I was wrong
and it was 11psi. So you no agree that I was correct all along. Now
let's deal with your other assertion that there was no steam bypass for
the turbine shall we?

You are busted.

Cheers

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.


You are contradicting yourself! That means either the old or the new
Doug position must be right but not both. Let me remind you, you said
that I was wrong in stating that there was a steam bypass valve on the
turbine.

I maintain that the bypass valve allowed steam back to the condenser.
Was I right after all? Well?

Cheers

Now as a stream plan expert can you tell us why did they not state the
inlet pressure in mmHg? What is vaccum normally measured in?

I still say the inlet pressure was 9psi! Care to admit I was right?

Cheers

DSK wrote:

Nav wrote:

I know the difference between a vaccumm and and 9 psi! The inlet to
the turbines was not a vacuum Doug!


If you don't believe me, try asking at

http://www.encyclopedia-titanica.org...919/91698.html

Or you could dig up a copy of "The Shipbuilder" magazine from 1911 which
discussed the Titanic's engineering plant in some detail.

Here's a clue- check the designed pressure drop across the turbine...
then look at some specs on operating main condensers...

DSK

Rick wrote:



Exhaust from Titanic's reciprocating engines, at a pressure less than
atmospheric, could be directed to the turbine or "bypass" the turbine
and exhaust directly to the condenser. The turbine was used to recover
heat energy from the main engine exhaust which would otherwise have been
wasted, and thereby increase the overall plant efficiency.


Yes I know that. It was Doug who said there was no bypass valve. He
seemd to think it was stopped by just closing a valve -but where he
thinks the steam from the main engines would go is anybodys guess.
While the inlet was subatmospheric, the steam would still be passed to
the condensor whose pressures were even lower. I think it's also pretty
sloppy to call the inlet prssure to a turbine a vacuum. Don't you guys
traditionally use mmHg for near vacuum pressures?

Cheers