any limits on mechanical seals?

Suppose you have a large space station (OK, let's say a colony) with a
rotating portion and a stationary portion, both pressurized, and with
constant traffic back and forth between them. Obviously you need a
large mechanical seal between them, and I have some questions about that
which I hope someone can answer:

- In general how likely is this to be a problem? Suppose we want a
corridor 20 m wide between the two portions -- how much bigger is that
than any seal we've built before?

- How can I estimate the leak rate through the seal?

- Can I characterize the problem only by the pressure difference between
the inside and outside? In other words, is a seal that holds 1 ATM
against a vacuum the same as one that holds 2 ATM against 1 ATM?

- What type of seal would you expect this to be? Radial shaft?
Labyrinth? Rotating face? Something else? (I only vaguely understand
these different types of seals, so even small insights will be
appreciated.)

Many thanks,
- Joe

On Wed, 05 Jul 2006 09:01:50 -0600, in a place far, far away, Joe
Strout made the phosphor on my monitor glow in such a
way as to indicate that:

Suppose you have a large space station (OK, let's say a colony) with a
rotating portion and a stationary portion, both pressurized, and with
constant traffic back and forth between them. Obviously you need a
large mechanical seal between them, and I have some questions about that
which I hope someone can answer:

- In general how likely is this to be a problem? Suppose we want a
corridor 20 m wide between the two portions -- how much bigger is that
than any seal we've built before?

Unknown, but I don't see any theoretical problems.

- How can I estimate the leak rate through the seal?

Depends on design. I see no reason it couldn't be designed to zero
leak.

- Can I characterize the problem only by the pressure difference between
the inside and outside? In other words, is a seal that holds 1 ATM
against a vacuum the same as one that holds 2 ATM against 1 ATM?

In terms of the basic mechanics and strength requirements of the seal,
I'd think so. The issue is that there may be problems on the vacuum
side from material sublimation or welding. Not to mention radiation
(and perhaps monatomic oxygen, if it's in a suitable
environment--e.g., LEO). Just off the top of my head.

- What type of seal would you expect this to be? Radial shaft?
Labyrinth? Rotating face?

Rotating face would be my first choice, but again, just off the top of
my head. Think of a giant (hold breath) O-ring. Just don't do it in
January. You'd probably heat it, particularly on the space side,
though rotisserie effect might ease the issues..

In article ,
h (Rand Simberg) wrote:

- How can I estimate the leak rate through the seal?

Depends on design. I see no reason it couldn't be designed to zero
leak.

That's a bold thought -- my understanding is that pressure seals are
assumed to always leak at some rate, and that it was just a matter of
how expensive you want to make it to get the leak rate lower. But
that's a stance I'll question more carefully now.

- Can I characterize the problem only by the pressure difference between
the inside and outside? In other words, is a seal that holds 1 ATM
against a vacuum the same as one that holds 2 ATM against 1 ATM?

In terms of the basic mechanics and strength requirements of the seal,
I'd think so. The issue is that there may be problems on the vacuum
side from material sublimation or welding. Not to mention radiation
(and perhaps monatomic oxygen, if it's in a suitable
environment--e.g., LEO). Just off the top of my head.

Thanks, that's what I was thinking too. So if we wanted to, say, build
working models of such seals on Earth, we could do it, and tests thereof
would be as accurate as tests of any other machinery intended for space
on Earth -- i.e., you just have to watch out for the standard effects of
the space environment.

- What type of seal would you expect this to be? Radial shaft?
Labyrinth? Rotating face?

Rotating face would be my first choice, but again, just off the top of
my head. Think of a giant (hold breath) O-ring. Just don't do it in
January. You'd probably heat it, particularly on the space side,
though rotisserie effect might ease the issues..

Thanks again. I confess that the space-environment thing makes me a
little nervous when elastic materials (like an O-ring) are in such a
critical path, but it's still worth looking into. Failure of an O-ring
in this case wouldn't be catastrophic; you'd just start venting
atmosphere at a prodigious rate, but it would still take many years to
depressurize an entire space habitat. No serious damage would result.

Of course, *replacing* the O-ring, or otherwise doing maintenance on
this giant rotating seal, could be a real PITA. Certainly something to
plan for up front.

I don't suppose anyone knows any mechanical seal engineers (if there is
such a thing) sufficiently interested in space to kick around some of
the details with me?

Thanks,
- Joe

P.S. It's probably a test of maturity to see how long you can use the
phrase "giant mechanical seal" before a silly image pops into your head.
(I didn't fare very well on this test.)

On Wed, 05 Jul 2006 10:20:38 -0600, in a place far, far away, Joe
Strout made the phosphor on my monitor glow in such a
way as to indicate that:

In article ,
h (Rand Simberg) wrote:

- How can I estimate the leak rate through the seal?

Depends on design. I see no reason it couldn't be designed to zero
leak.

That's a bold thought -- my understanding is that pressure seals are
assumed to always leak at some rate, and that it was just a matter of
how expensive you want to make it to get the leak rate lower.

Well, zero is a small number. Let's say negligible. How often do you
you have to fill your car tires (which contain double atmospheric
pressure)?

It also depends on what you're trying to keep from leaking and what
temperature it's at (e.g., O-rings and hot gases, or hydrogen or
helium, which will seemingly tunnel through anything). But we're just
talking about an atmosphere of air here.

Joe Strout wrote:
Of course, *replacing* the O-ring, or otherwise doing maintenance on
this giant rotating seal, could be a real PITA. Certainly something to
plan for up front.
For maintenance you could start up the stationary part or stop the
moving part. Then you just put in a temporary inner seal and work
on the O-ring to your heart's desire.

Or the connector consists of a tube with an O-ring on both ends, then
you can couple the tube either to the still or to the moving part and
always have one O-ring at rest relative to the tube and the connected
part. Then maintenance ought to be easy too.

You can minimize leaks further by pressurizing the connection
only when needed.

Also, for long term survival I think that the air locks pose the bigger
problem. And accidents of course.

OTOH, earth loses a bit of air too (http://tinyurl.com/nxqqu) but during
the last few billion years it wasn't much of a problem, so, depending on
the size of your station, a few spoonful per day may be tolerable.

Lots of Greetings!
Volker
--
For email replies, please substitute the obvious.

In article ,
Volker Hetzer wrote:

Joe Strout wrote:
Of course, *replacing* the O-ring, or otherwise doing maintenance on
this giant rotating seal, could be a real PITA. Certainly something to
plan for up front.
For maintenance you could start up the stationary part or stop the
moving part. Then you just put in a temporary inner seal and work
on the O-ring to your heart's desire.

Well, yeah, except that we're talking REALLY large parts here -- it
would take months to spin down the habitat, and the same amount of time
to spin it up again, during which your 10,000 residents are getting
mighty grumpy. Spinning up the stationary portion would be easier, but
may not be possible, as it wouldn't be designed to take the loads.

Or the connector consists of a tube with an O-ring on both ends, then
you can couple the tube either to the still or to the moving part and
always have one O-ring at rest relative to the tube and the connected
part. Then maintenance ought to be easy too.

That's an interesting idea. Seems to me it would double the leak rate,
and the failure rate, but it may be worth it for easy maintenance.

You can minimize leaks further by pressurizing the connection
only when needed.

I don't think so. Both modules are always pressurized, and there isn't
a lot of point to making the connection double as an airlock. Assume
the connection is needed pretty much constantly anyway.

Also, for long term survival I think that the air locks pose the bigger
problem. And accidents of course.

Could be, but one issue at a time please.

OTOH, earth loses a bit of air too (http://tinyurl.com/nxqqu) but during
the last few billion years it wasn't much of a problem, so, depending on
the size of your station, a few spoonful per day may be tolerable.

Quite so; I don't expect the leak to be a problem, but it should be
estimated and figured into the resupply needs.

Best,
- Joe

Joe Strout wrote:
Suppose you have a large space station (OK, let's say a colony) with a
rotating portion and a stationary portion, both pressurized, and with
constant traffic back and forth between them. Obviously you need a
large mechanical seal between them,
FWIW, there is an alternate method, assuming you are willing to let
your "constant" traffic travel in discrete chunks.

In the center of the junction between the two (on the axis of
rotation), you have a sealable "elevator". To go from the spin section
to the no-spin section, you enter the elevator, close the hatch. The
elevator is undocked, spun down and docked to the no-spin section.
Going the other way, you spin up. With careful design of the
(non-rotating) seals and hatches, the atmosphere loss at each
docking/undocking can be very small. The volume containing the elevator
shaft can be made such that while it isn't completely sealed, it leaks
at a low enough rate that if the elevator or one of the adjoining
segments depressurizes rapidly, it doesn't leak down too fast to do
something about it. You should be able to create a long lasting
rotating 'seal' that keeps the leak reasonably low without too much
trouble.

If you put hatches on the both the spin and no-spin ends of the
elevator, the range of travel just needs to be enough to unmate from
one side and mate to the other.

If you want to have multiple elevators (obviously not all located
exactly on the axis of rotation), the situation gets more complex. They
look a bit more like train cars, but it could be done.

Whether any of this is a win depends how hard large rotating seals in
vacuum really are.

On Wed, 5 Jul 2006, Joe Strout wrote:

Suppose you have a large space station (OK, let's say a colony) with a
rotating portion and a stationary portion, both pressurized, and with
constant traffic back and forth between them. Obviously you need a
large mechanical seal between them, and I have some questions about that
which I hope someone can answer:

Lets say the stationary protion is in the center of a wheel. Instead of
this dubious mechincal arrangement, it would be easier for cargo arriving
at the center to match angular rotation prior to landing.

Joe Strout wrote:
In article ,
Volker Hetzer wrote:
Or the connector consists of a tube with an O-ring on both ends, then
you can couple the tube either to the still or to the moving part and
always have one O-ring at rest relative to the tube and the connected
part. Then maintenance ought to be easy too.

That's an interesting idea. Seems to me it would double the leak rate,
and the failure rate, but it may be worth it for easy maintenance.
Not necessarily double. Since only one of them has to be in operation,
they would, when alternated regularly, only accumulate half the duty-hours
each.

Planning an SF story? :-)

Lots of Greetings!
Volker
--
For email replies, please substitute the obvious.

In article ,
Joe Strout wrote:
- How can I estimate the leak rate through the seal?

Depends greatly on the technical details. The fast answer is that it can
be made arbitrarily small if you're willing to work hard and accept
compromises. For example, you can use a double seal, with vacuum pumps
running continuously to scavenge air that leaks past the first seal before
it gets past the second.

- Can I characterize the problem only by the pressure difference between
the inside and outside? In other words, is a seal that holds 1 ATM
against a vacuum the same as one that holds 2 ATM against 1 ATM?

The pressure difference is all that the seal sees... except that vacuum
can have harmful effects on some seal materials, which is a different
issue but possibly a significant one.

- What type of seal would you expect this to be? Radial shaft?
Labyrinth? Rotating face? Something else? (I only vaguely understand
these different types of seals, so even small insights will be
appreciated.)

You wouldn't use a labyrinth seal for this, at least not as the primary
seal -- labyrinth seals are what you use when rubbing contact is
unacceptable (e.g., because of high rotation speeds) and you're willing
to tolerate a certain amount of leakage.

My thought would be to make the tube from (say) the stationary side a few
meters bigger than the tube from the rotating side, and put bearings and
seals between them. Why? So you can maintain the bearings and seals!
The large difference in diameter is so people can walk (well, sort of,
since spin "gravity" will be minimal at such a short radius) and work
between the tubes.

In particular, it's not that hard to have multiple bearings set up so that
you can replace parts of any of them without stopping rotation. The
bearings are rollers against the inner tube, supported inside the outer
tube by struts, so that you can just step between the struts to get
through the first bearing ring and work on the next one. To replace a
roller, just jack it down -- the struts telescope a little bit to permit
this -- and take it out and swap in a new one.

Seals obviously need a continuous wall supporting them, but it's still
possible to work on them if you're clever.

At the outer end, a pair of labyrinth seals; because they're non-contact
seals, they doesn't wear out or need maintenance. Normally, they have no
air pressure across them, because the sealing is being done by the inner
seals -- a pair of shaft seals, each supported against the inner tube by a
wall running up from the outer tube. The walls have doors, but they have
airtight seals and are normally shut. There are vacuum pumps scavenging
leakage air from the volume between the inner seals.

When the leakage rate gets too high, you pressurize the space between the
inner seals and the one between inner and outer seals, open the doors, and
go in and fix the inner seals. The outer seals carry the pressure
meanwhile, with the vacuum pumps scavenging from the space between them.
There'll still be some loss, but it won't be much. When you're done,
close the doors, and have the pumps clear out first the space between
inner and outer seals, and then the space between the inner seals. You're
back in business.

This sort of arrangement is how you need to think for long-lived hardware
that's going to be *permanently* in service -- it has to be maintainable.
With proper design (including provisions for replacing all parts subject
to wear) and regular maintenance, this sort of heavy equipment can have an
essentially infinite service life.
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