Re: [PSUBS-MAILIST] Seals again

[MerlinSub@t-online.de (Carsten Standfuß)]

My 6 cents, 

I know 6 methods to protect a engine compartment against the 
seawater pressure: 

1.) For shallow water applications - up to about 300 feet :  

Simmer ring (simmer gasket) like in my Aquazep-scooter. 
Useful and simple, cheap - but you have to use two of them. One,
the outside to protect the other one from dust, sand, mud etc. 
The rubber lip is sensitive against sand or other hard materials. 
The engine pressure hull had to hold the full outside pressure. 

2.) For shallow and medium water deeps up to about 1000 feet : 
Air pressure compensate engines. The seal can be simple as possible 
just water- and gastight. It is not nessesary that the engine housing 
is pressure tight. Needs a pressure bottle, a gauge, a regulator
most parts can be used from scuba gear. Usefull if you have more
than one engine and outside batteries. I use one bottle with one
old second hand regulator for fife engines and three batteries. 
A leak is indicate by airbubbles or indicator gauge. 

3.) For shallow, medium and deep waters up to 36000 feet : 
Oil compensate engines. The simplest solution. Needs only a water/oil
tight
seal, no pressure tight engine hull, no regulator - just oil and a
compensator
bag - like a hairsoap box. But you lost some 15-20% efficenty of the
engine
because the motor has to rotate in the oil. Maybe its help if you give
the 
rotor parts a streamline form with GRP. Also Oil has a good head
transfer
to the outside hull for cooling. 

4.) For shallow and medium deeps: 
Ceramic shaft seal of the ground joint type. Only as complete factory
work.
Expensive but pressure tight. For big applications and shafts
and higger outputs. Needs good bearings and small tolerances.  

5.) For medium and deep waters : 
Magnetic coupling. Absoulte tight under every circumstates. Needs also a 
pressure tight motor house. Best solution but maybe not for higher
output
power engines. 

6.) For shallow waters: gland seal - out of time.., cheap

I used point 3.) and later 2.) in combination with a water protect ball
bearing
in Sgt.Peppers. I will use number 4.) on the outside end of the shaft 
and number 1.) inside the pressure hull shaft end with a oil filled
shaft in CSSX. 

Two days before I finished the side-rudder (5,6 x 2,3 feet..) of CSSX
and last weekend I suba dived in the baltic. I used : 
Two 7 Liter airtanks, one 4 Liter saftey bottle, another 0,5 liter
bottle on my diver-bouancy system. All together with 3 independent (nice
word) 
regulators. Its a heavy equipment but I dive alone and I am still
alive..

Carsten



Steve Lindblom schrieb:
> 
> Stan wrote:
> >You make some very good points, so I begin to wonder why not the best of
> >both... seals and pressure compensation?   Overpressure is easy enough to
> >avoid, which eliminates a the blowout you describe as a drawback.  I just
> >find it hard to believe that pressure compensation is a bad thing.
> 
> Not a bad thing, just quite possibly an unecessary thing. Like wearing a
> parachute on an airliner. Or maybe a better analogy might be to, when you
> can't decide whether to wear a raincoat or a parka, wearing both. Why is
> the KISS principle so suspect on this list? Note that neither of the
> pressure compensation systems mentioned here are benign - both add
> significantly to task loading or complexity/failure points.
> 
> You say "overpressure is easy enough to avoid". How? By adding even more
> components?
> 
> >In the debate over pressure compensation or no, I wonder what the shaft seal
> >life expectancy is in each case.  It would seem to me that the more pressure
> >differential on the seal, the shorter the operating life... probably by
> >orders of magnitude, I would think.
> 
> We don't have to speculate on what the life might be, because these seals
> are used in so many applications that their performance is well documented
> and extensive engineering data is available. In my search for seals I've
> focused on DPVs, because I'm lazy, and checking out what they use is a
> cheap way to get someone else to do the engineering for me. Expensive DPV's
> function at depths to 400', and the cheap ones down to 150-180'. Seal life
> doesn't seem to be a problem. If it is, well, the seals are cheap and easy
> to replace, so it can be done routinely as prophylaxis. What more do you
> want?
> 
> If there was a record to demonstrate that seals would not do the job alone,
> then I'd applaud the ingenuity of those who have come up with compensation
> systems. As no one seems to have spent any time at all on the simple
> solutions before heading merrily off to the complex, one has to wonder.
> 
> We had a guy die recently in the techdiving community who many of us knew
> personally. He had, once upon a time, had a scary out-of-air emergency, and
> (rather than resolving to watch his gauge more closely in the future, or
> any number of other low-tech solutions) embarked in a bizarre quest to
> build an absolutely safe SCUBA rig, which evolved over the years, as he
> strung one mechanical fix on top of another, until he had a SCUBA rig the
> size of a steamer trunk, that weighed several hundreds of pound, with eight
> separate tanks and god knows how many hoses regulators and valves. He died
> in it.
> 
> The reason I mention this, is that after he died, many of us who'd been
> laughing at his rig behind his back realized how inevitable in hindsight
> his death was, and how we all bore some responsbility for his death, in
> that we just treated his rig as a joke rather than trying to talk him out
> of it. Not that it probably would have made any difference, but for the
> sake of a clean conscience one has to try.