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Re: [PSUBS-MAILIST] Seals again
----- Original Message -----
From: "Duncan Milne" <milned@teckcorp.ca>
To: <personal_submersibles@psubs.org>
Sent: Thursday, October 05, 2000 7:13 PM
Subject: RE: [PSUBS-MAILIST] Seals again
> Pat,
> Steve is just pointing out that it is important to realize
> that SOME redundancies can actually INCREASE the overall risk of failure.
Thanks, Duncan; I think I know what Steve is trying to do.
With you're stated experience, I'd expect you to understand that pressure
compensation systems can be about as simple and reliable as it gets, and
therefore, don't fall into the category of redundancies that necessarily
increase overall risk of failure beyond the value of the additional safety
and functionality they provide.
Let's say there are two subs on the bottom, nosed into and entrapped by a
mesh of cables, and needing to apply reverse thrust to escape. Both subs
have motors equipped with Steve's "double shaft seal" system, but are still
subject to possible leaks at the electrical coupler and nosecone O-ring;
and, a flooded housing might cause a short rendering the motor inoperative.
One boat is equipped with pressure compensation, the other is not. Which
one would you rather be in?
The main objection seems to be that pressure compensation might blow out the
seals. That would require the failure of a simple relief valve of the type
proven acceptable in the handling of explosive gasses, and which is about as
reliable as anything made by man. It can also be backed up with a redundant
valve, making it virtually failure proof. But even if that did happen on
my NAUTILUS MINISUB, the excess air pressure will also keep the water out of
the housing while I abort the dive and surface. No problem!
There are a lot of other reasons why it's better to have pressure
compensation than to rely on a shaft seal system alone, but I've got to go
to work right now.
VBR,
Pat