[PSUBS-MAILIST] Hatch interlock

Thu Mar 3 08:36:14 EST 2016

On March 2, 2016 11:22:11 PM MST, Stephen Fordyce via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>Hi Sean,
>I really like the idea of vacuum monitoring for hatch leaking - nice
>one.
>Also Alan a good idea for a single central transmitter monitoring the
>whole
>thing.

This defeats the purpose of the system, other than to provide a global "go or no go" interlock signal.

>A couple of thoughts from me:
>- as well as the vacuum breaker valves on the actual hatches, you'd
>need
>individual manual isolation valves in the 1atm chamber so you determine
>which was leaking, and in case one leaks.

My idea was to use one solenoid valve per channel to connect or isolate each hatch from the vacuum pump, and one transducer per channel, so that information is immediately available.

>- You could protect the transducer from over pressure with a relief
>valve
>in the 1atm chamber (installed on the 1atm side of isolation valves)

Any such relief valve would continue to vent into the 1 ata space until it wasn't 1 ata anymore. Pressure leaking past an o-ring will continue to do so until the differential is equalized. I really think full-range transducers are the way to go here. Convolution has a particular penalty in vacuum systems that you don't encounter in positive pressure gas systems, in that you can't easily diagnose and localize  leaks by snooping for them, since they leak to the inside. The other advantage of the full-range transducer is that it enables you to automatically determine which of the two seals has failed, since the vacuum space will (eventually) rise to the pressure of the failed side.  Such an event is only a remote possibility though. Keep in mind that an o-ring which seals initially is highly unlikely to subsequently develop a leak in service. What you're really trying to catch with this system is when you get a pinched or damaged o-ring, or a hair or other debris under the seal, where
a pinhole leak path exists that isn't immediately obvious, but won't permit the vacuum to hold.

>- I'm not sure how likely it really is, but if you got salt water into
>the
>vacuum space the water would quickly evaporate, leading the salt
>residue to
>accumulate over time

This is true. The vacuum pump itself has a water trap, but the piping would need to be flushed as a matter of routine maintenance, and the hatch flanges wiped clean if salt buildup became an issue.

>- oil lubricated vacuum pumps, or at least the ones I've used, at only
>moderate vacuum can create smoke/oil vapour which would be
>uncomfortable in
>a sub. Using a diaphragm pump or similar you'd probably achieve a much
>poorer vacuum but no atmospheric contaminants

CO2 isn't the only thing you can scrub out of the air. In addition to a caustic scrubber, permanganate or activated carbon filters can remove hydrocarbons and other contaminants, and I will be including such a scrubber stage in the air processing. That said, it is definitely still wise to eliminate any known sources of contamination, so further research into vacuum pumps looks to be in order.

>- a vacuum switch would probably be cheaper and more likely to survive
>the
>over pressure, at the cost of less detailed info, ie. This one is good
>to
>200m
>http://www.suco-tech.com/product.php?p=44&c=12

Also true. The fundamental difference being that the vacuum switch is just an on-off indicator, while a pressure transducer can provide the leak rate. Maybe that isn't necessary. I'll have to think about this further.

>- actually on reflection, I sell digital gauges for SCUBA gas mixing
>(shameless plug:
>http://m.ebay.com/itm/Electronic-Digital-Pressure-Gauge-for-SCUBA-Blending-Nitrox-Trimix-Oxygen-/262307605334?nav=SEARCH
>) and they are -1 to 400bar(g) in 0.1 bar increments, which would be ok
>for
>both the vacuum and over pressure, so it can't be too hard, and
>requires an
>ADC resolution of only 12 bits - if 10 levels of vacuum measurement
>would
>be ok?
>- I have some very reasonably priced transducer samples coming from the
>same supplier - if they work out ok I could send you one or at least
>the
>specs.

I don't require a local display, as I'm custom-building the HMI. Separating the transducer from its signal conditioning also allows me to vary the resolution (and expense) of the ADC according to the application. I also will only employ transducers with NIST-traceable calibrations, that can be readily recalibrated annually, for example.

>- For a quick and easy way of turning high res analogue signals into
>useful
>digital data I can highly recommend this Arduino shield:
>http://rascalmicro.com/docs-precision-voltage-shield/
>Brandon has made a couple of 18-bit versions for me, which are good
>enough
>for 0.1% readings from oxygen cells and the like. I've been meaning to
>order some more.

Nothing about this project is taking the quick and easy path.  All of my DAQ / signal conditioning requirements will be implemented with NI C-series modules on the cRIO - also calibratable and NIST-traceable.

>Well that turned into a bit of a ramble, hopefully some is useful.
>
>Cheers,
>Steve

Thanks for your thoughts. They prompted me to think about a few things.

Sean

>On 3 Mar 2016 3:52 pm, "Sean T. Stevenson via Personal_Submersibles" <
>personal_submersibles at psubs.org> wrote:
>
>> That's a smart idea. I would also tie that valve actuation
>mechanically
>> into the actuation mechanism for the hatch dogs, so it remains
>intuitive
>> and doesn't require a second action.
>>
>> Sean
>>
>>
>> On March 2, 2016 9:05:54 PM MST, Alan James via Personal_Submersibles
><
>> personal_submersibles at psubs.org> wrote:
>>>
>>> Yes, didn't think of the pressure you get when you open the door.
>>> You would need to have a valve to release the vacuum before being
>able
>>> to open the door, perhaps this valve could be a 3 way valve, letting
>air
>>> in between the o-rings
>>> & closing the flow to the transducer simultaneously. This would stop
>a
>>> low pressure transducer
>>> being destroyed & stop the vacuu! m pump cutting in as it would
>maintain
>>> a vacuum in the line.
>>> Cheers Alan
>>>
>>> ------------------------------
>>> *From:* Brian Cox via Personal_Submersibles <
>>> personal_submersibles at psubs.org>
>>> *To:* Personal Submersibles General Discussion <
>>> personal_submersibles at psubs.org>
>>> *Sent:* Thursday, March 3, 2016 4:32 PM
>>> *Subject:* Re: [PSUBS-MAILIST] Hatch interlock
>>>
>>> Sean,  If you are only vacuuming that 1" or so circle in-between the
>O
>>> rings will the vacuum be enough to hold it?   Seems like you'd have
>around
>>> 280# pounds with a 10 Hg vacuum.   or you could  have close to a
>total
>>> vacuum?   25 Hg  ?  then you could get upwards to 700# +  ,  If the
>O rings
>>> squeezed all the way down you might get metal to metal, then would
>y! ou
>>> still reap the benefit of that area acting as a force?   Seems like
>there
>>> would be very little volume of vacuum.
>>>
>>> Brian
>>>
>>> --- personal_submersibles at psubs.org wrote:
>>>
>>> From: "Sean T. Stevenson via Personal_Submersibles" <
>>> personal_submersibles at psubs.org>
>>> To: Personal Submersibles General Discussion <
>>> personal_submersibles at psubs.org>
>>> Subject: Re: [PSUBS-MAILIST] Hatch interlock
>>> Date: Wed, 02 Mar 2016 14:53:19 -0700
>>>
>>> I will have one transducer per hatch, so that I can track the
>interlock
>>> status and seal integrity per hatch. Each transducer is therefore
>exposed
>>> to whatever pressure exists at the flange between orings, which in
>the case
>>> of the lockout hatches must necessarily include the pressure at full
>>> lockout depth, because those flanges are exposed to full pressur! e
>when
>>> the lockout is operated.  This means that in order for this
>conceptual
>>> design to work, I must accept a larger range, lesser resolution
>measurement
>>> for those hatches, but it occurs to me now that in every case, I
>would need
>>> to accommodate the maximum anticipated pressure on either side of
>each
>>> hatch, if I expect to be able to track progressive seal leakage
>without
>>> damaging a transducer. Ergo, only the 1 atm spaces could make use of
>0 - 15
>>> psia transducers. The rest would have to be 0 - 250 psia or
>whatever, and I
>>> may require higher resolution signal conditioning (24 bit?) to eff!
>>> ectively measure the range below 1 ata.
>>> Sean
>>>
>>>
>>> On March 2, 2016 1:19:54 PM MST, Alan James via
>Personal_Submersibles <
>>> personal_submersibles at psubs.org> wrote:
>>>
>>> Sean,
>>> why not leave the transducer in the 1 atm compartment & just attach
>it
>>> to the pipe from the compressor that would run through the wall to
>the
>>> various
>>> compartments & sealing flanges. There are of course other
>complications
>>> with
>>> releasing the vacuum pressure on the individual hatches.
>>> Alan
>>>
>>>
>>>
>>> ------------------------------
>>> *From:* Sean T. Stevenson via Personal_Submersibles <
>>> personal_submersibles at psubs.org>
>>> *To:* Personal Submersibles General Discussion <
>>> personal_submersibles at psubs.org>
>>> *Sent:* Thursday, March 3, 2016 8:35 AM
>>> *Subject:* Re: [PSUBS-MAILIST] Hatch interlock
>>>
>>> It is not the personnel transfer hatch that presents the problem,
>but
>>> rather the egress hatches in the lockout chamber, as those flanges
>will!
>>> see the lockout pressure in normal operation.  I can get away with a
>larger
>>> range pressure transducer for those hatches, but then I lose
>measurement
>>> resolution.
>>> Sean
>>>
>>>
>>> On March 2, 2016 9:32:49 AM MST, Alan James via
>Personal_Submersibles <
>>> personal_submersibles at psubs.org> wrote:
>>>
>>> Sean,
>>> that sounds a good idea. Nuytco set the o-ring on the deep worker
>hatch
>>> externally with a vacuum
>>> pump, but with your twin seal idea you could do this from within the
>sub
>>> without decreasing the hull pressure.
>>>   If the transducers are just monitoring the vacuum between the o-!
>rings
>>> can't the transducer be
>>> m! ounted on the 1atm side of the diver lockout hatch & not be
>exposed to
>>> diver lockout pressure?
>>> Alan
>>>
>>>
>>>
>>> ------------------------------
>>> *From:* Sean T. Stevenson via Personal_Submersibles <
>>> personal_submersibles at psubs.org>
>>> *To:* "personal_submersibles at psubs.org"
><personal_submersibles at psubs.org>
>>>
>>> *Sent:* Thursday, March 3, 2016 1:41 AM
>>> *Subject:* [PSUBS-MAILIST] Hatch interlock
>>>
>>> Doing some further design on my lockout submersible project, I came
>up
>>> with a novel way to implement hatch interlocks, which doubles as a
>seal
>>> condition monitor, ! and a means of establishing a preliminary seal
>in the
>>> absence of a pressure differential without relying on the hatch dogs
>to p!
>>> rovide the initial o-ring squeeze.
>>> My design entails two o-rings per hatch (vessel has six hatches:
>cabin
>>> loading / escape, outer lockout loading / escape, inner lockout
>loading /
>>> escape, inner lockout egress , outer lockout egress, and transfer).
>These
>>> o-rings are concentric face seals, each residing within a half
>dovetail
>>> groove for positive retention of each o-ring when the hatch is
>opened or
>>> manipulated. The grooves are oriented such that the flat face of
>each half
>>> dovetail faces the intermediate space between the two rings. This
>>> intermediate volume is not isolated, but rather connected (on the
>sealing
>>> flange side) to a vacuum transducer, and piped through appropriate
>valving
>>> to a vacuum pump. When the hatch is closed, this intermediate space
>is
>>> pulled to vacuum (as strongly as the pump allows), then locked off,
>and the
>>> strength of this vacuum is measured by the transducer and
>continuously
>>> monitored. The interlock is clear as long as the va! cuum holds, a!
>nd
>>> activates the moment the seal is rele! ased, instead of relying on
>some
>>> arbitrary movement of the hatch to indicate that it is open.
>>> Apart from the obvious expense, I see a potential problem with
>exposing
>>> those vacuum transducers in the lockout hatches to high pressure,
>>> necessitating either a less sensitive transducer that will withstand
>the
>>> pressure, or some means of isolating the transducer when the
>pressure
>>> approaches the limit of its range - I'm still working this out in my
>head,
>>> but I thought I would share anyway.
>>> Sean
>>>
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