[PSUBS-MAILIST] G.L. summary Pressure Hull

Wed Dec 11 14:45:55 EST 2013

It seems that the need for sensing could be eliminated (as far as the present question goes) by positioning the handle for the pressure relief valve in such a manner that it would interfere with releasing the hatch latch unless the relief valve handle was in the open position.  In other words, you have to move the pressure release handle out of the way before you can get to the hatch release.  

Having a manual control handle doesn't preclude the relief valve from being automatic.  It just means the manual portion leading to the auto portion was open.  It seems like a good idea for the manual valve to be opened at some reasonable depth prior to surfacing so any overpressure is free to flow to the auto relief.  You don't want overpressure to blow the hatch open when you're still two feet below the surface.  If the relief system is entirely manual, you have to wait until surfaced to open the valve unless the overpressure is particularly high.

Back to the sensing issue:  Several months ago we discussed automatic overpressure valves vs manual OP valves.  I favored automatic (AOP) since it's difficult in an overpressure situation to intermittently, manually vent as the sub ascends and the outside pressure decreases.  However the set cracking pressure on an AOP valve automatically and continuously senses the pressure differential and reacts accordingly.  Consider that even 1 or 2 psi multiplied times the number of square inches on your hatch adds up to a huge amount of pressure.  George K. and others have experienced overpressure without even realizing it until the hatch blew.

How about this setup:  Small, onboard compressor with line running to the manual OP valve (by the hatch latch), then to the AOP valve.  Latch the hatch, leave the manual OP valve open, run the compressor until you get the desired vacuum inside the sub.  Turn off the compressor and the AOP will close itself.  Wait to see if air leaks back into the sub.  If it doesn't, you're sealed and good to dive.  The manual OP valve mainly serves as a cutoff if the AOP should fail and leak.  Whether you were diving with the manual valve opened or close, it should at least be opened when the ascent reaches a certain level (another discussion).  If you set the cabin pressure gauge to 0 before you closed the hatch and monitored it while running the compressor, you know how much vacuum you pulled then, and now you can tell if you have an overpressure situation or not.  Additionally pulling the slight vacuum helped ensure against an OP situation.  Sure, an atmospheric pressure change could have occurred while you were submerged, but the difference shouldn't be too much.  An unbalanced life support system could change the cabin pressure as well.  When you surface you're more likely going to have under-pressure and need to admit air to equalize so you can now open the hatch.

A vacuum on the support boat instead is great, but given the difficulty of doing some tasks from the support boat to a sub floating next to it, I want to try the miniature compressor on a shop dive and see how it works.

Would love to hear corrections and comments from those with more operational experience.

Jim

-----Original Message-----
From: Joe Perkel <josephperkel at yahoo.com>
To: personal_submersibles <personal_submersibles at psubs.org>
Sent: Wed, Dec 11, 2013 8:54 am
Subject: Re: [PSUBS-MAILIST] G.L. summary Pressure Hull

Alan

No definitive answer but we can postulate that both sensing and actuation are involved. 

On sensing, differential pressure on either side of the hatch is translated to a impulse to actuation.

On actuation, a method of locking out the operator from physically un-dogging the hatch, is the only way to meet the standard as per your O.P.

Joe

Sent from Yahoo Mail for iPad

                                                            From:                                                        hank pronk <hanker_20032000 at yahoo.ca>;                            
                                                            To:                                                        Personal Submersibles General Discussion <personal_submersibles at psubs.org>;                                                                                                     
                                                            Subject:                                                        Re: [PSUBS-MAILIST] G.L. summary Pressure Hull                            
                                                            Sent:                                                        Wed, Dec 11, 2013 2:36:26 PM                            

Alan,
Is this equalization from overpressure inside the hull? or external pressure.  
Hank 

  On Wednesday, December 11, 2013 1:23:21 AM, Alan <alanlindsayjames at yahoo.com> wrote:

Can anyone enlighten me on what sought of mechanism
is used that renders the opening of the hatch possible only after
equalisation, as per G.L. Requirements?
Thanks
Alan

Sent from my iPad

On 11/12/2013, at 3:01 PM, Alan James <alanlindsayjames at yahoo.com> wrote:

Hi,
I'm not covering much in this section because if I started on construction
regulations, I would just be duplicating what was printed.
G.L. wants you to have manufacturer certificates for every non structural screw.
Any company you contract to do your welding has to have their facilities, personal
& supervision approved by G.L..
Some points of interest are.... hatches need to be opened from both sides, they need
to be able to be secured open, they need to be designed in a way that opening of the
hatch is only possible after equalization. Two measures are required to guarantee that
the hatches are closed & secured before diving; one measure is to be visibly noticeable.
Mechanical measures for stopping a banging of the hatch is required.
Any external through hulls are to be protected against damage by appendages or covers.
If you are doing calculations on a computer you have to prove that the program is suitable.
Alan

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