[PSUBS-MAILIST] Hydraulic idea

Sun Nov 27 11:28:05 EST 2016

The commercial accumulators are heavy because they are designed for high
service pressures.  If you know for a fact that your vessels will only
ever see e.g. 1100 psi, you can get away with something lighter.  The
concept is exactly the same.  Pistons require tight tolerances and
surface finishes - the bladder is the easier solution.  Since the
bladder will never see any delta-P across it (disregarding it's own
elasticity), it doesn't need to be that substantial - even an inner tube
might work.  Just something to allow you to completely purge all air out
of the oil volume, and which is materially compatible with the oil, and
which will maximally expand within the pressure vessel.

As an alternative to your constant pressure accumulator, you could go
with a sealed accumulator and a small hydraulic pump which is capable of
high pressures but not necessarily high flow rates.  For example, empty
the accumulator of oil, and precharge the bladder to 1000 psi.  This
ensures that just before going completely empty, it would still be
supplying fluid at 1000 psi.  If you do this, you can use a pressure
gauge on the gas side of the accumulator to monitor the oil level, so
you pump oil in until the gauge reads 3000 psi, and you then know the
vessel is 2/3 full of oil.  You can recharge the accumulator over time
at a much lesser flow rate than is actually demanded by the manipulator,
because the accumulator supplies the high flow necessary for operation.

Sean

On 2016-11-27 08:44, hank pronk via Personal_Submersibles wrote:
> Sean,
> I agree completely except the operating pressure is  not as high as
> you suggest.  I know for a fact Gamma ran at 1,100 psi from the
> maintenance records.  You do make a point that I missed.  Without a
> bladder, I could leak oil into the water when relieving the pressure
> while surfacing.  I was considering an accumulator either  a piston or
> rubber bladder type, but they are very heavy.   I would like to figure
> out how to use this idea with a water pressure tank with bladder for
> the light weight.  
> Maybe this is not the way to go, but an interesting brain exercise.   
> I do have a small hyd  pump that is pretty light and makes 1,100 ps,i
> it is just such a pain in the butt. Mind you with external batteries
> and external pump it will not be as bad.
>
>
> On Sunday, November 27, 2016 7:38 AM, Sean T. Stevenson via
> Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>
>
> Hank, what sort of total oil volume are you considering?  The
> immediate problem I see is the loss of pressure in your HP supply,
> given the necessary operating pressure of the hydraulic system. 
> Regulating a cylinder at e.g. 4000 psi down to 2000 for drive pressure
> is still not going to last very long, and you also have the problem of
> pressure increase in the receive tank if it is not vented reducing
> your total available delta-P (i.e. you have a drop in pressure on the
> drive side combined with an increase in pressure on the receive side -
> it may not take long to become ineffective).  If that isn't an issue
> for you, then there's no reason it wouldn't work.  You are essentially
> powering your system from a precharged accumulator, and draining into
> another one at lesser charge pressure.  I would incorporate a check
> valve between the supply accumulator and the control valve, so that
> pressure pulses are not transmitted upstream and no sponginess is
> evident.  You might also want to take a concept from commercial gas
> charged accumulator designs and incorporate some sort of bladder
> inside each pressure vessel to physically separate the oil from the
> charge gas.  If you insist on using air (I still prefer nitrogen),
> this would prevent the mixing of high pressure oxygen with oil, and
> also allow you to vent the receive tank to the water without worrying
> about oil contamination in either direction.  It also makes the system
> insensitive to vessel attitude.  Then your pumbing becomes simple -
> one tank is charged and one is vented, and when all the fluid is moved
> to the receive side, you just switch the direction.
>
> Such a system gets its energy input from the compressor that charges
> the HP air or nitrogen source, versus having an on-board hydraulic
> pump drawing energy from the batteries.  The advantage of the former
> is that the energy input is decoupled from the vessel and need not be
> carried, but you only achieve constant power for as long as you can
> maintain constant precharge pressure, which will be a function of the
> pressure setting and the respective volumes of both the hydraulic
> system and the HP gas source.  The advantage of the latter is the
> ability to run continuously without having to switch directions, and
> that energy storage in batteries is probably more efficent than energy
> storage in compressed gas.
>
> Sean
>
>
> On 2016-11-27 05:47, hank pronk via Personal_Submersibles wrote:
>> Alan,
>> Think of it as a hydraulic system, not an air system.  The arm is
>> identical in every way except it is powered by air pushing the oil
>> instead of a pump.  It would not be spongy because the cylinders are
>> full of oil.  The air never leaves the pressurized oil tank.    When
>> the  valve is activated, the oil moves as if it had a pump.  Instead
>> of the oil returning to the pump reservoir, it is sent to a holding
>> tank that is a HP bottle.  The air in the receiving tank compresses
>> as the oil flows into it.  Nothing is vented, it can not vent because
>> I need to maintain a balance between the two tanks for buoyancy.  As
>> the oil leaves the pressure tank, the tank gets lighter.  At the same
>> time the receiving tank gets heavier, so they are balanced.  
>> I am not worried about space, it is weight I am thinking about and
>> complexity.  Of coarse if you were doing construction with the arm it
>> would need to be electric.  But to grab one gold bar it can be air
>> driven.  Air drive saves battery power also.   Having said all that,
>> it may have a problem I have not expected, so speak up and save me
>> some trouble anyone.  
>>  I don't see a problem with a motor submerged in WD40, my vertical
>> thrusters are full of it.   I could use something else as long as it
>> has a low enough viscosity for the motor to run.  
>> Hank
>>
>>
>> On Saturday, November 26, 2016 8:03 PM, Alan James via
>> Personal_Submersibles <personal_submersibles at psubs.org>
>> <mailto:personal_submersibles at psubs.org> wrote:
>>
>>
>> Hi Hank,
>> Wouldn't you have to exhaust the air as in a pneumatic system?
>> Also air would have to displace the volume of oil in the cylinder
>> as the piston moves in & out, so there would be marginal benefits
>> over a pneumatic system. It would also be spongy like a pneumatic
>> cylinder because any bouncing force on the cylinder would compress
>> the air
>> that is the source for the movement.
>> You were talking about saving space with this idea, but if compressed
>> air was a good form of power we would have pneumatic thrusters
>> instead of batteries & electric motors..
>> I once did the maths on how much energy was stored in a dive tank;
>> can't remember the result, but there was at least 3 x more energy in
>> a battery of an equivalent size.
>> WD40 as hydraulic oil? It is flammable with a reasonably low flash point.
>> Also at that viscosity you would be more prone to leaks wouldn't you?
>> https://www.wd40company.com/files/pdf/wd_40tec16952473.pdf
>> Cheers Alan
>>
>>
>> ------------------------------------------------------------------------
>> *From:* hank pronk via Personal_Submersibles
>> <personal_submersibles at psubs.org>
>> <mailto:personal_submersibles at psubs.org>
>> *To:* Personal Submersibles General Discussion
>> <personal_submersibles at psubs.org>
>> <mailto:personal_submersibles at psubs.org>
>> *Sent:* Sunday, November 27, 2016 4:59 AM
>> *Subject:* Re: [PSUBS-MAILIST] Hydraulic idea
>>
>> Sean,
>> The receiving tank will take the full depth pressure and be
>> large enough  to take all the oil without building up excess air
>> pressure, no need to vent off, since it is only receiving oil and
>> displacing air.  I need a balance with two tanks to maintain neutral
>> buoyancy.
>> I am not worried about oxidation of the oil because the oil is not
>> going through a pump and the flow rate is so small.   I ordered air
>> cylinders for the manipulator with a 5\8 rod to reduce the back
>> pressure.  Gamma's manipulator operated at 1,100 psi when the sub was
>> at 1,000 feed of depth.  The arm will loose power, but I don't expect
>> that to be an issue, because the oil tank will be powered from a
>> separate bottle of air.  
>> I have to work with what I have to keep the cost in check, so I can
>> modify an open centre valve by blocking the final pressure port
>> drain.  I also have some HP tanks.  If it does not work out easily, I
>> have a few electric pumps I can use.  
>>
>> If I go electric, I intend to submerge the motor pump unit in the oil
>> reservoir with a bladder top to compensate.  That means I will use
>> WD40 as hydraulic fluid.
>>
>>
>> On Saturday, November 26, 2016 6:25 AM, Sean T. Stevenson via
>> Personal_Submersibles <personal_submersibles at psubs.org>
>> <mailto:personal_submersibles at psubs.org> wrote:
>>
>>
>> How do you intend to control the pressure on the receive tank? Just
>> vent it through a check valve to the water? You need to design
>> carefully to prevent contamination in either direction.
>> I wouldn't use air as the drive gas under high pressures, in order to
>> prevent oxidation of the oil. Charge with nitrogen if you intend to
>> do this.
>> To generate the same drive capability as a hydraulic pump, you are
>> talking about very high precharge pressure - approaching the pressure
>> at which HP bottled gas is supplied, unless you can source e.g. 6000
>> psi nitrogen and regulate it down to 2500-5000 depending on your
>> manipulator requirements. In any case, if your cylinders are single
>> acting, or even dual acting with a single rod, you have to contend
>> with the force from the ambient water pressure, so your receive tank
>> pressure needs to be this at minimum, and with a pressure reservoir
>> source instead of a pump, that available delta-P is further reduced
>> the moment you demand any fluid from the system (i.e. HP bottle
>> pressure will drop), so your manipulator becomes weaker over time.
>> Just a few things to think about.
>> Sean
>>
>>
>> On November 25, 2016 5:25:47 AM MST, hank pronk via
>> Personal_Submersibles <personal_submersibles at psubs.org>
>> <mailto:personal_submersibles at psubs.org> wrote:
>>
>>     Hi All,
>>     I have an idea to replace the hydraulic pump for my new
>>     manipulator with a air over hydraulic system.  It is quite
>>     simple, the hydraulic oil reservoir is a hp tank that can be
>>     pressurized from a designated HP supply.  The oil return goes to
>>     another HP tank  to receive the oil.  This eliminates the pump
>>     completely and that is a dream.  The manipulator can go through
>>     54 complete extensions and retractions, that is 54 complete
>>     movements of all functions.  
>>     After the oil is used up, the oil can be returned to the pressure
>>     tank by reversing the air flow.
>>     Hank
>>     ------------------------------------------------------------------------
>>
>

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