[PSUBS-MAILIST] right side up compensator

Sat Mar 28 17:00:27 EDT 2020

The primary purpose of a compensation mechanism is to control the differential pressure (dP) between the inside of an enclosure and the surrounding environment.  This is important both structurally, because the service dP will dictate the required vessel shapes, wall thicknesses, etc., as well as for both dynamic and static sealing elements across the pressure boundary, which will have particular performance constraints that must be met.  Since pressure compensated systems are always at or near the surrounding seawater pressure by design, there is no design requirement for pressure tolerant housings in such systems.  A pressure compensated system need only provide effective isolation from the surrounding environment, and whatever pressure capability is required to accommodate design bias pressure and incidental operational loads.

Secondary to controlling dP, a compensation mechanism is often required to perform additional functions, such as facilitating convective and conductive cooling within an enclosure, lubricating dynamic seals, lubricating bearings or other mechanisms within the enclosure, and providing effective dielectric insulation of contained electrical systems.

A compensation mechanism must also provide some capacitance in order to accommodate changes in instantaneous oil demand as a result of temperature changes in any or all of the enclosure, the compensating oil, and the contained equipment.  The nature of operational heat sources, heat sinks, and the thermal coefficients of expansion of the various components within a pressure compensated system will all contribute to variation in demanded oil volume as the actual enclosure volume, volume displaced by contained equipment, and oil density change with temperature changes of these components.

There is also a volume change associated with the absolute ambient pressure, which in a compensating oil is due to a combination of the effect of its own bulk modulus in conjunction with the volume change associated with both entrained and dissolved air within the oil.  Even the internal volume of a rigid enclosure is affected by the external absolute pressure, albeit to a much lesser extent than the volume change of the compensating oil.  The air effect is significant enough that there is an argument to be made for initially pumping compensation volumes to vacuum and then filling them with initially degassed compensating oil, as well as providing the facility to periodically vent any accumulated gas bubbles (which can be spontaneously driven out of solution) out of the system to best approximate 100% non-compressible behaviour.

Finally, an important function of a compensation system to to prevent the ingress of seawater, which will rapidly cause corrosion problems to compensated equipment if it occurs, in addition to compromising dielectric insulation and oil viscosity.

In accordance with these requirements, it is advantageous to isolate the compensating oil completely from the surrounding seawater environment, both to prevent the inadvertent contamination of the compensating oil with water (or other external contaminants), and to minimize the probability of leaking compensating oil into the surrounding environment.  This is typically accomplished by incorporating a flexible membrane element of some kind into the pressure boundary, such that the surrounding seawater pressure is effectively communicated to the compensating oil while maintaining physical isolation of the two environments.  With no additional bias pressure capability incorporated by design, such an embodiment could entail a simple flexible bladder or collapsible tube subject to sea pressure, which would ensure that the oil within the compensated volume is at that same pressure.  Design volume of such a bladder would need to be large enough to fulfill the anticipated maximum oil demand in both directions, meaning that it must not initially be filled to 100% capacity, but rather to some intermediate value which accommodates receiving fluid as well as delivering it.

There are a number of operational reasons why it may be advantageous to impose some amount of positive pressure on the compensation system, over and above the ambient sea pressure.  Doing so ensures that the compensation effectiveness is insensitive to vessel attitude, can act to energize elastomer seals which may not otherwise exhibit acceptable squeeze or initial sealing effectiveness under zero differential pressure, and in the event of a seal or housing leak, ensures that fluid flow occurs from inside the compensated volume out to sea, and not the other way around, which can have serious consequences.  Inside to outside leakage may have environmental consequences, but these can be managed both by oil selection, and by limiting the maximum available compensation volume.  In what is perhaps its simplest embodiment, a positive pressure compensation system may comprise a flexible membrane element which exhibits some stretch, and which therefore applies pressure as a result of its own elasticity when deformed.  This is analogous to a bladder type accumulator with no precharge pressure being inflated beyond its initial undeformed volume.  The obvious disadvantage to this system is the fact that available compensation volume and bias pressure are necessarily linked.

More advanced embodiments of positive pressure compensation systems are analogous to hydraulic accumulators in their typical embodiments, comprising either bladders or pistons which can be loaded externally either by physical springs or by gas pressure.  In the former case, spring force will depend on displacement, and thus vary with compensation demand, but this can be managed within acceptable limits through appropriate spring selection.  In the latter case, compensation overpressure may be controlled to a continuous value above ambient with a pressure sensing regulator that delivers gas at the desired pressure to the precharge side of the compensator.

While low-cost open compensation systems are possible, such designs suffer from the very real problem of contamination of the compensation oil by moisture, even if the oil and ambient seawater are not in direct contact.  Air can of course contain and transport moisture, and will readily do so and contaminate oil systems in contact with air with both moisture and with oxygen if not prevented by design.  To that end, gas pressurized compensators / accumulators are best charged with dry nitrogen as opposed to air where possible, lest a leak of gas from the precharge side to the fluid side unnecessarily admit oxygen or moisture to the oil.

Sean

‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
On Saturday, March 28, 2020 2:19 PM, Øystein Skarholm via Personal_Submersibles <personal_submersibles at psubs.org> wrote:

> Oil Expansion
>
> Since I am from Europe I have to use celsius to not confuse myself.
>
> Let's say Alan fills his motor AND compensator with 8,2 gallons of oil. The oil and surrounding temp is 20deg C.
>
> He then brings it out in the sun and it warms up to 40deg C
>
> During the dive it cools down to 6 degrees C
>
> Expansion during heating to 40 deg C = 8,31Gallon Total Volume
> negative expansion during diving 6 deg C = 8,12 Gallon  Total volume
>
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> Virusfri. [www.avg.com](http://www.avg.com/email-signature?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=webmail)
>
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>
> lør. 28. mar. 2020 kl. 20:35 skrev Alan via Personal_Submersibles <personal_submersibles at psubs.org>:
>
>> I like the ROV Oystein, well done.
>> That compensator would work well for Brian.
>> Brian's problem is that he made his own thruster & housing which is a bit
>> of a beast in that it needs gallons of fluid to compensate it.
>> His concern is to have enough room for oil expansion which is an unknown
>> quantity. Personally I think he will get more expansion with it sitting out of
>> water in the sun than running underwater where oil circulating against the
>> large housing will keep it cool.
>> Alan
>>
>> On 29/03/2020, at 7:57 AM, Øystein Skarholm via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>>
>>> Again, NEVER open the oil to contact the seawater. You WILL get water into the oil! Have the oil closed. Unless you have a piston in your riser tube, the air will flow straight out and water will come i.
>>> <image.png>
>>> The idea to the left.....and the result to the right. Just trust me on this.
>>> The below picture shows one of the ROV I have built. The canister in the center contains computers and power supply ++ 600Volt DC
>>> The whole canister is filled with Shell Diala S4 oil (In the progress of changing to MIDEL ( Not so aggressive to the rubber and plastic parts.
>>> I use a small bladder compensator for this (its far too big really, but came cheap off the shelve from an auto dealer shop) The little tee lets me inject oil through a check valve
>>> until I get 0,2bar overpressure. The red cap in the end lid is the main filling bulkhead.
>>>
>>> http://www.avg.com/email-signature?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=webmail
>>> Virusfri. [www.avg.com](http://www.avg.com/email-signature?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=webmail)
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>>>
>>> lør. 28. mar. 2020 kl. 15:14 skrev Jon Wallace via Personal_Submersibles <personal_submersibles at psubs.org>:
>>>
>>>> I'm thinking we need someone to give a technical talk on this at our next convention.  I'm not only confused but I guessing others are as well.  I thought the simplest approach, from previous discussions, was just to use a length of hose connected to the oil filled motor on one end and open to water on the other end.  See attachment.  This allows expansion of oil into the hose (blue), but traps a gap of air (white) as water (green) enters the hose during a dive.  Water compresses the trapped air the deeper you go, so designing a large enough gap is important because that air gap can get quite small and potentially result in mixing of the oil and water.  At 300 feet for example, the air gap is only .652 inches (16.5mm) when using a 30 foot (9 meter) hose.
>>>>
>>>> My understanding for the need of a bladder is in a closed system, meaning no hose open to the water, to have an expansion point (the bladder) for any increased oil volume due to heat whether it be motor operation or just sitting in the sun while out of the water.
>>>>
>>>> Jon
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>>>
>>> --
>>> Vennlig hilsen
>>> Øystein Skarholm
>>> 91369599
>>
>>> <Compensator.jpeg>
>>
>>> <SDI ROV.jpg>
>>
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>>
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>
> --
> Vennlig hilsen
> Øystein Skarholm
> 91369599
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