Underwater Shaft Seals

I doubt that any shaft seal scheme is totally bulletproof. A double mechanical seal could work, but it depends on being able to remove any leakage through seal 1 before it penetrates seal 2.

i have considered flat faced mechanical seals but the pressure applied to the faces due to depth will vary . so design has to reduce affective area where force ( water pressure ) is applied from the outside and prevent metal spalling , excess friction and stop lubricant being ejected from the contact faces.

lubricant injection from inside is possible but needs to vary in response to changes in water pressure

I am not familiar with all details (such as API standard seal designs), but there are schemes of balancing lines that can equalize the fluid pressures on either side, leaving always the spring pressure as a differential.

i hear you . but with oil / grease inside and saltwater outside , the balancing lines would need a diaphram canister inline to keep them separate ?

There is a tried and true submarine technology that avoids the problem of keeping the lubricant separate from the seawater..

Lignum Vitae has long been successfully used as shaft bearing material at depth in seawater. the self lubricating properties allow it to be exposed to sea water without need for additional lubrication. This allows a variety of solution for sealing the shaft inboard of the lignum vitae bearings.

What kind of bearings? Basically it is better if one avoids the problem than try to solve it.

Depending on the loading some plastic bearings could work very well with water and avoid whole problem. You should compute the p*v factor IN Water and see it temperature stays within acceptable limits. In water the bearing will be a lot more cooled by the fluid which is not the case in surface applications.

thanks but no , shaft is a 60,000 Newtons radial load , 240 mm diameter cylindrical roller bearing.

i like your thinking of removing the problem rather than having to solve it , but the bearing is required.

bearing cavity is grease filled

i have scope in designing the seal to suit

something used in deepwater drill rigs might be adaptable but still reading through 10 zillion internet pages to find one

Yes you need to borrow the thoughts of a real Sub-Sea Engineer (deep water drill rig) OR directional drilling tool engineer.

In years past, there were real Sub-Sea engineers out there and easy to find, now a days most are dead or retired sitting on the beach :-)

I know of (1) left, he is a French guy, Michel Capdeville, currently working for Ensco in Brazil. Look him up on face book and tell him Tim from Mexico sent ya!

you can never underestimate the value of years of experience in the field , those guys have seen a million things tried and busted before finding their solutions.

pity we seem to be losing so much old school knowledge , the new generation know it all before they leave school.

or do they spell it skool these days

Not sure, but I think they just use kool these days....

Yea thats where we are though, its a pity but true. My area of expertise is with the mechanical side of deep water drilling, mainly the power generation. These days, if there is no where to plug a lap top into the engines CPU the poor little guys that call themselves technicians are lost.

Same I suppose in all fields these days.

Tim

Found these spring loaded hydraulically balanced spit bearings, I think they are used on subs...I think they use labyrinth seals on the outside and these split ones on the inside...

http://www.mt-online.com/component/content/article/168-march1999/376-new-split-seals-cut-maintenance-costs.html?directory=90

http://www.strategypage.com/militaryforums/462-1518.aspx

You might try these people that build the deep dive vehicles...

http://www.silvercrestsubmarines.co.uk/rovsinfo.html

silvercrest - tick

thanks for your help . very very useful

have a nice day.

I had once this info but now, after a while looking for, i got this:1)hard rubber bushing,up to 50 bar (you ask 60),2)wooden arrangements,up to 30 bar;i think (my memory could fails) cork was used for these purposes (water lubed joints for submersed shafts).In every cases the construction, profile etc. is for so specialized engineers.-

Is pressurisation a viable option?

I have to agree on the side comments regarding new "talent".

Drop them in the deep end.

pressurisation... i have a design for that but it needs to match very closely the pressure exerted on the external seal area which is exposed to water pressure.

so . design of such requires very delicate balancing , or..... self balancing of pressure differential .

re new talent...... we have had it soft for a very long time , necessity causes us to strive for solutions , and for survival , i guess they will sink or swim , with or without our knowledge pool.

thanks for reply

You need not worry about the balancing of the pressure when you are using grease, as long as the supply side can stand the pressure (60bar+) and also that it is OK that a little bit of the grease escape into the water.

supply side cant withstand 60 bar

even if it could , what happens when the machine comes up from 600 meters to near the surface , it will start spitting out the entire contents of the grease cartridge to equalise the internal pressure buildup

but we can use biodegradable grease from SIMALUBE which is great

also if there is 60 bar acting on the seals outer face that is alot of pressure on the seal contact faces so i am concerned about the affect of that pressure creating metal spalling or tearing the seal itself

:)

ps...... what kind of car is that in your photo ? i have a friend rebuilding a 1907 Fiat

thanks

Hi again

the pressure in the grease and around the bearing is only due to the water pressure at whatever depth it is sitting at any moment. the grease feed is just a screw mechanism that you give a turn now and then to replace the old grease with fresh. You will need a balanced seal on the inside of the bearing (low pressure side) and a double labyrinth seal on the outside (saltwater side). Between the two labyrinth groups a drain line should be to remove the mixture of old grease and saltwater.

The grease does not expand and the grease feed does not suddenly start to feed just because you take the assembly out of the water so there is no need to make any elaborate pressure matching machinery for that purpose.

I believe the car is a Ford T, but I did not get the photo from anyone I know so I don't have any info on it - could be fun to play with thoug.

thanks . the grease supply to the harsh environment seal is an autogreaser cartridge , Simalube or Alemite both make them , they supply pressure to force the grease in with about 7.5 PSI constant feed and can be set to last 1 , 2 , 3 , 6 or 12 months feed rate but they dont tolerate pressure underwater and would need a housing to protect them.

Could you tell us a little more about the device, please? is it like a submarine with air and crew inside or is it just a motor, gearbox and remote controls. I have worked with a couple of powerful electric water mover pumps where the motor was directly coupled to a propeller and the whole assembly submerged. The water was prevented from entering by having the entire assembly flooded with an incompressible dielectric fluid that was kept balanced via a feed from above. I don't have details about the seals, but I may be able to get them if you are interested.

Installing the auto greaser cartridge in a pressurized housing does not seem too hard a problem and that would probably do the trick for you.

Doesn't have to be balanced if it vents off at a predetermined differential.

Bubble bubble hiss.... should stop you blowing out the seals.

Or could the OP use a magnetic bearing? The electro-magnets and control unit can be totally sealed.

The control and power supply are more complex than installing a mechanical bearing but no maintenance for the next 20 years...

You still need some sealing to prevent water from moving up the shaft but corrosion is not a problem anymore. Bilge pumps can be used for the leaks.

As for the force, it shouldn't be a problem as it is the same technology used for the levitating trains.

Shock loads can be an issue but most controllers can react within fraction of milliseconds. Usually faster than the shaft will take to move out of the acceptable position range.

What about a mechanical seal as used in automotive water pump, OR

see http://www.ahpseals.com/products/high_pres.php - 20,000psi 1,300bar would probably meet your requirements

thanks . that could work if i kept it purged with water , there is a tendency for grit / sand etc to accumulate inside the seal body where the gap is , it builds up over time and goes hard like concrete , abrading the shaft and seal and stops the seal lip from flexing , possibly also damage the spring.

in that example the pressure is inside a pump , in my case the pressure is outside from salt water so the seal has to face the other way

i think they are available with a stainless steel spring so if they have 140mm ID seals then its not out of the question , just a matter of keeping it clean of abrasive debris

usually in a pump the seal is fixed and shaft rotates , however if i make the seal rotate with the shaft and the seal lip touch the fixed bearing housing then the centrifugal force will throw the debris out of the seal.

cheers.

If you placed a washer outboard of the seal, ID 1/2mm greater than the shaft, OD a neat fit into the seal housing and held in place against the seal with an internal circlip into the seal housing - that should protect the seal against the buildup you mentioned while allowing water pressure against it

How about this idea? Surround the shaft closely as in the previous post, and pump fresh water through at slightly greater pressure than the surrounding ocean. This should minimize particle intrusion and corrosion, but it would depend on the amount of fresh water available. (Oil or grease could leak outward into the ocean, resulting in pollution.)

Yes. Submarine vessel manufacturers know this. Make some calls, perhaps.