Vacuum in Telemetry cans.

[Anonymous]

Beautifully put Subseaaddicted. Not much more to add than that…..

[xxx]

Totally agree Subseaaddict

The technical issues governing pod vacuums are pretty complex. The main reason for pulling a vacuum is to test the seals in air. Air molecules enter a vacuum at a velocity of 500 meters per second and so any leak would be detected quickly in air. A leak under the water is a different matter and the speed the water enters the can is governed by a complex relationship but basically is due to pressure differential and the size of the leak. Of course the pressure in the can will change depending on the temperature inside the can which as the electronics/electrics heat up changes with time. So if you have a small vacuum in the can then the rise in temp will not raise the pressure as much as with no vacuum. When it all works properly the pressure differential is what gives a good seal while at depth. With a small vacuum the temp in the can reduces this differential less than with no vacuum. At depths of greater than 60m there would be no chance of pressure differential reduction due to temp in the can having much effect as the pressure under water varies at density x acceleration due to gravity x depth but at depth below this there could be a chance of the pressure in the can exceeding the water pressure and leaks could occur.

As an aside this physics is also the reason that burton plugs can "leak" at shallow depths but seal as you go deeper.

[spanker]

wasn’t being hostile but i still say the sllep thing works better

[rovbionic]

well said subseaaddict
Fired the queston to the Supervisor and it reminded him of a just installed vacum sensor that wasn’t enabled yet through diagnostics…cheers lads.

From my understanding it basicly confirms the tlmy can has a seal prior to comming off deck and not as a substitute for a water alarm.

As far as slaging such topics…..it is the better tech who bats such topics off the guru’s etc.

Hell …hadn’t seen the ole Charles Gas Law re psi/temp/volume relationship since I jumped out of bells.

Having said that….watched a few lads debate the "which to pull a vacum first" scenerio on a XL tlmy can…the port for the o ring lid or the can itself…twas white mans magic but they asked…no harm…right!

[Andy Shiers]

That is a most interesting theory 🙂
The deepest , there abouts in the Middle east is approx’ 88m and the temp at depth of 40m is very warm 😯
Having a vacuum caused no end of problems , one being no airflow through the power supplies which in turn overheated and shut down.
The manufacturer had had the brains to put thermo fuses in but it’s still a pain as ya have to recover or dock it and turn the power off to reset them 😕
Water ingress or Vacuum Alarm , Two of the finest ways of seeing the ROV crew leap into action from playing The Eagles greatest hits, reading or having a snooze on the back transformer to the headless chicken or fox in the Chicken Run 😆

[rovnumpty]

vacuums!

subsea has a lot to answer for.

Had lots of problems in W. Africa with overheating due to ex-subsea bods pulling- and leaving- vacuums on pods.

Then at subsea, in greenwell base no less, I worked on an old pioneer. When I asked the base supervisor what the fans in the pod were for if you pull and leave a vacuum, he replied " for cooling!" in the kind of exasperated voice reserved for stupid questions.

"but there’s no air for them to push round" had escaped before I could stop it.

"eh. well. hmmm. it’s not really that much of a vacuum? I think. Go and fit those lights!"

My two pounds (inflation you know) worth –

Test the pod seals with a vacuum – WITHOUT the instruments running. – A lot of capacitors and and ICs aren’t designed to be run in a vacuum.

Then let them back to atmospheric pressure slowly.

Then put the caps back on.

[Donnie Cameron]

I agree vacuums should not be present in telemcans when working. Only use them to check for pod seal integrity. The reasons have all been posted and I must say I’m pretty impressed by the arguments the ladies have come up with.

[Andy Shiers]

Just because you call ya self POR and wear womans clothing ya self Pondlife 😀

[Donnie Cameron]

I would expect nothing less from you numb nuts!
Did you find your helmet? Bet it was right where I left it! 🙄
See you that side mate!

[TheBaron]

Time to get my tuppeanuts worth. Another major benefit of keeping a partial vacuum in the pod is the reduction of condensation issues. Pods on deck in some of the warmer climes can reach temperatures of up to 75 °C. When you dip your ROV into water that is less than 37 °C (this is the warmest water temp I’ve seen recorded on a TS dip), there is a rapid cooling of the air in the pod. As we all know (being engineers?!!), when vapour (air, in our case) is cooled or compressed, or both, we get condensate. Think about a cool glass of beer in the summer….only in reverse. By having a partial vacuum, you are reducing the amount of vapour present in the pod.
My thruppeanuts worth, if you are a smart cookie and have to do a battery change on a divers watch, try and do so in a container of nitrogen. Unless the watch is used below temperatures of 77Kelvin (-195.8°C), our biodegradable friends should have no condensation issues whilst reading their watch, unless they are really, really, really deep!

[Che]

Ha all this chat reminds me of when i was a trainee many moons ago being told to replace the end cap on a tel -m can and i was there for funckin ages trying to get the end cap and the can face to face so i could get the securing ring on and nip it up , but every time i got it and reached over to get the spanner to nip the nut up the funckin thing would shift , it was only after an age that i suddenly had not a Eureka moment but a Doh! moment when i realised that the vent plug was closed and as i pushed the end cap into place it generated enough pressure to push it off again when i took my hand off it.
Needless to say i said nowt about it in the t-shack and am only telling now cos i’m much older and a wee bit wiser . Sometimes you just cant see the wood for the trees!

[Andy Shiers]

Where is it you come from ?

[Che]

Fort William,The Peoples Republic Of Lochaber, Scotland!

[TheProphet]

I Fort William but he was bigger than me and i ended up with a black eye 😆

[Sarcy]

Boyles Law? What about Sods Law? If you really don’t want it to happen it will but the vaccum – check the spelling SSaddict – and water sensors will also fail and your pod will flood anyway.

[Author]

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