emptiying ballast tanks in submarines

[Nicknick]

a continuation from:

@Scott Kenny

https://www.secretprojects.co.uk/threads/design-challenge-around-the-world-from-east-to-west-or-pole-to-pole.42374/post-640842

It's just a little bit wasteful to fill the ballast tanks with 40 bar of air (at 400 m depth). If a water pump would be used, it would be enough to fill the volume with only 1 bar, so that only 2.5 % of the air mass would have been required. Powering an hydraulic pump would also require much less energy than compressing the air.

Of course, this method would have drawbacks, the pump can be clocked by things floating in the water, the time required for emptying the tank will be longer and you need reliable energy supply to drive the pump. Seems like the air tanks as an emergency system would be needed anyway.
It is a good idea to put the air tanks within the water tanks, so when they leak, they will push out the water which is a good safety feature.

 

[Hobbes]

If you remove the water from the ballast tank and fill the void with air at 1 bar, the tank walls have to be as thick as the pressure hull.
It's also hard to envision a hydraulic pump that can move a few hundred tons of water in a few minutes.

 

[Nicknick]

The pressure inside the balast tanks is a good point, this would eliminate the weight advantages compared to the air system.

I allready mentioned the speed problem, but with the very high burst power of battery electric system I believe this would be a minor problem.

 

[Hobbes]

These are the specs of a pump designed for pumping concrete.
- working pressure: 85 bar, twice of what you need
- capacity: 120 m3 per hour
- power required: hard to find, but I expect it takes a few hundred kW.

 

[Nicknick]

So lets calculate it..

120/3600*85*100000=283 kW (ideal case)