Fluid dynamics

[Stu Jackson]

Scott pretty much nailed it, and Maine Sail's last note about two tanks one emptying into the other is correct, too. That's why we always suggest that with two water tanks only one be activated by opening only its valve at one time, or the pump could suck air. On our boat our two tanks are essentially both on the same side of the boat, but one is higher than the other. By opening both valves when filling the higher aft tank, I can fill both tanks! Oddly enough the fill port on the aft tank is a wider diameter than the other one, so it makes it much easier to fill, too.

Whether it's fuel or water, the dynamics are the same.

The easiest test you could do would be to take two plastic cups, drill small holes in the sides at the bottom and glue a straw between them. Then fill one up and see what happens.

All a vent is is like having the top of the cups open.

Good luck.

 

[wufibugs]

Without knowing the fluid engineering, this is essentially what we did when converting the old Jag XJ6 with dual tanks from a manual switchover to a common feed system that emptied the tanks simultaneously.

 

[thinwater]

This situation is common in refineries and tank farms every day. The question statement isgrossly incomplete. Many variables, including viscosity, pipe runs, pipe size and on and on.

a. Some of the practial answers may have forgotten anti-syphon valves, which introduce a whole new variable; it isn't so much where the pipe fills from as the finer details.

b. If diesel there is probably a return pipe; that can make a major difference.

 

[RichH]

Brian -

If this is a fuel tankage system, using 'cross over pipes' to affect a constant level between both tanks, I think, would be totally against CG rules. Typical CG regs for marine fuel tanks is that there shall be NO 'bottom exit nozzles' nor 'drains' on such tanks and that all fuel delivery is via 'diptubes' (pumped discharge over the side walls of the tank and with NO 'bottom connections'); the diptubes always connected to 'tank shut off valves' (ON the tank or higher in elevation of the top of the tank) to enable the POSITIVE shut down of the fuel exit. All connections on fuel tanks MUST be on the TOP of the tanks. This is to prevent/minimize leakage/spills of fuel into the bilge ... and to outside the boat to create 'a sheen on the water'.

If I remember such CG fuel tankage rules correctly, if you had a spill from such a 'cross over arrangement' and there were environmental clean up charges, your marine underwriter would probably DENY YOUR CLAIM and NOT cover such costs; and, the full monetary charges of potentially many thousands and thousands of dollars would be in YOUR exclusive financial lap.
Multiple marine fuel tanks are usually always independently connected by 'transfer pumps', not 'cross over pipes' connected to the bottom nor sides of the tanks; marine fuel tanks by the rules never have any connection on other than the TOP of the tank. All 'transfer' between tanks MUST be by 'pumping'.

For water tanks, cross-over pipes from the bottom of the tanks is OK; AND, the cross-over pipes should be quite large in diameter so that the tanks quickly equilibrate the level of the water in multiple (large) tanks so as to not create 'trim' problems of the vessel.

 

[Brian D]

Rich, originally I was asking a generic question on fluid dynamics. But it seems most everyone wanted to get specific about their answer. It was all good. Although generic in nature, I was wondering if my fuel tanks would equalize or not. So that is why I was asking.

Fuel is a fluid. Granted its properties differ from other fluids, but it is still a fluid and must abide by rules of physics. Hence the general nature of the question.

I have tandem tanks, one is 1/2 full and the other empty. The valve on the empty tank is closed, but this morning I opened it. (Both valves are right at the top of the tank and before the T fitting.) So I was wondering if due to the unequal pressure between the two tanks, fluid would transfer from the 1/2 full to the empty. My tanks meet the requirements you stated above.

I will go back in a day or so and see if there has been any change. If yes, then I will be somewhat happy. If no, there is no loss. I will just fill the other tank and start using it until I know there is no air in that fuel line, then I will start using both tanks.

Thanks to everyone who answered. Really got me thinking about fluids. I might just start playing with empty water bottles and tubing to see what happens.