Unbalanced fuel tank reading

[jim.howes]

Hi your photo is as your schematic drawing. A really clean and tidy layout, well done. Jim

 

[mmassimilla]

So this is what's on the bulkhead.
The fuel supply shutoff valves are elsewhere in the cockpit.
The original poster will have something similar.

Hi just to make sure I understand. If the valve at the bottom is horizontal then the fuel coming from tank 1 can go to engines 1 and 2 and the fuel coming from tank 2 can go to engines 1 and 2. Does that rebalance the tanks’ fuel levels? Imagine I have tank 1 at 10% and tank 2 at 90%. Question 1. Do they eventually both go to 50% if that valve is horizontal? Then above that valve we see the return system. Fuel is going up in this case. So coming from engines 1 and 2 it hits the return crossover valve first. If that’s horizontal then return fuel from engine 1 can go to both tanks 1 and 2 and return fuel from engine 2 can go to both tanks 1 and 2. After that, return fuel goes up to the other 2 valves at the top. If one of them is horizontal then the return fuel cannot go to that tank. In that case, if the crossover valve is horizontal then fuel can still go to the other tank. If the crossover valve is vertical then you can have pressure build up and a big issue. Correct? I have an SC35 and last weekend I saw one tank level going from 50% to 10% and the other from 50% to 80% but I am suspecting there is an issue with the sensors reading the fuel levels rather than the flow of the return fuel. Because such imbalance could only happen if the return crossover valve is horizontal and one of the return valves above is also horizontal, which would mean return fuel from both engines would go to one tank, which then gets filled up over time while the other gets depleted. Am I correct? Thanks in advance.

 

[jrudge]

Hi just to make sure I understand. If the valve at the bottom is horizontal then the fuel coming from tank 1 can go to engines 1 and 2 and the fuel coming from tank 2 can go to engines 1 and 2. Does that rebalance the tanks’ fuel levels? Imagine I have tank 1 at 10% and tank 2 at 90%. Question 1. Do they eventually both go to 50% if that valve is horizontal? Then above that valve we see the return system. Fuel is going up in this case. So coming from engines 1 and 2 it hits the return crossover valve first. If that’s horizontal then return fuel from engine 1 can go to both tanks 1 and 2 and return fuel from engine 2 can go to both tanks 1 and 2. After that, return fuel goes up to the other 2 valves at the top. If one of them is horizontal then the return fuel cannot go to that tank. In that case, if the crossover valve is horizontal then fuel can still go to the other tank. If the crossover valve is vertical then you can have pressure build up and a big issue. Correct? I have an SC35 and last weekend I saw one tank level going from 50% to 10% and the other from 50% to 80% but I am suspecting there is an issue with the sensors reading the fuel levels rather than the flow of the return fuel. Because such imbalance could only happen if the return crossover valve is horizontal and one of the return valves above is also horizontal, which would mean return fuel from both engines would go to one tank, which then gets filled up over time while the other gets depleted. Am I correct? Thanks in advance.

Quite a thread resurrection

If the valve at the bottom is horizontal then the fuel coming from tank 1 can go to engines 1 and 2 and the fuel coming from tank 2 can go to engines 1 and 2. Does that rebalance the tanks’ fuel levels?

YES - probably !

The above is unusual in that the balance pipe is normally between the tanks but this is after the filters. So it is very much an alternative feed as opposed to a balancing pipe. It lets you select to run off one tank should the other have a leak / be contaminated.

It will very likely balance the fuel system over time but the filter are " in the way" which will slow it down.


Imagine I have tank 1 at 10% and tank 2 at 90%. Question 1. Do they eventually both go to 50% if that valve is horizontal?

See above

Then above that valve we see the return system. Fuel is going up in this case. So coming from engines 1 and 2 it hits the return crossover valve first. If that’s horizontal then return fuel from engine 1 can go to both tanks 1 and 2 and return fuel from engine 2 can go to both tanks 1 and 2.

YES but assuming both are running at the same approx speed the pressure will break similar so if open of closed would not make much odds.


After that, return fuel goes up to the other 2 valves at the top. If one of them is horizontal then the return fuel cannot go to that tank. In that
case, if the crossover valve is horizontal then fuel can still go to the other tank

YES - but be VERY careful that the return fuel always has a path to a tank. If not you will end up with a ruptured fuel line or worse.


. If the crossover valve is vertical then you can have pressure build up and a big issue. Correct?

Yes- as above both engines must have a clear path to a return tank.

I have an SC35 and last weekend I saw one tank level going from 50% to 10% and the other from 50% to 80% but I am suspecting there is an issue with the sensors reading the fuel levels rather than the flow of the return fuel. Because such imbalance could only happen if the return crossover valve is horizontal and one of the return valves above is also horizontal, which would mean return fuel from both engines would go to one tank, which then gets filled up over time while the other gets depleted. Am I correct?

Yes - the return fuel rate is far higher than the engines consumption rate so messing about with return fuel can move a lot of fuel very rapidly - even to the extent of over flowing as tank.


The only reason to ever play with return fuel would be

1. a need to correct a large fuel imbalance ( feedl from both return to one)

2. a contaminated / leaky tank that you were isolating. ( feed from one - return to same tank


In normal operation leave as above. My boat has a tank to tank crossfeed which is always open which is useful as I fill mostly to one tank and also the gen runs 24/7 and the fuel stays in balance by itself.

If you have significant fuel movements as you describe you need to understand why. If it is a return full issue one tank can and will overflow and the other will draw air and stop. You want neither.

 

[wonkywinch]

re: leaving engines etc to the experts - if you plan on going out to sea, it's definitely worth doing an engine maintenance training course. Boats usually break down because of a fuel issue or a cooling issue. Its well worth your while learning how to deal with it, and will make your boating life much easier and happier (and cheaper ;-))

Couldn't agree more. I'd also recommend doing it with Sea Start as it's much more of a practical real world course rather than blindly following the RYA syallabus.

I commented on my course "do you think 90% of breakdowns are due to poor maintenance?". They replied, "no, 90% are caused by lack of understanding of basic principles, fuel, air, ignition".

PS only just spotted date of OP!