Fuel tanks filter and pump setup

[geem]

I want to feed the engine from a tank that I can drain easily, and stick my arm into and reach all the corners to give it a good wipe from time to time. I feel like filters are a vital part of a full hygiene system, but not all of it. The idea of little microorganisms doing filthy things obscured behind baffles kind of gives me the willies.

But you may get diesel bug in your main tank. You need to be able to clean it. Your day tank will be fed from your main tank. It's all one system and needs to all function not just a day tank.
My 500L single tank has a 600mm diameter access lid. I can climb in. It has a sump that collects any dirt. A 1" access port in the lid allows the insertion of a polishing kit to vacuum the sump of dirt, diesel bug or water. I do this every few months. Having a day tank following this tank would achieve little

 

[noelex]

Most fuel problems on yachts show up during rough conditions (often the worst time) when agitation has stirred up deposits at the bottom of the main tank.

One of the main advantages of a day tank is that normally all the fuel the engine will consume is transferred in settled conditions before any offshore sail. In addition, the fuel passes through a very fine (typically 2 micron) filter even before entering the tank. The day tank is small, designed to be easily cleaned, and can have a large, simply drained sump at the bottom to remove any water and debris. This just requires a turn of a tap. These factors keep the day tank very clean. In turn, this helps ensure a clean fuel delivery to the engine even in rough conditions.

The day tank is typically located above the engine, ensuring the positive pressure to the fuel system. This means the engine is not dependent on the lift pump to suck up fuel from a lower tank. Bleeding and changing filters is also easier.

Many boats manage without a day tank. There are many important steps helping to achieve a clean and reliable fuel supply. Like Geem suggests a polishing system is a very helpful addition, but if like the OP you have the opportunity to fit a day tank it is a worthwhile addition.

 

[geem]

Most fuel problems on yachts show up during rough conditions (often the worst time) when agitation has stirred up deposits at the bottom of the main tank.

One of the main advantages of a day tank is that normally all the fuel the engine will consume is transferred in settled conditions before any offshore sail. In addition, the fuel passes through a very fine (typically 2 micron) filter even before entering the tank. The day tank is small, designed to be easily cleaned, and can have a large, simply drained sump at the bottom to remove any water and debris. This just requires a turn of a tap. These factors keep the day tank very clean. In turn, this helps ensure a clean fuel delivery to the engine even in rough conditions.

The day tank is typically located above the engine, ensuring the positive pressure to the fuel system. This means the engine is not dependent on the lift pump to suck up fuel from a lower tank. Bleeding and changing filters is also easier.

Many boats manage without a day tank. There are many important steps helping to achieve a clean and reliable fuel supply. Like Geem suggests a polishing system is a very helpful addition, but if like the OP you have the opportunity to fit a day tank it is a worthwhile addition.

The OP seems to be replacing his whole fuel tank system. He has relatively small fuel storage requirements. Something like 360L in total. If he designed the main tank to be accessible with a sump, polishing system and good filters, he would not need the added complexity of a day tank

 

[Dockhead]

This is interesting. So, I was trying to match pump speed to filter speed; I don’t know what I’m imagining about a powerful pump, sucking the filter paper out of the filter or collapsing the canister or turning the glass bowl inside out or something. You reckon that the filter can take a powerful suction through it, without stressing it and without compromising its filtering, and that it would do its job properly at its rated rate rather than being forced into fast poor filtering?

I was thinking 30 microns between big tank and day tank, 10 between day tank and engine, and the manufacturer’s recommended secondary filter on the engine. I would have expected a 2 micron filter at the first pass to get blocked up quickly and easily, and maybe be more effort than it’s worth. I’m not expecting pristine diesel in the day tank, just good enough that the primary and secondary filters can happily feed the engine. I’m interested in other schools of thought though!

You're right that pump and filter should be matched. A Racor 500 (and you can buy cheap Chinese knockoffs) will flow 227l/h. The Facet pumps flow about 90. I would pick a combination which would allow me to fill the day tank in an amount of time I will have patience to be nearby watching, if I were you.

I would definitely advise you to use the finest filter you can get to fill the day tank. Why would you use a coarser filter there? You have the luxury of stopping and changing filters when you're just filling the day tank, which you may not have when you're running the engine. So you've got nothing to lose by using your finest filter there. If it clogs, then you need to polish the fuel or clean the main tank out.

I don't have a day tank, unfortunately, and I run a double Racor 500 with vacuum gauge and changeover valve. I keep a 2 micron filter in one side and a 30 in the other, and normally run the 2. The plan is to switch to the 30 if the vacuum ever gets too high, change out the 2, and then switch back. That will load the secondary filter with whatever debris was not getting through the 2, but only briefly. I have fortunately never once clogged the 2 micron filter (in 15 years and 4000 hours with this setup on this boat), but you can't be too careful. I have had fuel problems on other boats, and it can be a nightmare, and dangerous.

A day tank is a very, very useful thing, for all the reasons which Noelex has laid out. Your idea about this is right. A polishing system is also highly worthwhile, and not expensive or difficult to install since you're doing all this other stuff.

 

[geem]

You're right that pump and filter should be matched. A Racor 500 (and you can buy cheap Chinese knockoffs) will flow 227l/h. The Facet pumps flow about 90. I would pick a combination which would allow me to fill the day tank in an amount of time I will have patience to be nearby watching, if I were you.

I would definitely advise you to use the finest filter you can get to fill the day tank. Why would you use a coarser filter there? You have the luxury of stopping and changing filters when you're just filling the day tank, which you may not have when you're running the engine. So you've got nothing to lose by using your finest filter there. If it clogs, then you need to polish the fuel or clean the main tank out.

I don't have a day tank, unfortunately, and I run a double Racor 500 with vacuum gauge and changeover valve. I keep a 2 micron filter in one side and a 30 in the other, and normally run the 2. The plan is to switch to the 30 if the vacuum ever gets too high, change out the 2, and then switch back. That will load the secondary filter with whatever debris was not getting through the 2, but only briefly. I have fortunately never once clogged the 2 micron filter (in 15 years and 4000 hours with this setup on this boat), but you can't be too careful. I have had fuel problems on other boats, and it can be a nightmare, and dangerous.

A day tank is a very, very useful thing, for all the reasons which Noelex has laid out. Your idea about this is right. A polishing system is also highly worthwhile, and not expensive or difficult to install since you're doing all this other stuff.

I can't agree with this. In my career I have designed diesel fuel systems for V16 generators for data centres where we stored 20,000L in the main tank and had 1000L day tanks on each generator. Those day tanks were far larger than the main storage tank proposed by the OP. Mimicking a fuel system from a massive diesel generator serves no purpose in my opinion. It just adds complexity with minimal benefits. It is possible, given a blank sheet of paper, to design a system that is simpler and more robust, with less pipework. Such a system would be designed to manage the whole fuel storage allocation to the same standards as is suggested here for the day tank.
If you intend to cruise long distance, all you fuel in all tanks needs to be kept stored to the highest standard. This means water, bug and dirt free. How are you achieving this by installing a day tank? If you are achieving fuel storage to this high standard, why do you need a day tank?