Diesel Fuel Tank Design
Tanks, Tests, Connections and Baffles
By Steve D’Antonio
Fiberglass And Alcohol
Fiberglass has gained some attention in the boating press in the last year or so for its unfortunate susceptibility to ethanol, an additive that’s now used in most gasoline. Ethanol is an alcohol that will attack and molecularly disassemble most ordinary fiberglass and even epoxy resins. Owners of gasoline–powered boats that are equipped with fiberglass tanks are in the unenviable position of having no choice but to replace their tanks with either metallic or alcohol–resistant fiberglass tanks. The latter are being manufactured by only a handful of fabricators.
The good news is that diesel fuel does not currently contain alcohol, so it’s not an issue for most boaters. However, there are two possible concerns for fiberglass as a tank material. The first is an alcohol–spiked diesel fuel, referred to as “E diesel,” which contains up to 15 percent ethanol. Although it’s not approved for use by any production marine diesel engine manufacturer, because of its status as a partially renewable fuel, there are plans afoot to implement wider acceptance. At the moment, E diesel is not widely used because several problems are associated with it: lower lubricity, which can result in engine damage, and a lower flashpoint (62°F versus diesel’s 130°F or higher). Ethanol used as an additive in E diesel will presumably have the same detrimental effect on fiberglass that ethanol–laced gasoline does.
The other concern is biodiesel, a diesel fuel manufactured from soybeans or other plants as well as waste animal fats. Alcohol is used in the biodiesel refining process, and thus there are some concerns that it could affect fiberglass, particularly in its not commonly available B100 or 100 percent biodiesel form. Biodiesel has already been proven to degrade some rubber hoses and gaskets; however, there are no widespread reports of its deleterious effect on fiberglass. The consumer advocacy group BoatU.S. is currently performing tests on fiberglass tanks to determine how they might be affected by both E diesel and biodiesel. We’ll keep you informed of the results as they are made available.
Can Your Tank Handle The Pressure?
It would be an understatement to say that leaky fuel tanks are undesirable. Therefore, it’s well worth the effort to ensure that your tanks are leak free both before and after they’ve been installed. Reputable, conscientious tank fabricators will pressure–test fuel tanks after they’ve been built. Then, the boatbuilder or boatyard should pressure–test the tank before it’s installed and once again after it’s been installed and plumbed (discovering that a tank has a leaky weld after it has been installed is less than ideal). With this approach, it’s unlikely that a leak that is a result of a manufacturing or installation flaw will go unnoticed. In my experience, 25 percent of new tanks fail their initial pre–installation pressure test. To be fair, in many cases the leaks are found around inspection port gaskets that simply require re–torquing. In some cases, however, welds or plumbing fittings show outright evidence of leaks.
The leak test procedure calls for applying a minimum of 3psi (or 1.5 times the maximum hydrostatic head to which the tank may be subject while in service, whichever is greater) for a minimum of five minutes for tanks of 50 gallons or less, with an additional minute being added for each increment of 10 gallons above 50 gallons. Tanks that are larger than 500 gallons must be tested for 50 minutes, with a half minute being added for each increment of 10 gallons over the initial 500 gallons. The test involves observing a pressure gauge; however, other means must be used to check for leaks. A soapy water solution should be sprayed over all tank joints, fittings, and welds. (Deck fills and vents are exempt from this portion of the test. However, in my opinion, if the tank is being tested in situ, these fittings should be properly tightened, capped, or plugged so that their gaskets, O–rings, and hose runs can also be tested). Ordinary diluted dishwashing detergent in a spray applicator can be used for this test. Bubbles indicate the presence of a leak. Avoid any solution that contains ammonia, as it can be harmful to brass components such as those commonly used in fuel systems.
Tank Connections
In an upcoming article, I’ll discuss fuel system plumbing in detail. For the purposes of this discussion on tanks, I’ll mention a few of the fittings that are confined to the tank. There are two primary ways to connect hoses to tanks. One involves permanently welding short pipe or tube sections to the tank, over which hoses are slid and clamped in place. This is a neat, easy approach that makes the tank turnkey for the builder; all he or she has to do is make the hose connections, and the tank is ready to go.
But there are drawbacks to this approach. Primarily, it means that the installer is stuck with the hose size and orientation in which the fittings have been installed. If a 90–degree vent fitting faces inboard instead of outboard, there’s little that can be done. The hose will have to be longer, and it may protrude into spaces where it’s not wanted in order to complete the run. Additionally, theses types of fittings do not endure stress very well. Fuel hoses are thick, stiff, and sometimes wire reinforced. They can heavily stress the tank fittings. If overstressed, welded tank tube fittings often crack or cause the tank to crack next to where they are attached. Once that happens, repairs, if possible at all, are difficult and often expensive (welding fuel tanks that are already in service requires “gas freeing” and other special precautions).
The alternative to permanent tank fittings calls for the installation of a component known as a welding boss. This is simply a female–threaded and flanged pipefitting that is made of the same material as the tank (aluminum, steel, etc.) and is specifically designed for adapting pipefittings to tanks. Once welded to the tank, it provides infinite attachment possibilities for a variety of pipe shapes, diameters, and orientations. Additionally, the welding boss serves to reinforce the tank adjacent to the fitting, reducing the likelihood of failure as a result of stress.
Baffles
Baffles are nothing more than vertical walls or dividers (although horizontal baffles may be used as well) that are installed inside fuel tanks when they are built. They serve several purposes. Primarily, they prevent fuel from sloshing around in the tank. Sloshing fuel is undesirable because it can, as a result of its free surface effect, degrade the vessel’s stability. (It’s worth noting that this cannot occur if the tank is full.) A large quantity of fuel moving across a partially filled tank in the direction of roll will increase the degree of list. When the vessel rolls back (if it rolls back), the fuel will move across the tank once again, increasing the list in that direction, and so on. In extreme cases, this could lead to a rolling oscillation and eventual capsize. (An example of this is detailed in the book The Serpent’s Coil by Farley Mowat. In the case described in the book, the free surface was the mud and gravel used as ballast in the hold of a former Liberty ship.)
Additionally, fuel sloshing around in a tank while the vessel is under way, particularly in unsettled weather, can lead to air entrainment and eventual engine stoppage as a result of air reaching the fuel injection system. Or, if the tank is nearly empty (admittedly less than ideal for operating in rough weather), the bottom of the pickup tube may become exposed, allowing air to be drawn into the fuel supply and ultimately into the engine. Baffles prevent or reduce all of the undesirable effects of fuel movement within the tank. Finally, baffles can be used to add support and rigidity, particularly for larger capacity tanks or for tanks that cannot be fully supported across their bottoms.
While baffles are not a requirement, ABYC guidelines do make note of them and offer several suggestions for their design. Baffles should be strongly considered for use in any tank that is over 30 gallons or over 30 inches long and especially for tanks that are mounted athwartships, in which moving fuel will have the most effect on a vessel’s roll. No tank is too small to benefit from the strength, rigidity, and fuel stability afforded by baffling. If baffles are installed, their total open area should not exceed 30 percent of the tank’s cross section in the plane of the baffle, and the baffle openings should be positioned such that fuel flow across the bottom of the tank and vapor flow across the top of the tank are not restricted. Essentially, the baffles should slow the fuel down, but they shouldn’t stop its flow or the free movement of vapor within the tank. Traditionally, baffles are placed 22 inches apart.