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ContinuousWave: Whaler Performance
Evaporation of Gasoline From Fuel Tank
|Author||Topic: Evaporation of Gasoline From Fuel Tank|
posted 04-29-2009 04:55 PM ET (US)
I use the boat about once a month and I think I get a lot of evaporation [of gasoline from the fuel tank]. How can I prevent this?
posted 04-29-2009 08:26 PM ET (US)
This is a very interesting question. Let me restate it:
What is the rate of evaporation for gasoline?
With my recall of Chemistry somewhat dimmed by the 41-years that have elapsed since my last study of that subject, I vaguely remember that above any liquid there is a vapor pressure. I suspect that the vapor pressure varies with temperature. To analyze this problem we have to pick a temperature. Let's say it is 25-degrees-C, or about 80-degrees-F.
Now we need to find the vapor pressure of gasoline at 25-degrees-C. Has anyone this information?
posted 04-30-2009 03:47 AM ET (US)
I gather you have an over the side venting system where you tie up plays a part in how your boat is effected heat and lots of wave action can cause gas loss by lots of sloshing and pressurising your fuel tank you could try calmer water or a shaded area or if you can get to the vent hose a simple gate valve would work
posted 04-30-2009 07:54 AM ET (US)
That is a very interesting observation from deepwater. I had not considered the effect of a lot of motion and sloshing on the rate of evaporation.
Back to Chemistry. Above the liquid gasoline there will be a vapor pressure of gasoline vapor that has evaporated. If the tank were sealed, once this vapor pressure was reached the evaporation would stop. Since most fuel tanks on small boats are vented to the atmosphere, it is possible for the vapor to escape. We know that gasoline vapor is heavier than air, so it does not seem reasonable that air could come down the vent hose and displace the gasoline vapor in the space above the liquid in the fuel tank. Therefore, in theory, it seems like once the gasoline evaporates and establishes a vapor pressure above the liquid, the process should be in an equilibrium, and no more gasoline should evaporate. There could be some exchange of gasoline vapor with the atmosphere at the vent, but given the small diameter of the typical vent hose, I would not expect that a large about of gasoline vapor would be lost via the vent.
A way to reduce loss from evaporation would be to reduce the volume of the space above the liquid in the tank. You could do this by keeping the tank full.
If there is a very significant loss of fuel from an unattended tank, the cause may be theft, not evaporation.
posted 04-30-2009 11:06 AM ET (US)
I seem to find that ethanol gas evaporates more quickly than gas from the past.
Any validity to that?
posted 04-30-2009 11:17 AM ET (US)
Yes, I see that E10 has a higher vapor pressure than normal gasoline in some listings of vapor pressure.
posted 05-01-2009 02:32 PM ET (US)
I was hoping that this discussion about evaporation rate of gasoline would elicit some replies from readers with an advanced understanding of physical chemistry and hydrocarbon fuels. Having made my hopes public, I will now pray for a response from a learned reader.
How can we calculate the rate at which gasoline will evaporate from a vented fuel tank?
posted 05-01-2009 05:56 PM ET (US)
This should answer you question. You just need to replace the vapor pressure of water with gasoline.
The gasoline vapors would push out of the tank as the temperature when up. If you had more space above the liquid gas I would assume you would keep more of the vapors in the tank and it would reduce the amount of loss gasoline, though very slightly.
posted 05-01-2009 05:57 PM ET (US)
The reason the gas vapors would push more vapors out of the tank as temperature rises is because the pressure in the tank will increase as temperature increases, due to evaporation, and would increase the pressure difference between the tank and the atmosphere.
posted 05-01-2009 09:40 PM ET (US)
There is a simple article located here - http://www.creditinfocenter.com/wordpress/2008/07/24/ increase-mileage-gas-tank-full-or-half-empty/
The related paper written for/by the EPA located here - http://www.epa.gov/ttn/chief/ap42/ch07/final/c07s01.pdf
Sorry, but it doesn't raise my curiosity enough to read the 123 page document.
Basically, I burn more gas by the time I leave my slip then the rate of evaporation for my tank during the entire season.
posted 05-01-2009 11:07 PM ET (US)
Yes, there is much more burnt fuel in while using the boat. I don't think evaporation is worth worrying about, unless you are Al Gore.
posted 05-02-2009 07:58 AM ET (US)
Reid Vapor pressure(RVP) is used to measure the evaporative pressure rate of gasolines. In the summer we use what is called low RVP. Our vehicles would not run well in the winter with these low RVP gasolines so the requirements are lifted in the fall.
The gasoline suppliers have to convert all of their Tanks to low RVP before June 1.
Low RVP fuel was mandated to reduce ozone depletion and is usually more expensive than conventional gas.
Low RVP gas used to be mandated in more populated areas, I'm not sure this is true anymore.
posted 05-02-2009 08:05 AM ET (US)
i consider myself in no way a learned person,,just a person that sees so ill just stay ahead by saying thanks jimh,,what i suggested was just an observation on my part,,i have seen gas sloshing in a tank with the vent screw open and vapors coming out with sputters of gas and running down the sides,,im sure somewhere out there under laboratory controlled conditions is an answer
posted 05-02-2009 08:21 AM ET (US)
Deepwater--I think your observation was cogent. When the fuel sloshes around in the tank it could expel some of the gasoline vapor out of the vent and draw in fresh air. As the air in the tank above the fuel changes, more gasoline has to evaporate to re-establish the vapor pressure equilibrium.
posted 05-02-2009 11:40 AM ET (US)
Let's assume that average gasoline (octane or 2-methyl heptane) has the chemical formula of c8H18 and a molecular weight of 114.2 g/mol. The vapor pressure is 10 mm Hg at 68 deg F.
10 mm Hg/ 760 mm Hg = .01316 atm.
The rate at which the outside air gets into the tank and the surface area of the liquid gasoline have an effect on the rate of evaporation and if the tank is just sitting there at equillibrium, the volume of outside air that might come in through a small vent hole is probably minimal.
posted 05-02-2009 11:42 AM ET (US)
In the references cited above regarding evaporation there seems to be confusion and lack of clarity if the tanks being discussed are truly openly vented to the atmosphere. Most small boat fuel tanks are openly vented to the atmosphere, which is very distinct from most automobile tanks or most fuel storage tanks which are not vented to the atmosphere.
If you pour gasoline into a pan and put it out into the sun, it will evaporate completely. A boat fuel tank is something akin to this, but with a much smaller surface exposed directly to the atmosphere. A boat fuel tank is typically vented with a rubber hose having an internal diameter of 0.5 inch connected to a vent with an opening of perhaps two holes with a 0.25-inch diameter. The hose is normally shaped into an inverted-U above the vent, making a sort of trap (intended to help prevent water from entering via the vent).
In a stable situation with no movement, gasoline vapors would rise from the tank liquid and settle above them. Since gasoline vapor is heavier than air, the air would remain above the vapors. This implies that gasoline vapors would not spontaneously be expelled from the vent. At the boundary between the gasoline vapor and the air some mixing might occur, and some gasoline vapor might migrate into the air and eventually to the vent for escape.
If you walk up to a boat that has been sitting motionless for a long time and put your nose to the fuel tank vent, I do not think you will get a strong smell of gasoline vapor. Generally you only get vapor escaping from the tank as the tank is filling, when the increasing volume of liquid in the tank forces the gasoline vapors out the vent.
posted 05-02-2009 01:25 PM ET (US)
I believe that there is a trend to extend regulations regarding the general prohibition of directly venting gasoline storage tanks to the atmosphere to small boats. I would not be surprised to see the EPA force small boat manufacturers to use non-vented fuel tanks.
In this regard I reproduce below a letter from the National Marine Manufacturers Association dated March 31, 2009:
Re: Guidance on the EPA Evaporative Emissions Regulations for Boat Fuel Systems (2nd Revision) March 31, 2009
This document is dated and designed to be updated as more questions arise and these questions are answered by EPA. As with any major rulemaking, it is often the case that EPA has to provide guidance and in some cases go back and publish technical amendments to correct discrepancies or provide better direction.
The following document is a first step in explaining this new boat builder requirement in a simple Question & Answer format. It has been reviewed by EPA. If you have any questions or comments please do not hesitate to contact me at firstname.lastname@example.org or 202-737-9757. Also, if you would like to review the rule more closely, which I recommend, it can be found at http://www.epa.gov/otaq/marinesi.htm
posted 05-02-2009 01:31 PM ET (US)
"We are also adopting new standards to control evaporative emissions for all vessels using marine spark-ignition engines. The new standards include requirements to control fuel tank permeation, fuel line permeation, and diurnal fuel tank vapor emissions, including provisions to ensure that refueling emissions do not increase."
posted 05-02-2009 01:33 PM ET (US)
From the NMMA letter mentioned above, more details are given:
4. Diurnal Controls and Carbon Canisters
What are diurnal controls?
Diurnal emissions are evaporative emissions that are released through the boat’s fuel tank vent due to the daily cycle of liquid fuel becoming fuel vapor during the daylight hours and condensing during the night.
EPA requires that the boat builder reduce diurnal emissions from the gasoline in the boat fuel tank. Boat builders will generally install either a carbon canister or a 1 psi pressure relief valve in line with the fuel tank vent. The marine canister manufacturer will provide the boat builder with the correct size canister specified for the volume of the fuel tank. There will be different canister size requirements for boats 26 feet and greater than for boats less than 26 feet. In addition boats with a beam greater than 8’6’’ will be treated in the same category as boats 26 feet and greater. These distinctions are based on the USF&W definition of a trailerable vs. non-trailerable boat. Boats that are non-trailerable would be more likely to be kept in the water and would have less diurnal temperature change, thus having less evaporative emissions and require less activated carbon and thus a smaller canister.
There will be several design changes and additional pieces of equipment that will need to be included in order to make the boat fuel system compatible with evaporative emission controls. It is critical to keep the carbon free of fuel and water and the canister will need to be self draining and accessible, Boat builders will need to work with the canister suppliers to insure proper installation of the canisters.
What are the dates when diurnal controls need to be installed in boats?
For permanently installed fuel tanks on boats, EPA has adopted a phase-in that begins July 31, 2011. In the period from July 31, 2011 through July 31, 2012, 50 percent of the U.S. market boats produced by each company must meet the diurnal standard and beginning August 1, 2012, all marine fuel tanks and boats must meet the diurnal emission standard. (except as noted below for small businesses) EPA defines a “company” as all boat brands produced under one corporation
For small business, small volume boat builders, EPA is allowing additional time for compliance so that boat builders can become familiar with installation of carbon canisters in their boats. To address this, EPA will allow small business boat builders to make 1200 boats without diurnal emission controls from July 31, 2011 until July 31, 2013. These allowances would be an alternative to the 50 percent two year phase-in. If a small business boat builder chooses this relief they must notify the EPA certification group that they plan to do this. A small business boat builder is defined as having less than 500 employees.
posted 05-02-2009 08:05 PM ET (US)
Ron--Many thanks for your reply.
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