Engine Compartment Blower - do you use it?

[Tedd]

I think there are some incorrect assumptions in @dlochner 's analysis. Here's what I get. I don't know if the 1 percent threshold @dlochner quoted is on a mass basis or a volume (i.e., molar) basis, so I've calculated both.

If the spec is on a mass basis I get 9.8 ml (0.0026 gallons) of diesel fuel vapourized to reach 1 percent in the volume that @dlochner estimated. If the spec is on a volume (molar) basis I get 68 ml (0.018 gallons).

I'll publish the details of the calculation if anyone's interested.

 

[Ralph Johnstone]

I'll publish the details of the calculation if anyone's interested.

Very interested. Yes, please do as it takes me waaaaaaaaaaaaaaaay back .

 

[Tedd]

My calculation is attached. I think I have this mostly right but if you think I've missed something important please let me know.

 

[dlochner]

I think there are some incorrect assumptions in @dlochner 's analysis. Here's what I get. I don't know if the 1 percent threshold @dlochner quoted is on a mass basis or a volume (i.e., molar) basis, so I've calculated both.

If the spec is on a mass basis I get 9.8 ml (0.0026 gallons) of diesel fuel vapourized to reach 1 percent in the volume that @dlochner estimated. If the spec is on a volume (molar) basis I get 68 ml (0.018 gallons).

I'll publish the details of the calculation if anyone's interested.

It was volume per the OSHA website. I have long since forgotten how to calculate volumes based on molar numbers. I do remember Avogradro came up with an important number (1.08x10^26?).

I thought my number, 1.76 gallons was high, that's why the disclaimers. On the other hand, I have had more than .018 gallons (2.3 ounces) of diesel sprayed around the engine compartment and I'm still here to talk about it. It was during those episodes of tracing fuel leaks I learned about the propane detectors.

 

[Tedd]

On the other hand, I have had more than .018 gallons (2.3 ounces) of diesel sprayed around the engine compartment and I'm still here to talk about it.

Great example of how empiricism can be dangerous! It's not hard to think of many reasons why that wouldn't result in reaching the OSHA threshold. How much of the fuel actually vapourized? What is the air exchange rate in the enclosure, compared to the rate of vapourization? What are the other relevant conditions (bulk temperature and point-source temperatures, for example) that weren't being met at the time? Air flow patterns relative to ignition sources?

If I can wax philosophical for minute, one of the down sides of our scientistic culture is an over emphasis on empirical evidence. Empirical evidence in the absence of an appropriate theory is basically useless.

 

[dlochner]

Great example of how empiricism can be dangerous! It's not hard to think of many reasons why that wouldn't result in reaching the OSHA threshold. How much of the fuel actually vapourized? What is the air exchange rate in the enclosure, compared to the rate of vapourization? What are the other relevant conditions (bulk temperature and point-source temperatures, for example) that weren't being met at the time? Air flow patterns relative to ignition sources?

If I can wax philosophical for minute, one of the down sides of our scientistic culture is an over emphasis on empirical evidence. Empirical evidence in the absence of an appropriate theory is basically useless.

Yes, and thus the several cautionary notes in my original post.

The part of the question that intrigued me the most was the effect of air exchange. If the air is exchanged every minute then it will be very difficult to achieve the LEL in the absence of an efficient vaporizing mechanism.

The atmosphere in an engine room is very dynamic in contrast to the atmosphere in an enclosed space like a fuel tank, making this conversation considerably more difficult. In an earlier post I mentioned the other combustable fluids we keep on board. It was my goal to put diesel fumes in context, something to be concerned about, yet lower on the list than gas for the dinghy, propane or alcohol for the stove, and the many solvents and lubricants we may have on board and which may be stored in a more contained space.

I was hoping to generate a more informed discussion, thank you @Tedd and @Ralph Johnstone for stepping up.

 

[Rick Webb]

We tended to run ours a half hour before docking and an hour after. Helps to get the heat out of the boat. When you shut down the engine the temperature in the engine compartment actually goes up for a while before it starts cooling down.

 

[Tedd]

It was my goal to put diesel fumes in context, something to be concerned about, yet lower on the list than gas for the dinghy, propane or alcohol for the stove, and the many solvents and lubricants we may have on board and which may be stored in a more contained space.

I'm inclined to agree. According to this source, fumes of any kind aren't in the top six most likely causes of a boat fire. But it may be that fumes are a contributing factor in some of the six, such as engine wiring harnesses (2), battery cables (3), or engine overheating (5).

 

[MikeHoncho]

@MikeHoncho Thanks for the reminding us that diesel is not as safe as we might to believe. It piqued my interest so I read up on the explosive properties of diesel. The Lower Explosive Limit (LEL) of diesel is 1% and the upper level is 6% according to OSHA. Below 1% by volume diesel will not ignite and above 6% it will not ignite.

Curious about what 1% would look like in my engine compartment, I calculated the rough volume at 41,472 in3.(about 2'x4'x3' = 24 ft3). A fluid gallon has 231 cubic inches. If my math is correct (and sometimes it is off) it would take about 1.79 gallons vaporized to reach a 1% level. (414.72/231= 1.79). That is quite bit of diesel.

Meanwhile, with the diesel running at cruising speed (using @Ralph Johnstone's calculation) the engine is consuming 31.42 ft3 of air per minute. In my engine compartment the air is being changed every 45 seconds (24ft3/31.42ft3 = .76).

If my logic is correct the engine would have to vaporize more than 1.76 gallons per minute to reach a 1% level. That's a lot fuel to vaporize. In order to attain that rate the injector pump would have to fail and spray diesel on the exhaust manifold or a very hot alternator. And then it would need an ignition source.

I think a fire or explosion from diesel inside our small engine compartments with small diesels is a low probability event, however that does not negate the possibility. Promptly dealing with and fuel leaks and a clean bilge and engine compartment might well be the best preventative measure. Incidentally, propane gas leak sensors are also sensitive to diesel vapors and will sound an alarm.

The more likely scenario for diesel explosion might be while fueling a near empty tank on a hot day. Vapors in the tank could reach explosive levels and a spark from static electricity generated while fueling could trigger an explosion. Boats with twin diesels of a larger hp might be more problematic as one diesel could spray fuel onto the other causing the ignition.

Another issue worth considering is how a small fire onboard could ignite the diesel and cause the fire to rapidly expand.

Note, the calculations here are rough back of the napkin calculations and not meant to be definitive. The goal is to put the issue Mike raised in context, yes it is a concern, no it is not as much of a concern as gasoline, propane, and other flammables onboard. Almost all of the organic solvents we use, Acetone, mineral spirits, alcohol, etc. are combustible and explosive at some level. Some are safer than others.

OSHA might show it at 1% but most other resources shows it at .7% in atmosphere. Your use of liquid state of matter for vapor is in error. Take your cubic inches of the total space and .7% of that is all you need to get it to the flammable range. I don’t know the physics of how much liquid would be needed to generate that .7 but I can tell you it’s not all about volume. It’s more about surface area as long as that surface area has sufficient volume. A comparison is gasoline has a flashpoint of -44f while diesels FP is around 120f. Gasolines LEL is 1.4% while diesels is .7.
Apply this data to your extrapolation of needing 1.7 gallons of diesel to get to its LEL. Take gasoline. Imagine a scenario were you are vaporizing 1.7 gallons of gasoline. How explosive would that be? Right very, diesel would be twice that at the same volume as long as it is vaporizing which requires a temp of 120 . Now it’s going to vaporize much slower and quiet possibly stay under the LEL due to dissipation. This is where the vent comes into play.

 

[MikeHoncho]

I'm inclined to agree. According to this source, fumes of any kind aren't in the top six most likely causes of a boat fire. But it may be that fumes are a contributing factor in some of the six, such as engine wiring harnesses (2), battery cables (3), or engine overheating (5).

My point was that a vent fan is in almost all cases, useless (for reasons of flammability) unless underway were you are generating enough heat to get the fuel to its flashpoint. Once there a fan could have some applicability. There are to many variables to vaporizing liquids to try to pre-plan if you can keep it under its LEL through passive ventilation like engine consumption. That being said, I don’t have a vent fan. I depend on passive ventilation ( how the boat was designed) and a high level of maintenance and inspection which is what most people with my vintage of boats do I think.

 

[MikeHoncho]

I think there are some incorrect assumptions in @dlochner 's analysis. Here's what I get. I don't know if the 1 percent threshold @dlochner quoted is on a mass basis or a volume (i.e., molar) basis, so I've calculated both.

If the spec is on a mass basis I get 9.8 ml (0.0026 gallons) of diesel fuel vapourized to reach 1 percent in the volume that @dlochner estimated. If the spec is on a volume (molar) basis I get 68 ml (0.018 gallons).

I'll publish the details of the calculation if anyone's interested.

I’m interested.

 

[Tedd]

Following on from @MikeHoncho 's comment, I was a bit pedantic above but for a reason. I've found that a lot of people have difficulty assessing situations that involve instability. (To take two examples relevant to boating: ignition of a fuel-air mixture; and trailer sway.) We're accustomed to risks that develop in a progressive way, with the danger increasing in rough proportion to the changing parameters. The risk of a snake bite is fairly proportional to the density of snakes in the area. But stability risks aren't like that. A tiny, possibly imperceptible or even immeasurable change in conditions can tip the balance into disaster. So arguments of the "it's never been a problem before" kind simply don't apply. You want to know where the threshold is and how close you are to or, at the very least, you want to know that you've taken appropriately prudent steps to be far away from it, factoring in your ignorance. (Meaning ignorance in a technical sense, not a pejorative sense.)

 

[dlochner]

In a broad sense we are talking about risk management. How much risk are we willing to take and do we know the factors affecting that risk. We often hear statements like "In XXX years of sailing I've never seen that problem, I'm not worried about it" because you haven't seen it doesn't mean it can't happen.

On the other hand, some risks like the one we're talking about here diesel vapors, bear learning about and understanding them so we know the risk involved. A similar issue is hydrogen off gassing from batteries while being charged. How explosive is hydrogen, very. What would take to sink a boat or start a fire from hydrogen off gassing? A lot unless the batteries are installed in a tightly filled chamber next to leaking gas tank.

BTW, I appreciate the discussion and I don't mind having my thinking clarified.

 

[Tedd]

A similar issue is hydrogen off gassing from batteries while being charged. How explosive is hydrogen, very. What would take to sink a boat or start a fire from hydrogen off gassing?

I saw a battery explode from hydrogen once, in a shop I worked in. Definitely not something you want to happen on your boat. The explosion itself is quite impressive, and the spray of battery acid makes a nasty mess, even if nobody is near by.

In this particular case the source of ignition was an arc welder--nobody warned the welder that a battery was being charged in that area. But you might get a sufficient ignition source from a short circuit, or even a loose connection, in the right circumstances.

 

[FastOlson]

I installed a compartment suction fan after installing an external alternator regulator. The alternator was running much hotter as it was working much harder with the external reg. and needed cooler air inside the engine housing.

View attachment 206875

Bingo! We have a winner!

 

[jssailem]

When you install a "compartment suction fan" you are evacuating the air from the space.
Did you also install a vent to replace the air.
Your diesel can not run in a vacuum.

 

[Ralph Johnstone]

Bingo! We have a winner!

What did I win ? What did I win ? I want to win something .

 

[Scott T-Bird]

I'm starting to use mine. I never had a blower on our Starwind. The engine compartment was open to a lot of space under the cockpit. So I was not in the habit of running a blower as I was with my inboard gas engine ski boat before starting. I didn't even notice the blower switch on our Catalina for awhile. The compartment is fairly confined. I moved the regulator outside the compartment and I started using the blower and now leave it on for a while after shutting the engine down. I think it has made a positive difference.

 

[Ralph Johnstone]

When you install a "compartment suction fan" you are evacuating the air from the space.
Did you also install a vent to replace the air.
Your diesel can not run in a vacuum.

As mentioned in post #11, the air comes from the bilge area which is in communication with the most aft section of the boat which is wide open to air ingress.

Air can also be pulled from the forward section of the hull between the stringers.

 

[jaysweet]

I use mine every now and then, but almost exclusively after shutting down the boat, to cool the engine compartment a bit. And usually, this is at the dock, where I have shore power.

Yeah: I know. This doesn't make much sense. Maybe I just feel obliged to use it every once in a while to prove to myself that it's useful. Using the blower while the engine is running for hours would make more sense. Or after shutting down, at anchor, when it's warm out, so the cabin isn't heated up as much.