I try to think of ways to use my real world knowledge to add things to the game. Last November at UCon, +Del Teigeler played in the Delving Deeper game I ran. His character poured his water down a tunnel that sloped gently downwards away from the party, then poured oil on it, which floated, then lit the oil. Del is a firefighter, so quickly used his actual knowledge to benefit the party.

A few years ago, I wrote an article about using the Rule of 9’s for hit placement, from my training as an EMT.

I decided to write about BLEVEs today. A Boiling Liquid Expanding Vapor Explosion is why pressure cookers have relief valves. Fill a pressure cooker with war oil and no relief valve, and you get a device that will cause a lot of damage. However, it has to receive constant heat until the vessel fails.  If you don’t want to read the linked Wikipedia article, see the video at the end of the article.

In a fantasy setting, these will require decent quality metal and the skills to craft them. They will have to have an airtight seal that will hold until the vessel fails. Poor quality metal and/or in adequate skills will lead to leaks that may cause a gout of fire on one end, but it won’t explode. Slightly better materials and/or skills could result in an explosion while those intending to use it are still in range. The fire has to be stoked and kept burning until failure.

The requirements for decent quality metal and adequate skills to manufacture would limit this to “higher” civilizations. for example, dwarves and gnomes could be expected to have such things. Some human nations may have them, as well as others with the adequate stuff. Goblins might try to use them, but have as much chance of hurting themselves as their victims.

Wizards might have them constructed and use heat and fire based spells to set them off quickly. This would depend on the GM’s ruling. A fireball with one or more “supplemental” fireballs  from these devices would be devastating in a battle.

These devices would work better as traps or “mines” to hold an enemy at bay while retreating. For example, a cluster of them could be placed in a building that is abandoned and burned to cover a getaway. A trap could cause a room to seal and a fire to light that heats such a device. If the party can’t extinguish the fire or get out of the room in time, they suffer the effects.

A dragon could have its minions construct these devices to be placed in all the narrow entrances to its lair. A blast of dragon’s breath would be sufficient to cause instant failure of the vessels.

The size of the vessels would indicate the area/volume that is affected and the damage.

The formula for the area of a sphere is: (4/3)  r³  Here is a link to a calculator from Google. It shows that a 20 foot radius (40 foot diameter), as from a fireball, has a volume of 33,510.32 cubic feet. A 10x10x10 room has a volume of 1,000 cubic feet. Thus a fireball will fill 33.5 ten foot sections of room/dungeon/underground.

Therefore, if your pressure vessels are spheres, you will need this formula to figure how much oil, or other substance, is in them. A 12 inch sphere (6 inch radius)  has a volume of 904.78 cubic inches or 0.52 cubic feet. A BLEVE would hurl heated vaporized and boiling oil plus shredded metal at least 100 feet, if not confined. (I don’t have any facts to back this up, but it seems reasonable. ) Within a close distance, in addition to burns from fire, one would have burns from boiling liquid.

Using water instead of oil will result in shrapnel, and scalding from steam and hot water.

If a metal with a melting point lower than that of the failure point of the pressure vessel is used, one has shrapnel and molten metal to deal with.

A 24 inch sphere (12 inch radius) has a volume of 7,238.23 cubic inches, or 4.19 cubic feet, basically 8 times the volume of a 6 inch sphere.

18 inches for the radius (3 feet diameter) equals a volume of 24,429.02 cubic inches is 1.77 cubic feet.  This is 27 times the volume of a 6 inch sphere (3 inch radius).

I built this spreadsheet so you can run the formula on different size increments. As you can see, a 60 inch sphere (5 feet, with 30 inch radius) is 1,000 times the volume of a 6 inch diameter sphere! If you plug in a different number in the yellow box (cell C2) the only thing that changes is the volume, all the other cells are steady. NOTE: The spreadsheet has been shared as read only, so you will need to copy it to make changes. I also left out the units, so you can plug in your numbers and get the results desired, just add units.

Damage would decrease with distance. For simplicity’s sake, let’s use 1 die of damage per 6″ of diameter within 100 feet. With half damage out to 100 feet.  Combine with a save for half, and one can easily take no damage at 100 feet. I would limit the size of a sphere to six feet in diameter. Time time and cost to construct larger and larger spheres makes it extremely impractical.

Here’s the Table:

(Note: Save for half damage.)

Diameter of Sphere	Cost in GP	Construction Time	Damage within 50’	Damage within 100’
6 Inches	1,000	1 week	1d8	Half
12 inches	8,000	2 month	2d8	Half
18 inches	27,000	6 months	3d8	Half
24 inches	64,000	1 year	4d8	Half
36 inches	125,000	1.5 years	5d8	Half
72 inches	216,000	2 years	6d8	Half

The type and number of dice used can easily be modified to suit your taste. Such as, d6 instead of d8.  One can easily modify this to have separate damage dice for shrapnel and the contents. For example, 1d6 for shrapnel and 1d6 for fire damage per 6″ of diameter.  If you want you could use different sized dice for each type of damage.

For more verisimilitude, pick the size of dice to role, such as a d12. Orient the d12 with the 1 up and the 3 facing you. Determine if this is facing the NPCs/monsters or the characters. Roll another d12 and determine which side on the first d12 is indicated. That would be the side that failed first. This would indicate the direction the contents go. The majority of the vessel would go in the opposite direction. If you roll 12, this indicates it fails on the lower right area.

I based the cost and time on how much bigger each size sphere is than the base 6 inch sphere. Since this is the OSR, you can adjust any part of this to suit your needs.

About my experience:

Way back in high school I was a Junior Firefighter. I took the EMT class in the Spring of my junior year of high school. When I turned 18, shortly after my senior year started, I received my EMT license and became a volunteer firefighter. I achieve firefighter II certification, via training at the station. I was not able to maintain licensing and certification when I went off to college.

Instead of getting the approval of the teacher who was on the board of the fire district to respond to fires and accidents, I got to make my own decision. I never had a call while at school my senior year. Once while still a junior firefighter, I got out of school to help with a major grass fire.  I don’t think they’d do that today.

I think this is a clip from one of the training films we had, or they re-used the narrator’s track. That guy narrated everything!

DISCLAIMER: This article is not an endorsement of making devices in the real world with the intent to harm others or otherwise break the law. It is merely a thought experiment using a snippet of my real world knowledge for inspiration to enhance a narrow aspect of fantasy role playing games (FRPGs). If you are not capable of understanding this, then you are not qualified to be part of any organization or agency overseeing such things.