7 replies to this topic

[StevenRS]

Just thought it was interesting, but how much pressure (psi) do you think Bp can generate? I mean, if you somehow ignited a completely enclosed container filled with loose meal, how much pressure do you think it would build up? I know that density, the ratio of ingredients, quality of ingredients, and maybe even the particle size would effect it, but it would be interesting to know how pressurized it would be the instant combustion finished.

I did a search, and I didn't find anything similar to this, and I am curious. Any thoughts?

[digger]

Just thought it was interesting, but how much pressure (psi) do you think Bp can generate? I mean, if you somehow ignited a completely enclosed container filled with loose meal, how much pressure do you think it would build up? I know that density, the ratio of ingredients, quality of ingredients, and maybe even the particle size would effect it, but it would be interesting to know how pressurized it would be the instant combustion finished.

I did a search, and I didn't find anything similar to this, and I am curious. Any thoughts?

It should be possible to calculate, as you know the volume of the vessel, the mass of the BP, hence you know the number of moles of gaseous products produced you could probably take a pretty good guess at the temperature. Hence assuming ideal gases you could calculate it using the formula PV=nRT to calculate the pressure assuming no deformation of your container.

[BrightStar]

It should be possible to calculate, as you know the volume of the vessel, the mass of the BP, hence you know the number of moles of gaseous products produced you could probably take a pretty good guess at the temperature.

Conkling in C.O.P. does it like this:

Assume that 200 milligrams (0.200 grams) of black powder is confined in a volume of 0. 1 milliliter. Black powder burns to produce approximately 50% gaseous products and 50% solids. Approximately 1.2 moles of permanent gas are produced per 100 grams of powder burned (the gases are mainly N2 , CO2 , and CO). Therefore, 0.200 grams should produce 0.0024 moles of gas, at a temperature near 2000 K. The expected pressure is:

P = (0.0024 mole) (0.0821 liter-atm / deg-mole)(2000deg) / (0.0001 liter)
= 3941 atmospheres

[Caramanos2000]

I think a bit under 10,000psi.

[digger]

Conkling in C.O.P. does it like this:

Assume that 200 milligrams (0.200 grams) of black powder is confined in a volume of 0. 1 milliliter. Black powder burns to produce approximately 50% gaseous products and 50% solids. Approximately 1.2 moles of permanent gas are produced per 100 grams of powder burned (the gases are mainly N2 , CO2 , and CO). Therefore, 0.200 grams should produce 0.0024 moles of gas, at a temperature near 2000 K. The expected pressure is:

P = (0.0024 mole) (0.0821 liter-atm / deg-mole)(2000deg) / (0.0001 liter)
= 3941 atmospheres

looks good to me, or in PSI 57930. It would have to be a preety strong container to hold that, (for clarity where you put deg in your formula deg is degrees Kelvin). Although I would doubt it would in real life achieve that pressure as heat transfer has been neglected

Edited by digger, 19 July 2007 - 10:36 PM.

[StevenRS]

Wow, I didn't think I would get a number THAT fast. This must be a really smart forum! Interesting to know though. But is 200 milligrams of meal 0.1 milliliter in volume?

[BrightStar]

But is 200 milligrams of meal 0.1 milliliter in volume?

Commercial corned / granulated BP in bulk has about the same density as water - ie 1g per ml. This is quite useful sometimes if you need a charge (say for lifting a shell) and don't have a balance to hand - a measuring cup borrowed from the kitchen can get you close

Assuming the BP under test was a solid puck, 2g per ml sounds about right. The often quoted optimal figure when pressing is 1.7g per ml.

Either way, it's a lot of of pressure... it illustrates nicely why pyro actuated emergency tensioners / releases / cable cutters etc are so effective.

[Andrew]

There have been some good answers so far.

However...

This is very difficult to calculate, first off no-one actually knows exactly what happens when BP burns. Even calculating the adiabatic reaction temperature is next to impossible. You also have to remember that Sulphur Dioxide and Trioxide also make up a significant mass fraction of the evolved gases. As these are many times the molecular mass of the simplest evolved gases making an extimation

BrightStar's estimation seems reasonable (definitely within an order of magnetude), although the only real way to do this is to do the following.

1. Put a predetermined amount of BP, in a sealed chamber (with a large head of volume).
2. Weight it before the burn.
3. Ignite the BP without affecting the mass change of the system. (Intense IR or a hot Nichrome wire in a v.small trail leading to the main bulk of BP).
4. Measure temperature and pressure of the chamber, and plot over time.
5. Allow the smoke to settle, or coat the sides of the container.
6. When the temperature is still above 60 degrees centigrade, vent the excess gas pressure and allow a normal atmosphere to return.
7. Weigh the apparatus again. (Now you has an empirically derived figure for the solid/gaseous product ratio).
8. Measure the adiabatic burn temperature by packing a tube with BP and measuring the temperature with a thermocouple.

Now armed with the gas characteristics, solid/gas mass fractions, adiabatic reaction temperature etc. one can now calculate what the theoretical maximum pressure is (I wouldn't mind doing that and publishing the results). As digger also mentioned heat loss is a big one that acts to lower the maximum achievable pressure. You also have to bear in mind that even if no heat is lost (an adiabatic reaction/process), the evolved products can do some funny sh*t under the pressures that will be generated, firstly the CO2 will be placed in what is called a supercritical state, and so too will most of the other gases, this is a very interesting field of research and some crazy things happen!

You also have to remember some important factors like:
1. The smoke will have a negligible effect on the pressure, it is suspended solid that floats in the gas due to the viscosity of the gas (colloidal physics are somewhat alien to the classical minded, you really need to delve into nano-science to truly understand the driving factors), and does not contribute a lot to the pressure. It's main effect will be to displace a tiny volume, but again not measurable.
2. Different chemicals have different latent heats, and heat capacities at different temperatures, it's not a case of three numbers to plug in for each molecule, over the temperature range we're talking about the heat capacity can change by orders of magnitude.
3. The reaction will take place that fast that very little heat will be lost during or immediately after the burn. Because of this, one might find that instantaneous pressures recorded immediately after the burn will be very close to the theoretical maximum.

I reckon form experience that BP will be more than capable of producing pressures in excess of 3,000 bar perhaps even as high as 10k bar (would like to do some work on this when I have time). Although the solid mass fraction of products will be the dominant limitation to this.



I'm doing some research in to spacecraft thrusters (for interplanetary travel and moon/mars lander missions) at the moment, mostly simple HTP monoprops and more complicated plasma and ion thrusters. Some of my work was into the safety aspect of using HTP, try this on for size:

If 85% HTP decomposes and LOOSES ALL EVOLVED ENERGY to return to ambient temperature, (given that no chamber relaxation takes place) the resulting chamber pressure is 2404 bar, assuming ambient temperature is 20 degrees. If adiabatic decomposition takes place (worst case), the pressure is in excess of 12k bar. Although the latter is an estimate as the physics at these temperatures, states and pressures are not empirically recorded.