The overall goal of this procedure is to measure the minimum pressure required for self sustained combustion in water-based emulsion explosives which is called the minimum burning pressure. The minimum burning pressure is a parameter that can guide the estimation of safe operating pressures for the manufacturing, handling and pumping of emulsion explosives. The main advantage of this technique is that it requires only 20 to 30 grams of explosives.
It can therefore be performed in a laboratory environment. The installation of MBP facilities in other laboratories would facilitate inter-laboratory MBP testing, which could lead to the establishment of standard reference material for performance validation. To begin preparing the test cell components, cut an 85 millimeter length of Nichrome wire.
Use needle nosed pliers to bend each end of the wire into a small loop. Use uninsulated butt end splice connectors and a crimping tool to splice the ends of the Nichrome wire into 50 centimeter lengths of number 14 American wire gauge solid core bared copper wire. Prepare 10 to 15 ignition wire assemblies in this way.
Verify that the resistance measured across each assembly is less than 0.5 Ohms. Next, obtain 10 to 15 small cylindrical steel pipes, each with a three millimeter wide slit machined along the long axis. Paint the interior of each cell with two coats of high temperature, non-conductive paint.
For each cell, cut off the narrow ends of two number zero neoprene stoppers to obtain 16 millimeter long stoppers. Punch a hole six millimeters in diameter through the center of each stopper. Verify that the holes are wide enough to accommodate the copper wire and splice connectors.
Then, insert an ignition wire assembly into the dried painted cell through the three millimeter wide slit. Thread a neoprene stopper onto each copper wire. Carefully insert the stoppers into each end of the cell without compressing or twisting the ignition wire.
Pull the ignition wire taut. Bend the copper wires by about 90 degrees at the ends of the stoppers to fix the ignition wire in place. Repeat this process to assemble the remaining test cells.
To begin preparing the cell for the test carefully introduce the emulsion into a test cell through the three millimeter wide slit. Once the cell is full, remove any air pockets by tamping down the sample with a small spatula and tapping the cell against a hard surface. Continue filling the cell and settling the sample in this way until no more sample can be packed into the cell.
Then, place the loaded cell horizontally in a remotely operated pressure vessel with the slit facing upwards. Connect the copper wires to the electrodes in the pressure vessel, ensuring that the wires do not contact the body of the vessel. Use a multimeter to verify that there is no electrical contact between each electrode and the body of the pressure vessel.
Then, close and seal the vessel. In a protected instrument room, verify that the data acquisition card is connected to the pressure transducer and the voltage signal across a high precision shunt resistor. Confirm that the shunt resistor is connected in series with a constant current source and that this series is connected to the pressure vessel electrodes.
Then, start the data acquisition software. Remotely close the pressure vessel outlet valve and begin pressurizing the vessel with argon. Once the vessel has reached the starting test pressure, close the inlet valve.
Monitor the system for five to ten minutes to verify that there are no significant leaks. Once the vessel has passed the leak check, open the inlet valve to adjust the pressure to the desired initial value and then close the valve. Flow a constant current of 10.5 amps through the ignition wire until the sample ignites, melting the ignition wire and halting the current flow.
Turn off the current power supply once ignition has occurred and the pressure starts increasing. During the test, the pressure may reach one or two minima or maxima before continuously decreasing. Wait ten minutes after the start of the continuous pressure decrease before performing additional manipulations.
At the completion of the test, open the outlet valve to vent the combustion gasses to exhaust. Set the argon regulator to about 50 psi and open the inlet valve. Allow a gentle flow of argon to circulate for two to three minutes to purge the vessel of toxic gasses, then allow the vessel to return to ambient pressure.
Lock out the constant current supply by disconnecting the AC power or with a lockout key before returning to the pressure vessel room. Put on a face mask with appropriate general purpose cartridges before opening the pressure vessel. Open the vessel and disconnect the copper wires from the electrodes.
Visually inspect the cell and pressure vessel to determine the extent to which the sample burned. Document the results in writing and with photographs. If the combustion front reached the test cell walls and little to no sample remains on the neoprene stoppers, the result is considered a go and the pressures should be decreased for the next test.
Otherwise, the result is considered a no-go and the pressure should be increased for the next test. Following the visual inspection, perform additional analyses using the recorded current and pressure data. Repeat the test with additional samples while gradually decreasing the pressure increments or decrements to precisely determine the MBP.
Clean the pressure vessel after each test. At 17.3 seconds into the test shown here the pressure began increasing from an initial average of 4.924 mega pascals. The dropping current corresponding to the burning of the ignition wire occurred at 19.1 seconds.
The maximum pressure of 6.095 mega pascals occurred at 33.7 seconds. Five identical AWEs with varying water contents were among the samples tested with this method. The MBPs for these emulsions were observed to increase with increasing water content.
Greater variance was observed in the MBP data for AWEs with lower water contents. This may have been caused by non-uniformity in the samples as their comparatively higher crystallization temperatures may make these samples more prone to crystallization when manipulated. The MBP test described here has been added to the requirements for the authorization of high explosives in Canada by the Explosive Regulatory Division of Natural Resources Canada.
Once mastered, a full MBP measurement consisting of ten to twelve experiments can be completed in two days if it's performed properly. While attempting this procedure, remember to always release vessel pressure and lock out the current source before returning to the pressure vessel room. Electrocution and projectiles from sudden decompression are your greatest hazards.
