Test Details
Test Purpose
This test is used to determine the critical diameter at which a shock initiation propagates through the sample confined within the pipe or process piece of equipment. In this test, a shock initiator is placed at the top of a pipe assembly filled with the test material. Pipes of varying diameters (with length of at least L/D = 4) are used to contain the test material. The test is performed by selecting a test diameter and completing three trials. A “+” reaction is one in which the witness place is shattered or holed. The test is concluded at each diameter by running a minimum of three successive trials which produce a “-” result at a diameter below a level which produces a positive reaction. This level is referred to as the critical diameter.
Example in-process applications where the goal is to operate under the critical diameter include nitroglycerin piping systems and rocket motors.
Special Instructions
| Special Instructions | Details |
|---|---|
| Definition of Critical Diameter | The critical diameter is the diameter that produces three negative results with at least one positive result (explosion) at the next higher increment. |
| Typical test methodology | The diameter of the test material is incremented until the critical diameter is determined (see definition above). |
| Application to process equipment | The critical diameter can be used in equipment design and to assess the explosion potential of existing process equipment. For example, if a substance in facility piping exceeds its critical diameter, the entire system will likely detonate if initiated, producing blast overpressure and fragments that may result in the loss of life. |
Key Parameters
| Key Parameter | Objectives | Origin | Specs |
|---|---|---|---|
| Booster | Provide a strong, repeatable, stable shock front to the top of the sample | 1:1 L/D (cylindrical or conical) allows basic point source initiation in the center to expand hemispherically, outputting a relatively planar 200+ kbar shock (maximum strength); actual value is 280 kbar. A conical shape can produce the same effect but uses 1/3 the explosive. | Composition C-4 or equivalent (e.g. Composition B), 2:1 L/D plus 1 in for the detonator; cylindrical boosters for samples under 4 inches in diameter and conical boosters for diameters 4 inches or greater. |
| Substance diameter and length | Sufficient length to permit stabilization or decay of the shock front in the propellant; sufficient diameter to identify in-process substances susceptible to detonation | Length – 3:1 L/D is required to run beyond any effects of the booster (established by testing in the 1950’s and 1960’s); 4:1 L/D is industry standard for critical diameter tests, enabling discernment of the shock wave generated by the propellant. | 4:1 L/D minimum |
| Confining medium | Provide confinement, increasing the susceptibility of the substance to detonation | Testing has demonstrated that confinement effectively lowers the critical diameter of the substance, increasing its susceptibility to detonation. For example, a confined test of 1.5-in ID could produce the same response as an unconfined 2.9-in ID. | Schedule 40 carbon steel (A53 Grade B) pipe of various diameters |
| Spacer | Separate the steel tube from the witness plate | MIL-STD-1751A Method 1041: The small stand-off (0.063-inch gap) between the acceptor and the witness plate was introduced to prevent the witness plate from shattering and thereby facilitate interpretation of test results. |
1.6 ± 0.2mm (1/16-in) thick |
| Witness plate specifications | Evidence of reaction type | MIL-STD-1751A Method 1041: Some laboratories prefer to use witness plates with larger length and width, e.g., 9″ square, to reduce the likelihood of shattering the witness plate. This is an acceptable variation. | Mild steel (e.g. SAE 1010 – 1026) witness plate large enough to completely overlap the sample base with thickness of 0.375 in. |
| Substance density | Determine whether the substance in its in-process form is sensitive to the test stimulus | Substance to be tested as near as possible to the in-process density | |
| Number of trials | Sufficient to give a reasonable level of confidence in the reproducibility of the test results | 3 trials |
Indicators
| Indicators | Detection Method | Assessment |
|---|---|---|
| Damage to the witness plate | Visual post-test inspection | Hole in the witness plate: [IP Division 1.1 if in an in-process diameter greater than the critical diameter] |
Example Photo of Witness Plate Not Holed (-)
Example Photo of Holed Witness Plate (+)
