Test Details
Test Purpose
This test is used to determine the critical height at which a flame initiation transitions to an explosive reaction (explosion or detonation). This test is sometimes referred to as a critical mass test because the results are dependent on the self-confinement provided by the mass of material. In this test, a flame initiator (bag igniter) is placed at the bottom of a pipe assembly filled with the test material. Pipes of varying lengths and diameters are used to contain the test material. The test is performed by selecting a diameter, and progressively changing the height of the pipe until the material transitions from burning to explosion or detonation. The diameter is then changed and the progressive height variation testing is repeated. Normally, a curve can be fitted using the data, to predict the critical height for other diameters as well. A “+” reaction, for explosion, is one in which the pipe is damaged. The test is concluded at each diameter by running a minimum of three successive trials which produce a “-” result at a height below a level which produces a positive reaction (explosion). This level is referred to as the critical height at that diameter.
Example in-process applications where the goal is to operate under the critical height include hopper and bin designs, feed systems, etc.
Special Instructions
| Special Instructions | Details |
|---|---|
| Definition of Critical Height | The critical height for a given diameter is the level that produces three negative results with at least one positive result (explosion) at the next higher increment at a given diameter. |
| Typical test methodology | The fill height of the test material at a given diameter is progressively increased or decreased until the critical height is determined (see definition above). The diameter is then incremented and the critical height determined for that diameter. Generally, 1-inch, 2-inch, and 3-inch diameter pipes are tested in lengths ranging from 3 inches to 24 inches in 3-inch increments (maximum). |
| Data extrapolation | Plotting the critical height versus pipe diameter test data permits identification of data trends for regression and interpolation/ extrapolation. For example, data may be fit to the equation: CH = a + bD^0.25, where CH is the critical height at the diameter D. NOTE: Larger diameter test pipes (e.g. 6-in) or in-process simulation tests may be required for data extrapolation to in-process conditions. |
| Application to process equipment | A plot of the critical height versus diameter can be used in equipment design and to assess the explosion potential of existing process equipment. For example, if the fill height of a substance in a steel cylindrical hopper exceeds its critical height for that diameter, it will likely explode if bottom ignited, producing blast overpressure and fragments that may result in the loss of life. |
Example plot
Test Variations
Typically the test is completed in a steel pipe of differing diameters and heights; however, the test can be completed with various types of confining media:
- Container construction
- Thick-cased steel (high confinement)
- Thin-cased steel (medium confinement)
- Plastics, textiles, fiberboard (low confinement)
- Surrounding medium
- Atmosphere
- Sand (displaceable medium)
- Supplemental venting
- External vent panels
- Internal vent tubes
Other specifics such as the temperature, density, processing stage, etc. can be set by the specific in-process configuration. Below are some photos of examples of in-process specific critical height tests as well as example results with single-base propellant.
In-Process Example Results with Single-Base Propellant
Key Parameters
| Key Parameter | Objectives | Origin | Specs |
|---|---|---|---|
| Confining medium | Observe an explosion or onset of explosion evidenced by the steel pipe and/or end cap (bulging, splitting, fragmenting, etc.) |
|
|
| Ignition stimulus | Ensure a sufficient amount of the material is ignited by flame at the bottom of the pipe |
|
|
| Bottom ignition | Allows the substance’s own weight to provide self-confinement | Ignition at the bottom of the pipe | |
| Fill height and diameter of the substance | Determine the fill height at which a substance at a given diameter transitions from deflagration to explosion; the relationship between fill height and diameter can be determined by testing different diameters | Generally, 1, 2, and 3-inch diameter pipes are tested in lengths ranging from 3 to 24 inches in 3-inch increments (max) | |
| 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 steel pipe or end cap | Visual post-test inspection | Bulging, splitting, or fragmentation of the pipe or end cap are considered a positive result (i.e. failure). In-process HD 1.1 for substances above the height given the in-process diameter, else in-process HD 1.3 candidate |
