What the Kst value expresses
Kst characterises how rapidly pressure rises during a dust explosion under standardised test conditions. It allows the severity of different samples to be compared and informs the design of protective measures. It is usually expressed in bar·m/s. A high value indicates a faster pressure rise; it does not, by itself, mean that an explosion is more likely.
Related concept: Explosive atmospheres.
How it is determined
The test examines a dust cloud in a vessel under controlled conditions. The maximum pressure-rise rate is normalised in relation to the vessel’s volume. This provides a comparable parameter, but the tested material must represent the actual dust.
Particle size, moisture and composition influence the result. Information from a product sheet or database may support an initial assessment, although it is not always sufficient to justify the design of a particular installation.
The expression is Kst = (dP/dt)max · V1/3, where V is the test-vessel volume and (dP/dt)max is the maximum pressure increase per unit of time.
Dust explosion classes
The usual classification distinguishes St 0, St 1, St 2 and St 3 according to the Kst range. St 0 means no explosion under the test conditions; the other classes reflect increasing severity. It is important to consult the test report and method rather than simply copy the class.
Even dust in the lower range can cause a serious accident when sufficient fuel, confinement and exposure of people are present. A class does not replace an assessment of the installation.
The usual ranges, with Kst in bar·m/s, are: St 0: Kst = 0; St 1: 0 < Kst ≤ 200; St 2: 200 < Kst ≤ 300; St 3: Kst > 300. They must be interpreted alongside the conditions and representativeness of the test.
Decisions it helps inform
Kst is used with other information, such as maximum explosion pressure, enclosure characteristics and operating conditions, to study venting, suppression or resistance. Propagation into connected ducts and equipment also matters. Addressing an isolated vessel without analysing the whole system may leave a hazardous transmission path.
Preventing ignition sources, cleaning and controlling accumulations remain necessary. Kst describes a potential physical consequence; it does not replace those measures.
Hypothetical example
A workshop modifies its dust extraction and adds an enclosed filter. The supplier proposes using a database value for a similar material. The practitioner finds that grinding produces finer particles and requests a review of whether the data are representative. The filter’s protection is specified using appropriate information and a discharge arrangement that does not expose workers.
Misinterpretations to avoid
Kst must not be confused with maximum pressure, minimum ignition energy or a concentration limit. Nor can a test without an explosion be interpreted as universal safety for every particle size or process. The parameters must be read together by personnel competent in explosion protection.
