Proving the Safety of CROHMIQ FIBC

Temperature
&
Humidity

The electrostatic properties of polymeric materials depend to a greater or lesser extent on the amount of moisture they are able to absorb from the atmosphere. There is too little moisture available at low relative humidity and so static charge tends to accumulate more readily and dissipate more slowly.

Some materials absorb a lot of moisture and at high relative humidity their resistance can be so low that spark discharges may occur if the materials are isolated from ground.

To ensure that CROHMIQ Type D bulk bags are able to maintain safe charge dissipation without becoming too conductive, it is necessary to test at both low and high relative humidity.

Charging current

The rate at which charge flows into and out of a bulk bag depends on the speed of filling and emptying and the chargeability of the product. Charging current is the charge flow rate expressed as ampere (A), or more conveniently micro-ampere (µA).

End-user data has shown that sustained charging currents of about 3 µA are possible, with transients sometimes even higher. It is necessary to test using charging currents that replicate those found in industry.

Minimum Ignition Energy (MIE)

MIE is the smallest quantity of energy in an electrostatic discharge required to cause the ignition of a gas, vapor or powder. MIEs are normally expressed in milli-joule (mJ). The severest incendivity test is to use a gas mixture with the lowest practically relevant MIE.

Methanol vapor has the lowest MIE of any gas or solvent that is likely to be present when bulk bags are emptied. The MIE for methanol is 0.14 mJ. Click here for MIE of other common substances.

Gas Composition

The gas selected for incendivity testing is mixed with air to achieve the required MIE. The ratio of gas to air must be controlled to tight tolerances in order to maintain the specified MIE throughout the test. Furthermore, the composition of the air used must also be controlled.

MIE normally changes with gas composition in a way similar to that shown below:

Small changes in gas composition can produce a dramatic increase in MIE, making it easier to pass the test. Precise control of the gas composition is essential if the severity of the test is to be maintained.

Gas Flow Rate

The gas probe used for incendivity testing is a grounded metal electrode surrounded by a shroud that directs a flow of gas in front of the electrode, as shown below:

As gas exits the probe it is diluted by the surrounding air. It is necessary to minimize dilution because it changes the MIE of the gas. If the gas flow rate is high enough, dilution will not be significant.

Number of Repeat Tests

Ignition of a gas by electrostatic discharges is a probabilistic phenomenon. For example, a 1 mJ spark might ignite a 0.1 mJ MIE gas every time, but a 0.1 mJ spark may only ignite the same gas 1 time out of 100.

Confidence of a pass in incendivity testing (i.e. no ignitions) can only be achieved by performing a statistically significant number of gas probe approaches. Furthermore, approaches should be made at different locations on all sides of the bulk bag under test, during filling and emptying.

(23 ± 2) °C / (20 ± 5) %RH, and

(23 ± 2) °C / (60 ± 10) %RH

Charging Current

(3.0 ± 0.2) µA negative polarity

MIE

(0.14 ± 0.01) mJ

Flammable Gas

Ethylene

Air

(21.0 ± 0.5)% Oxygen, balance Nitrogen

Gas Composition

(5.4 ± 0.1)% Ethylene

Gas Flow Rate

(0.21 ± 0.04) litre/s

Gas Composition Monitoring

IR Ethylene Gas Analyzer providing constant, real-time monitoring

Calibration Checks

Before each test series:

• Mass flow rate of pellets
• Charging current
• Gas Analyzer checked using reference gas with composition traceable to NIST standards

Number of Repeat Tests

At least 50 on each side, top and bottom (>200 per FIBC)