On November 20th, 2017, a solvent explosion occurred at a cosmetics manufacturer in the United States. The surveillance video shows the transferring of a flammable liquid (hexamethyl disiloxane, flash point -8°C) from a 200-liter drum into a RIBC. The liquid transfer was carried out without any local ventilation. At the filling opening of the RIBC a liquid pool has formed. When the employee wiped the surface of the RIBC with a cloth in the area of the filling opening, an explosion occurred.
Figure 1: Pictures of a surveillance video depict the accident
The technical conditions as shown and the action of the employee led to the prerequisite conditions for an explosion - fuel, oxygen as an oxidant and an effective ignition source – were present simultaneously.
Important rules of explosion protection avoidance of a flammable atmosphere or avoidance/removal of any effective ignition source) have not been respected in this case. Open and non-ventilated handling of flammable substances, with flashpoints well below the ambient temperature, will quickly lead to a local explosive atmosphere, as it happened in this example. An adequate locally ventilation system that adequately removes the escaping flammable vapors, would have been a measure against the formation of a potentially explosive atmosphere.
Once such an explosive atmosphere is formed, a variety of ignition sources can cause the explosion. In this example, the ignition of the explosive atmosphere occurred when the employee, wanted to wipe off spill material with a cloth. In absence of other obvious ignition sources, it is assumed that an electrostatic discharge provided the necessary ignition energy.
Electrostatic discharges occur during charge separation, in case of e.g. flow of liquids in pipelines or friction of insulating surfaces to each other. In the above event, the following effective discharges are conceivable to trigger the explosion:
Spark discharge between the charged and non-grounded operator and the conductive metal grid of the RIBC.
Spark discharge between the charged and non-grounded RIBC grounded metal grid to the operator
Spark discharge between the drained and non-grounded drum to the RIBC's metal grid
Brush discharge from the surface of the RIBC (due to by rubbing of the cloth) to a conductive object (metal grid or employee)
For this reason, we like to address an important but often underestimated aspect: the requirements for containers used for flammable substances and its use in potentially explosive atmospheres. Technical regulations such as IEC/TS 60079-32 and TRGS 727 describe in detail the necessary technical and organizational requirements to avoid ignition hazards due to electrostatic charges. In general, containers made of insulating materials cannot be used in zone 0 areas and in zone 1 and 2 their use is restricted. An upper scale limit of 5 liters for a non-conductive container is often used, so that the surrounding, insulating surface is still sufficiently small to prevent the accumulation of effective charge amount. If containers with a nominal volume ≤1m3 are used, such as RIBC (see Figure 2) with insulating liner, the following technical requirements and organizational measures apply.
Figure 2: Scheme of a RIBC in use with flammable substances (Source: TRGS 727)
1. Lid should be conductive and grounded or dissipative and grounded or insulated but not dangerously charged
2. Insulating liquid container
3. Non-combustible or flammable liquid, with the exception of group IIC and IIB liquids with MIE <0,2 mJ
4. Conductive grid, grounded, fitted on all sides to the liquid container, grid shape so that the enclosed partial areas A ≤100 cm2 or
5. Conductive or dissipative full-surface coating with earth contact
6. Outlet fitting conductive and earthed
7. Liquid in contact with earth, here e.g. over the outlet fitting
8. Metall pallet
9. Conductive or dissipative floor
10. Ground terminal
The TRGS 727 also states minimum requirements for insulating containers with conductive coating for the above operations, i.e. filling, emtying, transport and storage In addition, RIBC intended for use in potentially explosive atmospheres (Ex-Zone) must be periodically inspected to show that the electrostatic requirements of the used RIBC have been met. Also when using RIBC with dissipative plastic inliner the conductivity must also be verified by measerements.
Above event showed clearly, that the risk potential of an open handling of an flammable substance has to be assessed as high. Measures should be taken to reliably prevent the formation of an explosive atmosphere. Employees should be adequately trained in handling flammable substances. The use of appropriate equipment and the implementation of applicable minimum requirements provide effective protection against ignition hazards due to electrostatic discharge.