Process safety is one of the oldest priorities in process industry. With time, the ignition source mechanisms are more and more understood and the current standards are much more accurate than their first versions. Due to this evolution, some characteristics that were allowed in the past may not be tolerable any more. Besides due to the ageing of the devices, it can happen that some properties do not comply with the last versions of the concerned standards. In that case, the end user has few choices and usually end up changing its device.
This presentation describes the study of old centrifuges (more than 30 years old) whose some characteristics appeared to be out of the ranges defined by the current standards. In that case, the centrifuges were supposed to be coated with a dissipative coating but some showed values of resistances to the ground several orders of magnitude higher than the values defined in the IEC 60079-32-1: 2013 (IEC, 2013). As resistance to the ground is a key parameter to avoid spark discharges, the user should change the centrifuges or recoat them with a proper coating. However, this standard describes that the threshold values for resistance were assessed to avoid that the electric potential du to static electricity build up exceeds 100 V.
The basket of the centrifuge was then modelled through an RC circuit and based on measured data on the centrifuges and on the specifications of the handled products, the voltage across this circuit was defined as a function of time. It was then possible to assess the required waiting time to let the voltage decrease below 100V.
The model was then tested against a real case experiment using the worse centrifuge. The results of the test were predicted by the model.