An operator failed to close the exhaust valve and the handhole, so there was little sealing effect of the nitrogen gas. Therefore, a combustible gas-air mixture (ranges of flammability 1-7%) of xylene that remained in the piperazine crystal (about 5%) was generated in the centrifuge near the crystal pull-out mouth. In addition, static electricity generated in the separation process charged the piperazine crystal. AS the drum was insulated by two polyethylene bags, static electricity accumulated in the piperazine crystal. It reached the minimum ignition energy (1.2mJ) of the xylene and ignited.