Causes of Static Charge from Liquid Flow and the Precautions Needed
Typical examples of accumulation
Electrostatic charge formation in tanks and vessels in which hydrocarbons or chemicals are pumped or stirred is widely recognized as a serious hazard. This electricity, which is generated in tanks during these normal operations, occasionally causes a spark in a tank vapor space. As statistics show, during 10 years in one state and one oil company only, 18 fires have been attributed to static electricity, causing damage and product losses of millions of dollars. Knowledge of particular chemical’s or hydrocarbon’s resistivity and the current generated by pumping or gravity flow into tank to calculate the voltage difference will help in deciding the likelihood of self-ignition or explosion. The method to calculate the current is given in detail in article written by Adam Zanker for Hydrocarbon Processing (March 1976). The precautions generally taken to avoid the static charge are to reduce the velocity of inflow to the tanks by providing one size larger diameter nozzle or adding a downcomer to the tank, which provides relaxation of the generated current. Additionally, use of antistatic additives to reduce the resistivity of the liquid, or providing padding of the liquid using an inert gas such that there is no air inflow will help reduce static charge formation.
Bustin, W. M., Culbertson. T. L. and Schleckser, C. E.."General consideration of static electricity in petroleum products." Proceedings of American Petroleum Institute, Section 111. Refining, Vol. 37 (111), p. 24-63. 1957.