(52as) Experimental Evaluation of Electrostatic Discharge in Multi-Phase Hydrocarbon Flows

The flow of hydrocarbons through pipes and hoses can lead to the generation of static electric charge. If there is an electrically isolated conductor in a hydrocarbon flow, the electrostatic build-up can be sufficient to result in a discharge capable of igniting flammable vapors. This is a well-known hazard in many industrial applications that is typically alleviated through the use of grounding and bonding. When assessing the potential for electrostatic discharge, three important parameters are the rate of charge generation, the rate of charge dissipation, and the capacitance of the isolated conductor. Combined these define the maximum achievable voltage and the speed at which that voltage is reached.

There are generally-used equations that quantify the rate of electrostatic charge generation in single-phase flows, but only limited information for multi-phase flows. Multi-phase flow (e.g., a hydrocarbon with air) can occur in the transfer of hydrocarbons for a variety of reasons. This could be due to a hole or breach in the line, or due to entrainment at a hose or pipe inlet if the hose or pipe is not fully immersed in the liquid to be transferred, such as during the start or end of transfer operations.

We performed experimental testing where liquid hydrocarbons were transferred under two-phase flow conditions (hydrocarbon and air) and measured the accumulation of static charge on an isolated conductor. A series of shop-vacs were used to draw liquid hydrocarbons from a tank through an isolated section of metal piping followed by a non-conducting polymer hose under a range of different flow rates. The position of the hose relative to the liquid surface of the tank was varied to vary the relative flow rates of liquid and air. The voltage on the hose was measured using a non-contact voltmeter. These tests were used to characterize the rate of charge generation, charge dissipation, and the maximum achievable voltage as a function of flow rate, the resistivity of the hydrocarbon, and the presence of water contaminants.

Results from these tests were extrapolated using a model to real-world operating conditions for industrial facilities, hydrocarbon transfer stations, or hazardous waste clean-up equipment to assess the potential for electrostatic accumulation and discharge.