Metals Minimum Ignition Energy
Currently, no test standard exists for quantifying the ignitability of metallic materials in oxygen. Metallic materials are consistently used at oxygen pressures significantly greater than their associated burn limits. Oxygen fires are rare in these applications; however, they still occur and the proximity to ignition and fire for applications that don't experience fire is always unknown. A new test method for quantifying the ignitability of metallic materials in oxygen has been developed by WHA. This test method uses an electrical discharge capable of delivering a quantifiable amount of energy to a conductive surface via a high-current electrical arc.
The High Current Arc Discharge System (HCADS) uses a series of 40 capacitors to store energy proportional to the predetermined voltage (V) across the capacitor bank. The capacitance (C) of the capacitor is 10 Farad and the energy stored is related to ½CV2. The HCADS was incorporated into WHA's existing promoted combustion chamber which is used for performing ASTM Standard G124. Using a remote control switch, the energy stored in the capacitors is released in the form of electrical current through a conductive circuit connected to the capacitor bank output leads. The remote control switch initiates current flow through a closed circuit until the total stored energy is dissipated or until the circuit becomes open. The promoted combustion chamber electrodes are attached to the capacitor bank output circuit to generate a high current arc at the surface of the test sample in pressurized oxygen (see figure at right). The capacitor bank output leads are connected directly to the chamber allowing current to flow out of the capacitor bank and through the test sample and electrode interface. After the current flow is initiated, the electrode is mechanically rotated away from the sample surface to draw an arc. The intensity of the arcing is regulated by the amount of voltage applied across the capacitor bank. The rotation of the electrode is performed by a compressed air actuator attached to a crank drive on the portion of the electrode that protruded through the chamber top.
An initial case example was performed in which arc ignition test results were used to evaluate the ignitability of aluminum-bronze and to help support a particle ignition risk analysis of aluminum-bronze industrial control valves.
|Electrode Configuration for 1/8 inch Diameter Rod Samples ||Electrode Configuration for Thick Coupon Samples|
|Arc Discharge Heat Affected Regions for (a) 95 ± 8 J and (b) 141 ± 13 J|