Static charges can build up for a number of reasons, including processes involving low-conductivity organic solvents, and operational practices such as introducing free-falling liquids and powders as well as excessive agitation. If the dielectric strength exceeds 500 V per mil of thickness, it can result in damage to the glass lining. The most affected parts of the vessel are generally located near high-velocity areas like the tips of agitator blades and the vessel wall opposite the blades. The damage usually appears as microscopic holes that go all the way down into the steel substrate; chipping may or may not occur. You can also usually see a discoloration, or "aura", around the pinhole. Reglassing is required to fix electrostatic discharge damage. To avoid putting your vessel at risk, keep your agitation speeds at a minimum and add materials through dip tubes so that they enter below the liquid level line.
Spark testing is the most commonly-used method for inspecting glass-lined equipment. The metal brush that is moved across the glass surface will generate a spark to indicate a defect in the lining. The most common problem faced with “DIY” spark testing is that personnel use excessive voltages (levels that should only be used by glass manufacturers when they are running quality checks on new equipment) or linger in one area too long. We normally recommend 6 KV for field testing, and the brush should also be moving over the surface. Furthermore, spark testing should only be used occasionally. It is always recommended that a qualified technician performs spark testing in glass-lined equipment. If someone at your facility has not been trained on the proper method for spark testing then a service specialist should be hired to execute the test on your behalf. When the procedure is mishandled, it can create pinholes in the glass that will look similar to electrostatic discharge damage. Spark testing damage is usually small enough that a three-piece plug will cover the problematic area, unless someone creates a number of pinholes over a large area of the vessel.