The problem of electrical discharge and subsequent explosive detonation of the ullage inside chemical storage tanks containing methane-infused fluids is becoming more widespread as the use of new non-metallic storage tanks proliferates. Such tanks are typically made of non-corrosive but otherwise insulating materials (either fiberglass resin, PVC, or similar insulating plastics), have no continuous metallic grounding conductors within or outside of the tanks, and are exposed to the electrical environment in the vicinity of lightning-producing thunderstorms. They are often used to store fluids used in hydraulic fracturing. The gas inside the tank above the fluid level (the tank's ullage) can contain a stoichiometrically explosive mixture of oxygen and methane or other similarly volatile hydrocarbon gas. Such a mixture is amenable to explosive detonation upon either arc or strong corona discharge within the tank.
Conventional metallic tanks form a Faraday cage of conducting material around both the fluid and potentially explosive ullage, thus ensuring that electric fields never approach appreciable values within the tank. However, the lack of a continuous conducting boundary resulting from the use of non-metallic tank walls permits electric fields to approach breakdown strength in response to a nearby lightning discharge. Furthermore, depending on the specific conductor geometry, enhanced local electric fields that exceed breakdown strength can occur near either small metallic objects or even dielectric objects within the ullage region of the tank. Such conductors include small boltheads or other metallic fasteners, as well as other electrically good conduction materials within the tank (e.g., including droplets of the fluid itself and the fluid surface corners). Enhanced fields at sharp conductors can occur during either the incipient or active phase of a nearby lightning strike, but gaseous dielectric breakdown and subsequent ullage ignition will likely only occur during a nearby strike event.
Since the fluid inside the tank is often laden with salts, it can be expected to be of moderate to high ionic content. As such, its conductivity can range from a low value of ˜0.001 S/m to values for heavily brackish water that can easily exceed a few S/m. Such fluids have relaxation time constants of less than ˜1 nsec, but for low saline contents (i.e., spring water) this time constant can be as long as ˜1 usec. In either case, these fluid masses behave as good electrical conductors on the time scale of an atmospheric electrical transient. They redistribute a surface charge event and thus effectively shield the charges on the fluid surface so they don't manifest itself within the volume of fluid itself.
However, the transient can produce a strong field within the ullage. It is well known that high electric fields occur where conductive materials form sharp corners or points. For example, the electric field around a simple spherical metallic object immersed in an otherwise uniform electric field will be up to a factor of three times as large due to the electric polarizability of the object. If the object is needle-like (for example, a rivet or long bolt) this field amplification factor can be significantly higher, readily approaching a factor of ˜10× for many common fasteners. The field amplification effect occurs not only around good conductors but also at the ends of long dielectric objects, albeit to a slightly lesser extent that depends on the dielectric constant of the object. For example, for a sphere of fiberglass with a relative dielectric constant of 4.2 the amplification would be a factor of approximately two times that of the external field.
Origin of Tank Explosions: It is hypothesized that the cause of recent explosions of ullage in fracture fluid storage tanks is the result of the above field amplification near sharp ungrounded metallic objects or sharp dielectric protrusions. Rapid increases in the external field of order 2MV/m per millisecond will cause field amplification on many small dielectric and conducting objects within a non-metallic tank. The rapidity of this field change does not permit time for charge to bleed off through the insulating tank walls, and thus to null out the applied external field from the lightning transient. A rapidly increasing field that exceeds the local dielectric breakdown strength at a location in the vicinity of a lightning strike can readily produce additional localized corona or even arc discharge by exceeding the breakdown strength of the gas mixture. Note that the breakdown strength of ullage gases may also differ from that of air, as well. For example, carbon dioxide hydrogen, and helium all serve to lower the breakdown strength of air, and can contribute to a somewhat lower overall breakdown strength if present in the ullage. The presence of such sharp conductors is thus to be avoided in order to minimize field enhancement anywhere within the ullage. Alternately, the use of proper shielding can reduce the likelihood of the transient producing high fields within the ullage.
What is needed in the art is a grounding system that is highly corrosion resistant, can be secured to a roof in a non-metallic tank, and then rest on the bottom of the tank like an anchor. The present invention provides a stainless steel tripod type electrode that readily rests on the tank bottom and is connected to a roof of the tank with a cable.