Patent Application
20130292263
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1. Field of the Invention
The invention pertains to cathodic protection of buried metal structures using continuous electric current from a single voltage source.
2. Background Art
Buried metal structures corrode due to voltage differences between the ground and the various different contact points of the structure. Because of the voltage difference, a continuous current develops in the structure that causes hydrogen to form in the cathodic points of contact, and iron corrosion in the anodic points of contact.
Eventually, the electron flow will corrode enough iron to weaken a buried structure and develop leaks or failures. The term “cathodic protection” refers to the practice of using current through the structure to combat the corrosion by artificially forcing the structure to be at a negative voltage in comparison to the surrounding ground connecting the negative terminal of a direct current voltage source to the structure and connecting the positive terminal to a sacrificial anode buried near the structure. The term “sacrificial anode” comes from the use of the anode—it is slowly eaten away as it provides a steady stream of ions to provide the cathodic protection.
When the applied voltage is correctly set, the electrons flow from the anode to the structure, and the structure does not corrode. The current flow must be set so that the structure remains cathodic (draws electrons to the structure from the surrounding ground), but not so cathodic that the electron flow is excessive, which causes the structure to become weak and brittle.
The cathodic protection industry struggles with development of a system to protect multiple structures, as each structure exists in a slightly different resistive ground network. A single voltage source results in different current flow to each structure. The various approaches include the insertion of variable resistors along the cable network to even out the current (Al-Mahrous, U.S. Pat. No. 7,192,513), use of multiple buried anodes of different values of resistance that provide different amounts of protection, and use of multiple anodes that are switched off and on to provide more or less resistance (Husock U.S. Pat. No. 3,143,670).
As these patents and the product offering of the cathodic industry demonstrates, the search for efficient protection of buried metal structures has been a fifty-plus year endeavor that continues to this day. Those in the industry must weigh many factors to select a proper product. Those factors include system hardware cost, system operational cost, system reliability, energy efficiency, and number and size of structures to be protected. There is no one-size-fits-all solution, as a pipeline with easy access to electric utilities will naturally call for a different solution than a single well in an isolated area.
The invention is a multiple-structure cathodic protective system that includes multiple voltage-controlled outputs to protect multiple structures with a common ground bed, or a single structure with multiple ground beds. Each output feeds an individual cable that is powered by pulse-width-modulated voltage power supplies. Current through the cables is monitored and the PWM control compensates to maintain the correct amount of current for maximum protection and long life.
In the current embodiment, a computerized control system monitors the current and voltage drop across each protected structure circuit, adjusting the current through each line corresponding to the drop by adjusting the voltage output. In this way, the system gives protection with a single bulk power supply and multiple power supply outlets, reducing cost while maintaining a flexible configuration that other single-source cathodic protection systems do not possess.
FIG. 1—A schematic diagram of the invention configured to protect multiple structures using a common anode bed.
FIG. 2—A schematic diagram of the invention configured to protect a single structure using multiple anode beds.
FIG. 3—An operational flowchart of the invention configured to protect multiple structures.
In the embodiment shown in
Each power supply card 31 converts power from the input power supply 17 to a voltage level calculated by the cards to deliver optimal current flow through a particular protected device by traveling through a common buried anode 19, through the soil to a particular protected structure, and then a cable back to the cathodic protection unit 11.
The system can be configured to operate in accordance with software programming permanently installed in the computer 13 such that no user interface is necessary, or be configured to accept user input during operation by an optional user interface 41. The user interface 41 also represents the ability of the system to report operating conditions, including the voltage and current output of each output power supply card 31, as well as any alarm conditions indicating a need for concern, such as an increase amount of current draw across a structure. Such reporting can be by digital output to a distant computer, a visual or audible alarm, or any other type of signal to operators using other currently available monitoring devices not discussed here. (In the current embodiment, output voltage of 2V or less causes an alarm.)
In most electrical circuits, a power source is set to provide a set voltage for a set load. In cathodic protection systems, changing soil and moisture conditions can radically impact the load impedance, potentially causing a set voltage to deliver too little current to protect the structure, or too much current which embrittles the metal structure being protected. To counter this, the current through each return cable is monitored by an internal current meter contained within each power supply card 31 and each card independently adjusts its output voltage in real time to maintain an optimal constant current at an operator-preset level. Cathodic protection systems vary considerably on the optimal current, from a few milliamps to 15A in some larger systems. The current embodiment can deliver 15A to a multitude of outputs.
While this invention has been described as it is currently built, the invention is not limited to the disclosed embodiments, but can be employed in various equivalent arrangements included within the spirit and scope of the claims. In particular, the change in configuration between protection of one multiple structures and one anode bed to a single structure protected using multiple anode beds is easily understood given the explanation provided to any person having ordinary skill in the art, as well as a configuration in which multiple structures are protected with multiple anode beds, though this configuration is not shown.
Among the possible variants, the invention can be simplified such that the main computer need not exist. In this embodiment, the input power converter 17 provides an intermediate bus voltage that each output power supply converts to maintain a current setting that is hard-wired into the card, or set by a physical control on the card. Similarly, a more exotic embodiment might include computer control to create a rotating duty of output cards to reduce energy consumption or detect circuit flow interaction.
The explanation contained throughout this specification discusses three-output units merely for discussion purposes. The invention is not limited to three outputs, but anticipates as many outputs as the input power can supply sufficient current to protect the target structures. In the current embodiment, four outputs and more are typical.
