US3424665A

US3424665A - Cathodic protection system

Info

Publication number: US3424665A
Application number: US500765A
Authority: US
Inventors: James A Mathews
Original assignee: Harco Corp
Current assignee: Harco Corp
Priority date: 1965-10-22
Filing date: 1965-10-22
Publication date: 1969-01-28
Anticipated expiration: 1986-01-28

Description

- Jan. 28, 1969 J. A. MATHEWS CATHODIC PROTECTION SYSTEM Filed Oct. 22. 1965 .N X Z P K: AA/ l g fi WWW/N Q INVENTOR JAMES E. MATHEWS 6 E o 3 ATTORNEYS United States Patent 8 Claims The present invention relates generally, as indicated to a protective system and, more particularly, to certain improvements in self-regulating cathodic protection systems especially of the type shown and described in US. Letters Patent No. 3,143,670, granted to Bernard Husock on Aug. 4, 1964.

In such prior patent, there is disclosed a relatively simple and inexpensive impressed current cathodic protection system which is adapted to provide a substantially constant current at any given anode even though there may be a rather wide variance in the resistivity of the soil or other electrolyte in which the structure to be protected, such as a gasoline tank, for example, is buried. This substantiallly constant current is obtained by placing a fixed resistor, which has a resistance at least ten times as great as the resistance between the anode and the protected structure, in series with a rectifier for converting a typical 115 volt A.C. source to direct current. Accordingly, even should the conductivity of the electrolyte between the anode and protected structure change rather drastically, it has been found that the current output will still deviate less than percent. Thus, by employing the protective system of the aforementioned Husock patent, it is possible to provide the minimum current flow which will protect the buried structure, since there is no danger of a substantial drop in such flow as operating conditions change.

However, in some cases it is desirable to provide a cathodic protection system which provides an even more constant current output despite fluctuations in the resistance between the anode and structure protected, which is a principal object of the present invention.

Another object is to provide an improved cathodic protection system of the type indicated which generates much less heat for longer component life than the resistor circuit of the aforementioned Husock patent.

Still another object is to provide such cathodic protection system which maintains the desired low constant current output at the anodes under a lower voltage potential in the vicinity of the protected structure.

Other objects and advantages of the present invention will become apparent as the following description proceeds.

To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims, the following description and the annexed drawing setting forth in detail a certain illustrative embodiment of the invention, this being indicative, however, of but one of the various ways in which the principles of the invention may be employed.

In such annexed drawing there is shown a circuit diagram of a preferred form of protection system in accordance with the present invention.

The preferred embodiment of protective system illustrated in the drawing is generally indicated at 1, it being shown schematically in a typical installation providing a substantially constant low current output at an anode 2 buried adjacent but out of contact with a metal gasoline tank 3 or the like which is to be protected from corrosion. The current for the protective system 1 may be supplied from an external 117 volt A.C. source connected to the leads 4 and 5 of the primary winding 6 of a 117 v. A.C. primary, 28 v. A.C. secondary, l0 va. step-down transformer 7.

The transformer secondary winding 8 is in turn connected to diagonally opposite corners 10 and 11 of a bridge rectifier 12 which in the form illustrated is a 1 ampere, v. PRV encapsulated rectifier comprised of four diodes 13 arranged in back-to-back relationship for creating a full-wave direct current output at the remaining two

corners

14 and 15. The current from the bridge rectifier 12 always flows in the same direction from the corner 14 and back to the corner 15. The output corner 14 of the rectifier 12 is connected to the emitter 16 of a power transistor 17 through an emitter bias resistor 18, while the base 19 of the transistor is connected through a Zener voltage reference diode 20 across the emitter bias resistor 18. In addition, the transistor base 19 is connected through a Zener diode reference resistor 21 to ground through an electric cable 22 leading to the corner 15 of the rectifier 12 and also to the tank 3 which is the cathode. The collector 25 of the transistor 17 is connected to the anode 2.

The emitter bias resistor 18, which in a typical installation may be a 68 ohms, 11 /2 watt resistor, determines the current flow between the base 19 and emitter 16 of the power transistor 17, thus helping to provide the correct current value at the anode 2. However, the low valve resistor 18 does not prevent fluctuations of current at the anodes 2, as when the load resistance between the anode and tank 3 fluctuates between wet and dry periods or for other reasons. Such current fluctuations are prevented by the Zener diode 20 because of its unique operating characteristics which permit it to accommodate changes in output current without a change in voltage, that is, so long as the Zener diode reference resistor 21 has a value which will cause the diode 20 to operate in its Zener region. In the preferred form shown, the resistor 21 is a 4.7K ohms, 2 watt resistor.

From the above discussion, it can now be seen that the transistorized current regulator 1 of the present invention is basically a grounded base circuit, with the collector current being essentially equal to the emitter current due to the high gain characteristics of the transistor 17. Moreover, the current supplied to the load will remain essentially constant until the load resistance (i.e., the resistance between the anode 2 and tank 3) is increased to a point where the voltage drop across the emitter bias resistor 18 is as large as the voltage drop across the transistor base resistor 21.

In actual practice, it has been found that this particular system will provide a 100 milliamperes nominal current at the anode 2 over a range of from 0 to 200 ohms variance in the load resistance, which is a much greater variance than is ordinarily found. Thus, by using the protective system of the present invention it is possible to provide the minimum current flow which will protect a buried structure, since there is no danger of a substantial drop in such flow as operating conditions change. Furthermore, this system operates at a very low potential at the anodes while still maintaining the desired constant current output, which is advantageous especially where there is a fire hazard as when the structure being protected contains a flammable product such as gasoline. Also, because the resistor 18 has a rather low resistance and the power transistor 17, due to its construction, permits maximum dissipation of heat, the system disclosed herein operates at cool temperatures for longer component life.

While only one anode and its associated components is shown, it should be understood that a plurality of such anodes may be provided, the exact number depending upon the amount of current needed to protect a given structure. Thus, if the current output at each anode is 100 milliamperes, and 800 milliamperes are required to protect a given structure, then it would be necessary to provide eight such anodes with associated components.

Other modes of applying the principles of the invention may be employed, change being made as regards the details described, provided the features stated in any of the following claims or the equivalent of such be employed.

I therefore particularly point out and distinctly claim as my invention:

1. A protective system for objects subject to corrosion comprising at least one anode adapted to be submerged in an electrolyte in which such object is located closely adjacent but out of contact with such object, a transistor comprising an emitter, a base, and a collector, said collector being directly connected to said anode, an emitter bias resistor connected to said emitter, said base being con nected through a Zener diode across said emitter resistor, and an electric cable adapted to be connected to such object as the cathode, said base being connected to ground through a Zener diode reference resistor, said Zener diode also being connected to ground through said Zener diode reference resistor.

2. The protective system of claim 1 wherein said emitter resistor has a low resistance and said transistor is a power transistor which permits maximum dissipation of heat, whereby said system operates at cool temperatures for longer component life.

3. The protective system of claim 1 further comprising a step-down transformer the primary winding of which is adapted to be connected to an A.C. source, a bridge rectifier having input terminals connected to the secondary winding of said transformer and output terminals con nected to said resistors, respectively.

4. The protective system of claim 3 wherein said bridge rectifier is comprised of four diodes arranged in back-toback relationship for creating a full-wave direct current output.

5. The protective system of claim 3 wherein said bridge rectifier is a 1.0 ampere, 100 v. PRV encapsulated rectifier for converting alternating current from a 117 v. A.C. source, said transformer is a 117 v. A.C. primary, 28 v. A.C. secondary, va. step-down transformer, said emitter bias resistor is a 68 ohms, 11 /2 watt resistor, and said Zener diode reference resistor is a 4.7K ohms, 2 watt resistor.

6. The protective system of claim 1 wherein there are a plurality of said anodes with associated transistors, emitter bias resistors, Zener diodes, and Zener diode reference resistors connected thereto as aforesaid.

7. A protective system for underground metal storage tanks and the like comprising a plurality of anodes adapted to be buried adjacent but out of contact with such tank, an alternating current source, a plurality of transistors each including an emitter, a base, and a collector, said collector for each transistor being directly connected to one of said anodes, an emitter bias resistor connected to each of said emitters, said base for each of said transistors being connected through a Zener diode across said emitter resistors, and an electric cable adapted to be connected to such object as the cathode, said base for each resistor being connected to ground through a Zener diode reference resistor, said Zener diode also being connected to ground through said Zener diode reference resistors, a step-down transformer having a primary winding which is connected to an A.C. source, a bridge rectifier having input terminals connected to the secondary winding of said transformer and output terminals connected to said resistors, respectively.

8. The protective system of claim 7 wherein said bridge rectifier is a 1.0 ampere, 100 v. PRV encapsulated rectifier for converting alternating current from 117 v. A.C. source, said transformer is a 117 v. A.C. primary, 28 v. A.C. secondary, 1O va. step-down transformer, said emitter bias resistors are 68 ohms, 11 /2 watt resistor, and said Zener diode reference resistors are 4.7K ohms, 2 watt resistors.

References Cited UNITED STATES PATENTS 3,143,670 8/1964 Husock 307- 3,242,064 3/1966 Byrne 204-196 3,258,612 6/1966 Rubelmann 204196 JOHN H. MACK, Primary Examiner.

T. TUNG, Assistant Examiner.

US. Cl. X.R.

Claims

1. A PROTECTIVE SYSTEM FOR OBJECTS SUBJECT TO CORROSION COMPRISING AT LEAST ONE ANODE ADAPTED TO BE SUBMERGED IN AN ELECTROLYTE IN WHICH SUCH OBJECT IS LOCATED CLOSELY ADJACENT BUT OUT OF CONTACT WITH SUCH OBJECT, A TRANSISTOR COMPRISING AN EMITTER, A BASE, AND A COLLECTOR, SAID COLLECTOR BEING DIRECTLY CONNECTED TO SAID ANODE, AN EMITTER BIAS RESISTOR CONNECTED TO SAID EMITTER, SAID BASE BEING CONNECTED THROUGH A ZENER DIODE ACROSS SAID EMITTER RESISTOR, AND AN ELECTRIC CABLE ADAPTED TO BE CONNECTED TO SUCH OBJECT AS THE CATHODE, SAID BASE BEING CONNECTED TO GROUND THROUGH A ZENER DIODE REFERENCE RESISTOR, SAID ZENER DIODE ALSO BEING CONNECTED TO GROUND THROUGH SAID ZENER DIODE REFERENCE RESISTOR.