US3361660A - Reference electrode mounting assembly - Google Patents

Description

Jan. 2, 1968 J. F. CHITTUM ETAL 3,361,660

REFERENCE ELECTRODE MOUNTING ASSEMBLY 3 Sheets-Sheet 1 Filed June 16, 1964 ZBRINE f FIG.5

INV ENTORS JOSEPH F. CH/TTUM HANK E. ENGLANDER FREDER/C W. SCHREMP BY f w 24 ATTO RN EYS 1968 .1. F. CHITTUM ETAL 3,

REFERENCE ELECTRODE MOUNTING ASSEMBLY Filed June 16, 1964 5 Sheets-Sheet 2 HIIMIIIII llllll I U p L '0 t a} "w: n .536 9 Q LL. I

INVENTORS JOSEPH F. CH/TTUM i| I HANK E. ENGLANDER FEEDER/C w. SCHREMP 5 I BY 1' I ATTORNEYS Jan. 2, 1968 J. F. CHITTUM ETAL 3,351,560

REFERENCE ELECTRODE MOUNTING ASSEMBLY 3 Sheets-Sheet 5 Filed June 16, 1964 II'I' I I I IIIIII II I III I I I I I I I I I I I I United States Patent 3,361,664) REFERENQE ELECTRODE MOUNTING ASSEMBLY Joseph F. Chittum, Whittier, Hank E. Englander, Costa Mesa, and Frederic W. Schremp, Fullerton, Califi, as-

siguors to Chevron Research Company, a corporation of Delaware Filed June 16, 1964, der. No. 375,461 4 Claims. (Cl. 204-195) This invention relates to electrolytic potential measuring apparatus. More particularly, it relates to a device for measuring the electrochemical potential of a steel tank in contact with a brine solution to determine the amount of cathodic protection that must be applied to the tank to prevent loss of metal by solution in the brine.

It is a primary object of the invention to provide an apparatus for measuring the electrochemical potential of the inner surface of a tank containing brine, which comprises a tubular member that can be inserted through an access opening, such as a gate valve that may be installed for such purpose or one that is already available, making electrolytic contact with the brine solution in the tank. The tubular connection permits direct electrolytic connection with the brine at a point near to, but displaced from the surface of the tank. Through this solution in the tubular member, the electrolytic connection is made to a reference electrode in a small body member connected to the tube, and this reference electrode is then connected to one terminal of a potentialindicating device. The other terminal is connected to the metal of the tank. By this arrangement, the cathodic potential then present at the inner surface of the tank can be measured.

It has long been known that it is desirable to measure the actual potential that exists between a brine solution and the inner surface of a tank, such as a crude oil storage tank or wash tank. However, this brine solution, or salt Water, is usually at the bottom of the tank under the crude oil. In tanks with a capacity of several thousand barrels, the major part of the tank may be filled with oil and only the bottom few inches or few feet will contain the brine. This small amount of brine, however, frequently requires that the tank be placed under cathodic protection to prevent electrolytic solution of the tanks iron into the brine. Protection of the tank is often provided by a sacrificial anode, such as a magnesium cylinder that is installed in the brine of the tank and electrically connected by copper cable to the tank wall. The anode is consumed or dissolved to generate the current that is necessary to bring about the required protective potential on the inner surface of the tank. The tank is protected by current from a direct current source, and it is essential that the current be regulated to produce a negative potential of the correct magnitude on the surface of the tank in contact with the brine to oppose the tendency for iron to go into solution.

It has been known that this negative potential between the tank and the brine can be measured by apparatus that includes a standard reference electrode, but it has been necessary, if the cost warranted measurement at all, to lower the electrode through an access hole in the top of the tank possibly down through a body of oil on top of the brine. A particular difficulty in this procedure is that the reference electrode may become coated with crude oil unless special precautions are taken so that an erratic reading is obtained. There is also a danger of losing the reference electrode in the tank. Additionally, the reference electrode may not be properly placed in the brine to give a correct reading. To complete the measurement, the potential of the cell made up of reference electrode and the inner surface of the tank must be determined through wires running back to the top of the tank and from the tank itself to measuring instruments.

In a preferred embodiment of this invention, the foregoing problems are eliminated by connecting a gross body member into a fluid tight engagement with a gate valve that has either been specially installed in the tank below the brine-oil interface or which is already present as a part of a water drop out, or separation system, connected to the tank. The gross body member includes a small body member containing the reference electrode and forming a fluid chamber connected to the elongated tubular member forming a needle, or probe, that can be injected through the fluid tight opening in the gate valve and the interconnecting flange member on the tank so that it terminates at a point a few inches Within the tank. The fiuid chamber contains such standard electrode that is best adapted to be electrically connected to the brine in the tank by fluid in the probe. In said preferred form, the brine in the tank flows out through said probe to fill the fluid chamber. The inside of the tank contacting the brine, the salt bridge" created by this fluid, and the standard electrode form a cell with a potential that is measured by a suitable high impedance voltmeter when such meter is connected across the cell. If the tank is under adequate cathodic protection, that is, a sufficient electrical current of the correct polarity is being supplied to the tank, the indicated negative potential will be a predetermined value suficient to prevent solution of the iron. However, if direct current is connected to the tank in the Wrong sense, reversed polarity, or if a sacrificial anode, such as magnesium or other metal having a more negative potential than iron, has been consumed, the potential-indicating means will indicate nonprotec tion. If a sacrificial anode has been consumed, this potential will be more positive than 0.78 volt when a saturated calomel electrode is used as the reference. If an external current source is used with the wrong polarity, the measured value might be several tenths of a volt less negative than the desired value because the external current acts in the same direction as the corrosion current between the steel and the brine.

In accordance with our invention, the potential of the inner surface of the tank can be measured quickly and conveniently at ground level, so that such measurements can be made often by untrained workers and with minimum hazard to the operator.

Further objects and advantages of the invention will become apparent from the detailed description taken in conjunction with the accompanying drawings which form an integral part of this application.

In the drawings:

FIG. 1 is a schematic, side elevation view of an oil storage tank to which a preferred form of the invention has been applied.

FIG. 2 is an enlarged view partially in cross section of the form of apparatus illustrated in FIG. 1.

FIG. 3 is a top plan view of the apparatus shown in FIG. 2.

FIG. 4 is an enlarged, cross-sectional view illustrating the probe, or needle, portion of the testing apparatus shown in FIG. 2.

FIG. 5 is a schematic, side elevation view of an alternate form of apparatus constructed in accordance with the present invention.

FIG. 6 is an enlarged view of the FIG. 5 apparatus in vertical cross section, and includes a detailed, cross-sectional view of a standardd, or reference, calomel cell as used in the present invention.

Referring now to the drawings, there is shown in FIG. 1 a typical oil storage tank ll of a type frequently used for temporary storage of produced crude oil. Such tanks normally require cathodic protection because in most oil production, salt water or brine is produced with the oil. The oil, of course, is less dense than the brine, so the brine settles as a stratum at the bottom of the tank, as indicated. The interface between the oil and the brine phases is indicated by dashed line 11. This interface is not necessarily a sharp plane, but may be an emulsion, or diffuse layer, of oil and water. The oil, of course, is nonconductive and tank 10 requires no protection from corrosion by the oil. However, the brine forms with the iron of tank 10 an electrochemical corrosion cell that results in solution of the iron in the brine and either evolution of hydrogen or consumption of oxygen in the presence of small amounts of acid or oxygen. Acid can accompany the oil and water from the well; oxygen is also introduced inadvertently when the oil and water are pumped into the tank from the well. To prevent such corrosion, it is common to provide cathodic protection that will polarize the inside of the tank with sufficient direct current of proper negative polarity so that suitable negative electrical potential is created between the iron and brine. This potential must be about 0.77() volt when measured with the standard saturated calomel electrode as reference. Such cathodic protection can be provided by the use of a sacrificial anode 12 which usually comprises a body 14 made of magnesium surrounding a copper core 16. Core 16 electrically contacts tank 10 through conductor 17. The arrangement shown is schematic of one form of cathodic protection frequently used where an external direct current supply is not readily available. In some producing fields, direct current is supplied from a rectifier (not shown) by line- 22 through insulator 23 to a permanent anode, such as conductor rod 24, immersed in the brine (FIG. The current is returned to the rectifier by line 20. The rectifier in turn is connected to a public utility power line.

The only practical way to find out if the protection means is functioning, prior to this invention, has been to measure the potential between the metal surface of'tank and the brine from the top. T 0 measure this potential, it was necessary to take the equipment to the top of tank 10, for example, by stairway 19, and then lower a potential reference electrode through an access opening (indicated in dotted outline by numeral 21). To contact the brine phase at the bottom of the tank, the testing apparatus must pass through the oil phase. Such a procedure has generally been so inconvenient and the results undependable that it has found little acceptance as a continuing maintenance technique. At best it is used only at infrequent intervals.

As distinguished from these previously known techniques, the present invention permits simple and rapid measurement of the electrochemical potential between metal and the brine at ground level. This measuring apparatus is designated as 25 in FIG. 1 and is more specifically shown in FIGS. 2, 3 and 4.

An important feature of the apparatus shown in FIGS. 1 to 4 is that there be available an access valve from the outside of the tank and directly through wall 27 into the brine phase in tank 10. In many field production tanks used for production metering, separation or storage of crude oil, or refined products, a'water knockout or withdrawal line is already available at the desired level. If so, it can be fitted to include a connector, such as flange 29 welded to wall 27 near the bottom of tank 16, and a gate valve 31 directly connected to the fiange. Where such an opening is not available, it is advantageous to install gate valve 31. This can be done when tank 10 is empty by boring access hole 33 (FIG. 2) in wall 27. If it is not convenient to empty tank 10, flange 29 can be installed by a technique known as hot tapping. Briefly,'this technique requires welding flange 29 to the outside of wall 27, attaching valve 31, and to the valve, attaching a packing gland (not shown in FIG. 2). A drill bit, proceeding through the packing gland, and the open gate valve, penetrates the tank wall. The bit is retracted through the into the brine phase. The prime purpose of probe 37 is to form a fluid conduit path, known as a salt bridge. A portion of the brine in tank 10 is admitted to body member 39 through probe 37 to contact reference cell 41. Probe 37, thus, completes an electrolytic connection from reference, or standard cell 41 to the inner surface of wall 27. The potential difference then existing between metal and brine with respect to reference electrode 41 is indicated by high impedance voltmeter 43. One input terminal of meter 43 is connected to electrode 41 by cable I 45; the other terminal is connected to a return circuit by lead 47 and alligator clip 49 which is clamped to any convenient metal surface connected to the tank, for example, lug 51 welded to the outside of tank wall 27. i

It is apparent that, except for flange 29 and gate valve 31, apparatus 25 is not intended to be integral on each tank. The portable portion of measuring device 25 is best seen in FIG. 2. As indicated, it is suitably connected to the tank installation by plug 50 whose pipe threads 52 engage the internal pipe threads of valve 31.

Plug 50, with probe member 37, extending through its center forms a fluid-tight seal so that the gate valve 31 can be opened to permit free access to the brine phase in tank 10. In the embodiment of FIGS. 14, probe 37 comprises a stainless steel tubing 53 and an insulating cover 55 that prevents electrical contact between metal needle 53 and the metal surface of valve 31, flange 29 and wall 27 of tank It). The purpose of this arrangement is to insure that the only internal electrical connection between tank and reference electrode is through the electrolytic solution that flows into the center passageway of needle 53 from tank 10. As best seen in FIG. 4, O-ring 56 seals probe 37 to plug 50 by frictionally surrounding cover 55. Plug 57 holds O-ring 56 in place.

To attach the measuring unit to valve 31, tubular member 59 including annular groove 62 rotatably mounts within collar 60 or plug 50. Screws 61 threaded into collar 69 act as keys in annular groove 62 to keep tube 59 coupled to plug 58. By this connection, plug 50 is free to rotate but when screwed into gate valve 31, makes a fluid-tight connection. Tubular member 59 guides body member 39 so that metal probe 37 will not touch the metal parts of the tank assembly. It also acts as a locking member for body member 39. Standard electrode 41, whose construction will be described more fully in connection with FIG. 6, is mounted so that it seals one end of body 39. This seal is made fluid-tight by O-ring 64. Tube 65 taps into the side of body 39, and with hose 66 and valve 67, controls the flow of brine from tank 10 into body member 39. The flow may be continuous, but need not be. Tube 65 also serves as a means for locking body 39 in a retracted position in notch 68, as indicated in FIG. 2, or in one of three notches 69 when probe 37 is inserted into tank 10. Notches 69 are used to control the exact distance that the forward end of probe 37 extends into the tank. They also permit the body member to be locked against the hydraulic force of the liquid head in tank 10 acting to expel probe 37. In the arrangement of FIG. 2, the outer end of tube 59 includes a mounting bracket 71 that suitably mounts a reel and voltmeter housing 73. Lead 45 may be wound and unwound on a feel (not shown) when body 39 is moved in tube 59. A unitary assembly of all the measuring system includes voltmeter 43 which also conveniently mounts on housing 73. 7

FIG. 6 shows an alternate form of apparatus that may be used to carry out the present invention. In this embodiment, pipe nipple 75 is threaded to gate valve 31 as a permanent part of the field installation. Cap member 77 mounts on nipple 75 and includes a perforation or opening 78 and a rubber or plastic diaphragm 79 that forms a fluid-tight connection with probe member 37A when it is injected into the brine phase of tank 10. In FIG. 6, body member 39A forms an enclosed chamber that can be filled prior to use with a saline solution, such as potassium chloride, indicated as 80. Body 39A is closed by cap 81. A rotatable piston actuator, indicated as 83, mounts on cap 81. Piston 83 permits solution 80 to be ejected from body 39A through probe 37A to complete an electrolytic solution path between calomel electrode 41 and tank 10. As indicated, piston actuating means 83 includes a handwheel 85 and a threaded member 87. Member 87 cooperates with threads in cap assembly 81 to move tapered member 89 inwardly to displace solution 80 through probe 37A. The volume of member 89 is calibrated to equal the amount of fluid required to fill probe 37A.

Reference, or standard, electrode 41 is preferably a conventional calomel electrode. It comprises a platinum wire 90 that is electrically connected to a mixture of mercury and mercurous chloride (generaly referred to as calomel) 91. Contact between wire 90 and calomel mixture 91 is usually made through a body of mercury 92. Mixture 91 is wetted by a potassium chloride solution 93 that fills annular space 94 between the inner tube 95 and outer tube 96. This fluid contact is preferably through a weephole 97 and glass wool fibers 98 that hold calomel mixture 91 in place. External connection to solution 80 is then through a wood fiber 99 mounted in the lower end of cell 41. As indicated schematically, solution 93 is normally saturated, and to assure that it remains so, crystals of potassium chloride 100 are indicated in the bottom of cell 41. An opening 101 in the side of wall 96 permits solution 93 to be replenished or replaced. A cover of rubber tubing or electrical tape 102 covers hole 101. It is preferably perforated with minute pin holes to assure pressure equalization between solution 80 in body 39A and the interior of electrode 41. Such an electrode generates a potential of +0246 volt with respect to the standard hydrogen electrode.

FIG. illustrates another way of measuring the potential between the brine and the tank. As there indicated, battery 105 and potentiometer 106 are connected to produce an IR drop whose potential will balance any potential sensed between measuring device 25A and tank 10. The null point is indicated by ammeter 109. Potentiometer 106 may be calibrated by adjustment of contact 110 on slidewire resistor 111 when double-pole, double-throw switch 112 connects the potentiometer circuit to another standard cell, designated as SC. In this way, the position of slide 107 on potentiometer 106 can be used to indicate the potential between the tank and brine.

From the foregoing description it will be apparent that the primary advantage of this invention is that the actual potential existing between the metal wall of a tank and a corrosive fluid, such as brine, may be readily measured Without special preparations or technically trained operators. These measurements may also be made with a minimum of risk since they are made at substantially ground level. And, a single unit is usable on a number of ditferent tanks.

Various modifications and changes in the apparatus will be apparent to those skilled in the art. Among such changes are the materials and the form of probe member 37. Depending upon the temperature and the condition of the corrosive solution into which the probe is inserted, certain plastics may be used as the probe member. For example, Teflon or nylon tubes have adequate rigidity at temperatures up to about 300 F. and are quite satisfactory for substitution in this arrangement. Additionally, the plastic probe member may have a reasonable amount of flexibility to pass obstructions in piping or valves that may already be available and usable as access means to reach a solution whose potential must be measured relative to a tank with apparatus constructed, as detailed in FIG. 2 or FIG. 6, and the plastic probes are naturally insulated.

It is also apparent that a solution contact can be made either by pulling brine from the tank or by adding an outside solution to the measuring probe.

Other changes falling within the terms of the appended claims are intended to be included therein.

We claim:

1. Potential-detecting apparatus for determining the potential existing between the inner surface of a tank and an electrolytic solution contained therein which comprises a body member adapted to contain an electrolytic solution,

a reference electrode in said body member and electrolytically connected to said solution,

an elongated tubular needle member connected to said body member for establishing an electrolytic solution flow path therethrough,

a coupling member adapted to be detachably connected to a fitting on the tank, said tubular needle member being in slidable and fluid tight engagement with an opening in said coupling member,

means for extending and retracting said tubular needle member relative to said coupling member so that an end of the said tubular needle member may be extended a known distance into the electrolytic solution in said tank,

and means for detecting the potential ditference between said reference electrode and said tank through said electrolytic solution to indicate the extent of cathodic protection then existing between said inner surface of said tank and said electrolytic solution.

2. Apparatus for measuring the potential between a brine solution and a steel tank to determine the extent of cathodic protection then existing on the inner surface of said tank which comprises an elongated housing, a coupling member rotatably mounted at one end of said housing and adapted to threadably engage a fitting on said tank that opens into a portion of said tank containing brine,

an elongated tubular member slidably engaged with said coupling member,

fluid sealing means between said elongated tubular member and said coupling member,

a body member connected to said tubular member, said body member being slidable within said housing member when said tubular member is extended or retracted with respect to said coupling member, said body member forming an enclosed chamber for receiving a portion of the brine from the tank through said tubular member,

means supporting a reference electrode Within said chamber, said reference electrode being electrolytically connectable to said brine in said tank through said body member and said tubular member,

and means electrically connected between said reference electrode and said tank for detecting a potential difference, said potential difference being representative of the cathodic protection of the surface of said tank in contact with said brine.

3. Apparatus in accordance with claim 2 with the addition of conduit means connected at a substantial angle to said body member,

an elongated slot formed in said housing means for receiving said conduit,

and notch means formed in said housing adjacent said slot to lock said conduit and said body member relative to said housing to prevent extrusion of said elongated tubular member by the hydrostatic head of fluid in said tank.

7 4. Apparatus in accordance with claim 3, with the addition of valve means for controlling the flow of brine from said tank through said body member, said valve means being located in said conduit.

References Cited UNITED STATES PATENTS 2,768,135 10/1955 Adelson 204195 2,788,380 4/1957 Epprecht 13623O 8 2,911,859 11/1959 Longley et a1 137 3 18 3,152,058 10/1964 Hutchison et al 204-196 3,229,711 1/1966 Leopold et a1. 137-318 5 ROBERT K. MIHALEK, Primary Examiner.

JOHN H. MACK, Examiner.

T. TUNG, Assistant Examiner.

Claims (1)

1. POTENTIAL-DETECTING APPARATUS FOR DETERMINING THE POTENTIAL EXISTING BETWEEN THE INNER SURFACE OF A TANK AND AN ELECTROLYTIC SOLUTION CONTAINED THEREIN WHICH COMPRISES A BODY MEMBER ADAPTED TO CONTAIN AN ELECTROLYTIC SOLUTION, A REFERENCE ELECTRODE IN SAID BOYD MEMBER AND ELECTROLYTICALLY CONNECTED TO SAID SOLUTION, AN ELONGATED TUBULAR NEEDLE MEMBER CONNECTED TO SAID BODY MEMBER FOR ESTABLISHING AN ELECTROLYTIC SOLUTION FLOW PATH THERETHROUGH, A COUPLING MEMBER ADAPTED TO BE DETACHBLY CONNECTED TO A FITTING ON THE TANK, SAID TUBULAR NEEDLE MEMBER BEING IN SLIDABLE AND FLUID TIGHT ENGAGEMENT WITH AN OPENING IN SAID COUPLING MEMBER, MEANS FOR EXTENDING AND RETRACING SAID TUBULAR NEEDLE MEMBER RELATIVE TO SAID COUPLING MEMBER SO THAT AN END OF THE SAID TUBULAR NEEDLE MEMBER MAY BE EXTENDED A KNOWN DISTANCE INTO THE ELECTROLYTIC SOLUTION IN SAID TANK, AND MEANS FOR DETECTING THE POTENTIAL DIFFERENCE BETWEEN SAID REFERENCE ELECTRODE AND SAID TANK THROUGH SAID ELECTROLYTIC SOLUTION TO INDICATE THE EXTENT OF CATHODIC PROTECTION THEN EXISTING BETWEEN SAID INNER SURFACE OF SAID TANK AND SAID ELECTROLYTIC SOLUTION.

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