US2758080A - Spring loaded galvanic anode assembly - Google Patents

Description

1956 J. w. BERNARD 2,758,080

SPRING LOADED GALVANIC ANODE ASSEMBLY Filed April 7, 1954 2 Sheets-Sheet l my a IN VENTOR J /m Ml Berna/'0 ATTORNEYS.

Aug. 7, 1956 J. w. BERNARD 2,758,080

SPRING LOADED GALVANIC ANODE ASSEMBLY Filed April 7, 1954 2 Sheets-Sheet 2 INVENTOR Ja/m W Bernard ATTORNEYS.

United States Patent 2,758,080 SPRING LOADED GALVANIC ANODE ASSEMBLY John W. Bernard, Midland, Mich., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Application April 7,1954, Serial No. 421,486 7 Claims. (Cl. 204-197) This invention relates to an improved galvanic anode assembly for use in the cathodic protection of metal vessels, andparticularly for use in vertical evaporators of the type commonly used in the production of Epsom salts and the like.

Cathodic protection of vertical and other evaporators has been accomplished ,for many years in the so-called fluid portion of the evaporators. This has been accomplished, for example, by suspending, in the fluid, consumable anodes of a galvanically active metal anodic to the vessel. The resulting flow of current maintains the vessel cathodic to the medium and minimizes corrosion.

Cathodic protection of the ,steam side of the evaporators, however, has been generally unsuccessful, mainly because builders and operators of these evaporators have been unwilling to reduce the number of evaporator tubes in order toprovide sufficient room to mount galvanic anodes of suliicient size to give adequate and long lived protection against corrosion. Because of the maze of tubes in the evaporators and the limited throwing power of galvanic anodes, considerable re-design of the tube distribution in evaporators wouldbe necessary to provide enough space between the tubes to permit the installation of anodes of sufficient diameter to provide long, efliective cathodic protection. However,'any re-design which materially lessens the number of tubes "in the evaporator also materially reduces the operating ,efiiciency of the evaporator.

Because of the above and other difliculties the corrosion and ultimate failure of evaporator tubes in the condensatecovered portions of the steam side of the evaporator has been considered a necessary evil. When a tube failed because of excess corrosion, the procedure usually was to shut down the evaporator and grind out the defective tube or tubes and replace them with new tubes which then were rolled into the upper and lower tube sheets of the evaporators. Such repairs are expensive because of lost operational time, cost of replaced materials, and the labor cost involved.

When cathodic protection in the condensate area has been attempted in the past, the results have been characterized by uneven protection, short life, and, in view of the above, by prohibitive cost.

It is, accordingly, an object of this invention to provide an improved galvanic anode assemblywhich is adapted for use in vertical evaporators and has a long, useful life.

A further object of this invention is to provide an improved galvanic anode assembly adaptable for use in the steam side of a vertical evaporator and which may be easily installed in and removed from such evaporators. An additional object of this invention is to provide an improved vertical evaporator having galvanic means whereby reliable, long lived, cathodic protection through out the condensate portion of the evaporator is economically achieved. a In accordance with the present invention, there is provided a spring loaded hollow galvanic anode which surrounds and is carried by a self-supporting core and base 2 member. The anode assembly is inserted through a tapped hole in the bottom evaporator tube sheet and is conductively and physically secured to the bottom tube sheet by means of a threaded base plug member to which the remainder of the anode assembly is mechanically and electrically secured. 1

The main electrical contact with the lower tube sheet is made via the spring loading member and through the core rod which supports the anode. Thus good electrical contact is provided between the anode and the lower tube sheet until the anode is almost entirely consumed. Insulation in the form of a film sleeve or coating between the core electrical conductor and the anode excludes the possibility of local corrosion between the two which might cause the anode to corrode and ultimately to break above the condensate level and thus create a high resistance path between the tube sheet and the portion of the anode which is in the condensate.

To provide reliable cathodic protection to the entire portion of the evaporator which is in the condensate, an array of detachably mounted galvanic anode assemblies is generally symmetrically disposed in the space between the tubes.

Because of the long useful life of galvanic anode assemblies made in accordance with this invention, and because of the ease of installation in existing evaporators without modification of the evaporator tube arrangement, economical and reliable cathodic protection of the steam side of the evaporator tubes is realized.

The invention, as well as additional objects and advantages thereof, will best be understood by reading the following detailed description in connection with the accompanying drawings, in which:

Fig. 1 is an elevational view, partly in section, of a spring loaded galvanic anode assembly made in accord ance with this invention;

Fig. 2 is an enlarged, fragmentary sectional view of the galvanic anode shown in Fig. 1;

Fig. 3 is an elevational view, partly broken away and in section, of a typical vertical evaporator in which galvanic anode assemblies in accordance with the present invention may be advantageously utilized;

Fig. 4 illustrates typical corrosion effects on evaporator tubes in the condensate area of an evaporator and shows the manner of mounting a spring loaded galvanic anode of the present invention between the tubes and on the lower tube sheet of an evaporator;

Fig. 5 illustrates evaporator tube-galvanic anode layout in accordance with this invention; and

Fig. 6 is an enlarged elevational view, partly in section, of an alternative form of spring loaded anode assembly made inaccordance with this invention.

Referring to Figs. 1 and 2, there is shown a spring loaded galvanic anode assembly 10 which is mounted through the lower tube sheet 12 of a vertical evaporator (of the general type shown in Fig. 3) by means of a threaded plug 14 whose smallest diameter is slightly larger than the diameter of the hollow galvanic anode 16. The plug 14 serves as the lower stop means against which the galvanic anode 16 is pressed. The galvanic anode 16, which may be composed of magnesium, aluminum, zinc or other suitable material which is anodic to the base metal of the evaporator which is to be protected, is supported by the conductive core rod 18. The threaded end portion 20 of the core rod 18 is screwed into the centrally disposed tapped hole 22 in the threaded plug 14, the tapped hole 22 opening on the enclosed interior portion of the evaporator. One end 24 of the galvanic anode 16 is held in physical contact with the end of the plug 14 because of the force applied by the spring 26 which is held under compression. The lower end 28 of the spring 26 is inserted (by a force fit) into an aperture 30 in the upper end of the galvanic anode 16 to provide good physical and electrical contact between the galvanic anode 16 and the core rod 13. The upper end 32 of the spring 26 is mechanically and electrically secured to the threaded upper end 34 of the core rod 18, by means of soldering, for example, to the washer and lock nut arrangement 36 which constitutes the upper stop means for the assembly. Some adjustment of the compression on the spring 26 may be made by changing the location of the lock nut and washer arrangement 36 on the core rod 18.

The galvanic anode 16 is insulated from its core rod 18 by means of an insulating coating 38, such as Heresite or plastic, for example, which extends along the core rod 18 at least as far as the galvanic anode entends above the lower threaded portion 20 of the core rod. Insulating and lubricating films or coatings of silicone, high temperature greases, or other insulating and lubricating compounds or materials may be used if they are capable of withstanding the ambient temperature and atmosphere without decomposing. The insulating coating is important in preventing unwanted galvanic action between the anode 16 and the core rod 18. The importance of preventing such action will be explained in greater detail later. The insulation could bea film or coating on the interior of the anode, the exterior of the rod, or be a sleeve disposed between the core and anode.

For a better understanding of the use to which the anode assemblies will be put, reference is made to Fig. 3 which shows a typical vertical evaporator 40. The evaporator 40 has a fluid inlet tube 42 through which the liquid to be evaporated is inserted and a settling portion 44 where solid material precipitated during evaporation settles out. An upper and a lower tube sheet 46, 48 are provided between the upper or inlet part of the evaporator 40 and the settling portion 44 thereof. Evaporator tubes 58 are disposed in an array between the upper and lower tube sheets 46, 48.

The evaporator tubes 50, which are usually made of steel and may number considerably more than 100 in a single evaporator, are commonly secured or bonded to apertures in the tube sheets by rolling in, which is similar to a cold welding process. Inasmuch as the rolling process takes place within the evaporator structure, it can be appreciated that the grinding out of defective tubes and the replacement with new ones by rolling is expensive and time consuming. A large diameter draft tube 52, which may be centrally disposed as illustrated or offset towards the perimeter of the evaporator, is likewise provided.

This invention is mainly concerned with preventing, in an economical manner, the corrosion and eventual failure, due to puncturing, of the evaporator tubes 50 and the lower tube sheet 48. Such failures usually occur in the condensate area adjacent to the lower tube sheet 48. While the material to be evaporated is circulated through the evaporator tubes 50 and the larger tube 52, steam is applied around the evaporator tubes 50 and the tube 52 through an inlet 54 in the peripheral or outer wall 56 of the evaporator 40. The liquid condensate from the steam, settling along the bottom of the tubes 50 and on the lower tube sheet 48, is the cause of the unwanted corrosion.

The circulation of the liquid through the small tubes 50 and back through the larger tube 52 is effected either by convection or by means of a circulating pump (not shown). The method of inducing circulation of the fluid is immaterial to the invention. An outlet tube 58, provided for the removal of the condensate, is usually disposed close to the lower tube sheet 48, with the result that a condensate layer, normally about one or two inches deep, is present on the lower tube sheet 48 and around the evaporator tubes 50 and the larger tube 52.

While corrosion of the type described above may be efli'ectively prevented by cathodic protection systems in which galvanic anodes are utilized, such protection has not heretofore been practicable from an economic standpoint for use in vertical evaporators having a maze of closely spaced evaporator tubes. When cathodic protection using galvanic anodes has been tried, the short effective life of the anodes and the .cost of installation and replacement has been found to be prohibitive in view of the results obtained. Long, thin galvanic anodes were required because of the close spacing between the tubes, and these were of no great utility since the only part giving eifective protection was the part which was within the condensate. This was usually only about the bottom two inches of the anode. The part of the small diameter anodes in the condensate would be consumed rapidly and the remainder would be of no use for cathodic protection purposes since no good, reliable electrical contact would exist between the steel of the evaporator and the anode, though the upper, unconsumed part of the anode might fallinto the condensate.

Fig. 4 illustrates the spacing of the evaporator tubes 50 (on an exaggerated scale) and shows the corroded portion 60 on the lower portion of each evaporator tube adjacent to the lower tube sheet 48 (where the condensate layer exists). For the sake of simplicity in the drawings, the tubes 50 are shown as being cut or broken off shortly above the corrosion area, but it must be appreciated that in practice the tubes are usually several (perhaps 12 to 18) feet long. A single spring loaded galvanic anode assembly 10 of this invention is illustrated in Fig. 4 for the purpose of showing its location, when mounted in an evaporator, with respect to the evaporator tubes 50.

A layout illustrating the use of a number of galvanic anodes in accordance with the present invention to provide cathodic protection to the steam side of a vertical evaporator is shown in Fig. 5. The anode assemblies 10 are disposed in the same triangular pattern as are the evaporator tubes 50. This arrangement provides adequate protection to the lower tube sheet 48 and all the evaporator tubes 50, yet provides a substantially uniform rate of decomposition of the anodes.

Because the anode 16 is spring loaded, as the end 24 of the anode which is in the condensate is consumed the remainder of the anode is pushed down into the condensate. Because of this, even though the diameter of the anode 16 may be small, its usable length is such that long, etfective life of the anode 16 is assured.

The end of the anode 16 which is in the condensate may become spongy as it is consumed. Therefore, the mechanical contact between the lower end 24 of the anode 16 and the plug 14 is not relied upon to provide a good electrical contact between the anode 16 and the evaporator. Instead, the primary electrical contact between the anode 16 and the evaporator 40 is made through the spring 26, which as previously mentioned is mechanically and electrically secured to the electrically conductive core rod 18.

Good electrical contact is provided between the anode 16 and the evaporator 40 from the top end of the anode 16, as previously explained, but local action between the metal (usually steel) core rod 18 in the presence of moisture and the anode 16 from the steam could cause the anode 16 to be consumed intermediate of its ends. If the anode 16 is consumed to the point where the anode breaks into two or more parts, the electrically conductive path between the anode and the evaporator through the spring 26 and the core rod 18 to the evaporator 40 may be broken. If the electrically conductive path through the core rod 18 is broken, the effectiveness of the anode 16 in providing cathodic protection to the evaporator may be greatly diminished. The reason for the diminished protection is that the physical contact between the anode and evaporator is usually not a good electrical contact due to the spongy rasped nature of the anode ends and also to insulating "coatings which may be formed on adjacent steel parts while the anode 16 is being consumed.

The insulation provided by thecoating 62 on the core rod 18 prevents local electrolytic action between the anode 16 and the core rod 18 and assures that good electrical contact will be kept between the anode 16 and the evaporator 40 until the anode 16 is almost entirely consumed. Materials such as silicones or high temperature greases which, when applied to the core rod, form both an insulating and lubricating film or coating may be used to advantage in many applications. The hollow core of the anode 16 should be of sufficiently large diameter to allow the core rod 18 to move freely within it. A spring loaded anode assembly in accordance with this invention requires only a single tapped hole in the lower tube sheet in order to mount the assembly in the evaporator. No modification of the disposition of evaporator tubes within the evaporator is required.

The length of the anode 16 of the assembly 10 is such that the anode will provide cathodic protection to the evaporator over a considerable period of time. When the anode is depleted, the assemblies may be easily removed and new anode assemblies installed.

The old anode assembly 10 may be repaired by installing a new anode 16 on the old core rod 18, thus reducing the overall cost of cathodic protection to the evaporator 40.

Another embodiment of this invention is shown in Fig. 6. In the spring loaded anode assembly 10a shown in this figure, the core rod 18a is a hollow rod which is composed of insulating material. The lower end of the rod 18a is screwed into or otherwise secured to plug 14a. The plug 14a has a slot 70 extending longitudinally thereof and aligned with the hollow core 66 of the core rod 18a when that member is screwed into the plug 14a.

Because the core rod 18a is made of insulating material, this member cannot be used to make the electrical connection between the spring 26 and thelower tube sheet 12. The electrical connection between the lower tube sheet 12 and the spring 26 is made by a con duct-or 64 such as a wire or thin rod which passes through the hollow core 66 of the rod 18a. The upper end of the conductor 64 is electrically connected to the upper end of the spring 26 by soldering, for example. The lower end of the conductor 64 is conductively connected to the plug 14a by means of a soldered connection 68, for example.

In the embodiment of the invention shown in Fig. 6, the insulating core rod 1 8a could, because of its thickness, be used for a considerable period of time (the life of several anodes) without the insulation between the anode 16 and the conductor 64 wearing through. Thus, from the standpoint of preventing local action between the anode 16 and the conductor 64, the embodiment of Fig 6 provides an excellent assembly onto which new anodes 16 may be added as needed.

When the anode assemblies 10 are initially installed in an evaporator, a protective coating builds up first on those portions of the tubes 48 in the condensate which are physically nearest to each anode. The protective coating, however, tends to increase the resistance between the tube and the anode and thus the effective protective power of the anode is increased so that in time the entire part of the tube which is in the condensate is protected.

While the period during which cathodic protection is provided is a function of the mass of the anode among other things, it is also a function of the specific resistivity of the condensate. For example, a spring loaded anode in which the galvanic anode member had inch outside diameter and was ten inches long would have a useful life of about 3.8 years in a condensate having a specific resistivity of rooo iaoo ohms. A similar electrode would have a useful life of only about "3.8 months when the specific resistivity of the condensate is around ohms.

If specific resistivity of the condensate is too high, it may be -'controll'ed and maintained within the desired 1000-1300 ohm range by adding small amounts of noncorrosive electrolyte to the condensate.

While this invention has been described in connection with evaporators, its use is not limited to such devices. It is adaptable for use in many devices in which the space available for mounting a galvanic anode in a fluid is limited in lateral dimensions but where the limitation of the length dimension is not so restrictive.

I claim:

1. A spring loaded galvanic anode assembly comprising an elongated anode having a bore extending axially therethrough, the length of said anode being substantially greater than the width thereof, a coil spring, a core member extending through said bore and axially through said spring in loose fitting relationship therewith, and stop members secured to each end portion of said core member, said spring and said anode being compressed between said stop members with an end of said anode abutting against one of said stop members, said last mentioned stop member being made of metal and comprising the mounting means for said assembly.

2. A spring loaded galvanic anode assembly comprising a hollow elongated galvanic anode, a coil spring having one of its ends mechanically and electrically secured to an end portion of said anode, an electrically conductive core member extending through the hollow part of said anode and axially through said spring in loose fitting relationship thereto, and electrically conductive stop members, one of said stop members being secured to each end portion of said core member, said spring and said anode being compressed between said stop members, said spring making an electrical connection with one of said stop members and an end of said anode abutting against the other of said stop members, the stop member which is adjacent to said anode being the mounting means of said assembly.

3. An anode assembly in accordance with claim 2, wherein said anode is separated from said core member by a sheath of insulating material.

4. A spring loaded galvanic anode assembly comprising a hollow elongated galvanic anode, a coil spring having one of its ends mechanically and electrically secured in a fixed manner to an end portion of said anode, a core member extending through the hollow part of said anode and axially through said spring, and electrically conductive stop members secured to each end portion of said core member, said spring and said anode being compressed between said stop members, the said stop members being electrically connected one to another.

5. An anode assembly in accordance with claim 4, wherein said core member comprises a hollow electrically insulating rod, and the electrical connection between said stop members is made through said rod.

6. A spring loaded galvanic anode assembly comprising an electrically conductive base plug member, an elongated generally cylindrical galvanic anode having an aperture extending between its ends, a coil spring disposed in axial alignment with said aperture and being electrically and mechanically connected to said anode, a core rod extending axially through said aperture and said spring, said core rod having one of its end portions secured to said base plug member, stop means adjacent to the other end portion of said rod for maintaining said spring in compression between said stop means and the end of said anode to which said spring is connected and thereby pressing the other end of said anode against said base plug member, and means for electrically connecting said spring to said base plug member.

7. An anode assembly in accordance with claim 6,

wherein said core rod comprises a conductive inner por- 1,608,709 tion and an outer insulating portion which prevents di- 2,656,314 rect electrical contact between said galvanic anode and said conductive inner portion of the core rod.

References Cited in the file of this patent UNITED STATES PATENTS 901,809 Harris et al Oct. 20, 1908 '8 Mills Nov. 30, 1926 Osterheld Oct. 20, 1953 FOREIGN PATENTS Great Britain Mar. 1, 1923

Claims (1)

1. A SPRING LOADED GALVANIC ANODE ASSEMBLY COMPRISING AN ELONGATED ANODE HAVING A BORE EXTENDING AXIALLY THERETHROUGH, THE LENGTH OF SAID ANODE BEING SUBSTANTIALLY GREATER THAN THE WIDTH THEREOF, A COIL SPRING, A CORE MEMBER EXTENDING THROUGH SAID BORE AND AXIALLY THROUGH SAID SPRING IN LOOSE FITTING RELATIONSHIP THEREWITH, AND STOP MEMBERS SECURED TO EACH END PORTION OF SAID CORE MEMBER, SAID SPRING AND SAID ANODE BEING COMPRESSED BETWEEN SAID STOP MEMBERS WITH AN END OF SAID ANODE ABUTTING AGAINST ONE OF SAID STOP MEMBERS, SAID LAST MENTIONED STOP MEMBER BEING MADE OF METAL AND COMPRISING THE MOUNTING MEANS FOR SAID ASSEMBLY.

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