The industrial products of greatest interest in the electrochemical field, that is chlorine and caustic obtained by electrolysis of aqueous solutions of sodium chloride, and hydrogen and oxygen obtained by water electrolysis, are produced in plants comprising a large number of electrolyzers electrically connected in series to an electric power source. When one of these electrolyzers needs maintenance, a suitable jumper switch means is connected to the electrolyzers immediately preceding and following the electrolyzer to be by-passed so that a low resistance path is formed which is preferentially flown by electric current. In this way the by-passed electrolyzer, wherein electric current no more travels, is shut down and removed from the row to be sent to maintenance. The electric current continues to travel through the electrolyzers circuit and, in correspondence of the point where the electrolyzer has been removed, it flows through the jumper switch means. When maintenance is over, the serviced electrolyzer is positioned again in the row and electrically connected through the relevant connections to the immediately preceding and following electrolyzers. Then the jumper switch means is disconnected following a sequence of operations allowing electric current to be fed again to the serviced electrolyzer.
The connection of the jumper switch means, which is the initial stage of the removal of the electrolyzer to be serviced, poses several problems. The first of these problem is related in particular to high current monopolar electrolyzers, wherein connection of the jumper switch means may cause a current shift which may damage the internal components of the electrolyzers immediately preceding and following the electrolyzer to be bypassed. This problem may be overcome as taught in U.S. Pat. Nos. 3,930,978-4,078,984 and in the European publication no. 0492551 A1, by using jumper switch means provided with multiple extension arms and positioned beneath the electrolyzers supporting base or above the electrolyzers, by means of cranes. Both configurations, which result from the presence of the multiple extension arms, permit also to solve the second problem typically affecting the jumper switch means of the prior art. These latter conventionally have the form of carts which can be moved along the supporting base of the electrolyzers and laterally with respect to the electrolyzers row. As the vertical size of said jumper switch means carts is remarkable, the electrolyzer to be serviced must be lifted and removed only by means of suitable cranes. This operation, besides requiring expensive equipment due to the remarkable weight of the electrolyzer, involves also a risk, as the electrolyzer is raised above both the rows of electrolyzers under operation and the operators themselves. For instance, leaks of liquids still present in the electrolyzer could have severe consequences.
If the jumper switch means is positioned beneath or above the electrolyzers, removal of one electrolyzer for maintenance may be made laterally by means of fork-lift trucks. However, positioning of the jumper switch means beneath or above the electrolyzers still involves very high costs due to more complex foundations or to the expensive structures required for the cranes.
A third problem is represented by the current reversal which crosses the electrolyzer to be bypassed when the electrolysis current is sent to the low resistivity circuit of the jumper switch means. This effect may be compared to the one characterizing the discharge of a charged condenser, when connected to a low resistivity circuit. The current reversal is particularly dangerous for the activated cathodes which are currently used in the electrolyzer, in particular for chlor-alkali and water electrolysis. The problem of reverse current has been faced in the prior art, for example see U.S. Pat. No. 4,589,966 wherein the jumper switch means are provided with suitably dimensioned resistor circuits which are inserted in a sequence with the progressive reduction of the current travelling across the electrolyzer to be by-passed and with the consequent reduction in its voltage. The voltage at the terminals of the jumper switch means, that is the contact points of the electrolyzer to be bypassed, is not brought to zero but it is kept at a minimum value, so-called polarization value, corresponding to the voltage spontaneously reached when current is no more fed. Under this condition, the electrolyzer is ready to function as a battery supplying current (reverse current) if the resistance of the external circuit, that is of the jumper switch means, is further reduced. To prevent this possibility, U.S. Pat. No. 4,589,966 foresees a suitable additional electrical circuit provided with other switches which are operated in a sequence. This circuit is undoubtedly effective, but it is expensive and complicated to be operated.