Experience with Long Term Operation with Demineralized Water Systems at Da Ne

Several papers deal with the problem of managing low conductivity water plant in accelerators [1,2], but outside of this field, only electric power generation plants (cooling of high voltage stator windings) seems to suffer from similar troubles. During eight years operation of the DA NE water cooling system some critical situations have been successfully managed by upgrading the demineralized water system. The collected data and the experience gained in the field of copper corrosion and related water treatment are critically revised. Typical values of water characteristics are: conductivity < 0.2 S/cm, dissolved oxygen <20 ppb, 6.8 < pH < 7.2. INTRODUCTION In the specific case of accelerator magnets, the leading corrosion process could be the electrochemical one. The Coulomb law (1) gives an estimation of the rate of metal mass (m) loss, knowing the expected current I through the electrolyte and the Faraday constant F: The water cooling system for the Da ne complex has been operated successfully since mid nineties. More than 1200 meters of piping convey low conductivity cooled water from pumping stations to end users in the accelerator buildings. The piping is made of AISI 316 stainless steel, jointed in place with high quality TIG welding. The end users are magnets and the RF structures. All their cooling passages are OFHC copper channels with stainless steel fittings, with no other metals included. A treatment system maintains the characteristics of the water, mainly the conductivity and the dissolved oxygen contents. I Fn M dt dm (M atomic weight, n ion charge) (1) Loss of copper of some mg to some 10 mg per year could be expected in good operating conditions. THE DA NE COOLING SYSTEM In figure 1 is represented the Main Rings cooling system. Note the position of the cushioned expansion tank, affecting the pressurization level of the whole secondary circuit, as well as the polishing on-line, whose insertion point plays a crucial role in its effectiveness. Some crucial improvements have been done during the early functioning period, sometimes following failure episodes, as reported in the next chapters. First, a short review of the topic of low conductivity water cooling in accelerators is presented. MAKE-UP POLISHING ON-LINE N2 T TO END USERS HEAT EXCHANGERS CORROSION IN ACCELERATORS In the accelerator cooling systems the copper corrosion in demineralised water is an unavoidable drawback but can be limited to a sustainable level. The corrosion rate depends on conductivity, dissolved O2 and CO2, pH, temperature, flow velocity and conditions, imposed electrical potential difference, galvanic potential difference (due to different metallurgy). The conductivity and other parameters can be maintained in set by polishing on line and controlled make up. Figure 1: Cooling system (secondary circuit) design. A simple hydrostatic device is employed to guarantee a stable and safe nitrogen pressure level. In addition, the oxygen inlet should be limited by adopting closed water circuit (sealed or inert gas cushioned, yet taking advantage by O2 pumping away with inert gas flow). Any possible depression in the piping should be checked. As a rule, it should be avoided any unnecessary stop and opening of circuits, as well as allowing dead leg in piping with differential aeration. Furthermore recommendable are vacuum deaeration and UV hygienization technologies, both on-line and on the make-up. The polishing system, as well as the final stage of the make-up one, consists in a row of mixed-bed ion exchanger resin bottles, with micrometric filtration stages against resin pollution and loss of containment of the sand-like resin particles. The make-up system has in series: mechanical prefilters, chlorineand ironremoving units, micrometric filters, reverse osmosis membrane units, again micrometric filters, a buffer tank, a booster/recirculating pump and mixed bed ion exchanger stage. All the components are redundant. Timers make self polishing of reverse osmosis Finally, it should be reminded that employing different metals and allowing unwanted electric potential difference are both possible cause of corrosion intake. Proceedings of EPAC 2004, Lucerne, Switzerland