Hide-out effect

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In steam boiler technology, the hide-out effect is a process of reducing salt in the water phase of high-pressure steam generators during operation. This term was first used by RE Hall in the United States in 1929. It was observed in the boiler water of drum boilers when using boiler feed water that was not salt-free. Only after the development of the ion exchange and the mixed-bed filter was it possible to produce boiler feed water that was salt-free and this effect could hardly occur.

Causes of the effect

The hide-out effect is that the concentration of some salts in the boiler water is noticeably reduced when the steam generator is started up. If, on the other hand, the boiler is switched off or the temperature and thus the pressure are greatly reduced, the concentration rises again to almost the same extent. This increase is called the hideout return effect .

In the past, changes in concentration were particularly pronounced for phosphates, silicates and sulfates in the boiler water. It was therefore long believed that chlorides were not subject to this effect. In high-pressure boilers at pressures> 200 bar in zones of high heat flux densities and, in addition, locally low mass flow, this change in concentration occurs even in modern systems, albeit with significantly less intensity.

The effect is triggered by several causes. One of the reasons for the changes in concentration is the accumulation of ions and salts on the inner metal surfaces, which consist of metal oxide layers. Further causes are probably changes in the solubility of the salts in water at very high temperatures and also reversible chemical reactions between salts and the metal oxides. The latter two theories were adopted by Tietz and Class , for example . The reaction of phosphates with metal oxides can proceed according to the following equation:

An increase in the NaOH concentration in the boiler water, which is to be expected according to this equation, is also observed in practice. In the hideout return, the equivalent concentration of the cations is often higher than that of the anions. The difference is compensated for by OH - ions. As already mentioned, the hide-out effect was a problem especially in the past with somewhat salty boiler water. These deposits could cause local overheating, which subsequently led to pipe tears . In modern systems, the effect can only be determined to a small extent.

This is different in the case of nuclear power plants with long operating periods between start-up and shutdown of the reactors. In pressurized water reactors , this effect is observed both in the primary and in the secondary circuit. However, since ultrapure water is used, the changes in concentration are relatively small and only occur in the µg / kg range with peaks of max. 0.1–0.7 mg / l for Ca 2+ , Mg 2+ , SO 4 2− and SiO 2 when a reactor is shut down. An increase in the chloride concentration is also observed, although this is significantly lower than in the case of the ions listed above. Despite the small changes in concentration, these are undesirable and disruptive. The reason for this is that metals such as 58 Co and 60 Co are also deposited in the deposits. In the circulating water, the radioactivity of the water increases undesirably strongly after the reactor has been shut down due to the hideout return with the release of these metals. With a special zinc salt dosage to the circulating water, the concentration of the radioactive elements can be reduced noticeably with success.

Individual evidence

  1. ^ RE Hall et al .: J. America Water Works Ass. 1929, 21, pp. 79-100.
  2. ↑ Iv. Dobrevsky, R. Winkler: To hideout in power plants. In: VGB power plant technology. 73, 1993, issue 2, p. 172.
  3. ↑ Heinz Lehmann: Steam generator practice, fundamentals and operation. 1st edition. Resch Verlag, 1988, p. 468.
  4. ^ HE Hömig: Physicochemical basics of feedwater chemistry. 2nd Edition. Vulkan-Verlag, Essen 1963, pp. 294-295.
  5. ↑ Iv. Dobrevsky, R. Winkler: To hideout in power plants. In: VGB power plant technology. 73, 1993, issue 2, p. 174.
  6. ↑ Iv. Dobrevsky, R. Winkler: To hideout in power plants. In: VGB power plant technology. 73, 1993, No. 2, pp. 172-174.
  7. ^ HW Rich, B. Stellwag, F. Böttcher, M. Juergensen, G. Holz, B. Markgraf, S. Schütz, M. Rübenich: Zinc metering in Siemens PWR plants. In: VGB Power Tech. 5/2010, pp. 44-49.

literature

  • HE Hömig: Physicochemical basics of feedwater chemistry. 2nd Edition. Vulkan-Verlag, Essen 1963, chapter 8.