9-11 April 2024
The quality of pure steam is determined by its condensate. This has the same quality characteristics as WFI (Water For Injection). The purity of the steam is usually very high due to the phase transition from liquid to gaseous. Nevertheless, in individual cases there may be increased measured values in the test point conductivity and/or TOC. What are the possible causes for this?
The conductivity check for the condensate of an ultra-pure steam generator is primarily intended to detect serious faults promptly. For example, a defective heat exchanger in the high-purity steam generator could lead to the transfer of heating steam to the clean side and contaminate it. Also, a crack in the pure steam sampling cooler could cause contamination of the pure steam condensate by cooling water.
Apart from these two obvious sources of error, there are also other possibilities:
In the feed water of the high-purity steam generator, for example, there may be steam-volatile substances without significant conductivity (e.g. humic substances), which decompose at the higher temperatures in the high-purity steam generator to form low-molecular, partially conductive substances. These can then cause an increase in conductivity and/or TOC values. This effect can be avoided by thermal degassing of the feed water or by adapted feed water treatment.
It is also possible for a negative pressure to develop if the clean steam system is switched off - e.g. as an energy-saving measure at the weekend. Due to this negative pressure, impurities can be "sucked" into the high-purity steam system via the condensate and/or waste water network (as liquid or also as gaseous substances). These then dry out on the surfaces, e.g. on the high-purity steam pipes. After the high-purity steam system has been started up again, it can take days or even weeks for these impurities to be flushed out / detached again. To avoid this effect, negative pressure should be avoided and cooling ultra-pure steam systems should be actively ventilated (e.g. with filtered, oil-free compressed air).
Another cause may be the test method. If conductivity and TOC are tested both online and at-line and in the laboratory, significant differences are to be expected. For example, an online conductivity value of 0.2µS/cm (25°C; closed system and continuous mode) could be displayed at-line as a 0.4µS/cm value (partially open system with discontinuous condensation) and in the laboratory as a 0.8µS/cm value (open sampling with bottle). The cause is contact with air and the absorption of CO2, which forms conductive carbonic acid or hydrogen carbonates in the water.