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Summer is over. What now for my UV system?

Many operators of aquatics facilities are asking that question. With the summer over, what must I do to prepare my outdoor UV system for the winter months or indeed, what is required should my UV system be shut down for an extended period of time?

UV System for a poolThere are several basic steps that should be taken when there is a long-term shutdown of a UV system to ensure that it is ready for the following season and will continue to operate for many years to come.

First, power to the UV system should be switched off. Both the front panel disconnect should be turned off and incoming power to the panel should be shut off. This will reduce the risk of transient voltages affecting the electrical control panel, eliminate the potential for an electrical shock and reduce energy costs.

If the electrical control cabinet is located in an environment where condensation is likely, it is advisable that power be left on or suitable action be taken to ensure condensation does not occur.

Next, water in the UV chamber and pipes should be drained and the chamber flushed with clean fresh water. Pool water has already been treated with various chemicals that are needed to protect the pool during the summer season, but those same chemicals can be harmful to the UV chamber. While there is a perception that stainless steel is not susceptible to corrosion, that is not the case and it can indeed corrode quite quickly.

If you operate a UV system in a saline environment rich in chlorides and you dose chlorine or hypochlorite and/or run it at an elevated temperature, your system may be at risk of accelerated crevice corrosion. Even systems constructed from 316L stainless steel could be at risk where crevices, such as screw threads, O-ring seats or behind gaskets, prevent the passive layer of stainless-steel replenishing itself.

Stainless steels rely on a thin, naturally occurring layer of chromium oxide to keep them `passive’ and in most applications and parts of the chamber, that oxide layer is continually replenished by oxygen carried in the water. However, in very tight crevices, such as screw threads or behind washers and O-rings, this does not always occur and when there are high levels of chloride ions or chlorine residuals, or the temperature is elevated, `crevice corrosion’ can develop.

Crevices are unavoidable in a UV chamber and crevice corrosion is serious because it is an accelerated form of corrosion, concentrated at a critical point. This means that in these environments, crevice corrosion can begin within a few months of start-up, leading to leakage soon after.

StandardWater Quality GuideLimitations
316L Stainless SteelChlorides (as Chlorides)<400 ppm
Free Chlorine<200 ppm
Temperature<86 F
SMO 254/Super DuplexChlorides (as Chlorides)<2000 ppm
Free Chlorine<5 ppm
Temperature<86 F

Figure 1 is a guide for the selection of two common materials used for UV chambers and shows how water quality can affect corrosion rates with temperature, concentration and the presence of trace elements. As corrosion rates vary widely, the table is intended only as a guide.

Draining the chamber is also important to eliminate the possibility of water freezing in the chamber and causing irreversible damage to the chamber walls, O-ring seals and quartz sleeves. If there is concern about residual water in crevices freezing after draining, it may be necessary to use dry air to blow water out of crevices, such as within screw threads.

Finally, draining will eliminate the possibility of the quartz sleeve and sensor window fouling due to stagnant water and residual chemicals.

  • Common access points that are used for maintenance, such as threads for sealing the quartz sleeve, sensor ports threads, automatic wiper gear boxes, etc. should have a small amount of oil or anti-seize applied. This will ensure that the potential for corrosion is reduced and also that any maintenance required in the ongoing year is not hindered by stuck threads.
  • The UV lamp should be removed from the chamber and stored in a cool dry place. It is preferable that lamps be placed flat on a smooth surface or returned to their original packaging, if available. This will ensure that lamps are not inadvertently damaged during storage.
  • Throughout the winter and as needed, the chamber and cabinet should be wiped clean, especially in corrosive environments.

Following these few simple steps will ensure that the UV system is not susceptible to damage during extended periods of shut-down and that start-up is carefree.

By: John Psaroudis