HC Legionella Ltd investigated temperature issues in hot water cylinders for a healthcare site, where temperatures were in a range conducive to bacterial growth; here, Authorising Engineer, Claire Jackson, explains how the issue was resolved
HC Legionella Ltd was asked to attend a healthcare site with temperature issues with its hot water cylinders, and therefore, temperatures were sitting in bacterial preferred growth range temperature profiles.
The system was designed to have a primary heat source to the hot water calorifiers from a primary coil and a secondary backup heat source from a 9kW immersion.
Due to ongoing temperature issues, the client had implemented the use of immersion back-ups to top up the system temperatures and regain control during investigation works; however, these were found to be constantly tripping out. Electrical testing showed that the immersion coils were no longer operating at peak performance. HC Legionella was asked to replace the immersion coils in one of the two hot water cylinders on site.
Finding the source of the problem
A manufacturer-spec immersion was sourced for the unit, and arrangements were made to shut down the cylinder, drain it and change the immersion. During this time, a full inspection of the cylinder was also to be carried out, as routine inspections had previously been carried out via drain-off sampling.
On arrival at the site, it was noted that there was discolouration of the water delivered at the outlets, and the temperatures were only achieving 43-45°C on the main circuit. As the cylinder was drained, it was noted that there was significant discolouration from the drain.
It was also noted that the cylinder was taking considerably longer to drain than expected for its size. As the water level in the cylinder started to lower, a water source was still filling the cylinder. All domestic water service valves were checked for soundness and were found to be holding water, so it was suspected that the primary coil within the cylinder had ruptured. The isolation valves to the primary circuit were closed, and the cylinder was drained.
When removing the immersion, it was found that this was in poor condition and was retaining a large amount of corroded sediment.
Once the cylinder was empty and opened for the immersion replacement, a full inspection was carried out using a camera system, the primary circuit was tested, and the cylinder internals were videoed.
The isolation valves on the primary circuit were carefully opened while the cylinder was empty, and internal access was gained. Heating system water was found immediately to be flowing into the hot water cylinder, with three holes found to be present in the primary coil.
Evidence of the primary coil ruptures could be seen in layers of material on the base of the cylinder, composed of scale, metallic compounds from the coil and sedimentation flushed from the heating system.
Implementing solutions
The cylinder could not safely be reinstated in this condition because the risk of bacterial contamination was too great. An emergency flush and disinfection of the hot water system was implemented, and point-of-use filters were installed on outlets to protect service users. At the same time, sampling was undertaken to ascertain full system health.
A new hot water cylinder had to be sourced and installed, leaving an inpatient site with limited hot water capacity for the interim period.
Upon replacement of the cylinder, the second cylinder was also subjected to a drain, and inspection also found a ruptured primary coil, taking this unit offline for replacement and causing further disruptions to the ongoing operation of the site.
During ongoing work to replace the cylinder, the primary circuit was tested to ascertain why the coils in the cylinder had ruptured. The primary circuit was heavily sedimented and contained no corrosion inhibitor or oxygen scavenger. No records were held of the last time the system was dosed.
Had suitable maintenance been undertaken routinely on the primary heating system, ensuring that the correct dose of chemicals was added, the requirement to spend £10k on new immersions, cylinders, disinfections, filters, and sampling may have been negated.
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