Twin trouble?

Kevin Belben talks about some of the problems that can occur when using twin water tanks – and he explains how to avoid them.

According to the government’s national surveillance scheme for Legionnaire’s disease in residents in England and Wales, there have been 311 confirmed cases in the 12 months to the end of September 2019, with 85 of these confirmed in September alone.

That’s not an insignificant number and the question has to be asked whether there is more that can be done to prevent the disease. We know that the Legionella bacteria develops in stagnant water, so the importance of water management should never be underestimated.

While most facilities are good at undertaking risk assessments and addressing the need for water monitoring, technical problems can occur that can create an issue in this area. Anyone familiar with water system management will know there is one such common problem that rears its head when it comes to balancing twin cold water storage tanks.

Some buildings have a tank with a division within it or two tanks that are sized and designed to serve the demands of the whole building independently of one another. It’s a strategy aimed at preventing down time if one tank or division has to be shut down for cleaning or maintenance. However sometimes it becomes clear that one tank is doing all the work, leaving the other exposed to a risk of stagnating water.

This happens because whilst the water is drawn off from a common header, you will sometimes find that the delayed-action float valves are installed at slightly different heights due to all sorts of influences, such as the accuracy of the height of the fixing holes, or even if the floor isn’t quite level. In addition, the distribution pipework is often not even. The consequence of this is that one valve will operating at a slightly different water pressure, albeit very minor.

In this twin tank scenario, you also have to take into account the orientation of the valve in the tank – the positioning of the two will vary, even if only by a degree or so. If therefore one valve comes on slightly before the other and keeps up with the demand from the building, the other filling valve will not activate. Once this starts to happen the problem becomes worse because generally valves prefer to be exercised, so the valve that operates more often becomes looser while the other becomes stiffer, further exacerbating the issue.

There are however things you can do to try and get the valves to operate in tandem. For example, it might help to rotate the valves slightly, as well as checking the height settings. If you rotate the dominant valve that is operating all the time marginally anticlockwise as you stand behind it looking into the tank, it will delay it slightly. If you ensure the valve that is not operating correctly is turned clockwise until the bubble on the spirit level is just touching the black line, this may do the trick to correct the imbalance.

Unfortunately, even when these minor issues have been overcome and the valves operate simultaneously, it is something that needs to be monitored because there is no guarantee that mechanical float valves will wear at the same rate.

For example, the Aylesbury KP Float Valve system and Tanktronic electronic tank management products were designed for precisely this purpose and have been utilised in many facilities successfully. It was developed with Keir Construction and uses two main control valves, one on each side to fill the thank, supported by two smaller pilot valves. The two main control valves are triggered by the pilot valves; whichever pilot valve opens first then opens both control valves simultaneously, thereby filling both sides of the tank.

This type of system is being used to ensure a constant supplies of fresh cold water from hospital storage tanks at the Garrett Anderson Centre (GAC), a self-contained building within the Ipswich Hospital complex. The GAC is supplied with water through a break tank on the ground floor that, in turn, feeds a storage tank in the roof through a booster set. Both tanks are divided into two, which provides easy maintenance and cleaning access while ensuring an uninterrupted supply of water to the 24/7 facility.

The use of twin tanks in substantial commercial premises undoubtedly has its advantages, so it is our job as water management specialists to offer advice and solutions where possible, to ensure this design approach remains a viable and safe option for our customers.

Kevin Belben is technical applications manager at Cistermiser and Keraflo

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