A better way of controlling stored water...an open-and-shut case for advanced float valves

Even products for very simple tasks can have far-ranging implications.Lawrence Burns considers float valves for controlling the water level in storage tanks and the benefits to health and safety and energy consumption.It is a truism of building services that it is important to use the right products for each job — products that perform the function required of them efficiently and cost effectively, while providing the end user with an acceptable cost of ownership. Increasingly, the choice of products is also influenced by health-and-safety considerations and, for controlling water levels in tanks, the water-supply regulations. At least, that is what is supposed to happen, but we all know that there are times when other factors come into play, such as capital cost. There are also occasions when people continue to use a product they feel comfortable with, even if it is not the most appropriate choice. Float valves A case in point is the specification of float valves — an essential part of any water system and one that is so familiar that it is often taken for granted. Yet, for many applications a conventional float valve does not offer the end user the best performance or value for money in the long term. If we consider some of the downsides of conventional float valves it is easy to see where there is room for improvement — because the design of a conventional float valve introduces inherent problems. For example, when a small amount of water is drawn off from the tank, the valve opens slightly and a dribble of water refills the tank. This process is often noisy, which in itself can become a nuisance at night in residential applications such as care homes. It can lead to water hammer and valve bounce to exacerbate the noise problem. It also increases the chance of wire drawing on the seat, limescale formation around the washer in hard-water areas and in a pumped system it can waste a lot of energy. Stagnation In addition, the water entering the tank can be at such low velocity that it does not mix with the other water in the tank, which can cause stagnation to develop in parts of the tank. Also, without this mixing, the temperature of the water in the cold water storage tank can rise to above 22°C in the summer, providing suitable conditions for bacteria to proliferate, such as, Legionella. This stratification and stagnation can be compounded during periods of low water consumption caused by variable occupancy. Schools, colleges, student halls of residence and partially occupied office buildings are all vulnerable to such health risks. Delayed action An alternative approach is a delayed-action valve that only opens when the water falls to a pre-set level in the tank and opens fully to achieve a fast refill at a velocity that will mix the water in the tank and prevent stratification. In buildings where variable occupancy is a consideration, the ability to adjust the water level in the tank, without the need for any tools, is also a major benefit. These are all features of the patented Aylesbury range of float valves. The K type uses a key-shaped float that can be precisely set in a number of positions on a brass float arm. The valve is actuated at a pre-determined difference in water level, driving it fully open or fully closed without water hammer. Inherent in the design is a non-adjustable operating differential in water level between opening and closing, of about 75 mm. The Aylesbury ‘KB’ type incorporates an actuator tube connected to a float-and-buoy assembly so that the maximum and minimum water levels in the tank are adjusted simply by lengthening or shortening a chain. This enables water levels to be set to suit occupancy levels and is used in a wide range of applications. As well as eliminating dribbling noises, water hammer and stratification, the Aylesbury valve design has important implications for energy consumption in pumped systems. With a conventional float valve, pump hunting is a frequent problem as the pump is often actuated every few minutes. Implications Even when pump operation is restricted to 20 actuations an hour, the booster set will run for at least three minutes. If a conventional valve fills the tank in less than this time the pump will continue to run against a closed valve and use energy necessarily. unnecessary energy. Also, for most of that time the valve is only partially open so the pump is operating against a high resistance to water flow. In contrast, the Aylesbury delayed-action principle actuates the pump only when the water level has fallen to the pre-set level which, depending on application and demand, might be only once every few hours. In theory, it is possible to have a system that only pumps at night using low tariff electricity, depending on water consumption and tank size. Inherent reliability The advanced engineering of the Aylesbury valve demands a slightly higher price, though not as much as many expect. However, the inherent reliability of the design reduces maintenance call outs, and the additional cost can be recouped by avoiding just one hour of a maintenance engineer’s time. It is for these reasons that several local authorities have used Aylesbury valves for over 10 years. They have been specified for Terminal 5 at Heathrow Airport and been extensively used in the Canary Wharf development and many major projects nationwide. In all these projects, value engineering has been important to the design, and specifiers and end users have recognised the added value and reduced cost of ownership of the Aylesbury design. Lawrence Burns is managing director of Keraflo Ltd, Griffin Lane, Aylesbury, Bucks HP19 3BP
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