Water & Wastewater Treatment

32 | JUNE 2017 | WWT | www.wwtonline.co.uk to install and have a smaller footprint and weight. All of this means reduced capital and operational costs. Q: What are the key requirements for the control system for dosing pumps? A: Safety in chemical dosing processes is a matter of accuracy, as dosing too little or too much can have significant consequences for the process outcome. This is especially true in water treatment applications, where inaccurate dosing can have serious implications, lowering the water quality and ultimately contributing to pollution or public health issues. Today's advanced diaphragm dosing technology is able to ensure that the dosing head volume is constant, and the dosing flow is directly correlated to pump speed. This allows for very accurate dosing. When you attain greater accuracy, not only will the pumps contribute to safer process outcomes, but in many processes, chemical consumption will be greatly reduced. Advanced diaphragm dosing has been shown to use 5 to 19% less chemicals than less accurate mechanical dosing pumps. Clever dosing pump designs will automatically control the number of points need to be taken into consideration. These include the chemical characteristics - such as viscosity and gassing properties - and the compatibility of the pump material for this; the capacity and pressures that the pump needs to perform at; the required degree of control and automation; the size and condition of the suction and discharge pipework conditions; and the degree of accuracy needed. It is important to remember that no two pump installations are the same, and therefore the pumps selected must be flexible and offer value-added features. Q: How has digital technology helped with the operation of dosing pumps? A: Digital technology has made a huge difference to the operation and control of dosing pumps in recent years. Traditional methods of control, such as variable speed drives and stroke control, have been replaced, while new systems offer improved turndown capability, improved accuracy and reduced chemical costs, and allow for the inclusion of remote monitoring and control capability via data transmission. They can also handle higher viscosity materials, deal better with gassing issues, use less power, are simpler and safer The Knowledge: dosing pumps pump to adjust to variable values. This means that even in installations where process parameters such as pressure, temperature and flow are unstable, the pump is able to automatically adapt and in addition will warn the user. In short, by selecting a sophisticated control system, the user can count on the dosing pumps to deliver reliable and safe dosing, even in changing or adverse circumstances, and with optional features that significantly improve overall system control. Q: What are the most common problems that can be experi- enced with dosing pumps, and how can they be avoided? A: There are a number of areas where issues can be experienced, o en as a result of incorrect installation, such as wrongly-sized pipework or the use of incorrect external components. Another problem area comes about where there is a lack of understanding about the acceleration head pressure, as this is necessary to correctly size the pipework. A poor maintenance and repair schedule will also lead to a loss of accuracy in the dosing application that will lead to a range of problems. Another common problem on larger installations is when pulsation dampers, which protect against pulsating flow and vibration, are either lacking or poorly commissioned. Q: How much main- tenance do dosing pumps require? A: The type of dosing pump that is selected plays an important part in the amount of maintenance that is required. The new generation of diaphragm dosing digital technology will require less maintenance than peristaltic dosing pumps. This can be partly explained by the fact that the tube in a peristaltic pump degrades quickly because of the constant squeezing action and the tube becomes dam- aged in low-flow and dry-run- ning conditions. This radically shortens the service intervals for a peristaltic pump. By contrast, in digital pumps, the PTFE diaphragm is resistant to virtually all chemi- cals, including acids, bases and organic and inorganic, ionic, non-ionic and corrosive agents of all kinds, so the diaphragm degrades only very slowly. This means that new gen- eration dosing pumps typically only require maintenance once per year (at the very least), or once every 2 years depending on usage and the chemical being dosed. If the chemical is particularly aggressive or abrasive, such as lime slurry, then it is important to check the dosing pump regularly for excessive wear. Grundfos's SMART Digital DDA model is an example of a dosing pump which uses digital diaphragm dosing technology

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