Getting to Grips with… dosing pumps
Dosing pumps are mission critical appliances that must operate reliably and efficiently, as the success of processes such as disinfection depend on achieving accurate dosing. Here, we look at some of the most important issues involved in operating and controlling these key pieces of equipment
by Heidi Berger, Business Director, Dosing and Disinfection, Grundfos
Q: What are the main applications for dosing pumps in water and wastewater treatment?
A: In municipal water treatment and wastewater treatment, dosing pumps are extensively used to add chemicals for chlorination (disinfecting harmful pathogenic microorganisms) for oxidation (assisting in aeration) and for pH correction or adjustment. Chemicals are also dosed to prevent corrosion and to make the water more stable.
Other typical applications are when dosing pumps are used to add chemicals for flocculation and coagulation purposes to remove particulate, dissolved metals and organic matter. They can also add chemicals that will remove odour, to change the colour of the water, or simply to improve the taste. Chemicals dosed can also remove lead from old pipes.
These are some of the typical applications, but there are many other occasions where dosing pumps can be called upon, such as fluoridation and phosphate removal.
The basic rule is that dosing pumps are an integral part of any and all water and wastewater treatment processes where chemicals need to be added.
Q: What types of pump are appropriate for these tasks, and what are the considerations when choosing between them?
A: Dosing pumps by definition are positive displacement pumps, and the most common types are diaphragm style, peristaltic or progressive cavity pumps.
When choosing between these different designs, a 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 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 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 experienced with dosing pumps, and how can they be avoided?
A: There are a number of areas where issues can be experienced, often 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 maintenance 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 damaged in low-flow and dry-running conditions. This radically shortens the service intervals for a peristaltic pump.
By contrast, in digital pumps, the PTFE diaphragm is resistant to virtually all chemicals, 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 generation 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.
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