Yorkshire Water, a principal water and wastewater treatment company in the U.K., had a problem with a lime-dosing application at the Ruswarp Water Treatment Works (WTW) near Whitby in northern England.
Hydrated lime or milk of lime is used to remove temporary hardness from drinking water and adjusts its pH for optimum purification conditions by precipitating the heavy metals in the form of insoluble hydroxides. Hydrated lime in the U.K. typically conforms to the new standard BS EN 12518: Class 1 Type A and is vital for the well being of millions of people.
Lime needs to be drawn from the mixing tanks and dosed directly into the main. In order to do this, Yorkshire Water needed a durable pump capable of pumping this abrasive media continually with little downtime and easy maintenance.
Lime Dosing & Pumps
Lime dosing is a classical Verderflex application with many installations in the water industry. The following describes a successful installation by Verder’s U.K. team, which addressed the problems of an abrasive variable viscosity product causing excessive pump downtime and continuously high maintenance costs.
Classical solenoid-driven diaphragm pumps were originally used to dose Kalic liquid lime, a viscous alkali solution added to the water supply to maintain a neutral pH value. The problem occurred when these pumps had to operate at low flow rates. Staff members were constantly needed on hand after the pumps became clogged with the viscous product and failed. As a solution, Verder recommended the replacement of these units with Verderflex peristaltic pumps.
Verderflex pumps are self-contained units that utilize a natural pumping principle. Also known as a hose pump, a Verderflex peristaltic pump draws fluids by compressing and relaxing the hose, operating similarly to one’s throat. The medium to be pumped never comes into contact with any moving parts and is entirely contained within the robust, heavy-duty hose, which consists of an inner layer, two to six reinforcement layers and an outer layer. A rotating shoe passes along the length of the hose, compressing it completely. When this is relaxed, a strong vacuum is created, drawing product in and along the hose. By adjusting the size of the hose, speed of the rotor and the number of shoes, the pump can be used for accurate measuring and dosing.
Pumps in this range can self-prime to a depth of 9.5 m and are engineered to prevent clogging. They can handle highly viscous liquids and offer precise flow control, making them ideal for dosing applications such as this one. These pumps are also easy to maintain because the hose is the only part of the pump that comes into contact with the product.
Installation & Results
Five Verderflex pumps have been installed at the Ruswarp WTW by Verder U.K. under the instruction of the consulting engineer Earth Tech/Morrison. The five pumps include two VF15s for pre-Kalic and two VF10s for post-Kalic dosing, while a VF25 recirculates the solution to storage tanks. The new pumps are controlled by the site’s programmable logic controller and utilize the existing inverters to give variable speed and flow as required.
This system has replaced the twin controls for speed and stroke adjustment that were fitted to the diaphragm pumps. The installation was carried out without interrupting the water supply. After the installation, no call outs have been experienced, dramatically reducing both downtime and maintenance costs.