INDUSTRY FOCUS WATER & WASTE TREATMENT SPOILT FOR CHOICE IN CHEMICAL DOSING APPLICATIONS


Ensuring that chemical dosing system used in water treatment is the right one for the application, site and process conditions is a key challenge for system owners, as Adrian Dawson, senior technical engineer at WES Group explains...


D


elivering chemicals accurately and in a controlled way is a fundamental part


of many industrial and water treatment processes. Fit for purpose, horses for courses, the right tool for the job, however you couch your terms, it is critical that you select an appropriate automated chemical dosing system for your application. Ironically, the fact that there is such a


large selection of alternative technologies and product variants to select from, can make it difficult to choose correctly. A typical dosing system comprises a


number of components, including a chemical storage tank, a metering pump, control system, and associated valves, pipework and accessories. Of these, the most critical is the pump; of which there are several types: Diaphragm pumps move a flexible


diaphragm to adjust the volume of an internal tube or cavity. A known volume of chemical is drawn into the cavity through an inlet valve and then pushed out into the chemical delivery pipework through an outlet valve. Chemical flow is in a series of pulses. An alternative to the basic solenoid


driven mechanical diaphragm pump is the Piston-Diaphragm (PD) or hydraulic diaphragm arrangement for more arduous,


high accuracy or higher pressure applications. The diaphragm is displaced by a secondary hydraulic fluid driven by a reciprocating piston. Sitting between these two pump types


are motor driven mechanically actuated diaphragm pumps, offering reasonable capital cost, simple maintenance and good life expectancy at the expense of lower accuracy and pressure capability compared to hydraulic diaphragm systems. Peristaltic pumps use a rotary motor


drive to move a series of rollers over a flexible tube inside a rigid housing. These squeeze the tube, forcing chemical out into the distribution pipework. Chemical flow is continuous, and the absence of


Selecting an appropriate automated chemical dosing system for your application is crucial


SOMETIMES, YOU HAVE TO BE IN IT TO WIN IT


If your process involves the manufacture or processing of liquids and you need to know the level of that liquid through your plant, then chances are you will start your considerations of the most suitable level sensor by looking at non-contact sensors. Keeping sensors out of measured fluid offers seemingly obvious advantages over immersed sensors such as


float or capacitive devices, particularly if the liquid has suspended solids or other matter that would clog up or accumulate on the sensor, making their data unreliable or inaccurate. However, the introduction of a new conductive level sensor from Gill Sensors means it is not the clear-cut


decision it once was. Here we take a look at some of the issues faced when using the most popular technology – ultrasonic – which Gill’s new sensor helps overcome. Temperature change affects the speed of sound waves, which alters the time-of-flight pulses from


ultrasonic sensors, affecting sensor accuracy. The Gill sensor remains stable across its operating temperature range of -40° to +85°C. Foam is a natural enemy of ultrasonic sensors because it absorbs the pulses from the sensor, degrading the


reflected signal making the sensor inoperable. The Gill sensor will penetrate any foam depth or density allowing it to measure the fluid level accurately underneath. To maintain a high quality return signal, ultrasonic sensors like a reflective surface to bounce off. In many


applications liquid surface can be turbulent, particularly if agitated. Due to its design, the immersed probe of the Gill sensor is unaffected by turbulence, can withstand lateral loads, providing an accurate output. Ultrasonic sensors must be mounted to avoid coating by material or condensation. Coatings dissipate the


signal, sometimes such that there is insufficient power to get the signal through the coating and back again. Regular cleaning can be one way to overcome this. Gill’s conductive level sensor has a smooth, non-stick FEP coating without any holes or cavities, meaning that it will not clog up or become coated with solids, giving reliable ‘fit-and-forget’ measurement performance - whatever it is immersed in. Gill Sensors & Controls


gillsc.com/level


The metering pump is the most critical component of a typical dosing system


internal valves makes the pump more tolerant of liquids containing solids. Peristaltic pump designs offer good


levels of suction lift – useful if the pump must be mounted above the storage tank or a long distance from it. Since the chemical is only in contact with the pump tube, peristaltic designs also simplify the task of ensuring compatibility between the pump and the chemicals it carries. However, they have limited discharge pressure capability and the pump tube, which can wear out, will need replacing periodically. Progressive cavity pumps use a helical


rotor turning inside a specially shaped stator. The motion of the rotor against the stator creates a series of fixed shape cavities that transport chemical through the pump to the distribution pipework. These pumps work well with slurries,


viscous materials and chemical products that are sensitive to shear, but have a limited range of capacity adjustment and the complex shapes and tight manufacturing tolerances required for their operation leads to high capital costs. The size of pump used for any dosing


application must be chosen with care to ensure accuracy can be maintained across the minimum and maximum dose rate requirements of the process. ‘Turndown ratio’ – the ratio between the maximum and minimum output of the pump – varies across different models and manufacturers of pump. Dosing system pipework also requires a


number of other components to facilitate safe, reliable operation and simple maintenance. These include switches, gauges, indicators and transmitters to signal the presence or absence of flow, flow rates and operating pressures to on- site operators and to transmit this data to


WES Group www.wes.ltd.co.uk


34 SEPTEMBER 2018 | PROCESS & CONTROL / PROCESS&CONTROL


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