20 Level Control
What level control solution should I use?
A frequently asked question when considering a pump system design is what method of level detection do you need? Jane Delicata, Controls Divisional Manager at fluid solutions provider T-T Pumps, looks at which options suit different applications and budgets.
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here are many different elements to bear in mind when choosing appropriate level sensors, but the key items are liquid/ medium for the level control to be installed in, the accuracy, criticality and reliability required and lastly, budget. LIQUID The first thing to consider is what medium the level control is to be installed in or is required to measure. If it is purely water, consider if this is potable (drinking water) or purely waste/grey water. Float switches are the most used product for systems in both clean water and wastewater applications as many variants are available to suit different requirements and offer a relatively inexpensive solution. Conductivity level control systems, such as probes, also lend themselves to largely clean or grey water applications, where the risk of solids adhering to the conductive rods is minimal. However, this would not be preferable for applications where the liquid is particularly viscous, or the solids’ content is high. For corrosive liquids, for example those used in process control, then ‘non-contact’ forms of level control such as ultrasonic (where an echo is bounced from a datum surface and timed until the signal is received back to determine the level) are most suited. This type of level control is also suited to very dense viscous mediums where contact with any measurement and control items would prove problematic. Various other forms of noncontact level control, such as radar, Quarter 4 2021
can offer solutions for applications where establishing a datum via ultrasonic echo would be difficult, such as tidal situations which have to deal with the constant movement of the surface in heavy seas. Working with powders presents other specific issues, for which tuning fork type level switches are an ideal choice. The level switch incorporates a piezoelectric crystal that vibrates the fork at its natural
Should extremely accurate level readings be required, then ultrasonic level control is the most obvious choice. frequency. When the fork comes in contact with a material, the vibration is dampened, and the switch changes state. ACCURACY AND RELIABILITY How critical is the level reading required? If it is simply to control an on/off action like turning on a pump or energising a solenoid valve, then floats or conductivity probes would
work well in this scenario. Should extremely accurate level readings, or perhaps the calculation of the volume of a tank’s contents be required, then ultrasonic level control is the most obvious choice, with factors such as temperature compensation and intelligence built into the units to cover a wide range of applications and scenarios. However, all this technology comes at a price, which leads us very nicely to the next point: the economics of purchase. PRICING Both float switches and conductivity probes are relatively inexpensive in terms of their cost to implement, with floats being the most costeffective as a rule. The use of conductivity probes will also require a control relay which could be costly to incorporate. If the budget allows, then ultrasonic ‘non-contact’ level control or tuning fork type level control is the budget-busting but highly effective, all-round choice balancing accuracy, life expectancy, and flexibility of the equipment. chevron-circle-right www.ttpumps.com www.bpma.org.uk
