Focus Flow & level control


Making sense of float switch selection


When selecting a float switch for a liquid level sensing application, a number of factors need to be considered. Simon Dear, of Cynergy3 Components, offers a guide to these factors, and expands on the choices available


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any industrial processes require devices that are able to sense the level of liquid stored within various types of


tank or chamber. The signals from these devices may be used to control the production process or to provide indication of the status.


One of the most reliable, well-proven technologies for liquid level sensing is a float switch. This type of switch com- prises a magnet contained within a float, as well as a magnetic reed switch contained within a fixed housing. The movement of the float, due to the changing liquid level, will cause the reed switch to operate (i.e. close or open) at a particular level. This tech- nology is based on a simple design that offers reliability without the need for the user to calibrate the switch. Whilst there are different methods available for selecting the right float switch for a particular application, the main factors to consider include:


Physical arrangement and style The choice of styles will depend on the physical arrangement of the tank, the available mounting positions and whether access is available to the inside of the tank. The main styles are horizontal/side mounting and vertical mounting. The horizontal/side mount- ing type normally has a fixed housing, which passes through the sidewall of a tank, with a hinged float attached to the fixed housing. Vertical mounting types normally have a fixed vertical stem, which is installed through the top or bottom of a tank, along which slides a cylindrical float.


It is also important to consider whether a build up of deposits from the liquid on the float body is likely. These deposits can, over time, accu- mulate and cause the float switch to fail to operate. Whilst particular types of float switch have been developed to limit the effects of this build up, the experience and knowledge of the switch manufacturer is critical here.


Material selection


It is critical to select a float switch that is constructed from the right materials


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There are various types of horizontal and vertical mount- ing arrangements


that are compatible with the liquids and temperatures of the particular applica- tion. Component damage as a result of incorrect materi- als selection can ultimately cause failure of a float switch. Typical float switch materials include: Nylon: suitable for many oils, diesel, organic chemicals and MEK-based printing inks.


Polypropylene (PP): suitable for many acids and alkali, detergents, inorganic and organic chemicals, oils and water.


Polyphenylene sulphide (PPS): suit- able for many of the more aggressive chemicals and higher process tempera- tures, up to 120˚C. Buna/Nitrophyl: suitable for many oils, diesel, petrol and water (non- potable applications). D300 foam (PVC): suitable for most hydraulic oils and many chemical solutions.


Stainless steel: suitable for most medical and food applications, chemi- cals, hydraulic fluids, fuel oils and applications with process tempera- tures up to 135˚C.


Selection of the most suitable mate- rials for both float switch and gasket


Vertical mounting types normally have a fixed verti- cal stem, which is installed through the top or bottom of a tank, along which slides a cylindrical float


The horizontal/side mounting type normally has a fixed housing


can be made by referring to a ‘Chemical Compatibility’ table. These tables provide a good indication of the suitability of the various float switch materials in a range of liquids. For some process liquids, it may be neces- sary to obtain a sample float switch in order to test the compatibility.


Electrical rating


It is important to understand the nature of the load that needs to be switched and to ensure that the float switch is capable of handling this load. The elec- trical ratings, which most manufactur- ers provide in their float switch specifications, are for purely resistive loads. Any loads that have either induc- tive or capacitive components should have the appropriate contact protection measure applied.


Cable specification


In applications where aggressive liq- uids may spill onto external wiring, it is important to specify particular materi- als for the cables used to connect to the float switches. There are standard, UL- approved cable types for the various float switches, as well as high tempera- ture, low smoke zero halogen (LSZH) and other specialised materials.


Custom engineered switches The operating environment is critical to the choice of float switch. A water tank for an industrial process may only require a simple plastic float switch. However, if the application is in a hazardous area, for example, a petrochemical storage tank where flammable gases, vapours or dust are present, a stainless steel, explosion- proof float switch will be required. Cynergy3 has custom engineered many variants of its float switches to match particular customer require- ments in a range of process manufac- turing applications. These include float switches for all types of environ- ment, including industrial process control, safe areas, intrinsically safe and hazardous areas (ATEX-certified), as well as WRAS-approved switches for drinking water applications.


Cynergy3 Components T: 01202 897969 www.cynergy3.com


Enter 220 JULY/AUGUST 2013 Process & Control


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