Electromagnetic flowmeter liners improve according to customer requirements
September 2003Flow Measurement & Control
Security Services & Risk Management
Expanding process parameters, coupled with the requirements of cleaning and sterilisation in the food and pharmaceutical industries has given rise to a number of new EMF (electromagnetic flowmeter) liners and liner technology. Liners are used to coat metal pipes, thereby electrically insulating the fluid from the pipe and allowing accurate flow measurement.
Because there is no liner technology that can be used for the entire range of nominal diameters from DN 2 to 2000, as well as the fact that each customer application is unique, Endress + Hauser has adapted four different liner materials: rubber, polyurethane (PU) and two different fluoro plastics - polytetrafluor ethylene (PTFE) and perfluoroalkoxy (PFA).
Historically, rubber has been the liner of choice and, as such, Endress + Hauser has decades of field experience with this type of liner. The steps to this process include sand blasting the metal pipe, applying a bonding agent and then the liner, inserting caoutchouc matting, followed by placing the measuring pipes in an autoclave, where the caoutchouc is vulcanised (at 145°C for three hours) into rubber. As a liner, rubber exhibits good resistance to water and alkaline solutions, but is not suited to oils or acids and fails to meet the hygienic requirements of the food and pharmaceutical industries.
The most recent liner technology to be introduced by Endress + Hauser is the fabrication of PU liners with the aid of a rotational casting process. Casting moulds are mounted on the flanges, followed by the injection of the two basic components - isocyanate and polyhydric alcohol - into the rotating measuring pipe. Once these components set, the liner is poured through a nozzle. PU liners are particularly suited to the fresh water and wastewater industries.
To manufacture a PTFE liner, PTFE tubes are inserted into the steel pipes - with the exterior diameter of the tube being slightly larger than the interior diameter of the pipe. The range of applications for this liner is, however, limited, as the liner is not mechanically bonded with the pipe wall. At large nominal widths, the liner could collapse due to negative pressure. Similarly, in cases of excessive fluid temperature, the tension in the liner could become too great, due to expansion. Nevertheless, this liner exhibits good resistance to chemicals and high temperatures (-40°C to 130°C), as well as CIP (cleaning in place) requirements of the food industry.
One of the problems of PTFE liners is that during fabrication tiny micropores are generated in the liner material. If the fluid enters a steam phase, steam could diffuse through the pores and condense on cooling. If the fluid is again heated, the condensed water will expand into steam, causing the micropores to stretch. Thus, if a process involves steam phases, coupled with frequent temperature variation, it would be better to opt for a PFA liner.
Liquid PFA at 400°C is isothermically injected into the pre-heated mould. A pre-defined cooling temperature ensures that no micropores are present in the material. To reduce tension in the liner, the pipes are tempered in an oven for 24-hours. PFA liner is resistant to chemicals and high temperatures (up to 180°C), meets CIP and SIP (sterilisation in place) requirements, and has received sanitary approval for Endress + Hauser's hygienic flowmeter Promag H.
Thus, with a variety of liners, Endress + Hauser is able to offer customers a cost-effective and tested liner for specific applications. Where more than one liner material could be used, tests will be carried out to determine which is better suited.
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