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ATEX EQUIPMENT Equipment


Digester Post digester


ATEX zone


Interior: gaseous atmosphere


Exterior: wall formed of a flexible membrane


Exterior: wall formed of a hard shell


Zone 2


Zone 2: envelope of 3m radius


Zone 2: envelope of 3m radius around openings such as portholes


Biogas storage tank


Valves of


digester/post digester/tanks


Interior Exterior


Spherical zones centred on the emission point


Zone 2


Zone 2: envelope of 3m radius


Zone 2 of 3m radius integrating a zone of 1 m radius


Combustion unit Interior of the combustion room


Buried condensate well Covered digestate trench the biomass,


requiring a costly restart, while too little can impair electricity and heat generation, potentially reducing revenue. With biogas containing a potentially explosive mix of gases including methane, we also need to ensure that any devices used to measure flow offer safeguards against damage to plant and equipment and injury to personnel. Under the ATEX Directive, equipment is designated by the type of potentially explosive atmosphere in which may be used – these are Group 1 for underground mines and Group 2 for surface industries. In Group 2, ATEX also defines categories of equipment, specified by their protection characteristics. It also designates the hazardous location zones they can be used in - zones 0, 1 and 2 for gases, vapours and mists. As specified by INERIS, most biogas plant processes come under Zone 2, where explosive atmospheres are likely to be infrequent and of short duration. This means that instruments intended for these areas should provide a normal level of safety.


THERMAL MASS FLOWMETERS OFFER PROTECTION


These problems can be overcome by using well installed thermal mass flowmeters


Technical room Interior Not classified Source: Institut National de l’Environnement Industriel et des Risques (INERIS) designed for use in


potentially explosive atmospheres. Capable of measuring mass flow directly, they can provide a mass flow rate in kg/h without any additional measurements or calculations. Thermal mass flow meters use a heated platinum resistor maintained at a constant over-temperature compared to an unheated platinum sensor kept at the same temperature as the gas flow. Heat is applied to the resistor so that a constant temperature difference exists between the resistors. Based on a known and constant gas composition, the mass flow can be determined according to the relationship between the heater current and flow rate - there is no need for additional pressure and temperature compensation. Because they have no moving parts, thermal mass flowmeters are ideal for handling the challenging conditions in biogas production, eliminating the need for maintenance to solve problems such as clogging.


Other advantages include a turndown ratio of 100:1, giving highly accurate


Interior – atmosphere of the condensate well


Interior – gaseous atmosphere


Not classified Possible failure Introduction of air Leak to outside Leak to outside


Introduction of air Leak to outside


Internal overpressure causing gas leak to the outside


Leak in the biogas supply


Zone 2 Zone 2 Accumulation of gas Accumulation of gas


measurement at particularly low and variable loads. This low


turndown is particularly important when working with flammable gases such as methane, as they need to be operated at a low pressure to reduce the likelihood of a fire. ABB’s FMT400 thermal mass flowmeter is


ATEX rated for biogas measurement applications, giving an added safeguard against the risk of explosion.


With their inherent operational benefits, plus a low cost of ownership through the elimination of peripheral temperature and pressure equipment and the reduced need for maintenance, thermal mass flowmeters offer the ideal solution for biogas measurement applications.


ABB www.abb.com


INDUSTRIAL COMPLIANCE | MAY/JUNE 2023 31


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