Technical paper by Jannik Meyer, Frank HolzLiquid chemical and petrochemical products are produced, processed or consumed at various locations within the EU. These products are transported by road, rail or waterways. Transport on European inland waterways are governed by the European Agreement concerning the International Carriage of Dangerous Goods by Inland Waterways, which was revised in 2019. This revision has tightened the requirements for explosion protection inside pumps. The inside of the pump must now meet the requirements for category 1 in order to be allowed to operate with an Ex-zone 0 inside. Twin screw pumps are used on tankers for loading and unloading. (EPS Services and Sales B.V.)Twin screw pumps are used in various industrial sectors to pump both high-viscosity and low-viscosity liquids as well as liquid-gas mixtures. They work on the principle of rotating displacement pumps. Their rotating, intermeshing feed screws transport the medium from the suction to the discharge nozzle of the pump. Ranges of application include the oil and gas industry as well as the chemical and pharmaceutical sectors. Flammable liquids are also often among the pumped media. Under certain operating conditions, for example when a tank containing a flammable liquid is emptied, an ignitable mixture can form within the pipeline and therefore also within the pump due to the admixture of ambient air.Conveying explosive liquid gas mixturesThe Directive 2014/34/EU, also known as the Atex Directive, regulates the requirements that devices for use in potentially explosive atmospheres must meet in the European Economic Area. Here, explosive atmospheres are classified into Ex zones from 0 to 2 based on their probability and frequency of occurrence. Until now, the product portfolio of ITT Bornemann from Obernkirchen in Germany has included twin screw pumps that are suitable for use in Ex zones 1 and 2 inside the pump. This product portfolio has now been expanded to include a series that is also suitable for the permanent conveyance of explosive liquid gas mixtures (“Ex zone 0 inside” the pump). This series therefore complies with equipment category 1G internal in accordance with the Atex directive and fulfills the highest level of safety.Making Atex-compliant pumps availableThe drive to take this development step was provided by the revision to the “Accord européen relatif au transport international des marchandises dangereuses par voies de navigation intérieures” (ADN) 2019 agreement for inland waterway transport in the European Economic Area. The Obernkirchen-based company's twin screw pumps have been used for many years in large numbers on various tankers for loading and unloading, including on various inland waterway vessels in Europe. With the ADN 2019, the European Commission has made a change to the classification of explosive atmospheres in the pipelines of these ships. The previous classification for “Ex zone 1 inside” was tightened to “Ex zone 0 inside” when the ADN 2019 regulation came into force.The European Agreement – ADNThe European Agreement concerning the International Carriage of Dangerous Goods by Inland Waterways (ADN) of May 26, 2000, which was signed by the United Nations Economic Commission for Europe (UNECE) and the Central Commission for the Navigation of the Rhine (CCNR), entered into force on February 29, 2008. With the ADN 2019, the current version of the regulation, which contains provisions on the transport of hazardous substances and the construction and operation of inland waterway vessels, was added to the Convention. To define certain regulations for different types of transported goods, these are divided into individual classes. Class 3, for example, includes flammable liquids. These include liquids such as petrol, cerosene and ethylene. The ADN 2019 regulates, among other things, which potentially explosive zones are to be defined in accordance with the Atex Directive (2014/34/EU) in and around tanks, pipelines and equipment on tankers that transport such flammable substances. At that time, however, there were no suitable Atex-compliant twin screw pumps on the pump market. As a result, the classification societies accepted that the previous pumps suitable for “Ex zone 1 inside”, including additional safety-related instrumentation, could still be installed. The manufacturer then set itself the goal of closing this product gap in the market and qualifying some of its twin screw pumps for use in Ex zone 0 inside.Focus on ignition hazards The twin screw pump is protected by an internal recirculation valve. (ITT Bornemann)In accordance with the Atex Directive, series of non-electrical devices can be qualified for use in an Ex-zone 0 (device category 1), for example, by means of a type examination. To do this, the manufacturer must present their product, including technical documents and physical tests, to a notified body. This notified body specifies the content and scope of the tests to be carried out in order to verify sufficient safety. In order to ensure the continuous safe production of these “Ex-zone 0 inside” pumps, the notified body has audited and certified the quality assurance in the production process in accordance with module D of the Atex directive.With twin screw pumps, particular attention must be paid to explosion protection to: - possible ignition sources due to mechanical friction, - mechanically generated sparks caused by the contact of metallic components and - the generation of compression heat.Avoid hot surfacesThe conveying elements of the twin screw pumps operate without contact, the rotating spindles are synchronized by an oil-lubricated transmission. The build-up of differential pressure by the pump generates forces on the rotors that cause them to deflect. The stress field, formed by the explosion-proof pump and the hydraulically efficient pump, is created by selecting the gap widths between the rotating conveying elements and the stationary housing. In the event of impermissibly high differential pressures and thus impermissibly high deflections of the spindles, contact could occur between the feed screws and the pump housing. This critical condition for explosion protection must be absolutely avoided under all operating conditions. A large gap width between the components would be a safe solution here. However, a large gap width results in poor hydraulic performance such as low volumetric efficiency and poor suction behavior. This is why the developers from Obernkirchen focused on this problem during product development.However, certain components in the pump cannot operate without mechanical friction. These include the mechanical seals, which seal the pump's delivery chamber from the atmosphere. The way the mechanical seal operates means that the seal faces, separated only by a very thin film of liquid, rotate continuously against each other. This rotation generates hydrodynamic friction in the gap, which leads to an unavoidable rise in temperature in the area of the seal faces. The maximum temperature rise had to be determined during development by means of test runs under different operating conditions and limited to a maximum permissible value. Sufficient lubrication and cooling of the seal faces must be ensured at all times to dissipate the frictional heat from the gap. As the “Ex-zone 0 inside” pumps are also intended for suction operation, it must be ensured that the sliding surfaces are supplied with lubricating fluid under these operating conditions.In order to monitor the temperature increase caused by the compression heat during suction and rest operation, the pumps are equipped with a fitted sensor. This sensor only needs to be integrated into the control unit by the operator. As part of the type examination, the pumps were tested on the test bench in Obernkirchen. (ITT Bornemann)A solution through cooperationIn cooperation with external specialists for plant safety and explosion protection, the developers analyzed the new twin screw pump for the “Ex-zone 0 inside” with regard to the requirements of the Atex directive and prepared a detailed risk assessment and ignition hazard evaluation. Together, they also prepared the technical documentation for testing by a notified body, in this case Dekra.Different sizes of the pump series were tested by Dekra on the test bench in Obernkirchen. Over several days, the developers ran the twin screw pumps under different operating conditions and recorded a large number of measured values. In some cases, the pumps were tested far beyond their designed load limits to meet the high safety requirements. For example, the developers carried out a test run lasting several hours in cavitation mode to prove the robustness of the machines. By carrying out various test runs until the pump reached thermal stability in all planned operating scenarios, they were also able to prove that no unacceptably high temperature rises occur. After evaluation of the measured values obtained by Dekra, the certificate of conformity for the use of the pump with the “Ex-zone 0 inside” was issued. The marking for “Ex-zone 0 inside” in accordance with VDMA 24227 indicates the device category of the pump. (ITT Bornemann)With the successful completion of the development work and certification of conformity for use in “Ex-zone 0 inside”, twin screw pumps are now available that fully meet the requirements of ADN 2019. It is no longer necessary to apply for an exemption permit from classification societies for the use of twin screw pumps with equipment category 2G inside on inland waterway vessels. The pump and its accessories underwent a final inspection before delivery. (ITT Bornemann)Other ranges of applicationIn addition to tank shipping, the use of “Ex-zone 0 inside” pumps could also offer tank terminal operators increased explosion protection. These twin screw pumps could also be of interest to the chemical industry, which previously had to rely on other types of pumps to meet such high explosion protection requirements.www.bornemann.com Read the full article
