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How to take off the pressure

Especially interesting for
  • road tanker manufacturers
  • haulage contractors
  • filling station operators

When filling mineral oil products into a road tanker, individual chambers of the road tanker may be damaged. The shell of the tank or the partition walls between two chambers may tear. The cause of this is presumably excessive pressure in some of the tanker chambers. PTB has investigated the causes and drawn conclusions on how to prevent such damage.

This vent valve froze after water penetrated into the vapour recovery pipe, it is, thus, no longer operational; this can lead to inadmissible excess pressure in the tanker chamber.

At a filling station, pursuant to legal provisions, road tankers for the transport of petrol may only be filled using the vapour recovery procedure. This means that each road tanker is equipped with a so-called “vapour-recovery hose”, a separate device via which each tanker chamber can “breathe out”. This ensures that the vapour/air mixture pushed out during the filling operation is returned to the vapour recovery system of the filling station, so that no excess pressure can build up inside the tanker.

Possible causes of damage due to excess pressure may be devices which are actually intended to serve for safety purposes themselves: the vent valves, which are supposed to ensure that the liquid cargo does not leak – even in the event of an accident or extreme tilt – or the flame arresters mounted in the vent valves which are supposed to prevent any flashback into the tanker chambers.

PTB determined the conditions of pressure, temperature and volume flow inside the road tanker occurring in real loading situations at filling stations. In further tests in a cold chamber, PTB investigated conditions which may lead to the freezing of the vent valve and of the flame arrester integrated into it. The results were then compared with the data obtained from real incidents.

As a matter of fact, some of the damage could be attributed to the freezing of the vent valves or of the flame arresters. Hereby, it is most probable that water penetrated first – either due to gate valves which had not been mounted correctly or due to cracks  caused by the plexiglass disk of a flame arrester becoming brittle. In other cases, damage was caused by the fact that the wrong filling quantity was selected or that the quantity remaining inside the tanker chamber was not taken into account for the filling, but also by technical errors on the part of the filling station or in the road tanker, by operating errors or by tampering.

Measures which could help prevent such damage in future can be found at the following site: www.dgmk.de/downstream/publikationen/link_informationen.html (in German). The table included in these data can also be downloaded directly as an Excel table. Compliance with these information sheets is considered as a voluntary commitment on the part of all tanker operators, manufacturers, repairers and filling station operators.


Dirk-Hans Frobese
Department 3.4 Fundamentals of Explosion Protection
Phone: +49 (0)531 592-3420
E-mail: dirk-hans.frobese(at)ptb.de

Scientific publication

Frobese, D.-H.; Pape, H.: Überdruck an Tankfahrzeugen. DGMK-Forschungsbericht 727, Deutsche Wissenschaftliche Gesellschaft für Erdöl, Erdgas und Kohle e.V., Hamburg, 2011