Pressure vessel

High-pressure fluid dynamic experiments are difficult to instrument due to the wall thickness needed. Furthermore, the safety-related licensing procedures of a pressurised facility are time-consuming and expensive. In order to circumvent these constraints, a technique was found to accommodate test sections in a pressure vessel for experiments under pressure equilibrium.

The test vessel is pressurised either with air for experiments at ambient temperatures or with nitrogen for steam experiments. For air or steam/water experiments, a tank placed at the end of the test  section separates the liquid from the gas. Air or exhaust steam of the experiment flow from this separator into the condenser (Fig. 1). Steam condenses there. At its cold end, the condenser is permanently connected with the inner atmosphere of the tank. This guarantees the full pressure equilibrium at all times.

Scheme of the pressure vessel
Fig. 1: Scheme of the pressure vessel with the "hot leg model" test section

The pressure equilibrium allows to perform pressurised experiments in light test sections. Thereby, the design of the components is not restricted by the mechanical strength. Furthermore, the test rigs can be equipped with large glass windows (Fig. 2) for observation and measurements of the two-phase flows with optical devices.

The test vessel of the TOPFLOW facility, in which the experiments take place, is a 6 m long horizontal cylindrical tank with an inner diameter of 2.44 m. It is made out of the steel 1.0565 with a thickness of 38 mm in the cylindrical part and is licensed for pressures up to 55 bar. The test vessel is connected with different auxiliary systems for the fluid supply. In order to assure a fast and full access to the test setup inside the pressure chamber, it is equipped with a fast operating full-size port on one side. The test facility itself can be moved on a rail track from the vessel to the service platform (Fig. 3), where parts can be assembled or dismantled.

hot leg with glass window
Fig. 2: Part of the hot leg model
with glass window

Pressure vessel with horizontal test section
Fig. 3: Pressure vessel with horizontal test section

The test vessel offers a free volume of about 2 x 2 x 5 m for divers test setups which can be operated in co- or counter-current flow. In case of the hot leg experiments, the test section is a hot leg model of a pressurised water reactor (PWR) being mounted between two separators (Fig. 3). Furthermore, experiments on the two-phase pressurised thermal shock (PTS) issue in a PWR cold leg are planned.

In principle, electronic measuring devices can be used inside the pressure vessel. However, this equipment has to be designed for these conditions and to be withstood pressures up to 50 bar and temperatures up to 50 °C or to be encapsulated. Special pressure proof flanges are available for the transmission of the measurement signals to the outside.