Activity responsible: Dr. Gino BOCCARDI
Address: ENEA C.R. Casaccia, Energy Department, Institute of Thermal Fluid Dynamics, Via Anguillarese 301 (S.P. 092), 00060 S. M. di Galeria RM, Italy
Phone: +39 06 3048.3664 Fax: +39 06 3048.3026
VASIB facility, designed to test two-phase flow through safety and relief valves, has a maximum operating pressure of 2.0 MPa, a maximum flow rate of 1500 kg/h, and an available electric power of 150 kW for the heating of the process fluid (water); vapour quality may range from 0 to 20 %.
Unlikely from most of available experimental facilities to study the two-phase flow through safety systems, the VASIB facility has been specifically designed to work as a closed loop, with the main aim of getting information on the effect of the back pressure on the mixture flow.
The facility can allow the testing of the two-phase flow through safety devices, with a back pressure different from the atmospheric value, using either actual safety valves (ø max = 10 mm) or reference geometries (convergent, divergent, or straight nozzles, etc). Besides the thermal-hydraulic characteristics of the VASIB facility allow to keep the test conditions in the loop without any time restriction, at low operating costs, thanks to the recovery heat exchanger.
To get these results, the facility can:
Click here to see the complete scheme of the facility.
Results obtained so far are reported below in.
1 st Campaign
The main aim of the first test campaign has been the measurement of the two-phase mass flow rate and the correlated parameters, in order to achieve a first assessment of design correlations available for the sizing of the safety values under two-phase flow conditions.
An actual safety valve has been used as a test section ; it has an orifice diameter of 10 mm, and is instrumented with pressure transducers in the nozzle and in the valve body.
The lift of the valve plug may be set and imposed manually, in order to ascertain its influence on the discharge conditions. In the table here under the operating conditions tested are reported
1a Test Campaign: test conditions
In the second test campaign the steam-water data obtained with the VASIB facility have been compared with available data from the literature, most of which obtained using air-water mixtures.
Using air, as the gas phase, is an easy and effective way to simulate steam in a liquid flow, but has some drawbacks, in particular when the back pressure is higher than the atmospheric pressure (main feature of the VASIB facility). Indeed, as the air density is not so dependent on the pressure, the steam density shows large variations even form small changes in the system pressure beside, the expansion of the mixture implies also large variations in the quality. In simple words, the continuous expansion of the steam-water two-phase flow implies also variations in velocity, pressure drop and quality which, in turn, have a feedback on the expansion. Consequently, the evolution of the phenomenon, depending on all these inter-related factors, turns out to be very complex and difficult to be represented by an-water modeling.
In the present tests, we used a
test section having a Venturi
shape as it is among the most studied geometries for such a problem.
In this way, the obtained data can be easily compared with similar data
obtained using an air-water mixture.
2nd Test Campaign: test conditions
The tests are being carried out on a commercial PSV; the original øor=10mm has been reduced to a ø or=5mm, ( test section ) in order to obtain a greater quantity of experimental data. The lift of the valve plug is set at the value reported by constructor.
These data are compared, with various inlet
parameters, with the predictions obtained using two method, one based
on the Homogeneous Non Equilibrium Model hypothesis (HNE) and the other
one on Homogeneous Equilibrium Model hypothesis (HEM). The
comparison between the model predictions allows a qualitative evaluation
on their performance in sizing of pressure safety valve in two-phase conditions.
In the previous campaign the geometry modification of PSV has reduced the number of possible tests and has not allowed a comparison with a real flow through a PSV. For these reasons a short campaign with the original orifice has be carried out; the results has confirmed the 3rd campaign evaluations.
In the table here under the operating
conditions tested are reported.
The lack of a reference standard represents a serious limit for PSV industrial applications in two-phase flow; in order to overcome it, different agencies are looking for a fairly simple correlation that considers all the two-phase flow aspects. Recently, correlations developed from the Homogeneous Equilibrium Model (HEM) have been considered the most interesting. For example, API (American Petroleum Institute) has edited in 2000 the API RP-520 "Sizing, Selection and Installation of Pressure Relieving Devices in Refineries” where a method for PSV sizing in two-phase flow, based on the HEM, is suggested.
In this campaign the tests are being carried out on a commercial PSV, øor=10mm, locked in the open position corresponding to the supplied discharge coefficients. The valve geometry has not been modified for obtaining the real working conditions. The experimental data have been compared to a lot of models, based on Homogeneous Non Equilibrium Model hypothesis (HNE) or HEM hypothesis, including the method suggested by API .
The mass flow-rate is calculated multiplying the model prediction for a theoretical nozzle by the discharge coefficient k s. Its importance is evident for computing the real mass flow-rate value; PSV manufactures supply and guarantee the coefficient for liquid (kl ) and gas (k g) while in two-phase conditions no information is directly available about ks because its value depending on the flow conditions.
In calculation a simple correlation that considers the inlet conditions and kl and kg, for discharge coefficient computing is proposed and an evaluation of its influence on mass flow-rate prediction has been done.
The test matrix proposed in the experimental campaign aims to gradually reach (or to approach as much as possible, based on the performance of the VASIB) conditions that limit the mass flow-rate through a safety device, which are low values of available pressure drop compared to flow rate and quality desired or critical flow conditions. The phenomenon variables are mass flow-rate, inlet pressure, inlet quality, outlet quality and back-pressure. In this experimental investigation, the inlet pressure, the mass flow rate and the inlet quality are kept constant: the back-pressure is measured while the outlet quality, as all the other quality values, is calculated by enthalpy balance.
To avoid using data affected by instrumentation errors, the tests with a pressure loss through the valve of less than 20 kPa have not been considered as this value is too close to the pressure measurement uncertainty zone. This screening involved a reduction to 132 in the tests evaluated. The tests carried out and considered for each inlet pressures are shown in the following table.
Examples of data evaluation with the method suggested by API (HEM) and with a model (Henry and Fauske) developed from HNE hypothesis .