03.11.2011
After completion of the reconstruction work and the general start-up of the petroleum test rig, all important subassemblies have now been investigated in detail with respect to their concrete influence on the measurement uncertainty of the test rig – in particular also the influence of the flow diverting mechanism on the newly realized flying start-stop operation.
The petroleum test rig is part of PTB's measuring equipment system for the realization of the units of volume and flowrate measurements of liquids in motion. As test medium, white spirit - which also allows measurements of liquids other than water to be metrologically traced back - is used. For comparability of the measurement results with those of standard test rigs working with water - in particular of the Hydrodynamic Test Field - the operation mode of the petroleum test rig was extended for flying start-stop operation. For this purpose, a special converter mechanism, consisting of two ball valves each, working in a push-pull manner (see Figure 1) - was installed in each of the two measurement leads. This allows the liquid flow to be switched rapidly between the measuring vessel and the bypass line at the beginning and at the end of a measurement so that the measurements on the meter under test can be carried out even at constant flow conditions of the meter without the influences during starting and stopping of the meter to be tested having to be taken into account.
Figure 1 shows one of the two flow diverting mechanisms. It consists of two ball valves which are currently rigidly coupled with a control shaft via a pneumatic actuator.
Due to the volatility of the white spirit, the complete flow conduction has to be realized inside a self-contained system which also encloses the flow diverting mechanism. This can be guaranteed by use of the ball valves.
In a first step, the ball valves were investigated while they were rigidly coupled. As expected, an influence - i.e. a retroaction to the indications of the test samples - resulted for each switching process due to (among other things) short-term increases in the resistance to flow and associated pulsed changes in the dynamic liquid pressure and in the flow. First of all, the magnitude of these effects could not be estimated, as it depends on the respective concrete test set-up and its resistance to flow.
These relations have now been verified experimentally for different pressures and flows with the aid of a turbine meter and a Coriolis flowmeter, and the deviation of the calibration results of the two devices was quantified (see Figure 2). For the examination, filling of the graduated cylinder was divided into several small single fillings. Thus, several switching processes were realized for the complete filling according to the number of partial fillings. This allows the deviations to be stapled and determined mathematically with the aid of a developed algorithm.
Figure 2: The figure shows the values of the experimentally determined relative deviation of the calibration results of a turbine meter and a Coriolis flowmeter for different flowrates and pressures.
The further task now is to develop a program which allows a failure-free and non-interacting switching process to be realized, in which the ball valves must be switched in a concerted and flowrate-dependent way so that a change in the flowrate does not take place.
Contact person:
Jörg Riedel, Dept. 1.5, WG 1.53, E-Mail: joerg.riedel@ptb.de