Water Concentrations and Interface Positions by Single Electrode Capacitance Probes (SeCaP™)

Schüller R. B.1, Solbakken T.2, Engebretsen B.3 and Halleraker M.4

1 Agricultural University of Norway, P. O. Box 5036, 1432 Ås, Norway, reidar.schuller@inf.nlh.no

2 Norsk Hydro ASA, P. O. Box 2560, 3900 Porsgrunn, Norway, trond.solbakken@hydro.com

3 Sentech AS, Trollåsveien 4, 1414 Trollåsen, Norway, bjorn@sentech.no

4 Sentech AS, Trollåsveien 4, 1414 Trollåsen, Norway, morten@sentech.no

ABSTRACT

The measuring range of SeCaP™ (Single electrode Capacitance Probe) systems is from 0 to 100% water. Unlike traditional capacitive systems, measuring between pairs of electrodes, the SeCaP™ system utilizes a single electrode for each measurement. As a consequence of this it does not “short circuit” in conducting water-continuous dispersions.

The system has been used for interface monitoring in the form of profile gauges in several multi-phase high- pressure hydrocarbon/water systems. One probe has been operational in the separator of the Norsk Hydro Multi Phase Flow Loop since June 1999, experiencing several operating periods of 100 bar pressure and 90ºC. It has also been used to control water/oil interface position in a high-pressure separator during several weeks of operation.

The SeCaP™ system has been evaluated on oil/water dispersions with water cuts varying from 0 to 100%. Circulating it through a colloid mill ensured the homogeneity of the mixture. The results from these tests show that the measuring system gives a continuous increasing signal from 0 to 100 % water. Additionally, the relationship between probe signal and water concentration is basically linear, and a simple two-point calibration is needed to convert to indication of concentration.

A traversable high pressure Titanium version has been tested in a 3” multi-phase flow line at high- pressure monitoring flow details in a hydrocarbon/water system. Results show that the system is fast enough to detect liquid slugs, wave phenomena and water distribution.

The construction of the electrodes has gone through several stages. A good design is to use a combination of Borosilicate glass behind the electrode and Sapphire between the electrode and the surrounding fluid.

A range of sensitivity tests have been performed in order to determine how deep into the surrounding fluid a SeCaP™ electrode can “see”. The electrode was submerged in air, water or oil. The distance to an adjacent fluid or material such as water, oil or metal boundary was varied. The results show that the electrode normally is capable of “seeing” typically 5 to 10 mm into the surrounding fluid based on a detection limit at 1% of full-scale reading. A salt-water film of 2 mm thickness “blinds” the electrode. Thus, the surface covering the electrode must be hydrophobic in nature, always ensuring that the surface is oil-wetted. In practise, the employment of Sapphire seems to ensure this.

Keywords capacitance, profiler, water, concentration