Method and device for oil-in-water measurement

This application claims priority under 35 U.S.C. §§119 and/or 365 to Appln. No. 199 59 005.2 filed in Germany on Dec. 8, 1999; the entire content of which is hereby incorporated by reference.

The present invention relates to the field of measurement of small oil concentrations in water. It proceeds from a method and a device for oil-in-water measurement according to the preamble of claims 1 and 6.

High-pressure separator tanks have recently been developed for offshore oil production which are suitable for separating the phases of sand, water, oil and gas directly at the seabed. According to statutory provisions, the water pumped back into the sea may have an oil concentration of at most a few 10 ppm. Although conventional capacitive or optical oil-in-water sensors can achieve such high measuring sensitivity in principle, it is not possible to achieve the required freedom from maintenance, long service life and pressure and temperature resistance.

Such a device for determining a low oil concentration in water is disclosed in U.S. Pat. No. 4,137,494. The measuring cell consists of an oil-absorbing diaphragm with electrodes mounted on both sides. Oil molecules contained in the water penetrate into the diaphragm, displace ions embedded there and increase the electric resistance of the diaphragm. The change in resistance is then a measure of the oil concentration in the water flowing through. In order to raise the measuring sensitivity, a water sample has to be pumped into a closed circuit repeatedly through the diaphragm in order to absorb all the oil there. The method has substantial disadvantages. As soon as the total amount of oil accumulated reaches a saturation value, the diaphragm must be disposed of or flushed with a solvent for the purpose of regeneration. Consequently, periodic maintenance intervals are stringently required, and this renders difficult or impossible installation at inaccessible sites, for example on the seabed. The measurement can also be falsified by a variable ion concentration in the water.

It is the object of the present invention to specify a method and a sensor for oil-in-water measurement in the case of which it is possible to implement a high measuring accuracy in conjunction with a greatly reduced maintenance requirement. This object is achieved according to the invention by means of the features of claims 1 and 6.

The solution according to the invention consists in a method and a device for oil-in-water measurement having a measuring cell whose impedance is a measure of an oil concentration in the water flowing through or in an accumulation filter, the measuring cell being automatically calibrated and/or flushed at recurring time intervals. During calibration, a current reference impedance value of the measuring cell is measured with the aid of a known reference liquid. The measuring cell is automatically cleaned of oil residues during flushing. Both self-calibration and self-flushing are carried out by a suitably designed or programmed electronic control system. Advantages are, inter alia, a reduction in signal drift, an improved long-term reliability and a long service life without monitoring or filter exchange. Overall, for the first time, an oil-in-water sensor is specified which is suitable for use at poorly accessible locations in oil production or the like.

In accordance with an exemplary embodiment, the measuring cell is operated in an alternating fashion in a long measuring cycle (for example 10 minutes) and a short flushing cycle (for example 1 minute) and/or an even shorter calibration cycle. The measuring cell preferably has two or more accumulation filters whose measuring cycles supplement one another such that a measuring signal can be determined in the measuring cell largely at any instant. This delivers a high degree of availability of the measuring cell for online measurements.

One exemplary embodiment relates to an oil-in-water sensor having a cross-flow filter whose output for oil-enriched water is connected to a capacitive measuring cell during the measuring cycle and whose output for cleaned water is connected to a capacitive measuring cell during the calibration cycle. The measuring sensitivity can be raised and the calibration liquid can be provided in the simplest way by the separating filter.

In another exemplary embodiment, there is arranged in a capacitive measuring cell an oil accumulation filter which can be flowed through in the reverse direction for flushing purposes by a valve-controlled line system. Owing to the reversal of direction during the flushing operation, oil residues or a filter cake on the filter outer surface, that is to say on the inlet side during the measuring cycle, can also be flushed out very efficiently with oil-contaminated water.

In an important exemplary embodiment, the oil accumulation filter is a cylindrical ring-type filter made from porous ceramic or polyethylene fibrid which has at least one radial and one axial connection. The cylindrical form creates a mechanically robust, compact and simultaneously large-area accumulation filter. In addition, it is possible by segmenting the electrodes on the outer and inner cylinder outer surface to implement a plurality of filters on a common ceramic body which run through measuring cycles which supplement one another in time, intermittent flushing cycles and/or cycles for determining offset. The self-flushing and self-calibration can thereby be implemented in a single component.

Further designs, advantages and applications of the invention follow from the dependant claims and from the description now following, in conjunction with the figures.

In the drawings:

FIG. 1 shows an oil-in-water sensor according to the invention with a cross-flow filter for oil enrichment for a capacitive throughflow measuring cell;

FIG. 2 shows an oil-in-water sensor according to the invention which has a capacitive measuring cell with an oil-accumulating ceramic filter;

FIG. 3 shows a serial arrangement according to the invention of two oil accumulation filters for simultaneous measurement and flushing and for independent offset measurement and

FIG. 4 shows a capacitive measuring cell according to the invention with a ring-type filter.

Identical parts are provided in the figures with identical reference numerals.

The subject matter of the invention is a method for oil-in-water measurement which is suitable, in particular, for measuring an oil concentration in water in a high-pressure separator tank. A measuring cell 7 , 12 is fed oil-contaminated water, and an electric impedance signal of the measuring cell 7 , 12 is measured. According to the invention, the measuring cell 7 , 12 is calibrated and/or flushed automatically at periodic intervals. Exemplary embodiments relating to the method are specified below with general reference to FIGS. 1-3.

During calibration, the measuring cell 7 , 12 is flowed through by clean water and a calibration signal is formed (FIG. 1 ). During flushing, the measuring cell 12 is flowed through backward by water (FIG. 2 ). According to the invention, the reverse flushing can be carried out even with oil-contaminated water. The oil-in-water sensor, for example for applications in the oil industry, is thereby significantly simplified and the everyday usefulness is improved. However, it is also possible for a solvent suitable for flushing to be fed from an additional tank (not illustrated). As an alternative, or in addition, the accumulation filter 16 , 16 a, 16 b of the measuring cell 12 can be cleaned by ultrasound.

A capacitive signal is preferably used in the measuring cell 7 , 12 as a measure of the oil concentration in the water. An oil concentration of the water flowing through is measured in a throughflow measuring cell 7 according to the invention. An accumulated oil concentration is measured in an accumulation filter 16 , 16 a, 16 b in an accumulation measuring cell 12 according to the invention. In both cases, an oil concentration in the water can be enriched in a separating filter 2 connected upstream by a known factor or a factor which can be determined.

The mode of operation of the sensor 1 is based on the fact that the capacitance is lowered with respect to clean water by the oil component in the water. With the accumulation filter 16 , 16 a, 16 b an absolute signal specifies the total embedded oil concentration and a differential signal specifies the instantaneous inflowing oil concentration.

An interval is intended to comprise a longer measuring cycle and a shorter calibration and/or flushing cycle. The measuring cell 7 , 12 can also comprise a plurality of accumulation filters 16 a, 16 b with alternating measuring and flushing cycles, the measuring cycles supplementing or overlapping, one another in time in such a way that an impedance signal of the measuring cell 7 , 12 can be measured permanently, and the flushing cycles alternate with one another in such a way that each filter 16 a, 16 b is flushed over an adequately long time.

In the exemplary embodiment according to FIG. 3, the measuring cell 7 , 12 has exactly two accumulation filters 16 a, 16 b, which are flowed through in series one behind another. In a first work operation (FIG. 3 a ), the first accumulation filter 16 a runs through a measuring cycle and the second accumulation filter 16 b runs through a calibration and/or flushing cycle. In a second work operation (FIG. 3 b ), the first accumulation filter 16 a runs through a calibration and/or flushing cycle and the second accumulation filter 16 b runs through a measuring cycle. In a further work operation (FIG. 3 c ) it can be that in order to fix an offset signal only one accumulation filter 16 a is flowed through and an offset measurement is carried out at the other accumulation filter 16 b . Thereafter, a second work operation (FIG. 3 d ) is carried out again, but now with a new offset at the filter 16 b.

Thus, in this configuration a current measuring signal is permanently generated in the measuring cell 7 , 12 , and at the same time the non-measuring accumulation filter 16 a or 16 b is flushed and/or its new offset value is determined as a measure of the total quantity of oil embedded. A high degree of availability and long service life are thereby ensured without manual maintenance.

An oil-in-water sensor 1 for carrying out the above method is also the subject of the invention. In accordance with FIGS. 1 and 2, the sensor 1 comprises means 4 , 6 a ; 10 a - 10 c, 101 for feeding oil-contaminated water to a measuring cell 7 , 12 which is connected to measuring means 19 , 19 a for impedance measurement. According to the invention, the sensor 1 further comprises means 2 , 5 , 6 b, 8 ; 10 b, 10 c, 11 a, 11 b, 102 for self-calibration and/or for self-flushing of the measuring cell 7 , 12 and control means 19 , 19 b for automatically activating the latter. Detailed exemplary embodiments now follow.

In accordance with FIG. 1, the means for self-calibration 2 , 5 , 6 b, 8 may comprise a cross-flow filter 2 with an inlet 3 for oil-contaminated water. The first outlet 4 for oil-enriched water and the second outlet 5 for cleaned water can be connected in an alternating fashion to the measuring cell 7 , 12 via controllable valves 6 a, 6 b. The cross-flow filter 2 advantageously has a second inlet 8 for clean water for the purpose of its own cleaning.

In accordance with FIG. 2, the means for self-flushing 10 b, 10 c, 11 a, 11 b, 102 may comprise two flushing lines 102 parallel to the measuring cell 12 , it being possible for the measuring cell 12 to be separated from a measuring line 101 by first valves 10 b, 10 c and to be connected to the flushing lines 102 in the back-flushing direction via second valves 11 a, 11 b. 9 denotes a feed line from a separator tank, and 10 a a pump. The control means 19 , 19 b advantageously comprise an electronic control system 19 and control signal lines 19 b which are configured for automatic control of the valves 10 b, 10 c; 11 a, 11 b.

In accordance with FIG. 4, one accumulation filter 16 of the measuring cell 7 , 12 is a cylindrical ring-type filter 13 which has an outer electrode 14 and an inner electrode 15 on the cylinder outer surface and a radial connection 17 a, 18 a and an axial connection 17 c, 18 c (FIG. 4 a ). The cylindrical filter 13 according to the invention is distinguished by a large filter surface, pressure resistance, compact design and the ease with which it can be installed.

The cylindrical ring-type filter 13 in each case advantageously has two half-shell outer electrodes 14 a, 14 b and inner electrodes 15 a, 15 b as well as two separate radial connections 17 a, 18 a; 17 b, 18 b (FIG. 4 b ). Moreover, it is also possible in a simple way for a plurality of radial connections for a plurality of partial filters 16 a, 16 b with appropriately segmented electrodes 14 a, 14 b, 15 a, 15 b to be implemented. Two axial connections going upward and downward can also be made.

The accumulation filter 16 , 16 a, 16 b of the measuring cell 7 , 12 is to be a ceramic filter 16 , 16 a, 16 b with a pore size which is water-pervious and oil-absorbing, or a polyethylene fibrid filter. The porous ceramic serves principally for mechanical deposition of oils or, in general, hydrocarbons or (toxic) additives. Hydrocarbons are also embedded in the polyethylene fibrid filter by their chemical affinity to the filter material. The separating filter 2 can be of corresponding design.

The invention specifies an oil-in-water sensor 1 which is capable of everyday use, has a long service life, is suitable for continuous real time measurements of oil contamination in the 10 ppm range, and can be used at inaccessible locations.

List of Reference Numerals

1 Oil-in-water sensor

2 Separating filter, cross-flow filter

3 Inlet

4 First outlet

5 Second outlet

6 a , 6 b Valves

7 , 12 Capacitive measuring cell

8 Second inlet

9 Feed line from separator tank

101 Throughflow line, measuring line

102 Reverse flow line, flushing line

10 a Pump

10 b , 10 c Valves

11 a , 11 b Valves

13 Capacitive ring-type filter, ceramic filter

14 Outer electrode

15 Inner electrode

16 Ceramic filter, plastic filter

16 a , 16 b Accumulation filter, oil accumulator

17 a , 17 b Radial inflow

18 a , 18 b Radial outflow

17 c Axial outflow

18 c Axial inflow

19 Measuring apparatus, electronic measurement and control system

19 a Measuring signal lines

19 b Control signal lines