BASIC SEDIMENT AND WATER CONTENT MEASUREMENT APPARATUS AND METHOD

Abstract: The basic water and sediment (BS&W) system compares a wet stream to a dry stream of the same oil in two electrical capa­ citance cells (9;19). A numerical zero offset is employed in the comparison to compensate for voltage output differences between the cells (9;19). The dry cell builds a small increase in its inherent offset from the wet cell because of its lower flow rate, which buildup is swept away when the wet fluid is run through both cells at a higher flow rate to determine the zero offset. To cancel the zero offset, the present invention alternates the flow to the two cells (9;19) so that each cell (9;19) experiences the same sequence of events alternately.

Description: The invention relates to a method and apparatus for measuring the sediment and water content of a wet stream, which is predominantly non-aqueous. A device for measurement of basic sediment and water (BS&W) in a predominantly non-aqueous stream (e.g. pipeline crude oil), is disclosed in EP-A-0,072,052 in the name of Applicant. This device is an improvement on capacitance type instruments of the art which are dependent upon the extent to which the intrinsic dielectric constant of the subject varies with time. (Vide, for example, US patent specification No. 4,184,952.) The gravity and physical composition of crude oil are two factors which determine its intrinsic dielectric constant. If one or both of these properties should vary, instruments measure the accompanying change in the dielectric constant as percent BS&W. This yields an inaccurate measurement of BS&W because instruments must be initially set to read zero BS&W as the intrinsic dielectric constant of the fluid. The capacitance type instruments of the prior art have no means for automatically correcting the zero BS&W setting to compensate for periodic variations in the oil properties mentioned. By comparison, the device of EP-A-0,072,052 in the name of Applicant provides for automatic compensation of BS&W measurements by producing a clean, dry sample of the line fluid for measurement of its intrinsic dielectric constant. In this way, the true BS&W content of the fluid is measured by finding the difference between the dielectric constants of the wet and dry streams. The improvement over the prior art represented by the invention of EP-A-0,072,052 in the name of Applicant is substantial, and the present invention provides an additional improvement which even further increases the efficiency and accuracy of the invention of EP-A-0,072,052 in the name of Applicant. Thus, it has been observed that the two capacitance cells of the BS&W instrument are identical, but not perfectly so, and therefore display a slight difference in voltage output when containing the same fluid. The cell testing the capacitance of a dry stream appears to build a small increase in its offset from the cell testing the capacitance of a wet stream, due to the necessarily low flow rates through the dry stream capacitance cell, which buildup is swept away when the wet stream is run at a higher flow rate through both cells to determine zero offset (the "wet-wet" check). The magnitude of the offset is somewhat uncertain during the periods between wet-wet checks. Accordingly, it is desirable to cancel the zero offset with improved certainty and eliminate the wet-wet checks. Therefore, it is an object of the present invention to provide an improved BS&W measurement apparatus and method and eliminate or reduce a tendency in the device to change zero offset between capacitance cells during operation. The present invention therefore provides a method for measuring the sediment and water content of a wet stream which is predominantly non-aqueous, comprising the steps of removing a sample of the wet stream and measuring its capacitance in a first capacitance cell; feeding the wet stream sample into an outer chamber containing an inner chamber having a wall formed of a filter; rotating the inner chamber while pressuring the wet stream sample from the outer chamber into the inner chamber; withdrawing a clean dry stream sample from the inner chamber and measuring its capacitance in a second capacitance cell; characterized by periodically passing the wet stream sample to the second capacitance cell and the dry stream sample to the first capacitance cell, so that each cell experiences the same sequence of events alternately. The invention also provides an apparatus for measuring the sediment and water content of a wet stream which is predominantly non-aqueous, comprising means for removing a sample of the wet stream and measuring its capacitance in a first capacitance cell; means for feeding the wet stream sample into an outer chamber containing an inner chamber having a wall formed of a filter; means for rotating the inner chamber while pressuring the wet stream sample from the outer chamber into the inner chamber; means for withdrawing a clean dry stream sample from the inner chamber and measuring its capacitance in a second capacitance cell, characterized by means for periodically passing the wet stream sample to the second capacitance cell and the dry stream sample to the first capacitance cell so that each cell experiences the same sequence of events alternately. In an advantageous embodiment of the invention the capacitance of the wet stream sample is compared with the capacitance of the clean dry stream sample to facilitate determining the basic sediment and water content of the wet stream. It can be remarked that EP-A-0,072,053 in the name of Applicant discloses a device to be used with at least one basic sediment and water monitor, which has at least a capacitance cell comprising concentric tubes through which a liquid to be monitored is flowed. However, the specific embodiment of the present invention has not been disclosed. In accordance with the present invention, a method and apparatus are provided for incorporating the water/oil emulsion separator in EP-A-0,072,052 into a complete system for determining basic sediment and water in crude oil, other hydrocarbons, or other substantially non-aqeuous systems. This invention is particularly concerned with a slight difference in voltage output within the two electrical capacitance cells of the system when containing the same fluid. The dry cell (containing a dry stream sample) appears to build a small increase in its offset from the wet cell (containing a wet stream sample) due to its necessarily low flow rate, which buildup is swept away when the wet stream is run at a higher flow rate through both cells to determine the zero offset (the "wet-wet" check). The magnitude of the offset is somewhat uncertain during the period between wet-wet checks. Accordingly, the present invention alternates the flow to the two cells so that each cell experiences the same sequence of events alternately. Advantageously, at least about half the time the first cell will experience a high flow rate of the wet stream while the second cell experiences a low flow rate of the dry stream. At least about the remaining half of the time the second cell advantageously experiences a high flow rate of the wet stream while the first cell experiences a low flow rate of the dry stream. The zero offset will cancel when an even number of these alterations is averaged as to water content measurement. The invention will now be described in more detail by way of example with reference to the accompanying drawing, in which the figure schematically shows the flow paths of various streams within the apparatus of the invention. With reference to the figure a sectional view of the present invention is provided with the flow stream shown schematically. Crude oil or other fluid containing a small amount of water is taken from a pipeline or other storage or transport via a line 10 and passed through a wet stream capacitance measurement cell 9, and then through a housing 11 supported by structures 6 and 8, and then into a wet stream chamber 12. The cell 9 determines the capacitance of the wet stream. From the chamber 12, the wet stream is forced through a filter 13 and into a hollow drive shaft 14. The hollow drive shaft 14 and filter 13 are spun by a motor or other drive means 115. While the present invention is not limited to the following theory, it appears that the resulting centrifugal force substantially prevents the water and sediment capable of otherwise passing through the filter 13, from entering the hollow shaft 14, and any water or sediment that may enter the shaft 14 is forced outwardly back into the chamber 12. Dry fluid in the hollow shaft 14 passes outwardly into the dry stream chambers 15 and 16. Dry stream from the chambers 15 and 16 then is passed via the lines 17 and 18 through a dry stream capacitance measurement cell 19 which determines the capacitance of the dry stream. As above noted, comparisons of the capacitances of the dry stream with the capacitance of the wet stream facilitates determining true BS&W content of the wet stream. A portion or all of the dry stream from the cell 19 is passed via the lines 7 and 20 into the spaces 21 and 22 separating the seals 23 and 24 and the seals 25 and 26, respectively, which separate the wet stream chamber 12 from the dry stream chambers 15 and 16. An additional minor portion of the dry sample stream may be passed via the lines 27 and 28 into the spaces 29 and 30, which separate the outer seals 31 and 32 from the atmosphere. The spaces 29 and 30 are at atmospheric pressure, and accordingly, the dry stream therefrom is passed via the lines 33, 34 and 35 to drain or disposal. Dry stream from the spaces 21 and 22 is passed via the lines 36 and 37 along with wet stream from the lines 38, 39 and 40 back to the pipeline or other storage or transport via a line 41. To divert the stream 10 to the cell 19, the valves 42 and 43 are closed whereby fluid passes through a line 44 and into the cell 19. The streams 17 and 18 then pass through a line 45 and into the cell 9. The valves 46 and 47 are also closed so that a stream 48 passes into the line 10 downstream of the valve 47 and a stream 49 passes into the line downstream of the valve 46. This effectively alternates the flow between the two capacitance cells 9 and 19. Control means (not shown) well known to those skilled in the art may be provided to control the times of opening and closing the valves 46 and 47.

Claims: A method for measuring the sediment and water content of a wet stream which is predominantly non-aqueous, comprising the steps of: removing a sample of the wet stream and measuring its capacitance in a first capacitance cell (9); feeding the wet stream sample into an outer chamber (12) containing an inner chamber (14) having a wall formed of a filter (13); rotating the inner chamber (14) while pressuring the wet stream sample from the outer chamber into the inner chamber; withdrawing a clean dry stream sample from the inner chamber and measuring its capacitance in a second capacitance cell (19), characterized by periodically passing the wet stream sample to the second capacitance cell (19) and the dry stream sample to the first capacitance cell ((9), so that each cell experiences the same sequence of events alternately. The method as claimed in claim 1 characterized by the step of comparing the capacitance of the wet stream sample with the capacitance of the clean dry stream sample to facilitate determining the basic sediment and water content of the wet stream. An apparatus for measuring the sediment and water content of a wet stream which is predominantly non-aqueous, comprising: means (10) for removing a sample of the wet stream and measuring its capacitance in a first capacitance cell (9); means (6, 8, 11) for feeding the wet stream sample into an outer chamber (12) containing an inner chamber (14) having a wall formed of a filter (13); means for rotating the inner chamber (14) while pressuring the wet stream sample from the outer chamber (12) into the inner chamber (14); means (15, 16, 17, 18) for withdrawing a clean dry stream sample from the inner chamber and measuring its capacitance in a second capacitance cell (19), characterized by means for periodically passing the wet stream sample to the second capacitance cell and the dry stream sample to the first capacitance cell, so that each cell experiences the same sequence of events alternately. The apparatus as claimed in claim 3, characterized by means for comparing the capacitance of the wet stream sample with the capacitance of the clean dry stream sample to facilitate determining the basic sediment and water content of the wet stream.

Inventors: DEMNY, FLORIAN CHRISTOPHER, STEWART, THOMAS LEE
Applicants: SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1986
Main classification: G01N33
IPC codes: G01N7, G01N19, G01N33, B01D37, B01D17, B01D33, G01N27