WO1998037395A1

WO1998037395A1 - Method and device for measuring fluid pressure

Info

Publication number: WO1998037395A1
Application number: PCT/NO1998/000052
Authority: WO
Inventors: Hystad Sigurd; Miracco Giovanni
Original assignee: Hystad Sigurd; Miracco Giovanni
Priority date: 1997-02-21
Filing date: 1998-02-23
Publication date: 1998-08-27
Also published as: NO970795L; AU6124898A; NO970795D0; NO309245B1

Abstract

There is referred to a process for the measurement of pressure in a fluid in a fluid-containing hollow body (30), such as in a fluid-conducting pipe line. The process is characterised in that the pressure is measured by means of sensors, preferably deformation sensors (10, 12, 14), which are mounted in a manner known per se in the surface (28) of the body (30) or the pipe line, and the measurements from the surface sensors (10, 12, 14) are calibrated relative to a pressure measurement which, by means of a second sensor (24), is executed directly in the fluid internally in the hollow space/the pipe line (30). There is also referred to an arrangement for carrying out the process, together with the application of the arrangement.

Description

METHOD AND DEVICE FOR MEASURING FLUID PRESSURE

The present invention relates to a process and arrangement for the measurement of pressure in a fluid internally in a hollow body. The invention also relates to a preferred application of the process and the arrangement.

The invention generally has to do with monitoring pressure conditions in pipe lines within the process industry, such as within chemical and petrochemical installations, and more especially within oil activity such as refineries and the like where hydrocarbons such as oil, gas or mixtures thereof are conveyed through pipe lines.

Even if the invention is mainly well suited for the measurement of pressure in pipe lines which convey fluids, it can also be employed for measuring fluid pressures in tanks, containers, and reactors and the like within for example process industry, chemical industry, petrochemical industry, and oil refineries.

Within certain such pipe line systems individually traditional pressure measuring equipment cannot be utilised as a result of prevailing safety regulations.

More specifically it is often not permitted according to such regulations, to conduct for example pressure measuring equipment directly through the metal material of the line. For example this applies to pipe lines (for hydrocarbons) which lead from an installation at sea and to a further treatment installation on land, or to intermediate installations at sea. The pipe lines which pass on or beneath the sea bottom, cannot be equipped with instrumentation which involves the metal material of the pipe being " broken through" between the last emergency shut-off valve on the installation at sea, and the first shut-off valve on land. If the pressure in such a hardly accessible loop of pipe is to be measurable, then alternative pressure measuring systems must be applied.

Such a method involves measuring the pressure by applying surface mounted sensors which record deformations in the pipe material as a result of temperature and pressure changes in the fluid which the pipe conveys. This involves the use of so-called strain gages which comprise loops of filamentary electrical conductors which are mounted to or in the surface of the pipe. The measurement is conducted by recording the drop in potential over the filament loop.

During stationary conditions in the flow of fluid in the pipe line, that is to say that the pressure and temperature are constant, a constant potential drop is recorded over such a filament loop. When the fluid pressure inside the pipe is changed (increases or declines) the metal of the pipe material is deformed correspondingly on a microscale, by being expanded or drawn together. This change is recorded via a changed potential drop over the filament loop as a consequence of the filaments of the loop being stretched or compressed in the longitudinal direction in step with the change of the pipe surface. When the filament for example is stretched, the cross-section decreases, the resistance increases, and the potential drop over the filaments increases. The loops are placed on the surface of the pipe both in an axial and circumferential direction of the pipe, and electrically insulated from the pipe material, so that changes in both these directions (really also in the radial direction) are recorded. In a corresponding manner temperature changes in the fluid can be recorded since similar structural changes occur in the material of the pipe wall.

Such measuring systems are known from US- patent specifications 2,420,148, 4,420,980, and 4,738,140.

The object of the present invention is to provide a new system for the monitoring of pressures in pipe lines. The process according to the present invention is characterised by the features which are evident from the following claim 1. Preferred embodiments of the process are evident from the dependent claims 2 - 3.

The arrangement according to the invention is stated in the claims 4 - 8, while the application of process and the arrangement for the recording of fluid pressures in fluid- containing hollow bodies are stated in the claims 8 - 10.

The invention shall now be explained further with reference to the accompanying Figures .

Figure 1 shows a schematic principle sketch of the system according to the invention.

Figure 2 shows the electrical circuit pattern in the system according to the invention.

Figure 3 shows a special and preferred application of the system according to the invention.

The principle for the system according to the invention is illustrated schematically in Figure 1. Three sensors (strain sensors) 10,12,14, which are mounted on the surface 28 of a pipe 30 (see also Figure 3) , are individually connected each via its signal amplifier 16,18,20 to a transmitter 22 (comprising a microprocessor CPU) . Further a manometer 24, which is coupled to the pipe 30, is connected to the transmitter 22. A transmitter 22 comprises/includes electronic circuits and associated data programs which are enclosed in suitable memory discs in order to be able to carry out the stated measurements, computations and communication with other units which are described below.

The manometer 24 comprises a pressure-transducer 24 which measures the real fluid pressure internally in the pipe line 30 at a distance from (that is to say at a location other than) that location where the surface mounted sensor (s) is/are mounted, and where such pressure measuring by penetration of the pipe wall is permitted.

A more detailed coupling is shown in Figure 2. Each circuit comprises the three strain gages (deformation sensors) 10,12,14 which are mounted to the surface 28 of the pipe 30 by means of a capsule 20 (see Figure 3) . The strain gages in the capsule 29 are electrically insulated from the pipe 30, but sense (record) any other physical change in the material 30 of the pipe. The three strain gages which constitute the sensors 10,12,14, are coupled together in a Wheatstone bridge 13,15,17 with which the changes in the potential conditions (in mV, millivolt) are recorded as a signal which is conducted via the respective amplifiers 16,18,20 which are mounted in the amplifier box, into the transmitter via the leads 21a, 21b, 21c. As the Figure shows, the signal is led from the pressure transducer 24 (via a coupling box 25) into the transmitter 22 via the lead 26. Since the metal expansion in the pipe material 30 is also temperature-dependent , there can also be coupled in connection with the sensors 10,12,14 a temperature sensor 32, and which emits signals about the temperature conditions (the temperature changes) internally in the line (that is to say via the pipe material) via the lead 34 to the transmitter 22. The object of this coupling, and of the invention generally, is that the output signal 40 from the transmitter shall give as correct a value for the fluid pressure as possible, and on the basis of the signal (s) which are exclusively recorded by the surface sensors

(strain sensors) 13,15,17, exemplified by the strain gages measuring system and the coupling according to the Figures 1 and 2.

If there is a larger distance between the pressure transducer 24 and the strain gages pressure measuring system, the transmitter ought also to pay regard to possible intermediate means in the pipe line, such as valves, bends and the like, and which can affect (change) the pressure between the two measuring points. The transmitter shall also correct for differences in pressure between the two measuring points which will arise if the measuring points are at dissimilar elevations, and static pressure differences are present caused by the fluid in the pipe. This means that the surface-sensed pressure can be corrected relative to the recording of the pressure of the pressure transducer (the reference) .

Besides as regards the use of surface-mounted sensors (strain gages) the transmitter is programmed so that it employs the two signals from the sensors 10,12,14 which lie closest together in value. If this difference is greater than a set limit, the computer will reject all the signals and warn that there is a fault in the strain gage system.

The output signal 40 from the transmitter can either be read locally (on a screen) on the instrument in the transmitter, or the signal 40 from the transmitter can for example be coupled to the remaining process control system of the installation where the fluid pressure at the current measuring location constitutes a parameter. The reference numeral 41 represents the signal input to the transmitter 22 of the status of the valve 50, that is to say whether it is closed or open. If it is closed the transmitter 22 records this via 41, so that the measurement by the transducer 24 is disconnected from the computation which the transmitter carries out. The measuring of the^' strain gages system thereby comes into force for the pressure.

That system which is illustrated in the Figures 1 and 2 is intended to be permanently in operation. That is to say that the signal from the surface sensors is calibrated relative to the reference signal from the pressure transducer at given intervals. According to a situation the reference signal from the pressure transducer can be excluded from processing in the transmitter. Thereby the calibration ceases and the surface-sensed measuring bridge signal comes into effect for the pipe pressure. Thereby there is obtained a measuring bridge signal which will be as accurate as possible, as a consequence of its being carefully calibrated.

In practice the surface sensors are mounted while there is a normal working pressure in the pipe(s), and since the pressure is rarely reduced to 0, the measuring system is thus calibrated in that small pressure variations normally arise during operation.

A preferred application of the system according to the invention is shown in Figure 3. The Figure illustrates a section of a pipe 30 of a pipe line system through which a fluid flows in one direction. The surface-mounted pressure measuring sensors 10,12,14 and the transducer 24 are arranged at a mutual distance, and via respective leads 21 and 26 to the transmitter. The output signal is shown at 40. The reference donor 24 and the sensors 10,12,14 are each arranged on their respective side of a valve 50. The arrow F indicates as an example the direction of flow of the fluid though the pipe 30/the valve 50. The valve can be closed for limiting or stopping the flow of the fluid through the pipe .

An instance where such a coupling is especially applicable, is where the surface sensors are mounted to that portion of a pipe where for safety reasons (mentioned above) it is not permitted or recommended to mount instruments which penetrate the pipe wall. This can be for example on a production platform where oil/gas is led through the pipe which is laid out along the sea bottom and upwards to the surface again to an installation on land, and comprises the pipe 30 on the one side of the valve 50.

The pressure transducer 24 is mounted into the pipe 30 on the secure side, that is to say in that portion of the pipe where instruments can be mounted which penetrate the pipe wall . The distance between the pipes ought to be the shortest possible. During operation the process fluid (for example oil/gas) is conducted through the pipe, generally at high pressures up towards 300 bars and temperatures of over 100°C. The sensor signals from the surface-mounted sensors are calibrated at given intervals. If the conveyance of the fluid through the line has to stop, a controlled closing of the valve 50 must occur on the installation at sea and of an equivalent valve system on the land side. When the valve 50 is closed, this is recorded in the transmitter via the signal of the valve- position from the process control signal and the pressure measurements from the sensors 10,12,14 exclusively indicate the pressure in the line at the point where these are mounted. Several sets of such sensors can be mounted along the pipe, so that a satisfactory survey of the pressure conditions along the line can be had. Installations beneath the sea surface will require special casings . By the process the method and the system according to the invention for calibrating the pressure measurement from the sensors, it is possible to produce a more dependable survey of the correct pressure conditions in the pipe lines between emergency closing valves than that which has hitherto been possible.

It will also be possible to reduce the number of executions in pipe lines on installations at sea and on land installations, something which will give increased safety.

Claims

Patent Claims

1. Process of measuring pressure in a fluid in a fluid- containing hollow body, such as in a fluid-conducting pipe line, characterised in that the pressure is measured by means of sensors, preferably deformation sensors, which are mounted in a manner known per se in the surface of the body or the pipe line, and the measurements from the surface sensor (10,12,14) is calibrated relative to the pressure measurements which are executed directly in the fluid internally in the pipe line.

2. Process in accordance with claim 2 , characterised in that a pressure transducer (24) is used for executing the direct pressure measurement, and the signals from the pressure sensor-systems (10 , 12 , 14 ;24) are compared/ processed in a transmitter (22) .

3. Process in accordance with claims 1 - 2, characterised in that the measurement with the surface- mounted sensors (10,12,14) is executed in a pipe line where penetration of the pipe wall is undesired or excluded, and where the two sensor systems (10 , 12 , 14 ;24) are mutually separated by a valve (50) , and when the valve (50) is closed, the transmitter (22) excludes the signal from the pressure transducer (the other pressure sensor) (24) , in order thereby to procure a pressure measured by the surface-mounted sensor (10,12,14), as the prevailing fluid pressure in the pipe circuit.

4. Arrangement of a system for measuring pressure in a fluid internally in a hollow body (30) , characterised in that it comprises : a first sensor (10,12,14) which is mounted in a manner known per se in the surface (28) of the body (30) , and which records/measures the pressure in the fluid via physical characteristics of the body (30) , together with a second sensor (24) positioned internally in the hollow space (30) in the fluid for measurement of the fluid pressure, and that the first (10,12,14) and the second (24) sensors are individually connected to a transmitter (22) which is continuously or periodically adapted to calibrate the measurement from the first sensor (10,12,14) relative to the second sensor (24) .

5. Arrangement in accordance with claim 4, characterised in that the first sensor (10,12,14) comprises a deformation sensor in the form of a strain gage system the electrical characteristics of which are changed in correspondence with expansions/contractions of the wall- forming material of the body.

6. Arrangement in accordance with claims 4 and 5, characterised in that the second sensor comprises a pressure sensing transducer (24) .

7. Arrangement in accordance with one of the preceding claims 4 - 6, characterised in that when the body is a pipe, expansions/contractions are recorded in the axial and circumferential direction of the pipe.

8. Arrangement in accordance with one of the preceding claims 4 - 7, characterised in that the transmitter (22) is adapted to be able to exclude the signal from the second pressure sensor (24) , in order thereby to procure a pressure measured by the first sensor (10,12,14) as the prevailing fluid pressure.

9. Application of the process according to claims 1 - 3, and the arrangement according to claims 4 - 7, in a system consisting of

1) a first circuit of a hollow body, such as a first fluid-conducting pipe line, in which the mounting of a sensor which involves penetration of the wall of the body, is undesired or excluded, and

2) a second circuit of a hollow closed body, such as a second pipe line, where such penetration is permitted, where

3) the first and second circuits are mutually connected via a closeable valve in the circuit, and

4) the first circuit comprises the first sensor (10,12,14), while the second circuit comprises the second sensor (24), the first (10,12,14) and the second sensors (24) are mutually connected as indicated in claim 1.

10. Application as indicated in claim 9, in that when the valve (50) is closed, the transmitter (22) excludes the signal from the second pressure sensor (24) , in order thereby to procure a pressure measured by the first sensor (10,12, 14), as the prevailing fluid pressure of the pipe circuit .

11. Application as indicated in claims 8 - 10 for measurement of the fluid pressure in pipe lines, tanks, containers, and reactors and the like within the process industry, chemical industry, petrochemical industry, and oil refineries.