AN APPARATUS AND METHOD FOR MEASURING THE PRESSURE INSIDE A PIPE OR CONTAINER

Abstract

An apparatus and method for measuring the internal pressure of a pipe or container is disclosed. The apparatus includes an acoustical transmitter (Tx.sub.1) mounted on a wall (1) of said pipe or container and a signal generator (2) connected to said transmitter and which is adapted to provide a signal to the transmitter. The signal from the transmitter is detected by two receivers (Rx.sub.1, Rx2) mounted on said pipe or container in a distance from said transmitter (Tx.sub.1). A processing unit (3) is connected to said transmitter and receivers, the processing unit being adapted to measure the travel time of an acoustical signal propagating between two receivers in the wall (i) and determine the pressure inside the pipe or container from said travel time.

Claims

1. An apparatus for measuring the internal pressure of a pipe or container, characterized in an acoustical transmitter (Tx.sub.1) mounted on a wall (1) of said pipe or container, a signal generator (2) connected to said transmitter and which is adapted to provide a signal to the transmitter, a first receiver (Rx.sub.1) mounted on said pipe or container in a distance from the transmitter (Tx.sub.1), a second receiver (Rx.sub.2) mounted on the pipe or container in a further distance from the transmitter, a processing unit (3) connected to said first and second receiver, the processing unit being adapted to measure the travel time of an acoustical signal propagating from the first to the second receiver in the wall (1) and determine the pressure inside the pipe or container from said travel time.

2. An apparatus according to claim 1, wherein the processing unit (3) is adapted to determine the pressure inside the pipe or container from the relation $P\propto \frac{2.Math.\Delta .Math..Math.\mathrm{Lt}}{\mathrm{\Delta \tau }.Math..Math.D}$ where P is the pressure, ΔL is the distance between the first receiver and the second receiver, t is the wall thickness, Δr is the travel time, and D is the internal diameter of the pipe or container.

3. An apparatus according to claim 1, wherein the signal generator is adapted to emit a signal exciting a thickness mode of said wall.

4. An apparatus according to claim 1, wherein the processing unit is adapted to determine the travel time by cross correlating the received signals.

5. A method for measuring the pressure inside a pipe or container, characterized in the steps of: transmitting an acoustical signal into a wall of said pipe or container, measuring the travel time of said acoustical signal when propagating between two points on said wall, and determining the pressure inside said pipe or container from said travel time.

6. A method for measuring the pressure inside a pipe or container, characterized in the steps of: transmitting an acoustical signal from an acoustical transmitter (Tx.sub.1) mounted on a wall (1) of said pipe or container, receiving the acoustical signal in a first receiver (Rx.sub.1) mounted on said pipe or container in a distance from the transmitter (Tx.sub.1), receiving the acoustical signal in a second receiver (Rx.sub.2) mounted on the pipe or container in a further distance from the transmitter(Tx.sub.1), measuring the travel time of the acoustical signal propagating from the first to the second receiver in the wall (1), and determining the pressure inside the pipe or container from said travel time.

7. A method according to claim 6, wherein the pressure is determined from the relation $P\propto \frac{2.Math.\Delta .Math..Math.\mathrm{Lt}}{\mathrm{\Delta \tau }.Math..Math.D}$ where P is the pressure, ΔL is the distance between the first receiver and the second receiver, t is the wall thickness, Δr is the travel time, and D is the internal diameter of the pipe or container.

8. A method according to claim 6, wherein the acoustical signal is exciting a thickness mode of the wall.

9. A method according to claim 6, wherein the acoustical signal is a sinc wave packet covering at least one thickness resonance frequency of the wall.

10. A method according to claim 6, wherein the travel time is determined by cross-correlating signal received in said points.

11. A method according to claim 6, wherein the acoustic signal is a sound pulse.

12. A method according to claim 11, wherein the acoustic signal is a burst pulse of a predefined number of periods.

Description

DETAILED DESCRIPTION

[0017] The invention is based on observing the behaviour of acoustic waves travelling in the pipe/container wall, and not as in prior art, by observing waves travelling through the medium inside the pipe/container.

[0018] A pressure difference between the inside and outside of the pipe/container will set up stress in the wall. For a cylindrical wall the stress a is given by:

[00002]$\sigma =\frac{\mathrm{PD}}{2.Math.t}$

[0019] Where P is the pressure difference, D is the internal diameter, and t is the wall thickness.

[0020] There is a nearly linear relationship between wall stress and phase velocity. Measuring phase velocity will provide a value of the wall stress from which the pressure difference over the wall may be deducted. Hence,

σ∝v.sub.p

[0021] where v.sub.p is the phase velocity in the wall of the pipe or container.

[0022] FIG. 1 is a schematic illustration of the apparatus for measuring pressure according to the invention. The inventive apparatus includes a signal generator 2 driving a transmitter Tx.sub.1 mounted onto the wall 1 of a pipe or container.

[0023] The transmitted signal may be a sharp spike, a square wave, or a burst pulse. The burst pulse may be a plain sine wave burst, a swept wave burst or a sinc burst pulse. It is preferred to use a burst pulse to avoid dispersion and to ease the detection in a noisy environment. It is further preferred to let the burst pulse excite one of the thickness resonance frequencies of the wall. There are several thickness resonance frequencies available, but here it is preferred to use the first harmonic (wavelength=t) due to the distance to other mode frequencies. The signal will propagate along the wall by several modes, notably as shear- and Lamb-waves.

[0024] The periodic signal is picked up by two receivers Rx.sub.1 and Rx.sub.2. The received signals are digitized and fed to a processing unit 3. The processing unit is adapted to determine the time shift between the receivers. The time shift may be determined by cross-correlating the signals from the receivers. Another option is to measure the time shift between identified zero-crossings in the signals, or measure when the signals exceed a specified threshold for determining the leading edge of the first arrival.

[0025] A general description of the method will be as follows; [0026] A sound pulse is transmitted from an acoustic contact transducer transmitter, Tx.sub.1 to a first acoustic transducer receiver, Rx.sub.1, and preferably also to a second acoustic contact transducer, Rx.sub.2 arranged on a pressure container or pipe. The pulse can be a so-called “burst” pulse of a predefined number of periods. It could e.g. be between 1 and 20 periods, preferably 5-10 periods. The pulse can typically have a centre frequency within the band from 50 kHz to 1000 kHz. The pulse should be adapted to the thickness of said pressure container or pipe, so that a clear pulse is received at said receivers, Rx.sub.1 and possibly also Rx.sub.2. [0027] The time lapse, τ.sub.1, from said pulse is transmitted at Tx.sub.1 until it is received at the first receiver, Rx.sub.1, is measured using a suitable time detection method, e.g. a cross-correlation or zero-crossing detection method. [0028] The time lapse, τ.sub.2, from said pulse is transmitted at Tx.sub.1 until it is received at the second receiver, Rx.sub.2, is measured using the same time detection method. [0029] The speed of sound through the wall of the container or pipe, V.sub.p, can be found through L.sub.1, the distance from Tx.sub.1 to Rx.sub.1, or, if two receivers are used, the distance between Rx.sub.1 and Rx.sub.2, ΔL:

[00003]$v_p=\frac{\Delta .Math..Math.L}{\mathrm{\Delta \tau }},\mathrm{where}.Math.\{\begin{array}{c}\mathrm{\Delta \tau }\equiv {\tau }_2-{\tau }_1\\ \Delta .Math..Math.L\equiv L_2-L_1\end{array}.$

If only one receiver is used, the following equations apply:

[00004]${\tau }_1=\frac{L_1}{v_p}+{\tau }_{\mathrm{EL}},.Math.\mathrm{Thereby}.Math.\text{:}$$v_p=\frac{L_1}{{\tau }_1-{\tau }_{\mathrm{EL}}},$

[0030] Where τ.sub.EL represents the delay time of the electronics, that should be accounted for.

[0031] It thereby follows that the method is equally suitable with the use of one or two receivers.

[0032] Now it follows that:

[00005]$\sigma \propto \frac{\Delta .Math..Math.L}{\mathrm{\Delta \tau }},\mathrm{or}.Math..Math.\frac{\mathrm{PD}}{2.Math.t}\propto \frac{\Delta .Math..Math.L}{\mathrm{\Delta \tau }}$

[0033] so that:

[00006]$P\propto \frac{2.Math.\Delta .Math..Math.\mathrm{Lt}}{\mathrm{\Delta \tau }.Math..Math.D}.$

[0034] By using a calibration procedure for a given container or pipe, it should be feasible to attain a pressure resolution of about 1 bar.

[0035] The transducers may be clamped to the pipe/container by any suitable method, e.g. a strap around the pipe/container, or strong magnets, and may include a suitable coupling gel to improve the acoustical coupling to the pipe/container.

[0036] The set-up shown includes one transmitter and two receivers. It is also possible to use one transmitter and only one receiver measuring the time shift for the signal between the transmitter and receiver. However, it may be difficult to achieve a satisfactory accuracy with such a set-up. The method is equally suitable with lamb-waves and shear waves.

[0037] In FIG. 1 the transmitter and receivers are located along the circumference of a pipe or container, but they could also be placed in the longitudinal direction of the pipe or container, or in any other possible direction.