GB2269900A

GB2269900A - Acoustic leak detection method for liquid storage tanks

Info

Publication number: GB2269900A
Application number: GB9217626A
Authority: GB
Inventors: Christopher David Hill
Original assignee: Individual
Current assignee: Individual
Priority date: 1992-08-19
Filing date: 1992-08-19
Publication date: 1994-02-23
Also published as: GB9217626D0

Abstract

The method is performed by affixing an acoustic sensor to an exterior surface of the tank under test. The pressure in the tank is then increased or decreased so that the pressure in the tank is not atmospheric and there is a differential pressure across the wall of the tank. Air is caused to flow through any hole that exists in the tank wall and the acoustic sensor picks-up the sounds generated by air flowing through the hole. The sensor can be attached to an elongate element which is itself fixed to the tank. The use of an elongate sound transmitting element, eg a ribbon, can be applied to other forms of testing. <IMAGE>

Description

ACOUSTIC DEEETION METhODS THIS INVENTION relates to acoustic detection methods.

It is known to detect a leak in a tank by decreasing the pressure in the tank so that air flows into the tank, and inserting an acoustic sensor into the tank on the end of a probe so that the acoustic sensor is in the ullage and picks-up the sound of air flowing into the tank. It is also known to check the status of bearings and other moving parts in machinery by attaching an acoustic sensor to the machinery and "listening" to the sonic and ultrasonic sounds produced by the moving parts. Early detection of an impending break-down of, for example, a bearing is possible.

The object of the present invention is to improve existing acoustic detection methods.

According to one aspect of the present invention there is provided a method of detecting whether there is a leak in the wall of a liquid storage tank which method comprises affixing an acoustic sensor to an exterior surface of the tank under test, increasing or decreasing the pressure in the tank so that the pressure in the tank is not atmospheric and there is a differential pressure across the wall of the tank whereby air flows through any hole that exists in the tank wall, and picking up by means of said acoustic sensor the sounds generated by air flowing through any hole.

In most tests pressure in the tank is reduced to cause air to flow into the tank.

If the hole is below the level of the liquid in the tank, then the sound has a first component which is generated by the air flowing over the edge of the hole, a second component generated as the bubbles detach from the hole, a third component consisting of the sound pressure waves generated by bubble vibration as the bubbles travel upwardly in the liquid, and a fourth component generated when the bubbles burst at the surface of the liquid.

If the leak is above the level of the liquid in the tank ie into the ullage, then the only component that is detected is the hiss of air through the hole.

The sensor will normally be placed on the top surface of the tank which is where access is most readily available, particularly in underground tanks.

It can, however, be located at any suitable place on the tank or on a part which is fixed to the tank. Such parts can comprise a fill pipe, a vent pipe or a suction pipe.

Experimental work has shown that an extension element such as a ribbon, tape, cable, wire or pipe suitably attached to the tank, or to a tank part, picks-up the sounds from the tank or tank part. The sensor can then be used to pick-up vibrations from the extension element.

Whilst he sensor is capable of generating an output in response to signals from 1 KHz to 42 KHz, the detecting instrument to which the sensor is connected is preferably tuned to pick-up sound in the audible range (up to 8 KHz), and in the ultrasonic range of 32 to 42 KHz, and particularly from 37 XHz upwards. This is because sounds in the audible range tend to be masked by traffic and other noises in the vicinity of the tank.

The output at ultrasonic frequency from the sensor can be converted by the detecting instrument to sound in the audible range so that the person conducting the test can listen to the sound being picked up. Alternatively or additionally the sound can be computer processed for display on a VDU and/or for recording as a hard copy.

The extension element referred to above can be used for convenience or where the immediate environs of the tank are hazardous and could possibly damage the sensor and the detecting instrument to which the sensor is connected. For example, there may be corrosive gases in the immediate vicinity of the tank.

In a specific test a general purpose acoustic detector was used to detect leaks into a 23000 litre storage tank which was half filled with petrol. The pressure in the tank was reduced to just below atmospheric.

The pressure reduction depends on the specific gravity of the liquid in the tank and the depth of the liquid. Clearly the pressure must be reduced to an extent which is sufficient to reduce the pressure at the point of the tank where there is the greatest depth of liquid to sub-atmospheric otherwise air will not be drawn in. Merely by way of example, a reduction of about 3 p.s.i is sufficient to achieve the desired result in most commercial storage tanks.

A valved probe was used to feed air into the tank. Air pressure in the probe was atmospheric and the outlet from the probe could be adjusted to simulate holes of 0.05mm, 0.lmm and 0.5mm diameter. The results of the tests are set out in the following table.

RANGE LOX. LEAK LEAK SIZE AMP.Db 1. ULTRASONIC Below .05 mm 017 2. ULTRASONIC Below .1 mm 033 3. ULTRASONIC Below .5 mm 035 4. AUDIO Below .05 mm 039 5. AUDIO Below .1 mm 053 6. AUDIO Below .5 mm 067 7. ULTRASONIC Above .05 mm 028 8. ULTRASONIC Above .1 mm 029 9. ULTRASONIC Above .5 mm 035 10. AUDIO Above .05 mm NIL-02 11. AUDIO Above .1 mm NIL-07 12. AUDIO Above .5 mm 006 Ultrasonic - means the detector was set so that it responded to signals in the 32 to 42 KHz range.

Audio - means that the detector was set so that it responded to signals up to 8 KHz.

Loc leak - indicates whether the incoming air entered the tank above or below the liquid level.

AMP Db - indicates the sound pressure levels in decibels.

It will be noted that for leaks above the liquid level, signals at ultrasonic frequencies are stronger and easier to detect (lines 7 to 9). The audio range signals are small (lines 10 to 12). For below liquid level leaks, signals at audio and ultrasonic frequencies are both satisfactory (lines 1 to 6).

Figure 1 to 4 show the traces obtained (sound pressure levels in Db V. time in seconds) under the following conditions: Figure 1 DETECTION IN AUDIO RANGE-LEAK INTO ULLAGE The ambient noise level is 40/48 Db and the size of the hole through which leakage took place is 0.05mm. The peak reading is mechanical noise as the valve was opened.

The reading then settled at about 60 Db.

This is sufficiently far above the ambient reading readily to be detectable.

Figure 2 DETECTION IN AUDIO RANGE-LEAK BELOW PRODUCT LEVEL The ambient noise level in this test was 41 43 Db and the signal level increased to 48/52 Db as soon as the leak commenced. Peaks of over 54 Db are not uncommon. The hole size was 0.lmm.

Figure 3 DETECTION IN ULTRASONIC RANGE-LEAK ABOVE PRODUCT LEVEL The ambient noise level in the Ultrasonic range is generally very small and quite often so small that it cannot be detected. The relatively steady "hiss" at 20-30 Db can thus readily be detected. The entire sound is generated where air is entering the tank.

The hole size was 0.lmm.

Figure 4 DETECTION IN ULTRASONIC RANGE-LEAK BELOW PRODUCT LEVEL The very uneven trace is due to the fact that various noise sources contribute to the composition of ultrasonic sound. Air flowing across the lip of the hole, bubbles breaking away from the hole, the pulsation of bubbles in the liquid and the bubbles bursting all make a contribution. Set against an ambient ultrasonic noise level which is between zero and one or two decibels, the sound from 1 Db to almost 40 Db can readily be detected.

The test at ultrasonic frequencies is to check on the result at audio frequencies where the result might be influenced by noise in the vicinity of the tank under test.

Whilst it is preferred that the general purpose acoustic sensor be placed on the wall of the tank, it can be placed on pipework or other components which are joined to the tank. The only requirement is that there not be an acoustic barrier, eg a rubber seal, between the tank and the pipework. The term tank wall thus includes not only the shell of the tank but also pipes which are connected to the tank. The detector can be held to the tank by magnetic means or gravitationally under the influence of its own mass.

All the external fittings of the tank under test can be checked by an airborne sensor which is placed close to each fitting in turn and, where necessary, carried entirely around the fitting.

According to a further aspect of the present invention there is provided a method of testing which comprises attaching an elongate extension element to the object under test so that acoustic signals generated by the object are transmitted to said element, and attaching an acoustic detector to said element at a location remote from said object thereby to pick-up the acoustic signal that has entered the element from said object.

The extension element is preferably a ribbon or tape of high tensile steel, the ribbon or tape being flat or arcuate in cross-section. The element can, however, be a cable, pipe or wire. Said sensor can be connected to a detecting instrument which converts the acoustic signal into a form in which it can be displayed on a liquid crystal display or on a VDU, and/or printed out as a hard copy and/or transmitted to earphones.

Said object can be a piece of machinery which includes bearings. The acoustic signal from bearings enables their state to be analyzed ie whether there is wear and/or misalignment and/or cracks in the races or other parts.

Claims

CLAIMS:

1. A method of detecting whether there is a leak in the wall of a liquid storage tank which method comprises affixing an acoustic sensor to an exterior surface of the tank under test, increasing or decreasing the pressure in the tank so that the pressure in the tank is not atmospheric and there is a differential pressure across the wall of the tank whereby air flows through any hole that exists in the tank wall, and picking up by means of said acoustic sensor the sounds generated by air flowing through any hole.

2. A method according to claim 1, wherein the pressure in the tank is reduced to below atmospheric to cause air to flow in.

3. A method according to claim 1, wherein the pressure in the tank is increased to above atmospheric to cause air to flow out.

4. A method as claimed in claim 1, 2 or 3, wherein the sensor is connected to a detecting instrument tuned to pick-up sound at frequencies of 32 to 42 KHz.

5. A method as claimed in claim 1, 2 or 3, wherein the sensor is connected to a detecting instrument tuned to pick-up sound at frequencies of up to 8 KHz.

6. A method of testing which comprises attaching an elongate extension element to the object under test so that acoustic signals generated by the object are transmitted to said element, and attaching an acoustic sensor to said element at a location remote from said object thereby to pick-up the acoustic signal that has entered the element from said object.

7. A method as claimed in claim 6, wherein said element is a metal ribbon.

8. An acoustic detection method substantially as hereinbefore described with reference to the accompanying drawings.