GB2215051A

GB2215051A - Detection of plug closure in cryogenic pipe freezing by monitoring acoustic emissions

Info

Publication number: GB2215051A
Application number: GB8802766A
Authority: GB
Inventors: Richard James Bowen; John David Turner; Mark John Burton
Original assignee: University of Southampton
Current assignee: University of Southampton
Priority date: 1988-02-08
Filing date: 1988-02-08
Publication date: 1989-09-13
Also published as: GB8802766D0

Abstract

The formation of a plug of frozen fluid within a closed vessel e.g. a pipe 3 during freezing is monitored by means of a sensor 7 disposed outside the vessel. The sensor detects stress waves transmitted in the vessel walls during plug formation and the state of development of the plug is determined from the sensed stress waves. The amplified and filtered signal from the sensor 7 is fed to both display/recording equipment 13, 15, 17 and also to a pulse counter 19 which records the frequency of acoustic emissions from the pipe. The technique is useful in the testing of pipes and vessels containing water-based fluids or oil. <IMAGE>

Description

DETECTION OF PLUG CLOSURE IN CRYOGENIC PIPE FREEZING This invention relates to a method of sealing pipelines known as cryogenic pipe freezing. More specifically the invention is concerned with detecting formation of a plug of frozen fluid within the pipe.

Cryogenic freezing is a technique that involves fitting a freezing jacket around a closed vessel and pumping a freezing material such as liquid nitrogen or a solid carbon dioxide/methanol mixture into the jacket. The freezing material cools the liquid in the vessel causing freezing, initially at the vessel walls and then extending radially inwards. As freezing continues, the plug of frozen liquid grows radially inwards until a fully developed plug is formed which seals the vessel.

Although the plug is commonly referred to as an ice plug, it should be understood that neither the technique described nor the invention are limited to pipe containing water based liquids.

Cryogenic freezing enables a pipe or other vessel to be tested up to the location of the plug. It also allows a section of say, a pipeline to be isolated by the formation of a second plug. The technique is very useful as it allows pressure testing of a vessel without any permanent change to the vessel. Furthermore it allows repair to a damaged section of, for example, a pipeline without the need to drain large sections of pipeline or shut down a whole plant.

The technique is applicable to pipelines carrying a variety of liquids and has been applied to pipe of diameters up to im. The technique has found use in a wide variety of applications from heating systems to industrial pipelines.

It is desirable to be able to monitor the development of a plug during a freezing operation and it is also desirable to be able to investigate the integrity of a fully developed plug before the pipe is opened, for example, for repair. Under difficult conditions, for example, a wide pipe diameter containing a high temperature high velocity flow, a long freezing time may be required which is expensive and so it is desirable to know when and if a plug is likely to become fully formed.

A number of methods of plug testing and plug monitoring have been proposed. For example if the cryogenic jacket is being applied near a flange it is common to loosen the flange bolts securing two pipe sections together to check if flow has ceased. Alternatively a pressure test may be applied across the plug.

All the known methods have the disadvantage that they require access to the fluid in the pipe which is both awkward and potentially dangerous if a plug is not fully formed.

As enclosed liquids freeze they may expand or contract and crack. The cracking can result in acoustic emissions. The formation of an ice plug during cryogenic freezing has been observed to be accompanied by such cracking. The inventors have appreciated that the cracking is transmitted as stress waves through the vessel and that the characteristics of the stress waves vary in accordance with the stage of development of the plug. These stress waves can be monitored to determine the state of development of a plug at any time during the freezing operation.

One aspect of the invention resides in a method of monitoring the formation of a plug of frozen fluid within a closed vessel during freezing, the method comprising sensing, by means of a sensor disposed outside the vessel, stress waves transmitted in the vessel walls during plug formation, and determining the state of development of the plug from the sensed stress waves.

The invention also resides in apparatus for monitoring the formation of a plug of frozen fluid within a closed vessel during freezing, comprising sensing means disposed outside the vessel for sensing stress waves transmitted in the vessel walls during plug formation, and means for determining the state'of development of the plug at a given time from the sensed stress waves.

The invention overcomes the aforementioned disadvantages of prior monitoring methods as access to the fluid in the vessel is not necessary to determine plug closure. Thus, the vessel does not need to be opened nor flange bolts loosened before it has been determined that plug formation has been completed.

In a preferred embodiment, the sensing and measuring of the generated stress waves is performed at a position remote from the cryogenic freezing. This is possible because the stress waves generated in the vessel travel along the vessel walls with little damping. Positioning the sensing means away from the cryogenic freezing apparatus has the advantage that special sensing apparatus that can withstand the very low temperature of the freezing apparatus is not required.

A method in accordance with the invention will now be described in detail, by way of example, and with reference to the accompanying drawings, in which: Figure 1 is a graph showing the variation in generated stress wave frequency with freezing time; Figures 2 and 3 are partially sectioned views of a pipe showing the initial and fully developed plug formations; and Figure 4 is a block diagram of apparatus for detecting stress wave and monitoring the state of development of a plug embodying the invention.

The invention stems from the observation that the formation of an ice plug in a pipe during cryogenic freezing results in acoustic emissions from within the pipe. The acoustic emissions are transmitted through the fabric of the pipe as stress waves which take the form of sharp impulses. It has been observed that the frequency of generation of the impulses varies during the freezing process giving a characteristic graph of frequency against time from which the state of plug closure can be observed. The characteristic graph is illustrated in Figure 3.

In Figure 3 two curves are illustrated. Curve A shows measured frequency of the stress waves against freezing time for a 20 cm diameter pipe with a 0.2 l/s flow rate and an initial fluid temperature of 17"C. Curve B (the points of which are marked by Xs) shows a similar curve for a 20 cm dimater pipe with a 0.4 l/s flow rate and an initial fluid temperature of 14 "C.

From Figure 1 it can be seen that there are three stages in the freezing process; stage 1, beginning at I and ending at II shows that acoustic emissions are initially generated at a high frequency and then decline sharply to a low value. Stage 2 (II to III) follows in which period the rate of acoustic emissions is at a minimum and almost unchanging. Finally, stage 3 (III to the end) which occurs when the point at which the plug closes is approached occurs when the emission rate increases sharply and then falls away again. The sharp increase from the minimum is the 'fingerprint' of the plug closing.

In addition to detecting the closure point, it is also possible to observe cases where plug closure will never occur, no matter how long the freezing process is maintained. This situation may arise when the initial flow rate or temperature is too high.

It is believed that the acoustic emissions which generate stress waves in the fabric of the pipe are produced by cracking of the plug as it forms. Figures 2 and 3 show the development of the plug in a pipe. When the plug is initiated (Figure 2) a layer of solid material 1 will form around the pipe wall 3. At this point the layer of ice is thin and the temperature gradient in the solid layer is steep. The steep gradient results in high thermal stresses. The stresses in the ice layer are relieved by the material cracking.

After the initial part of the plug has formed the cracking rate decreases sharply as the layer thickens and the temperature gradient decreases. The cracks that form are randomly directed. When the plug closes (Figure 3), there is a rapid growth of ice at the centre of the plug in the axial direction. The rapid growth is accompanied by a rapid change in temperature gradient and an increase in the cracking rate.

Referring now to Figure 4, the stress waves generated by the cracking can be measured by a sensing device 7 applied to the outside surface of the pipe. A piezoelectric strain gauge fabricated from poled homopolymer of vinyledene fluoride film (PVDF) is a suitable sensing device. The sensing device 7 can either be glued or taped to the pipe. As the generated stress waves are barely attenuated along the pipe it is convenient to attach the strain gauge to the pipe some way from the cryogenic freezing apparatus. One metre has proved a suitable distance, although distances of three or four metres may be possible. At these remote positions the strain gauge need not be capable of withstanding the extreme temperatures experienced around the freezing jacket 1.

The strain gauge 7 produces a charge signal output in response to a stress wave caused by a acoustic emission. This output signal is converted to a voltage signal and amplified by a charge amplifier 9. The amplified voltage ouput of the charge amplifier 9 is then filtered by a high-pass filter 11. This filter removes mains interference to produce a signal suitable for further processing.

At this point the signal can be displayed, for example at an oscilloscope 13 or recorded by a tape recorder 15 for future analysis. As an alternative or complement to the oscilloscope, the output could be displayed on a plotter 17.

The output from the high-pass filter is also fed to a pulse counter 19 which records the frequency of emission of the acoustic pulses from within the pipe. The counter may be reset at given intervals, for example 100 seconds, and the counter value at that time recorded. A plotter may be coupled to the output of the pulse counter 19 to plot the counter value at each 100 second intervals to produce a graph similar to Figure 3. For experimental purposes it may be desirable to include a spectrum analyser 21 and/or a pulse height analyser 23.

There are many variations to the detecting and monitoring apparatus illustrated in Figure 4 that may be made and will occur to the person skilled in the art. For example, there are many different types of sensing device that may be used instead of the PVDF strain gauge. For example, a piezoelectric accelerometer may be used in which case a threaded stud is attached to the outside of the pipe for mounting, the device. Another alternative would be to use a simple acoustic microphone. However, this latter example has not proved very effective due to the amount of background noise picked up in addition to the acoustic pulses.

The method may be applied to water based fluid and also other fluids such as oil.

The method and apparatus embodying the invention provide a very simple system for monitoring the formation of a plug. They obviate the need to open the pipe or loosen connecting flanges in order to check whether the plug is fully formed and so increase the safety of pipe testing and pipe maintenance. Furthermore they can warn the user of conditions in which a plug will not close within a time period which is economically acceptable.

Claims

CLAIMS:

1. A method of monitoring the formation of a plug of frozen fluid within a closed vessel during freezing, the method comprising sensing, by means of a sensor disposed outside the vessel, stress waves transmitted in the vessel walls during plug formation, and determining the state of development of the plug from the sensed stress waves.

2. A method according to claim 1 wherein the sensor is disposed at a position on the vessel remote from the formation of the frozen fluid plug.

3. A method according to claim 1 or 2 wherein the sensor is a strain gauge attached to the outer surface of the vessel.

4. A method according to claim 1 or 2 wherein the sensor is an accelerometer attached to the outer surface of the vessel.

5. Apparatus for monitoring the formation of a plug of frozen fluid within a closed vessel during freezing, comprising sensing means disposed outside the vessel for sensing stress waves transmitted in the vessel walls during plug formation, and means for determining the state of development of the plug at a given time from the sensed stress waves.

6. Apparatus according to claim 5 wherein the sensor is attached to the pipe at a position remote from the zone of the pipe in which the frozen fluid plug is formed.

7. Apparatus according to claim 5 or 6 wherein the sensing means is a strain gauge.

8. Apparatus according to claim 5 or 6 wherein the sensing means is an accelerometer.

9. Apparatus for sealing a pipe comprising a cryogenic freezing jacket for location around the pipe at a location to be sealed, a source of freezing material for introduction to the freezing jacket, and apparatus for monitoring the formation of a plug of frozen fluid within the pipe according to any of claims 5 to 8.

10. A method of temporarily sealing a pipe, comprising the steps of applying a cryogenic freezing jacket to the outside of the pipe at a location to be sealed, introducing a freezing material into the cryogenic jacket to form a plug of frozen fluid in the pipe and monitoring the formation of the plug according to the method of any of claims 1 to 4.

11. A method for monitoring the formation of a plug of frozen fluid within a closed vessel during freezing, substantially as herein described with reference to the accompanying drawings.

12. Apparatus for monitoring the formation of a plug of frozen fluid within a closed vessel, substantially as herein described with reference to the accompanying drawings.