GB2295018A

GB2295018A - Determining the integrity of mechanical seals

Info

Publication number: GB2295018A
Application number: GB9422612A
Authority: GB
Inventors: David Malcolm Parkes; Graeme Neil Crisp; Christopher Robert Thornhill
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1994-11-09
Filing date: 1994-11-09
Publication date: 1996-05-15
Anticipated expiration: 2014-11-09
Also published as: GB9422612D0; GB2295018B

Abstract

A method and system 10 for determining the integrity of mechanical seals using radio frequency radiation comprises a source of rf or microwave radiation 52, a receiver 54 and a programmed computer 90. A seal between a ships metal door 30 and a metal deck 32 is formed by a gasket 40. The source generates ultrawideband rf radiation on one side of the door and the receiver on an opposite side of the door detects radiation passing through the seal. A storage oscilloscope 80 measures and stores a signal from the receiver, and the computer generates a measure of the integrity of the seal from data stored in the oscilloscope. <IMAGE>

Description

Method and system for determining the integrity of mechanical seals The invention relates to a method and system for determining the integrity of mechanical seals, and more particularly the integrity of mechanical seals between metal structures.

In many situations, it is vital that the integrity of mechanical seals between metal structures be maintained.

One particular example is the sealing of bow doors on rollon-roll-off ferries. It is possible to make visual checks of mechanical seals on such ferries. These visual checks are generally performed intermittently and inspection in detail of a whole seal may be expensive and time consuming. It is often difficult to provide continual monitoring of the efficiency of such seals. It would be advantageous to provide remote testing and monitoring of a seal's integrity.

It would also be advantageous if such testing and monitoring could be performed continually.

It is an object of the invention to provide a method of determining the integrity of a seal between metal structures.

The present invention provides a method for determining the integrity of a seal between two metal structures comprising the steps of: (i) generating radio frequency electromagnetic radiation; (ii) directing the radiation towards the seal; (iii) receiving radiation transmitted through the seal; (iv) generating a signal dependent upon the received radiation; and (v) processing the signal to obtain a measure of the seal's integrity.

The electromagnetic radiation may have a frequency bandwidth in a range between 100 MHz and 50 GHz. The frequency range may be between 200 MHz and 20 GHz. Preferably the frequency bandwidth is from 300 MHz to 4 GHz. The electromagnetic radiation may be pulsed, thereby enabling time of flight measurements to be made which in turn provide information as to the location of regions of poor sealing.

The electromagnetic radiation may be generated and received by respective TEM horn antennas positioned on opposite sides of the seal. The signal generated in response to received radiation may be measured and stored by a digital storage oscilloscope or other signal measuring devices.

An alternative aspect of the invention provides a mechanical seal integrity measuring system comprising: (i) means for generating radio frequency electromagnetic radiation; (ii) means for receiving said radiation after transmittal through a seal; (iii) means for generating a signal in response to received radiation; and (iv) means for generating a measure of seal integrity from said signal.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawing in which: Figure 1 schematically illustrates an arrangement of apparatus for performing the method of the invention; Figure 2 is a schematic sectional view of a transmitter element incorporated in the Figure 1 arrangement; and Figure 3 shows a plan view of the Figure 2 transmitter element.

Referring to Figure 1 there is shown apparatus indicated generally by 10 for monitoring the integrity of a seal between a ship's metal door 30 and a ship's metal deck 32.

The seal is provided by a carbon loaded rubber gasket 40 attached to the door 30 and in intimate contact with the deck 32. A radar signal indicated by an arrow 50 is directed from a transmitter 52 towards the door 30. A receiver 54 on an opposite side of the door 30 to that of the transmitter 52 detects the radar signal 50 if it propagates through the seal. The transmitter 52 and the receiver 54 are each positioned five metres from the door 30.

The radar signal is an ultrawideband pulsed radar signal. It has a bandwidth which is selected in operation so as to reduce interference effects and so that high accuracy time of flight measurements are obtainable. Ultrawideband radar signals have a bandwidth within a frequency range which has a lower frequency limit of 200 MHz and an upper frequency limit of 20 GHz, though this range may extend between 100 MHz and 50 GHz.

Referring to Figure 2 and Figure 3 also there is shown a more detailed illustration of the transmitter 52. The transmitter 52 is a simple TEM horn antenna which comprises an antenna plate 60 which is affixed by soldering to a central conductor 62 of a coaxial bulkhead feedthrough 64.

The antenna plate 60 is a sheet of copper in the form of an equilateral triangle having sides lof length 25/cos 300 cm, or approximately 29 cm. One apex 65 of the triangle is attached to the central conductor 62 so that one edge 66 of the triangle is parallel to the door 30. The antenna plate 60 is held facing the door 30 so that the plane of the copper sheet forms an elevation angle o with the plane of the deck 32 such that o = 130. The apex 65 is held 4 mm above the deck 32. The connection between the central conductor 62 and the antenna plate 60 is made as close to the deck 32 as possible. The deck 32 acts as a ground plane for the transmitter 52 and it must be ensured that the antenna plate 60 is not so close to the deck 32 that discharge between the antenna plate 60 and the deck 32 occurs.The antenna plate 60 is supported by polystyrene supports such as a support 67. The receiver 54 is similar to the transmitter 52 but faces in an opposite direction to the transmitter 52. The use of a TEM horn antenna provides a dispersion free source of broadband radiation.

The transmitter 52 is connected via a co-axial cable transmission line 68 to a pulsed source 70. The transmission line 68 is a semi-rigid, high quality coaxial transmission line such as RG141. An elevation angle of 130 is chosen to ensure a good match between the antenna plate 60 and the transmission line 68. In practice, the elevation angle is adjusted and a time domain reflectometer is used to obtain an optimum match between the antenna plate 60 and the transmission line 68. The transmission line 68 is optimally matched to the antenna plate 60 when the amplitude of reflected signals measured by the time domain reflectometer.

The pulsed source 70 is a step generator which provides a pulsed signal comprising a step fall in output voltage to -2 kV with a fall time of 100 ps followed by a relatively slow recovery to 0 V. The source 70 provides an output signal with a repetition rate of 1000 Hz. A 100 ps fall time gives a theoretical radiation frequency maximum of 5 GHz. In practice, the transmitter 52 generates pulses of rf radiation with a maximum frequency of approximately 4 GHz, a useful frequency range extending down to 300 MHz and a pulse width of 150 ps.

The receiver 54 is connected by a transmission line 72 of RG141 coaxial cable to a transient data capture unit 80.

The unit 80 is a Tektronix TEK 11801A sampling oscilloscope.

The purpose of the unit 80 is to record any signal generated by the receiver 54 in response to received radiation emanating from the transmitter 52. The unit 80 passes captured data to a computer 90 for data processing and analysis. In order to synchronize the unit 80 with the source 70, the source 70 provides the unit 80 with a trigger signal via a trigger line 92.

In operation, the transmitter generates ultrawideband radio frequency (rf) radiation which is transmitted towards the seal. With perfect seals, in which the carbon loaded gasket 40 attached to the door 30 is in intimate contact with the deck 32, little or no signal is received by the receiver 54.

If the seal is not complete rf radiation is transmitted through the seal and pulses are detected by the receiver 54.

As the seal is progressively breached, the magnitude and shape of the signal received by the receiver 54 changes according to the size of the breach. The computer 90 is programmed to generate an alarm signal when the data received by the unit 80 exceeds a threshold value, thus giving a warning to an operator that the seal has been breached.

In situations where a gasket such as the gasket 40 is not present, or the gasket is not carbon loaded rubber and is rf radiation transmissive or minor breaches of the seal can be tolerated, it would be necessary to carry out controlled trials to determine the received signal characteristics when an acceptable seal breach is exceeded. The computer 90 would be programmed to compare the received signal with a library of stored signals obtained during the trials. The library would contain signals obtained from acceptable seal breaches. If the received signal did not correspond to a stored signal, an alarm signal would be generated.

As well as the magnitude of a received signal, the time of flight between the signal being transmitted and received would enable a degree of information to be obtained as to the location of a seal breach. The unambiguous location of such a breach would require the use of a number of transmitting or receiving antennas together with the a computer suitably programmed to process the differing times of flight between the antennas into a measure of position.

Whilst the apparatus 10 has been described as incorporating an ultrawideband rf source, the method of the invention is not limited to such a source. Alternative rf sources include narrowband sources. Pulsed rf sources producing radiation with pulse widths of between 50 ps and 50 ns may be used. The method of the invention may also be performed with rf radiation of substantially lower frequency, for example 10 MHz. The method of the invention may be suitable for monitoring the integrity of metal closures in a variety of situations, including aircraft and screened rooms.

Claims

1. A method for determining the integrity of a mechanical seal comprising the steps of: (i) generating radio frequency electromagnetic radiation; (ii) directing the radiation towards the seal; (iii) receiving radiation transmitted through the seal; (iv) generating a signal dependent upon received radiation; and (v) processing the signal to obtain a measure of the seal's integrity.

2. A method according to Claim 1 wherein the electromagnetic radiation has a frequency bandwidth in a range between 100 MHz and 50 GHZ.

3. A method according to Claim 2 wherein the radiation has a frequency bandwidth in a range between 200 MHz to 20 GHz.

4. A method according to Claim 3 wherein the radiation has a maximum frequency of 4 GHz.

5. A method according to Claim 1 or Claim 4 wherein the electromagnetic radiation is pulsed.

6. A method according to Claim 5 wherein the pulsed radiation has a pulse width in the range 50 ps to 50 ns.

7. A method according to Claim 6 wherein the processing of the signal includes determining the time of flight of received radiation so as to determine a location of a breach in the seal.

8. A method according to Claim 1 wherein the radiation is generated and received by respective TEM horn antennas on opposite sides of the seal.

9. A method according to Claim 1 wherein the signal generated in response to received radiation is measured and the measurements temporarily stored by signal measuring means.

10. A method according to Claim 9 wherein the signal measuring means is a sampling oscilloscope.

11. A system for determining the integrity of a mechanical seal comprising: (i) means for generating radio frequency electromagnetic radiation; (ii) means for receiving said radiation after transmittal through a seal; (iii) means for generating a signal in response to received radiation; and (iv) means for generating a measure of seal integrity from said signal.