It has been proposed in WO 00/22387 to measure the density profile of a medium by providing an axially distributed array of sources of ionising radiation, e. g. 24'Am which is a source of low energy y-radiation, to give collimated beams of said radiation and an axially distributed array of detectors disposed so that the medium under study extends between the sources and the detectors. By monitoring the radiation received by the detectors, the amount of radiation absorbed by the medium from each beam can be determined and so variations in the medium density can be detected.

One disadvantage of the system of WO 00/22387 is the need to employ radioactive materials which consequently imposes health and safety issues in order to ensure safe working.

We have devised an alternative system that does not employ radioactive materials. In the present invention ultrasonics are employed. It has been proposed in US 4446736 to test the integrity of internal linings on a pipe by transmitting an ultrasonic wave through the pipe wall and monitoring the wave if any reflected from the opposite wall. It is indicated in that reference that, if the lining was intact, the wave would be absorbed and so little or no reflected wave would be detected.

In the present invention, rather than reliance upon absorption of the sound wave, use is made of the fact that the velocity of sound is different in different materials. Thus the velocity of sound in water is of the order of 1500 m/s while the velocity in air is only about 340 m/s (the precise values depend on factors such as temperature and pressure). Hence by measuring the velocity of sound at different locations in a medium, differences in the composition of the medium at those locations can be determined, and in particular the location of phase boundaries can be determined.

It has been proposed in GB 1524303 to determine the boundary between liquids of different densities by means of an elongated member along the length of which a series of housings is disposed with spaces between each housing into which spaces the medium under study can enter through perforations in the wall of the elongated member. Each housing comprises an ultrasonic transmitter and detector and the end of each housing acts as a reflector to reflect waves from the transmitter of the next adjacent housing back, through the medium in the space between the housings, to that next adjacent housing. One disadvantage of this type of arrangement is that the vertical resolution is limited by the need to provide a vertical space between each of the housings.

In the present invention, the reflectors are displaced laterally from the axis of the elongated member.

Accordingly the present invention provides apparatus for monitoring the composition of a medium comprising an elongated member for insertion into said medium, said member having an array of transmitters and receivers of ultrasonic waves disposed at intervals along at least part of its length, and reflector means to reflect transmitted ultrasonic waves to the receivers, said reflector means being supported by, but laterally spaced from, said member whereby, when said elongated member is inserted into said medium, the medium occupies the space between said elongated member and said reflector means, and said transmitters, receivers and reflector means being disposed so that transmitted ultrasonic waves pass through said medium en route to the receivers, and monitoring means providing a signal dependent upon the time taken for an ultrasonic sound wave to travel from a transmitter to a receiver associated therewith.

The apparatus employs an elongated member, preferably in the form of a tube, in which the transmitters and receivers are mounted. Preferably there is a receiver associated with each transmitter, and the transmitters and receivers are disposed as arrays along at least part of the length of the elongated member. A single transducer may be arranged to act as both a transmitter and a receiver, or separate transmitters and receivers may be employed. The frequency of the ultrasonic waves is preferably in the range 20 to 50 kHz, particularly in the range 20 to 40 kHz.

In use, the elongated member is disposed, preferably substantially vertically, within the medium under investigation. It will be appreciated that the transmitters and receivers need only be disposed over the length of the elongated member over which a variation in the medium being monitored is anticipated. For example if the apparatus is used to locate the phase boundaries in an oil/water separator, it is only necessary to have the receivers and transmitters along that part of the elongated member that traverses the anticipated boundaries. Thus in normal operation, in a separator of height 4 m, if the oil/water boundary is within the range 1 to 1.5 m from the bottom of the separator and the oil/air boundary is normally in the range 2.5 to 3 m from the bottom of the separator, and it is desired to monitor the location of both boundaries, then, if the elongated member is disposed substantially vertically with its lower end on the bottom of the separator, it is only necessary to have transmitters and receivers disposed over the ranges 1 to 1.5 m and 2.5 to 3 m from the bottom of the elongated member. The transmitters and receivers are preferably, but not necessarily, disposed at equal intervals over the desired part or parts of the length of the elongated member. Preferably the transmitters are disposed at intervals in the range 2 to 10 cm, preferably 2 to 5 cm, over the part, or parts, of the length of the elongated member where monitoring is required.

The apparatus also includes reflector means supported by, but laterally spaced from, the elongated member. The reflector means may be a single strip reflector extending for at least

that part, or parts, of the length of the elongated member that is provided with transmitters and receivers. Alternatively there may be separate reflectors associated with each transmitter and/or receiver. The reflector or reflectors may be shaped so as to focus the reflected sound waves on to the appropriate receiver. In use, the elongated member is inserted into said medium so that the medium occupies the space between the elongated member and the reflector or reflectors. The apparatus is preferably inserted into a vessel containing the medium through a port in e. g. the roof of the vessel. In order that the apparatus can be inserted via ports of diameter less than about 15 cm, the distance between the elongated member and the reflector is preferably less than about 12 cm, and is particularly in the range 5 to 10 cm. Since the spacing of the transmitters and receivers is longitudinal, i. e. along the length of the elongated member, whereas the reflectors are laterally displaced from elongated member, the resolution of the apparatus is largely independent of the sensitivity. The latter is determined by the spacing of the reflectors from the transmitter/receivers, whereas the resolution is determined by the spacing between adjacent transmitters and receivers. The spacing between adjacent transmitters and receivers may be significantly less than the spacing between the transmitter/receivers and the reflectors, and is only limited by the physical size of the transmitters/receivers and the need to minimise"cross-talk", i. e. a receiver receiving reflected waves from a transmitter other than those associated with that receiver.

The ultrasonic waves are preferably transmitted as pulses and the time taken between transmission and receipt of a pulse is monitored. This time is thus indicative of the total time taken for the ultrasonic wave to travel from the transmitter, through the medium to the reflector and from the reflector through the medium to the receiver, and hence is dependent on the velocity of the wave through the medium. By calibration by measuring the times with different materials, such as oil, water and air, as the medium, the monitored time can be used as a indication of the nature of the medium at the location of the transmitter and receiver. By measuring the times at transmitter/receiver combinations at different spatial locations along the length of the elongated member, the location of interfaces or boundaries between different materials of said medium can be determined.

The invention is of particular utility in conjunction with an oil/water separator. Thus an oil/water separator may be provided with an inlet for an oil/water mixture and separate outlets for separated oil and water phases and provided with a monitoring apparatus in accordance with the invention, with the elongated member is disposed substantially vertically in the vessel with an array of transmitters and receivers disposed along a length of the elongated member embracing the expected oil/water boundary. Preferably the elongated member is also provided with an array of transmitters and receivers disposed along a length of the elongated member embracing the expected gas/liquid boundary. The rates of flow to the inlet and/or from the outlets may be controlled in response to the monitored levels of the oil/water and/or gas/liquid boundaries.

The invention is illustrated by reference to the accompanying drawing wherein Figure 1 is a diagrammatic elevation of apparatus in accordance with one embodiment of the invention shown in combination with an oil/water separator.

Figure 2 is an elevation of part of the apparatus of Figure 1, and Figure 3 is a section along the line III-III of Figure 2.

In Figure 1 there is shown in section an oil/water separator vessel 10 provided with an inlet port 12 to which a mixture of oil and water to be separated is supplied, a weir 14, and outlet ports 16,18 from which separated water and oil phases are removed from the vessel. A gas vent (not shown) may also be provided.

The supplied oil/water mixture initially tends to form a foam region 20 adjacent the inlet port 12. This gradually collapses forming an oil/water emulsion region 22 which separates with time into an oil layer 24 and a water layer 26. The oil layer spills over the weir 14 into an oil outlet zone 28 from which the oil phase is removed through outlet port 18. The water phase is removed through outlet port 16.

It is desirable to maximise the throughput. However, there is a risk that if the oil/water mixture is fed too fast, the separation will be insufficiently complete and the foam layer and/or the emulsion may reach one or both of the outlet ports. It is generally not practical to monitor the boundaries of the phases by optical means, e. g. sight glasses, and so to monitor the location of the phase boundaries, a probe 30, constituting apparatus in accordance with the invention, is deployed vertically through a port 32 in the roof of the vessel 10.

The probe 30 is a hollow tube 34 in which, for parts of its length, are mounted arrays of transducers capable of transmitting and receiving pulses of ultrasonic sound waves. The probe 30 is shown in Figure 1 as having two separate arrays 36,38. Array 36 spans the oil/gas interface while array 38 spans the oil/water interface. Each array is of sufficient length to embrace the range of heights in the vessel where that interface is liable to occur. Each array comprises a plurality, for example 5 or more, of the transducers 40 spaced at intervals along tube 34. Laterally spaced from tube 34 is a reflector 42 held in position by a series of struts 44 fastened by means not shown to tube 34. The reflector 42 is a strip having a curved section of such shape that pulses of ultrasonic sound waves impinging thereon from a transducer are reflected and focussed upon the transducer from which the pulse emanated.

Electrical circuitry, not shown, is connected to each transducers to provide the pulses and to determine the time delay between transmission of a pulse from a transducer 40 and receipt of its reflection from reflector 42. In order to avoid cross talk, it is preferred that pulses are sent from each transducer in turn rather than simultaneously.

By calibration, it is thus possible to determine whether the medium in the space between the transducer and the reflector is oil, water, gas, or mixtures thereof, e. g. a foam or an emulsion. By recognising that the nature of the medium at the level of one transducer is a certain phase and that the medium at the transducer next above the one transducer is a different phase, it is self evident that the phase boundary lies at a level between the levels of the two transducers.