FOULING PROBE

Abstract: A stream of fluid from a system to be monitored is passed through a tube of a fouling probe. One or more heaters surround the tube and heat it so that the resistance to heat flow through the tube wall can be measured. Heat transfer is assisted by a heater mandrel between the heaters and the tube. The space between the inner wall of the mandrel and the outer wall of the tube is filled by an alloy that expands on cooling to provide substancially uniform heat transfer.

Description: FOULING PROBE This invention relates to a fouling probe and more particularly a probe for biofouling. There are a number of instances where problems can arise in flow systems in which fouling can occur. For example in heat exchange systems such as those which employ a cooling water circuit, bacteria " in the cooling water can grow on the inner surface of the circuit. This build up can seriously affect the performance of the heat exchanger system by reducing system flow and heat exchanger efficiency. Bacteria related problems can also arise in enhanced oil recovery systems where large volumes of water are injected into the oil bearing strata to flush out oil. The temperatures existent in such systems provide conditions for bacterial growth and fouling builds up in the water supply to the oil bearing strata. Fouling shed from the system can be forced into, and cause blockages within the oil bearing strata. Yet another example of fouling due to bacterial growth is in the water intake lines of desalination plants. The consequence of such growth in those systems is that the bacteria can grow on filters or the like reducing their throughput or even blocking them completely. The accumulation of biomass is dependent on physical, chemical and biological parameters in the process stream. The further problem with fouling of the kind described is that it occurs in closed systems where it is not readily detectable until it has reached a level at which performance of the system indicates that something is wrong. Attempts to deal with this difficulty for example by introducing a biocide into the system are rather hit and miss since they are based on guesswork as to the extent, if any, of the biofouling present. It is known to use a fouling monitor which measures the resistance to heat transfer through the wall of a heated tube. If the probe or monitor is to give the most reliable information it is best if the tube through which the process stream is led is the same cross-section as the tube in the system being monitored. So the probe must be capable of accommodating tubes of different sizes. It is also essential, if the data obtained from the monitor is to be relied on, that there is good heat transfer between the heaters surrounding the tube and the tube walls. If there are air gaps between the heater and the tube then the heat transfer may not be as expected and the results obtained from the monitor will not correspond to what is happening in the system proper. The present invention has been made with the aim of dealing with this problem. According to one aspect of the invention there is provided a fouling probe for a liquid system comprising a tube for receiving a stream of fluid from the system to be monitored, one or more heaters for heating the tube and means for measuring the resistance to heat flow through the wall of the tube characterised in that the tube is surrounded by said one or more heaters, heat transfer is assisted by a heater mandrel between the heaters and the tube, and the space between the inner wall of the mandrel and the outer wall of the tube is filled by an alloy that expands on cooling thus providing substantially uniform heat transfer. According to another aspect of the invention there is provided a method of determining fouling in a liquid system by means of a fouling probe of the present invention. The invention is based on the re-creation within the probe of the conditions or substantially the same conditions in the part or parts of the system being monitored wherein fouling is expected to occur. Fouling can be expected to increase resistance to heat flow and measuring this resistance in the probe will indicate whether fouling is building up in the system being monitored so that appropriate steps can be taken. In a preferred embodiment of the invention the tube is made of metal, such as titanium, which preferably does not corrode although in some cases a tube of similar material to the process plant which may corrode is desirable. The resistance to heat flow through the wall of the tube is preferably obtained by measuring the temperatures of the fluid stream, for example as it enters and leaves the heated tube and the temperature of the heated tube while the heater power applied to the tube is maintained constant. A build up of fouling in the tube effectively increases the wall thickness and reduces the heat transfer through the wall to the fluid stream. A preferred way of measuring the heat transfer through the wall of the probe is by means of appropriately located temperature sensors such as thermocouples. For example, thermocouples can be located to measure the bulk fluid temperature and the temperature of the metal tube at the location where fouling build up is expected, e.g. a the downstream end of the tube. The heat transfer coefficient of the assembly is given by the equation:- t - t 1 = 1 2 ϋ Q/A Where U is the heat transfer coefficient, t, is the temperature of the tube, t 2 the temperature of the bulk fluid and Q/A the heat flux. Fouling is measured in terms of resistance to heat transfer, the fouling resistance R which is part of the total thermal resistance, R. The thermal resistance R is the inverse of the heat transfer coefficient; thus R = _. So the fouling resistance R 1 1 U f,- = -ϋ f _ —u ± where f refers to the fouled condition and i the initial clean condition. The local fouling resistance is given by Thus provided the heat flux is maintained constant the only measurements that need to be made are the temperature of the tube and the bulk fluid temperature. If desired a plurality of tubes can be connected together in series or parallel each tube being set to operate at a different temperature. In this way the probe of the invention can monitor a range of conditions occurring in the system under investigation. One tube or set of tubes may act as a control system against chemical treatment of another tube or set of tubes. Biocidal effects on bioaccumulation on process equipment surfaces are either estimated from laboratory trials or long term site evaluation. The former method can be limited by closeness to reality, the latter by delays in feedback. The statistical BAES ("BioAccummulation Efficiency in Service")-line of the present invention overcomes both the above by considering the real time fouling resistance response of two side stream fouling probe surfaces run in parallel (or series), one subject to biocide, the other being the control. The BAES-line - o - for a totally uninhibited system would be 100% whereas a totally inhibited system would have a BAES-line near to 0%. BAES-lines would be specific to any particular system at the biocide dose, operating temperature and Reynold's numbers found in practice. The BAES-line can be calculated from the time outputs as follows:- 100 . t _ N t=l ϋ t where t = the duration of the test U, = the uninhibited fouling resistance N = number of data points I. = the inhibited fouling resistance. Other mathematical methods may be equally valid. Biocide treatments at unit cost or unit dose rate could be compared in real-time in terms of BAES-lines specific to the system. The technique could similarly be employed for the investigation of other treatments intended to keep the bore of process equipment clean. For example scale inhibition, asphaltene precipitation inhibition. A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which:- Fig.l is a longitudinal section through a tube; Fig.2 is a diagrammatic representation of a probe comprising three tubes in series and appropriate logic- Fig.3 is a graph of fouling resistance against time; and Fig.4 is a histogram showing the amount of fouling at different temperatures after 500 hours. Fig. 5 is a graph of fouling resistance against time. Referring to the drawings the probe in its simplest form comprises a tube 10 of non-corrodable metal such as titanium. The tube 10 is surrounded by a heater mandrel 12 any space between the mandrel and the tube being filled with a low melting point alloy which expands on cooling thus ensuring that as far as possible the tube is in uniform heat conducting relationship with the mandrel. The alloy may be a bismuth containing alloy, e.g. Rose's Metal. The ends of the tube 10 pass through end plates 16. Annular end packers 18, for example of plastics such as polytetrafluoroethylene providing good heat insulation, are fixed to the end plates by screws 20 which extend through the packers into the mandrel 12. Seals 22 are provided around the tube 10 where it passes through the end plates. A number of heaters 24, three in the illustrated embodiment of Fig.l are disposed around the mandrel along substantially the whole length thereof. The heaters are, in turn, encased in thermal insulation 26. Couplings 28, 30 having through bores in register with tube 10 are fixed to the end plates for connection of the probe to other equipment. In the embodiment illustrated the coupling 28 is female and coupling 30 is a complementary male coupling so that the assembly can be connected at one or both ends with another like assembly. In the embodiment shown in Fig.l the left hand end is intended to be the inlet end. The coupling is provided with a thermocouple 32 for monitoring the temperature of fluid entering the tube 10. A second thermocouple 34 is provided for monitoring the temperature of the tube 10. In use the assembly is connected to a stream of fluid, for example heat exchanger liquid which is led through tube 10 at a constant rate, preferably the same flow conditions as in the equipment that the probe is simulating. The heater is then adjusted to heat the liquid to a temperature which corresponds to the temperature that is predicted in the said equipment. A fouling build up, for example due to growth of bacteria in the tube 10 will be signalled by a rise in the fouling resistance which is desired from the continuous measurement of the temperature of the tube and the temperature of the liquid as explained hereinbefore. If it is desired to monitor parts of a system at different temperatures then a plurality of assemblies 40 of the kind described with reference to Fig.l can be fitted together as illustrated for example in Fig.2 where three assemblies 40 are connected in series. Each of the assemblies can be set to operate at a different temperature and the fouling resistance for each assembly obtained in the same way as already described. If desired the fouling resistance can be displayed graphically as shown in Fig.3 and Fig. 5. Fig 3 shows the effect when the assemblies in the arrangement of Fig.2 are set to operate at 25°C, 30°C and 35°C. After about 150 hours a build up of fouling begins which gets progressively worse. In practice steps would be taken to deal with the build up of fouling in the system being monitored before it reached the condition shown after 500 hours in the graph of Fig.3. If desired when a build up of fouling has been observed in the assembly of the invention it can be removed by means of a plunger 42 (Fig.l) which can be inserted in the tube to push the fouling from the tube walls and out of the other end of the tube. Preferably the end 44 of the male coupling 30 is sized to receive a container 46 for collecting the fouling pushed out of the tube by the plunger. A useful container for this purpose can be the cylindrical plastic containers for photographic film which have a close fitting cover. The container can be provided with a breather hole 46 in the side wall so that when it is fitted to end 44 air displaced by the plunger can escape. The fouling removed from the wall of each tube can be weighed. Fig.4 shows the total mass of fouling from each of the three units of Fig.2 after having built up the fouling resistance as shown in Fig.3. The invention is not confined to the above described embodiments and many modifications and variations can be adopted. The invention can be used to monitor a wide range of problems including scaling, chemical fouling, precipitation or any other process that deposits a layer on the inner walls of process stream's tubes or pipes. In addition the invention can be used to monitor corrosion which may arise for example in process streams such as those involving electrochemical techniques. In this case, the tube 10 is segmented into typically two or more rings and each individual segment separated from adjacent rings by suitable electrical insulation. Each ring can form part of an electrochemical test arrangement where electrically connected and insulated wires run from the separate rings through the annular space to emerge from the outer wall of a coupling 28 or 30. This arrangement thus allows for the simultaneous measurement of fouling and corrosion on the tube 10 inner wall.

Claims: CLAIMS 1. A fouling probe for a liquid system comprising a tube for receiving a stream of fluid from the system to be monitored, one or more heaters for heating the tube and means for measuring the resistance to heat flow through the wall of the tube characterised in that the tube is surrounded by said one or more heaters, heat transfer is assisted by a heater mandrel between the heaters and the tube, and the space between the inner wall of the mandrel and the outer wall of the tube is filled by an alloy that expands on cooling thus providing substantially uniform heat transfer. 2. A fouling probe as claimed in claim 1 further characterised in that the tube is made of metal. 3. A fouling probe as claimed in claim 1 further characterised in that the tube is not intended to corrode in the fluid stream. 4. A fouling probe as claimed in claim 1 further characterised in that the tube is made of titanium. 5. A fouling probe as claimed in claim 1 further characterised in that the alloy is a bismuth containing alloy such as Rose's Metal. 6. A fouling probe as claimed in claim 1 further characterised in that the means for measuring the resistance to heat flow are temperature sensors such as thermocouples. 7. A fouling probe as claimed in claim 6 further characterised in that the temperature sensors are located to measure the bulk fluid temperature and the temperature of the tube at the location where fouling build-up is expected such as at the downstream end of the tube. 8. A fouling probe as claimed in claim 6 further characterised in that the temperature sensors are located to measure the temperatures of the fluid stream as it enters and as it leaves the heated tube while the heater power applied to the tube is maintained constant. 9. A fouling probe as claimed in claim 1 further characterised in that a plurality of tubes are connected together in series or parallel, each tube being set to operate at a different temperature. 10. A fouling probe as claimed in claim 1 further characterised in that a plurality of tubes are connected together in series or parallel, one tube or set of tubes acting as a control system against chemical treatment of another tube or set of tubes. 11. A method of determining fouling in a liquid system by means of a fouling probe as claimed in any preceding claim. 12. A method as claimed in claim 11 wherein two or more fouling probes are connected together in series or parallel, one fouling probe being subjected to biocide, another acting as a control.

Inventors: EDEN ROBERT DAVID, EDEN, ROBERT, DAVID
Applicants: EDEN ROBERT DAVID, EDEN, ROBERT, DAVID
Application year: 1992
Main classification: G01N25
IPC codes: G01N25, G01N33