Contamination detection in a gas detector

Description

[0001] The present invention relates to gas detection, especially related to reliable gas detection in oil and gas installations. In particular, the present invention relates to a gas detector cell unit and to a method of detecting a reduction in circulation through a protecting filter of a gas detector cell unit.

[0002] It is a well known problem that poisonous gases may occur in oil production or similar facilities, and optical sensors are commonly used for monitoring such environments by monitoring the absorption spectrum or fluorescence spectrum of a gas, e.g. as described in US5,218,422 A and US5,886,247 A. The measurements are then performed by leading the gas into a measuring cell and transmitting light at a certain wavelength or range of wavelengths through the cell and either directly detecting the absorption on selected, typical wavelengths or measuring the fluorescence spectrum at a detector, often using Fabry-Perot interferometers to select the wavelengths being specific for the gas to be detected. There are, however, some gases that are difficult to detect using optical measuring means as the spectrum coincides with the spectrum of another, often harmless gas. One example of such a gas is H 2 S which is difficult to distinguish from water, but is common in oil production and may be very dangerous. NO x gases may also be difficult to detect in a reliable manner.

[0003] One other problem with the known solutions is that in order to provide a fail safe detector, the power use and number of moveable parts should be kept at a minimum and thus complex systems e.g. incorporating pumps, as described in US5,886,247 A, should be avoided. Another complex system for monitoring gas is described in US2007 / 0285222 A1 where detector heads, e.g. for detecting hydrogen sulphide is positioned at different levels in a column, thus separating the gases to be detected. Another solution related to the use in oil and gas production is discussed in US7,705,988 B2, where it is stated that the reliability of optical hydrogen sulphide detection is low and thus the detection is linked to the amount of methane and the known ratio of hydrogen sulphide in the flow.

[0004] There are a number of different measurement principles that may be used for hydrogen sulphide / H 2 S detection, like catalytic, electrochemical and MOS, but typically these sensors will not be able to detect functional failure. This means that the sensors may stop working, but the users get no information about this.

[0005] US 2011 / 0054811 A1 discloses tracking gas flow along a transfer function of a filter to detect clogging.

[0006] Furthermore, US 7,034,943 B1, which is considered to represent the closes prior art, discloses a device for accomplishing noise reduction in a photoacoustic gas detector in which a target gas passses through openings into a measurement cell, a protecting filter being associated with the openings so as to be in fluid communication between an outside area and the measurement cell; the detector further including a loud speaker configured to impose on the protecting filter a pressure function, the gas detector cell unit being adapted to detect a reduction in circulation through the protecting filter based at least in part on a change in a pressure transfer function of the protecting filter over a predefined time.

[0007] Other prior art disclosure is made in US 6,964,694 B2, and US 2009 / 0128819 A1.

[0008] Thus there is a need for a simple optical gas detector providing reliable results for noxious gases, and where the detector is able to verify its own functional status. This is obtained as described in the accompanying claims.

[0009] The present invention thus provides a solution where the operation of the sensor is verified by detecting possible clogging which results in reduction or complete stop in the reduction in the circulation in the unit and through the membrane.

[0010] According to a first aspect of the present invention there is provided a gas detector cell unit according to claim 1.

[0011] The present invention further provides a method of detecting a reduction in circulation through a protecting filter of a gas detector cell unit according to claim 8.

[0012] Particular embodiments of the present invention are defined in appended dependent claims 2-7 and 9.

[0013] The invention will be described more in detail with reference to the accompanying drawings, illustrating the prior art and embodiments of the invention by way of example. Figure 1illustrates the optical system including the gas detection cell according to the prior art. Figure 2illustrates a catalyzing membrane which may be used in an embodiment of the invention. Figure 3a,billustrates schematically a detection cells according to the invention. Figure 4a,4billustrates the membranes of figures 3a,3b as seen from the front. Figure 5illustrates an embodiment where the catalyst constituting the membrane is a wound string covering the opening. Figure 6,7illustrates alternative layouts of a catalyst string constituting a membrane. Figure 8illustrates the system according to the preferred embodiment of the invention with an explosion proof encapsulation. This kind of encapsulation would typically be consisting of a flame arrestor such as a sinter filter allowing the gas to enter, but not explosion or a flame to propagate through; and a solid encapsulation around the whole system.

[0014] Figure 1 illustrates a measuring system according to the prior art related to fluorescence measurements where a light source 1 emits light through a lens system 2, the light being focused 3 in a cell 6 preferably having an inner surface being coated so as to avoid light being reflected therein. In the focus point 3 a fluorescence reaction is obtained, the light therefrom propagating through a second lens system 4 to the optical sensor being capable of detecting the wavelength and intensity of the received light.

[0015] The cell 6 is usually closed except for the opening with the catalyst membrane leading the gas into the cell. The cell in the illustrated example also being provided with windows 7 for letting the light into and out of the cell, the windows being angled to reduce the reflections from the light source inside the cell.

[0016] Other measuring systems may be used, e.g. for measuring the spectrum of the light transmitted directly through the cell, the important aspect being that the system includes a cell confining a fluid, especially a gas, to be measured.

[0017] According to one embodiment of the present invention the enclosed volume constituting the cell 6 is provided with at least one wall or wall part as illustrated in figure 2, being provided with openings for letting a target gas move through it. The nature of the openings may differ, but are chosen so as to allow diffusion of the target gas.

[0018] In figure 2 the openings are made in a silicon membrane 10 being perforated 11 with openings through which the gas 9 may move. In general the perforated wall 11 in the cell as illustrated in figure 3a, while figure 3b illustrates the use of a net 12 or other material. Figures 4a and 4b illustrates the perforated wall or net in figures 3a and3b , respectively, while figure 5, 6 and 7 illustrates the use of one or several crossing of a wire or catalyst string 13 over an opening 17 in the wall 18, thus illustrating that the area has to be sufficiently open to allow the gas to move through.

[0019] In the illustrated embodiment, the membrane 10,11,12,13 is provided with a catalyst so as to provide a chemical reaction. The catalyst usually being heated for providing the reaction. In a preferred embodiment of the invention for detecting H 2 S a catalyst is chosen so as to convert the gas into SO 2 , which is easy to detect in a suitable optical system.

[0020] As stated above the catalyst in the system may be of different types depending on the gas conversion. In the case of H 2 S the catalyst may be made from FeCrAl alloys (i.e. Kanthal) heated to a suitable temperature, e.g. between 300 and 500 degrees Celsius to obtain close to 100 % conversion. At lower temperatures, i.e. 200 degrees Celsius, less than 100 % of the gas is converted, but the sensor principle still works. In one embodiment, the heating element is a woven mesh of FeCrAl alloy, and the heating is performed by connecting the membrane to an electric power source, and turn the power to obtain the required temperature. The FeCrAl alloy wire thickness may be in the range of 0.05 mm to 0.2 mm. FeCrAl alloy is very well suited for this kind of combined heating, as the surface for certain kinds of FeCrAl alloy may withstand temperatures of above 1500 degrees Celsius, allowing several years of lifetime operated below 500 degrees Celsius.

[0021] The FeCrAl alloy woven mesh may be coated with Fe 2 O 3 or Cr 2 O 3 to improve conversion of H 2 S to SO 2 . Other types of coating like CuO or Cu 2 O, also work, and at higher temperatures a surface of Al 2 O 3 can be used. Oxides and sulfides of Co, Mo, Ni, W, V, Al and Mn are also possible candidates as catalytic converters. For other types of gases, the FeCrAl alloy woven mesh may be coated with different types of coatings.

[0022] In another embodiment, the FeCrAl alloy is not a woven mesh, but a wire wrapped around a frame as illustrated in figure 5. Typically, a FeCrAl alloy wire with diameter around 0.1 mm and period of 0.15 mm may be used. The advantage with this approach over the woven mesh approach, is that the resistance of the heater increases, and this may be advantageous in applications where the available current is limited.

[0023] Preferably the membrane is pretreated to not absorb the H 2 S during the measurements. Further, the membrane goes through a burn in of the surface to obtain stable operation.

[0024] The structure illustrated in figure 2 is a silicon membrane being provided with a heating layer and silicon dioxide or silicon nitride surfaces. The silicon membrane is provided with a number of holes to let the gas through at the same time as the gas is heated, and the gas reacts with the catalyst. The catalyst may be coated onto the silicon dioxide or silicon nitride surface, or the surface may be provided with a layer of Al 2 O 3 (by i.e. Atomic Layer deposition), and then coated with the above mentioned catalysts. The heater may be made of other types of semiconductors or ceramic compositions as well.

[0025] The cross section of the position where the heated catalyst is placed may typically be in the range 1 to 100 mm 2< , depending on the response time required by the sensor. The response time is given by the volume of the optical sensor chamber cell 6 and the amount of gas able to diffuse through the converter. A small volume will typically give a fast response time, since less gas needs to be converted, and a small volume will also require less power for the same reason. A reasonable response time can be obtained for volumes up to 4cm3. Typically, the optical sensor chamber cell 6 has a volume of less than (1 cm) 3< and more typically (5 mm) 3< .

[0026] The fill factor (i.e. wire to hole ratio) of the heater substrate (wire, membrane or woven mesh) is typically in the range of 50 %, allowing diffusion and conversion of the gas in less than a few seconds. With lower fill factor, the time required for the conversion, will increase, but the sensor principle still works, only with slower response.

[0027] Calibration of the offset can be performed by turning off the temperature of the heated catalyzer for a period of time to cool down the converter to a temperature where conversion don't take place (this takes typically between 0.5 to 100 seconds), and measure the signal response. This signal response cannot be from the targeted gas, since the converter is turned off, and this offset signal can be subtracted from the baseline to obtain good calibration of the zero gas level. This zero level calibration can be done by software from time to time, typically once a week or once a month.

[0028] In the case of a H 2 S sensor, the presence of SO 2 may give an error in the calibration of the zero level. This may be improved. E. Xue, K. Seshan, J.R.H. Ross: "Roles of supports, Pt loading and Pt dispersion in the oxidation of NO to NO2 and of SO2 to SO3", published in Applied Catalysis B: Environmental 11 (1996) 65-79, describes how a platinum (Pt) catalyst heated to about 300 degrees C can be used to convert SO 2 to SO 3 . By converting the gas from SO 2 to SO 3 , we can remove SO2 from the measurement, and thereby perform a zero calibration of the instrument online. This will be done by heating the platinum catalyst to around 350 degrees C at the same time as the H 2 S to SO 2 catalyst is turned off. When steady state is obtained, a measurement of the remaining signal amplitude will be performed, and this amplitude, also called an offset, will be subtracted from the following measurements to obtain good calibration of the zero gas level.

[0029] For a sensor used in hazardous and explosive environments, the surface temperature must be kept below a certain limit. Typically, a sintered filter 14 is used to isolate the high temperature catalyst 15 from the surrounding environment to avoid explosions or ignition, and is illustrated in figure 8. The function of the sintered filter is also to prevent a spark, flame or explosion to propagate from inside the detector enclosure 16 to the outside. A possible function failure may be that the sintered filter 14 is partly clogged and thereby limits the gas diffusion through the sintered filter. This will give an increase in response time for the sensor, and in the worst case, where the sintered filter is totally clogged, the sensor will give no response at all. It is therefore important to identify such clogging, and output a signal indicating the failure.

[0030] In general, a functional failure where a sinter filter is clogged can be identified by the change in the transfer function or impulse response of the sinter filter. According to the present invention, this transfer function is measured by imposing a step and measure the response. A step response in form of an increased pressure is imposed at a given time, and the transfer function or the step response of the sinter filter can be measured with a pressure sensor. Typically, the time response will increase when the clogging of the filter increase. The step response can be made by several methods, but according to the present invention a loudspeaker changes the volume inside the sinter filter. Other methods which do not form part of the claimed invention include a rapid change in temperature giving a rise in pressure in the volume inside the sinter filter, or letting in or out some gas changing the pressure, i.e. from a chamber with a different temperature. Instead of measuring the impulse response of the sintered filter, parts of the transfer function can be sampled by measuring the frequency response on one or several frequencies. The sintered filter will work as low pass acoustic filter, letting though slow variations in pressure (sound) and filtering away higher frequencies. In an embodiment of the invention, we can modulate a small load speaker with a frequency close to the frequency f0 given by the time constant of the sintered filter. We measure the signal response inside the measurement cell, where the sintered filter is the only way to the outside. The change in amplitude or / and phase can be used to estimate the time constant of the system, and thereby verify that the sintered filter is letting the gas in. Several frequencies may be used to improve the accuracy of this method.

[0031] The functional failure of the sintered filter may also be measured by another method which does not fall within the scope of the invention. A working sintered filter is letting gas through, and the outside gas is much colder than the gas inside the cell, due to the heating of the catalyst. This means that the gas temperature close to the sintered filter will be lower when the sintered filter transmits gas, while the temperature will increase when the sintered filter is clogged. The accuracy of the method can be increased by measuring the gas temperature on several positions, including the outside temperature. Further, the gas temperature inside the sensor may be modulated or stepped, to measure the transfer function or impulse response as described above.

[0032] As an example, NOx can be converted to N 2 O with platinum (Pt), palladium (Pd), rhodium (Rh) catalysts, or combinations of these, as described in "Emissions of nitrous oxide and methane from alternative fuels for motor vehicles and electricity-generating plants in the U.S.", ucd-its-rr-03-17f, December 2003, T. Lipman and M. Delucchi. When NOx is converted to N 2 O, N 2 O can easily be detected by optical methods like infrared spectroscopy.

[0033] It takes longer time and more energy to convert a large volume of gas than to convert a small volume of gas. For fast response sensors and low energy consumption, the volume of converted gas must be as small as possible. This favors to types of optical detection: a) Fluorescence detection, and b) Photo acoustic detection, which both works fine on very small gas volumes.

[0034] In some cases, where a pulsed source like a Xenon flash lamp is used, the total number of pulses available through the lamps lifetime is limited. In other cases, the available power is limited, and we want to save energy by using as little power as possible. To avoid false alarms, the following procedure has been invented. When a pulsed source is used to detect the gas, the pulsed source is typically operated at a given frequency, and the signal processing unit estimates a gas level at or above a predetermined alarm limit, the sensor performs a verification of the estimated gas level by increasing the frequency of the pulsed source by a predetermined factor (i.e. 30 times), and average these results over a limited period of time (i.e. 0.5 seconds) which typically is much shorter than the sensor response time. The averaged result (or a modified version of this incorporating previous measurements) is then given as the sensor output, to avoid false alarms.

[0035] The clogging is detected using detection means adapted to analyze the transfer function or impulse response of the sinter function by applying a pressure variation or acoustic signal inside said housing, and comparing the resulting pressure variations in the housing with reference measurements based on a clean, open sintered filter.

Claims

1. A gas detector cell unit comprising a measurement cell for optical gas detection and a gas cell (6) having an outer housing (16) with at least one wall provided with openings for letting a target gas therethrough into the measurement cell and a protecting filter (14) associated with the openings so as to be in fluid communication between an outside of the outer housing (16) and the measurement cell; the gas detector cell unit further including a loud speaker configured to impose on the protecting filter (14) a pressure function, the gas detector cell unit being adapted to detect a reduction in circulation through the protecting filter (14) based at least in part on a change in a pressure transfer function of the protecting filter (14) over a predefined time; characterized in that the gas detector cell unit further comprises a catalyst for converting the target gas to an easily detectable gas, wherein the catalyst is arranged inside the measurement cell2. A gas detector cell unit according to claim 1, wherein a reduction in circulation through the protecting filter is detected based at least in part on a change in at least one of amplitude and phase of the pressure function resulting from the change in the pressure transfer function of the protecting filter (14) over time.

3. A gas detector cell unit according to claim 1or claim 2, further adapted to output a signal for indicating system failure.

4. A gas detector cell unit according to claim 1or claim 2, wherein the pressure transfer function of the protecting filter (14) is in response to a pressure variation inside the outer housing (16).

5. A gas detector cell unit according to claim 1 or claim 2, wherein the protecting filter (14) is a sintered filter.

6. A gas detector cell unit according to claim 1or claim 2, wherein the pressure transfer function of the protecting filter (14) is based at least in part on a predefined step pressure increase imposed in the gas detector cell unit.

7. A gas detector cell unit according to claim 1 or claim 2, adapted such that the detecting of the reduction in circulation through the protecting filter (14) based at least in part on a change in the pressure transfer function includes sampling parts of the pressure transfer function by measuring the frequency response at at least one frequency.

8. A method of detecting a reduction in circulation through a protecting filter (14) of a gas detector cell unit, comprising the step of providing a gas detector cell unit according to any of the preceding claims, using the loud speaker to impose on the protecting filter (14) a pressure function, and detecting a reduction in circulation through the protecting filter (14) based at least in part on a change in at least one of amplitude and phase of the pressure function resulting from a change in a pressure transfer function of the protecting filter (14) over a predefined time.

9. A method according to claim 8, wherein the step of detecting a reduction in circulation comprises for a change in at least one of amplitude and phase of the pressure function resulting from a change in a pressure transfer function of the protecting filter (14) over a predefined time, and identifying a reduction in circulation through the protecting filter based at least in part on detecting changes in said at least one amplitude and phase of the pressure function.