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This invention relates to process control instruments, and more particularly, to a guided wave radar probe for use in emulsion measurement applications.
Process control systems require the accurate measurement of process variables. Typically, a primary element senses the value of a process variable and a transmitter develops an output having a value that varies as a function of the process variable. For example, a level transmitter includes a primary element for sensing level and a circuit for developing an electrical signal proportional to sensed level.
Knowledge of level in industrial process tanks or vessels has long been required for safe and cost-effective operation of plants. Many technologies exist for making level measurements. These include buoyancy, capacitance, ultrasonic and microwave radar, to name a few. Recent advances in micropower impulse radar (MIR), also known as ultra-wideband (UWB) radar, in conjunction with advances in equivalent time sampling (ETS), permit development of low power and low-cost time domain reflectometry (TDR) instruments.
In a TDR instrument, a very fast pulse with a rise time of 500 picoseconds, or less, is propagated down a probe that serves as a transmission line in a vessel. The pulse is reflected by a discontinuity caused by a transition between two media. For level measurement, that transition is typically where the air and the material to be measured meet. These instruments are also known as guided wave radar (GWR) measurement instruments.
One type of probe used by GWR level instruments is a coaxial probe. The coaxial probe consists of an outer tube and an inner conductor. When a coaxial probe is immersed in the liquid to be measured, there is a section of constant impedance, generally air, above the liquid surface. An impedance discontinuity is created at the level surface due to the change in dielectric constant of the liquid versus air at this point. When the GWR signal encounters any impedance discontinuity in the transmission line, part of the signal is reflected back toward the source in accordance with theory based on Maxwell's laws. The GWR instrument measures the time of flight of the electrical signal to, and back from, this reflecting point, being the liquid surface, to find the liquid level.
GWR probes are frequently used in tanks where multiple layers of fluids can exist, or in applications with highly viscous liquid. One example of such an application is in the oil and gas industry. Well fluid containing crude oil, water, sand and other impurities enters a separator tank as a mixture. This is generally illustrated in
The objective of the GWR probe in such applications is to accurately measure several levels, including, the top of the oil layer, the bottom of the oil layer (i.e., the top of the emulsion layer) and the top of the water layer (i.e., the bottom of the emulsion layer). There are several difficulties when using GWR measurement instruments in interface applications or with viscous fluids. GWR is commonly used to measure fluid interface levels where the dissimilar dielectric properties of adjacent layers produce a reflection from the transmitted signal at the boundary. However, interface detection becomes more difficult when a thick emulsion layer is present and the dielectric properties of the fluid changes gradually. It has been observed that a small percentage of water in oil creates a significant difference in the dielectric properties compared to oil alone. A small percentage of oil in water behaves much like water alone. Therefore, it is more difficult to discern the interface between water and an emulsion of water with a small percentage of oil compared to the interface between oil and an emulsion of oil with a small percentage of water. As such, it is more difficult to detect the bottom of the emulsion layer than the top of the emulsion layer.
U.S. Pat. No. 9,546,895, owned by Applicant herein, describes a method to go beyond time of flight, and profile impedance versus distance. That method uses a sharp-edged step instead of a narrow pulse. The method then analyzes the waveform taking into consideration the well-known relationship between impedance and wave velocity. Material properties can cause estimation errors in the upper layers which then propagate to lower layers. Also, the lower layers may comprise water mixed with a small amount of oil, so the reflected signal is very small, which means the measured emulsion has a great sensitivity to the voltage, leading to errors.
The present invention is directed to solving one or more of the problems discussed above in a novel and simple manner.
As described herein, a guided wave radar probe for use in emulsion measurement applications uses both top-down and bottom-up measurement signals.
In accordance with one aspect, a three phase guided wave radar measurement instrument for measurement of an emulsion comprises a hydrocarbon layer atop an emulsion layer of hydrocarbon and water atop a water layer. The instrument comprises a probe defining a transmission line, the probe comprising a process connection for mounting to a process vessel, an elongate rod extending downward from the process connection to extend into a process liquid, a top connector at a top end of the elongate rod, and a bottom connector at a bottom end of the elongate rod. A top excitation circuit is connected to the probe top connector for generating excitation signals on the transmission line and receiving a reflected signal from the transmission line, the reflected signal comprising a top-down waveform of probe impedance over time. A bottom excitation circuit is connected to the probe bottom connector for generating excitation signals on the transmission line and receiving a reflected signal from the transmission line, the reflected signal comprising a bottom-up waveform of probe impedance over time. A controller is operatively connected to the top excitation circuit and the bottom excitation circuit. The controller alternately operates the top excitation circuit and the bottom excitation circuit and profiles content of the emulsion responsive to analysis of the top-down and bottom-up waveforms to determine interface levels between air and the hydrocarbon layer, between the hydrocarbon layer and the emulsion layer and between the emulsion layer and water.
In accordance with another aspect, there is described a guided wave radar measurement instrument comprising a probe defining a transmission line, the probe comprising a process connection for mounting to a process vessel, an elongate rod extending downward from the process connection to extend into a process liquid, a top connector at a top end of the elongate rod, and a bottom connector at a bottom end of the elongate rod. A top excitation circuit is connected to the probe top connector for generating excitation signals on the transmission line and receiving a reflected signal from the transmission line, the reflected signal comprising a top-down waveform of probe impedance over time. A bottom excitation circuit is connected to the probe bottom connector for generating excitation signals on the transmission line and receiving a reflected signal from the transmission line, the reflected signal comprising a bottom-up waveform of probe impedance over time. A controller is connected to the top excitation circuit and the bottom excitation circuit. The controller alternately operates the top excitation circuit and the bottom excitation circuit and profiles content of the emulsion responsive to the waveforms by transforming the waveforms into impedance relative to distance, converting the transformed waveforms into effective dielectric relative to distance, determining mixture content of the emulsion at select distances responsive to the effective dielectric at the select distances and developing an output representing mixture content relative to level units.
In accordance with a further aspect, there is disclosed a radar transmitter for emulsion measurement comprising a probe defining a transmission line for sensing impedance. A first excitation circuit is connected to a top of the probe for generating downward travelling excitation signals on the transmission line and receiving a reflected signal from the transmission line. A second excitation circuit is connected to a bottom of the probe for generating upward travelling excitation signals on the transmission line and receiving a reflected signal from the transmission line, each of the reflected signals comprising a waveform of probe impedance over time. A controller is operatively connected to the excitation circuits. The controller profiles a section of waveform from each of the excitation circuits and combines information on the sections to determine positions of layers of fluids in a tank, wherein the first pulse circuit provides information about an interface from air into a first fluid layer, and from the first layer to a second layer, and the second pulse circuit provides information about an interface between a lowest layer and the second layer.
Other features and advantages will be apparent from a review of the entire specification, including the appended claims and drawings.
This application describes a method which supplements the methodology disclosed in Applicant's U.S. Pat. No. 9,546,895, the specification of which is incorporated by reference herein, by providing a second signal which travels upwards through water and reflects from a layer of emulsion floating on the water.
As described more particularly below, a radar transmitter for emulsion measurement comprises a probe defining a transmission line for sensing impedance and two excitation circuits. A first excitation circuit connects to the top of the probe for generating downward travelling excitation signals on the transmission line and receiving a reflected signal from the transmission line. A second excitation circuit is connected to the bottom of the probe for generating upward travelling excitation signals on the transmission line and receiving a reflected signal from the transmission line. Each reflected signal comprises a waveform of probe impedance over time. A controller is operatively connected to the excitation circuits. The controller profiles a section of waveform from each of the two excitation circuits and combines the information to determine positions of layers of fluids in a tank. The first excitation circuit provides information about the interface from air into the first fluid layer, and from the first layer to a second layer. The second excitation circuit provides information about the interface between the lowest layer, which is typically water, and the layer above, typically an emulsion of water and the upper layers.
Referring initially to
Referring to
As is described, the controller causes the probe 42 to generate and transmit step excitation signals. A reflected signal shows actual impedance along the transmission line.
The control circuitry of the process instrument 20 may take many different forms. This application is particularly directed to the probe 42, as described below. It should be noted in
As described previously, well fluid provided at the inlet 26 may contain crude oil, water, sand and other impurities. The fluids stratify to produce an oil layer 46 and water layer 48 separated by an emulsion 50. The water is to the left of the weir 30 in the orientation shown in
The process instrument 20 uses stepped radar in conjunction with equivalent time sampling (ETS) and ultra-wide band (UWB) transceivers for measuring level using time domain reflectometry (TDR). Particularly, the instrument 20 uses guided wave radar for sensing level. While the embodiment described herein relates to a guided wave radar level sensing apparatus, various aspects of the invention may be used with other types of process instruments for measuring various process parameters.
The probe 42 is able to transmit and receive excitation signals from both ends when used in connection with a signal circuit having two TDR circuits. A “top-down” circuit sends a signal down the probe 42 from the top and detects signals that are reflected back to the top. A “bottom-up” circuit sends a signal up the probe 42 from the bottom and detects signals that are reflected back to the bottom. The ability to transmit from the bottom-up has the advantage of improved detection of the emulsion layer bottom. Such a system is described in Applicant's co-pending application Ser. No. 16/278,368, filed Feb. 18, 2019, the specification of which is incorporated by reference herein. As described below, the transmission cable for the bottom-up transmission line runs through one of the ground rods, which is tubular.
The probe 42 may be as described in Applicant's application Ser. No. 16/507,672, filed Jul. 10, 2019, the specification of which is incorporated by reference herein. The probe 42 has a center rod which may be of stainless steel or other metal. Nickel alloys, such as Hastelloy or Inconel, may be used for corrosion resistance. The rod has PFA sleeve. Other fluorocarbon materials, such as PTFE, or other electrical insulating coatings may be used. The purpose is to allow maximum signal penetration through the process as described in Applicant's U.S. Pat. No. 9,360,361.
Referring to
The ground rods 70, 72, 74 and 76 are metal tubes, such as stainless-steel or the like, connected to the probe case 60. The fourth ground rod 76 is adapted for carrying a coaxial cable 84 used for bottom-up measurement. The ground rod 76 is secured as by welding to a cylindrical connector 90 connected to the probe case 60 in alignment with a passage 92 in communication with the probe housing 62 and is electrically connected via a terminal 94 to the pulse circuit 58.
The bottom case 80 is cylindrical and of stainless-steel and receives a PTFE gland bushing 96 which captures a bottom end of the center rod 68. A pin 98 is connected at one end to the center rod 68 and at the opposite end to a coax connector 100 connected to a bottom end of the cable 84. The cable 84 passes through a vertical opening 102 in the bottom probe case 80 which receives a cylindrical adapter 104 for connecting the fourth ground rod 76 to the probe bottom case 80.
Referring to
The digital board 116 is also connected to the analog circuit 114 which includes the pulse circuit 58 which is connected to the probe rod 68. The controller 110 includes ETS circuits which convert real time signals to equivalent time signals, as is known.
Guided wave radar combines TDR, ETS and low power circuitry. TDR uses pulses of electromagnetic (EM) energy to measure distance or levels. When a pulse reaches a dielectric discontinuity then a part of the energy is reflected. The greater the dielectric difference, the greater the amplitude of the reflection. In the measurement instrument 20, the probe 42 comprises a wave guide with a characteristic impedance in air. When part of the probe 42 is immersed in a material other than air, there is lower impedance due to the increase in the dielectric. When the EM pulse is sent down the probe it meets the dielectric discontinuity, a reflection is generated.
ETS is used to measure the high speed, low power EM energy. The high-speed EM energy (1000 foot/microsecond) is difficult to measure over short distances and at the resolution required in the process industry. ETS captures the EM signals in real time (nanoseconds) and reconstructs them in equivalent time (milliseconds), which is much easier to measure. ETS is accomplished by scanning the wave guide to collect thousands of samples. Approximately eight scans are taken per second.
The general concept implemented by the ETS circuit is known. A pulse circuit generates hundreds of thousands of very fast pulses of 500 picoseconds or less rise time every second. The timing between pulses is tightly controlled. The reflected pulses are sampled at controlled intervals. The samples build a time multiplied “picture” of the reflected pulses. Since these pulses travel on the probe 42 at the speed of light, this picture represents approximately ten nanoseconds in real time for a five-foot probe. The pulse circuit converts the time to about seventy-one milliseconds. As is apparent, the exact time would depend on various factors, such as, for example, probe length. The largest signals have an amplitude on the order of twenty millivolts before amplification to the desired amplitude by common audio amplifiers. The controller converts timed interrupts into distance. With a given probe length the controller can calculate the level by subtracting from the probe length the difference between a fiducial reference and level distances. Changes in measured location of the reference target can be used for velocity compensation, as necessary or desired.
A “pulse” excitation signal is commonly used in guided wave radar systems. With pulse excitation and equivalent time sampling the received signal produces an echo waveform that displays changes or transitions only in the transmission line (probe) impedance it is measuring. Pulse excitation cannot tell the absolute impedance (50, 55, 60 ohms etc.) of the transmission line it is measuring.
“Step” excitation is a signal that “steps” from one voltage level and stays at that level for a time period greater than the total measurement time of the system (several hundred nanoseconds). After this time, the voltage returns to its original level, and after a delay, the step signal repeats. The reflected signal processing is the same as with pulse excitation; equivalent time sampling techniques are used to detect the reflected signal on an expanded time scale.
The important difference between pulse vs. step excitation is that while pulse excitation only produces a waveform indicative of impedance changes along the probe, step excitation produces a waveform much more indicative of the actual transmission line impedance along the probe. That is, the detected waveform recovered from step excitation can be used to estimate the actual, true impedance along the probe.
In the illustrated embodiment, there are two TDR circuits. One is for the top-down signal and the other is for the bottom-up signal. The waveforms are sent from the analog circuit 114 to the digital board 116 in the control housing 52.
The block diagram in
The software algorithm used therein and in the present application to perform this compensation on an emulsion is called TDR inversion. This method takes a TDR waveform as produced by the instrument and mathematically converts it into N small segments consisting of transmission line models built as equivalent sections of R (resistance), L (inductance) and C (capacitance). The model produces the equivalent of electrical length for each segment, thereby allowing conversion of the waveform into actual length vs. impedance data.
A summary of how the device works is as follows: 1. Obtain waveform scan of tank via TDR (probe impedance vs. time); 2. Use TDR Inversion software technique to transform TDR curve into impedance vs. actual distance; 3. Convert this curve into effective dielectric vs. distance; and 4. Convert curve into percent of oil/water vs. distance. In accordance with the invention, the controller profiles a section of waveform from each of the two excitation circuits and combines the information to determine positions of layers of fluids in a tank. The first excitation circuit provides information about the interface from air 201 into the first fluid layer 202, and from the first layer 202 to the second layer 203. The second excitation circuit provides information about the interface between the fourth layer 204, which is typically water, and the layer above 203, typically an emulsion of water and the upper layers. As is apparent, the information can be used differently in the controller 110.
U.S. Pat. No. 4,774,680 discusses water-in-oil versus oil-in-water emulsions. It shows that two emulsions with the same percent fluids can have drastically different dielectric constants. This patent also shows that this occurs at an indeterminate area around fifty percent water. As a result, the algorithm in Applicant's U.S. Pat. No. 9,546,895 requires additional information to estimate the true percent water much beyond the fifty percent area. The methodology described herein avoids that problem, and simply assumes what is on the bottom is water, and finds the level where there is some material other than water, whether that be an oil-in-water or water-in-oil emulsion.
Because oil has a much lower dielectric than water, the water dominates in the fluids' effect on the TDR reflection. This translates to very small TDR voltage differences between pure water and water with, for example, 20% oil emulsified in the water. Since the desired signal is so small, various artifacts like multiple reflections can swamp the desired signal.
In
A reverse TDR step generator 30R is connected to the transmission line 305, resulting in measured waveform 3WR. Segment 302 leads to TDR waveform segment 302R which is flat. The segment 301 reflection comes much later in time, and so cannot affect the segment 302 response when using the bottom-up method described herein.
Thus, as is apparent, using a TDR signal from both directions eliminates the issue of multiple reflections interference in finding the small oil in water signal. The flat segment 302R response is valuable, since small deviations are easy to see in a known flat signal.
Applicant's US Publication US20190257935 describes using a bottom-up connection to look for motion in the TDR waveforms. That method relies on the TDR signal down in the water to be completely tranquil, even when fluids above are moving. The
The application illustrated in
The method to find the cursor 503 is to maintain a waveform 502, a fast-decay slow-attack filtered version of the waveform 501.
The diodes 606 and 607 are ideal since they are simple “if” statements in software. It can be seen that the signal on the 609 follows the input signal on the line 601, with some RC lag. Furthermore, when the signal 601 is above the signal 609, the lag is the resistor 607 and the capacitor 605, and when the signal 601 is below the signal 609, the lag is the resistor 604 and the capacitor 605. In this design, the resistor 607 is much greater than the resistor 604, so that the signal 609 follows the signal 601 downwards, but when the signal 601 deviates upwards, the signal 609 lags behind. The difference operation in the summer 608 sends the distance between 601 and 609 to the comparator 610, which then sets the signal 612 high when the difference exceeds the threshold 613. The threshold is used by the microprocessor 118 to find the cursor 503 of
Thus, as described herein, the guided wave radar probe is used for measuring levels in tanks where multiple layers of fluids can exist and uses both top-down and bottom-up measurement signals.
It will be appreciated by those skilled in the art that there are many possible modifications to be made to the specific forms of the features and components of the disclosed embodiments while keeping within the spirit of the concepts disclosed herein. Accordingly, no limitations to the specific forms of the embodiments disclosed herein should be read into the claims unless expressly recited in the claims. Although a few embodiments have been described in detail above, other modifications are possible. For example, the logic flows depicted in the figures do not require the particular order shown, or sequential order, to achieve desirable results. Other steps may be provided, or steps may be eliminated, from the described flows, and other components may be added to, or removed from, the described systems. Other embodiments may be within the scope of the following claims.
As is apparent, the functionality of the analog circuits, could be implemented in the microprocessor 118, or any combination thereof. Accordingly, the illustrations support combinations of means for performing a specified function and combinations of steps for performing the specified functions. It will also be understood that each block and combination of blocks can be implemented by special purpose hardware-based systems which perform the specified functions or steps, or combinations of special purpose hardware and computer instructions.