Flowmeters For Water And Wastewater Applications

Transit-time and Doppler flow measurement technologies were developed in the early seventies as methods of measuring flow non-intrusively. These technologies have evolved and developed far beyond their initial designs. However, this evolution took many years of research and development to overcome problems on applications that were once beyond the capabilities of these flowmeters. This paper will present details specific to the advancements in the transit-time and Doppler flowmeter fields. Use of these flowmeters in the water and wastewater industries is specifically addressed.

Overview

The single biggest development that has occurred in this field is the ability of a transit-time flowmeter to handle applications with a significant level of solids and or gases. Flow measurement applications within water and wastewater systems have varying levels of suspended solids or entrained gases. Typically, Doppler and magnetic flowmeters are used on these applications. However, with this development, transit- time flowmeters are finding an increasing acceptance. It is important to recognize that while transit-time and Doppler flowmeters have some similarities, their flow measurement principles are actually quite different. A Doppler flowmeter requires the presence of suspended solids or gas bubbles in the flowstream to properly make the flow measurement. This is not a requirement of a transit-time flowmeter. However, to many people in the water and wastewater industries, the term "ultrasonic flowmeter" solely means Doppler because they are not familiar with transit-time flowmeters. "Ultrasonic flowmeter" does not accurately and effectively describe these two diverse flow measurement techniques with specific emphasis on the transit-time approach.

Doppler

A doppler flowmeter transmits an acoustical signal into the flowstream at a specific frequency. This signal reflects off particulate, wither solids or gas bubbles. The reflected signal is detected by the receive transducer. The flowmeter compares the transmit and receive frequencies and computes the frequency shift. This frequency shift is proportional to the liquid's flow velocity and is known as the Doppler effect.

While the Doppler technique is suitable for certain water and wastewater applications, it is important to recognize its limitations. The basic limitation of a Doppler flowmeter is its dependence on non-flow parameters for its ultimate performance and accuracy. These non-flow parameters are the liquid sonic conductivity and, more importantly n water and wastewater applications, the presence and distribution of reflectors in the flowstream. While there are optimum levels of suspended solids on most water and wastewater applications, the distribution of the solids within the flowstream is beyond the operator's control and is more dependent on the liquid's flow velocity and piping layout. Doppler flowmeters can perform some averaging of the received signal to offset the effects of a skewed flow profile but not to the degree required to maintain an accurate flow measurement under all conditions. Therefore, the flowmeter's performance is dependent on the randomness of the velocity, the location of the reflectors in the flowstream and the variability of the solids loading.

The primary difference between good Doppler flowmeter and those of lesser quality is in the way they reject non-flow related noise. The detected frequency shifts (noise) can be the result of sources beyond simply the liquid's flow velocity. Mechanical vibrations or EMI/RFI interference can be sources of noise that create various frequency shifts detected by the flowmeter. Only higher quality microprocessor based Doppler flowmeters have the ability to reject this non-flow noise. The most successful way of filtering out non-flow noise is by using a fast Fourier transform (fft) spectral analysis. This analysis breaks down the flowmeter's receive signal into the specific frequency components. In doing so, the flowmeter can then reject the low frequency non-flow noise generated by mechanical vibrations and the high frequency non- flow noise generated by EMI/RFI interference. This allows the flowmeter to simply use the flow related noise in determining the instantaneous flow-rate.

A Doppler flowmeter offers the benefit of simple inexpensive installation, especially when compared to magnetic flowmeters or venturi tubes. However, like all flowmeters, it is crucial to recognize and abide by the limitations of the technology. It would be ideal if a flowmeter was able to provide the unique benefits offered by a Doppler meter that also avoids the Doppler's limitations. For many applications, a microprocessor based transit-time flowmeter is such a device. However, before the advancements and developments in the transit-time technology is addressed, the basis for the technology should first be presented.

Transit-Time

A simple analogy to explain the transit-time flow measurement principle is the college physics "rowboat in the stream" problem. A rowboat crossing a stream at an angle to the stream's current will take less time to cross when going with the current than when going against it. This time difference is proportional to the velocity of the stream's current. To relate this to a transit-time flowmeter, simply replace the rowboat with an acoustical signal. An upstream transducer (T1) sends a signal to the downstream transducer (T2) that in turn sends a signal back. When there is no flow, the time to go from the T1 to T2 is the same as the time going from T2 to T1. However, when there is flow, the effect of the liquid's flow velocity on the acoustical signal is to assist the signal in the up to downstream direction and hinder the signal in the down to upstream direction. This creates the time difference by which the liquid's flow velocity and ultimately the flowrate is determined. The following equation is used as the basis for determining the liquid's flow velocity. Vf = Kdt/TL
Where: Vf = Flow Velocity
K = Calibration Factor, in units of Volume/Unit time
dt = Measured Upstream minus Downstream Transit Time = Difference
TL= Measured Average Liquid Transit Time

Early transit-time flowmeter designs, and to some extent, certain designs that exist today, were limited to applications that were relatively free of entrained gases and/or suspended solids. In addition, it was important that the liquid characteristics, such as temperature or chemical composition, remained fairly constant. If an application had varying liquid characteristics or the presence of solids or gases, then it was unlikely that a transit-time flowmeter would provide adequate operation. These limitations existed for two main reasons.

1. A change in the liquid characteristics, such as temperature, would have a direct effect on the liquid's sonic velocity (Vs). Sonic velocity is the measure of speed the liquid will conduct sound. A change in sonic velocity would have a direct effect on the refraction angle as shown in Figure 3. If there is a sufficient change in sonic velocity, and subsequencially the refraction angle, then the signal from one transducer would not be received by flowmeter inoperative.
2. The signal sent from one transducer to the other was a single pulse of acoustical energy. The presence of entrained gases or suspended solids would obstruct and interfere with the single pulse and would therefore prevent it reception by the respective transducer. If this condition remained present for multiple cycles, the flowmeter would enter a fault condition and would cease operation.

With the advent of the microprocessor and its incorporation into the transit-time flowmeter, this technology has taken a major step forward. The microprocessor has allowed the transit-time technology to improve its signal discrimination and computation capabilities to a point where problems due to varying li2quid characteristics, entrained gases or suspended solids can easily be overcome. The most dramatic improvements in the technology are described below.

Another major development in the transit-time technology is the ability to transmit multiple pulses instead of a single pulse as described earlier. This approach, known as MultiPulse Transmission, transmits as many as one thousand pulses per second between the two transducers. As shown previously, the loss of any signal due to the presence of gases or solids in a single pulse transmission system is extremely detrimental. However, the loss of signal due to these same gases or solids in a MultiPulse system is insignificant because the amount of signal data is so high, especially when compared to the single pulse system. Because of the developments described above, transit-time flowmeters are now being used on applications that previously had been beyond their ability. Transit-time flowmeters are routinely usee on applications such as the following:.

• Raw Sewage
• return activated Sludge
• Waste Activated sludge
• Primary Sludge
• effluent
• chemical Additives

Beyond the ability to operated on these most common water and wastewater applications, a transit-time flowmeter can provide information on the process well. Diagnostic routines inherent to the flowmeter are used for detection of pipe wall build-up or for determining the level of aeration in the flowstream. The ability to detect pipe wall build-up is a direct result of measuring the liquid's sonic velocity. Continuous and accurate measurement of a liquid's this information is used as a diagnostic tool.

A liquid's sonic velocity is determined base on the average up and down stream liquid transit time of the acoustical signal. In addition, there are some fixed times in the overall signal transmission. The fixed times are the times that the signal spends in each transducer block and the pipe wall. These fixed times are quantifiable. The only variable is the actual time in the liquid. Water's sonic velocity is directly dependent on its temperature. This relationship is well known and us used to establish a particular application's expected sonic velocity. Therefore, if the measured sonic velocity differs from the expected, there is an indication either incorrect pipe data was entered into the flowmeter or the pipe wall has been altered due to scaling or build-up. If the initial measured sonic velocity matched the expected and then began to change over time while the water temperature remained constant, except for possible seasonal variations, the only possible cause would be due to a change in the pipe wall. The measured sonic velocity can trigger an alarm output when it exceed a specific set point or an analog output for trending purposes. This diagnostic capability is used in the Great lakes region for zebra mussel infestation detection in outfall and intake lines. This ability is also used for detection of build-up due to chemicals, minerals or grease.

Aeration in the flowstream has a direct effect on the stability of the receive signal. A stable receive signal indicates zero to low levels of entrained gases or air. Conversely, an unstable receive signal would indicate a potentially significant level of entrained gases. The amount of entrained gases would be proportional to the level of instability of the signal. It is important to note that the instability does not affect the accuracy of the flowmeter. This diagnostic capability is used for cavitation detection of pumps or valves. Either an analog or alarm output is used to remotely monitor a particular application's aeration level. This ability is a direct result of the development of the MultiPulse transmission System and is used at lift and booster pump stations for monitoring and scheduling of pump repairs and maintenance.

The information presented so far is based on a clamp-on type flowmeter as opposed to the wetted transducer design that will be reviewed later. A unique attribute of the clamp-on non-intrusive type transit-time flowmeter is its suitability for portable applications. The obvious benefit of a portable meter of this kind is its ability to be easily installed at a previously unmetered location for temporary measurement. The meter is also used as part of infiltration/inflow studies, conversation studies, accuracy verification of existing conventional intrusive meters, pump performance testing for flowrate By utilizing a microprocessor, a transit-time flowmeter now has the ability to communicate with remote equipment. While older designs and non-microprocessor based flowmeters have or had the ability to provide analog or alarm outputs, the current generation of microprocessor based meters can also communicate digitally. This ability allows these meters to be installed at remote locations throughout a collection or distribution system and then report back to a centralized station at a main office or treatment plant. Transmitted information is not limited to just flowrate. Information such as sonic velocity, aeration level, signal strength, process or fault alarms can also be transmitted to the central station for monitoring or report generation. Communications can also be bi-directional for interrogation of remote meters on demand. These meters can also internally log information at selected intervals for later retrieval.

With the developments presented here are significant, it is important to recognize that not all transit-time flowmeters are identical. The following is a summary of the features that transit-time should possess if the meter is to be used on water and wastewater applications.

1. The meter should be microprocessor based.
2. The meter should use a multiple pulse signal, not a single pulse.
3. The meter should use the wide beam approach, not shear mode, and have the ability to generate multiple frequencies to properly match the specific pipe conditions.
4. The meter should automatically and continuously measure the liquid's sonic velocity.

There are transit-time flowmeters available today, even microprocessor based instruments, that do not possess all these features. In addition, as previously stated, these developments are specific to the clamp-on design, not a wetted transducer type meter. While there is some commonality between the two designs, there are also some important differences. Figure 5 is an illustration of a wetted transducer.transit-time flowmeter.

The main advantage associated with a wetted system is the high signal strength the system can achieve since there us no pipe wall for the signal to penetrate. However, there are also several significant disadvantages when compared to a clamp-on system. Like all wetted intrusive flowmeters, the meter is difficult to install and maintain with potential for transducer fouling due to coating. Coating problems can be significant on water and wastewater applications due to the presence of minerals, chemicals or grease. While the wetted design improves the signal strength, it also results in a slower system. flowrate. This sluggish response is due to the effects of an echo chamber created by the signal. After each single signal transmission, it is necessary to wait for the echo to subside before the next transmission is made. This negatively effects the response time of the system and limits its data density. Data density is the amount of valid flow information obtained by the flowmeter for a given period of time. The higher the data density the better the meter's performance would be. Another disadvantage associated with a wetted system is that the transducers act to distort the flow profile. This fact caused several problems.

1. An unwanted flow profile distortion resulting in a decrease in accuracy.
2. Potential for the transducers to cause cavitation, especially at higher flowrates. Not only can the cavitation be destructive to the transducers can also prevent the meter from making the flow measurement.
3. While the angle between the transducers and the pipe is fixed, the system's calibration factor is not. This is due to the distorted flow profile and the fact that calibration is a function of the angle between the signal beam and the flowstream.

Conclusion

The advancements that have occurred in the transit-time flowmeter field have allowed this type of meter to be used applications previously thought to be beyond their ability. These meters can also provide more information than just flowrate and can be used for permanent and portable applications. However, when reviewing the suitability of a meter for a given application, understanding that there are differences between transit-time flowmeter designs is of extreme importance. As previously summarized, if a meter is to be installed on a water or wastewater application, it should possess the features described in this paper. In addition, when evaluating the potential use of a transit-time meter versus a doppler meter for a particular application, the inherent limitations of the doppler meter should be recognized and abided by.

While the developments in this field have been significant, the potential for continued improvement is vast. New features and advancements in the technology are occurring every day. With increased acceptance of these meters and the continued development, the water and wastewater industries will come to recognize the importance role that transit-time flowmeters will play in these industries for years to come.

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Edited by Ian Lisk.