Advancing vortex durability and intelligence for energy measurements

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1 White Paper Advancing vortex durability and intelligence for energy measurements Peter Kucmas Executive Summary Vortex sensor design has practically eliminated sensitivity to process vibration and temperature changes allowing these instruments to operate in applications that were before considered hostile with greater turn down ratios and higher accuracies. With the current focus on energy conservation vortex meters are playing a significant role in mapping energy consumption in manufacturing plants, campuses, heating and cooling districts and process applications world wide. About the Author Peter Kucmas was born in Prague Czech Republic (formerly Czechoslovakia) and immigrated to the United States in He possesses a BS in Electrical Engineering from Boston University. Peter joined Endress+Hauser in 2008 as a Flow Product Manager for vortex, Coriolis, electromagnetic, thermal dispersion, ultrasonic and DP flow products. He has previously held positions where his primary focus was on steam, cryogenics, high temperature liquids, multi-phase, water, wastewater, refined products, heat transfer fluids, LPG/LNG, crude oil, ethylene gas, flare gas, bio gas and others. Working closely with R&D on new applications expanded his understanding of profile effects, viscosity effects as well as multi-phase flow conditions effects on flow measurement, accuracy and repeatability. During that time, he participated closely on many tests and experiments of new and developing technologies utilizing ultrasound. CP 992D/24/ae/11.08 White Paper US/INDD CS

2 Advancing vortex durability and intelligence for energy measurements Introduction Advancements in sensor design and electronic signal detection continue to expand the functionality of the vortex flow measurement principle. Rather than one type of technology obsolescing another, trends indicate that most flow measurement technologies are continuing in expanding their boundaries of reliable and accurate operation. The vortex flow measurement principle is no different with its new DSC (Differential Switched Capacitor) sensor design. A vortex flowmeter for gas service also offers a unique capability that is only shared with the Coriolis flow measurement principle: Theory and test reports state that a water calibration is directly applicable to gas measurements. This physical law allows the vortex to have lower operating costs over its lifetime as compared to other flow technologies that require calibration in more expensive gas facilities. Some benefits of using a vortex flowmeter may include having identical instruments for steam, liquids and gases, saving on spare parts and training. Improved sensor design with a larger turn down ratio is another benefit to cope with seasonal flow changes, demand and production dynamics. Since the vortex meter is a single integral unit there is no possibility for impulse lines plugging like with delta pressure flow measurement as well as reducing the number of possible leak points. Lastly, today s vortex meters offer in-situ verification capabilities that allow the end user to extend required calibration cycles, reducing down time and reducing overall operating costs. From an installation and operation perspective a vortex meter offers a combination of flexible analog, digital and Figure 1. DSC sensor with integrated temperature diagnostic outputs that offer a wider set of process and sensor data allowing the end users to typically better understand their process and many times save on installation costs. For most applications the vortex typically has a very tolerable pressure drop and the meters come in a variety of line sizes to accommodate most applications including one and two line size reductions. Development and improvements Early generations of vortex flowmeters were typically very sensitive to vibration caused by motors, pumps, control valves and other mechanical noise. The end result was that the instruments erroneously translated the vibrations into flow measurement. Eliminating sensitivity to vibration, fluid pulsation, hydraulic noise and rapid temperature changes have been key focus points of manufacturers, resulting in the new DSC sensor design. Vortex shedding theory of operation A fluid moving around a body experiences a drag force, which is usually divided into two components: frictional drag, and pressure drag. Frictional drag comes from 1513 LeonardoDaVinci 1912 Theodor vonkarman Figure 2. The natural phenomenon of vortices formation as observed by Leonardo Da Vinci and Theodor Von Karman 2

3 friction between the fluid and the surfaces over which it is flowing. This friction is associated with the development of boundary layers, and it scales with Reynolds number. Pressure drag comes from the eddying motions that are set up in the fluid by the passage of the blunt body. This drag is associated with the formation of a wake and it is usually less sensitive to Reynolds number than the frictional drag. Formally, both types of drag are due to viscosity but the distinction is useful because the two types of drag are due to different flow phenomena. Frictional drag is important for attached flows, and it is related to the surface area exposed to the flow. Pressure drag is important for separated flows, and it is related to the cross-sectional area of the body.[1] Depending on the shape and location of the obstruction, the flow is diverted as it begins to flow past the bluff body creating a stagnation zone and a variety of pressure zones and velocity zones. Immediately after the flow passes the widest portion of the obstruction a low pressure zone is created, forming a wake. d Sensor Figure 3. Typical function of a vortex flowmeter A vortex meter requires certain design criteria for best operation. The least effective obstruction would be something streamline, dominated by viscous drag, a small stagnation point upstream, attached flow along the body and a minimal wake downstream. With a streamline case the flow follows the contours of the body and generates only tiny vortices. An example of this shape is the classic tear drop. The most effective design of a vortex flowmeter is to have an obstruction act as a bluff body. This can be characterized by a defined stagnation zone that is unstable, flow divides and diverts around the obstruction, vortices begin to form by rolling up of the shear layers around the shoulders of the obstruction which are dominated by pressure drag. The tail end of the bluff body needs to be designed in such a way that the formed vortices do not reattach, thus creating strong flow signals. We can try to visualize the entire process of vortices formation past a bluff body by comparing it to a flag on a flag pole in varying wind conditions. In a no flow situation, assuming a full pipe, the fluid whether gas or liquid encompasses the entire volume around the bluff body but no vortices are being generated because the fluid is not moving. In our example the flag hangs limp on the flag pole. When low flow occurs the fluid moves past the obstruction dominated by frictional drag but the fluid remains attached without any organized vortices being created downstream of the obstruction. In our example, the flag may have some slight indistinguishable movement (see Figure 4, Example 1). As moderate flow begins the fluid passes the bluff body, begins to separate creating a wake under the influence of frictional drag as well as under the influence of pressure drag. The flow can not yet organize into vortices so the vortex meter still indicates V Dpp zero flow. In our example the flag shows some disorganized typically non repeatable movement (see Figure 4, Example 2). Vortex shedding occurs when a flow velocity is reached dominated by pressure drag (based on the fluid s properties) typically Reynolds number greater than The physical result is that shedding vortices form alternatively and never simultaneously around the bluff body. It is also important to note that at a given flow velocity each vortex swirl contains the same amount of material much like a positive displacement pump. This physical property is the reason that repeatable and accurate measurements can be made using this technology. The frequency at which the vortexes form is directly proportional to the flow velocity and the properties of the fluid. The faster the flow becomes the more frequent the vortices are shed. In our example the flag waves back and forth faster and faster as the wind increases (see Figure 4, Example 3). It is interesting to note that the flag behaves similarly to a splitter plate causing some re-attachment of flow vortices, an undesirable effect in flow measurement which is minimized in the design of the vortex sensor. The shed vortices can be detected by using a variety of sensors that can feel relative change in pressure over time. The change in pressure is then converted into an electronic signal that is processed and calculated into velocity and volumetric flow (see Figure 5). Example 1 Example 2 Example 3 Figure 4. Flow conditions created around a bluff body Piezoelectric sensor techniques The two most prevalent obstacles facing sensor design for vortex shedding meters are predominantly 1) vibration most typically due to process machinery, and to 2) rapid temperature changes. Most of today s vortex meters utilize a piezoelectric crystal to detect the pressure changes that occur behind the bluff body. A piezoelectric crystal is one of the most delicate ways to measure pressure; unfortunately this sensitivity is also extremely responsive to mechanical stress either caused by vibration or by temperature Figure 5. Typical vortex bluff body and sensor design

4 changes. When a piezoelectric crystal is being vibrated due to mechanical noise the effect of the G forces can be perceived as vortices and reported as flow. Additional mechanical stress can be imposed on a piezoelectric crystal from sudden temperature changes such as cryogenic applications or hot steam. In both cases the changes in temperature or vibration cause the crystal lattice to either contract or to expand and create a voltage spike that can again be interpreted as flow. Depending on the amplitude and frequency of these spikes, some may be filtered out by the electronics while others may be passed on. In software, immunity to noise can be increased by raising the detectable signal threshold but this in turn reduces the meter s turn down ratio. Vibration effects on a piezoelectric vortex flowmeter can be seen most at zero flow conditions when the zero cut off limit is turned off. Vibration of the piping can be interpreted as flow unless suppressed by increasing the detection threshold. In order to try and maintain the flowmeter turn-down ratio, rather than raising the flowmeter threshold, a PLC is used to tie the position of a related process valve to an acceptance/rejection routine for passing the data from the vortex to the control system. Although possible, it is a complicated work-around and relevant only to that specific meter. More importantly, the vibration that is seen at zero flow is typically superimposed on the actual flowing conditions and will exhibit itself as noisy and erratic measurements. One possible way to abate mechanical noise is to position the vortex meter in the appropriate orientation within a piping system. Most vortex flowmeter manufacturers know which axis (X,Y,Z) is the most sensitive to vibration. Sometimes orientation of the meter with the least sensitive axis to the process vibration can reduce the effect. Although all of the above examples can help reduce a vortex s sensitivity to vibration, none of the examples can help with respect to rapid process temperature changes. DSC Sensor The most current evolution in vortex flowmeter sensor design comes from a balanced capacitive configuration otherwise known as a Differential Switched Capacitance (DSC) sensor. In brief this sensor minimizes if not completely eliminates the effect of rapid temperature changes, even as great as 300 F/sec and vibration effects of 0 to 500 Hz up to 1G in all three axis. The DSC sensor (Figure 6: seen in blue) is made from a single piece of steel that has three very unique aspects: 1) The Flag is the portion of the sensor that is located in the process behind the bluff body where the low pressure zones of the vortices impart their forces, causing the sensor to slightly deflect from side to side. Some sensors also offer an integral temperature measurement directly built into the flag. The extremely robust design allows the sensor to withstand a water hammer impact of up to 320 ft/sec without any permanent damage or effect on accuracy. 2) The flag is integrated into a metal diaphragm that is designed to be flexible yet be able to withstand full process pressure and rapid temperature changes. The diaphragm is strictly a boundary between the process and the signal detection element and is pressure tested to over psia. 3) Above the diaphragm is a tuning fork type device that is the exact mass equivalent of the flag below. The tuning fork (capacitive sensor) is housed in the cavity of the instrument. The instrument cavity has mounted within it capacitive collector plates (Figure 6: seen in red). Function of the DSC sensor As flow begins to shed vortices the flowmeter flag responds by moving toward the low pressure zone. At the same time the tuning fork of the sensor moves in the opposite direction of the flag. One side of the tuning fork comes closer to the capacitive plate creating a larger capacitance while the second side of the tuning fork moves away from the capacitive plate creating less of a capacitive charge. The changes in the capacitance seen as a sine wave with frequency proportional to flow velocity are registered, measured and converted into flow. The elegance of the DSC sensor comes from the mass balance which makes it impervious to vibration from 0 to 500Hz up to 1G in all three axes. The simplest example is to hold a pen in a vertical position at either the top or the bottom. Moving the pen left to right (simulating pipe vibration); the response of the non held portion of the pen is to follow the motion being imparted. This would be an example of a piezoelectric sensor interpreting pipe vibration as flow. If the same pen was to be held in a vertical position but exactly in the mass center and left/right motion was imparted as was done in the experiment before the reaction of the pen would be vastly different. The observer would note that even though the pen is moving left/ right the top and bottom of the pen move in unison. The DSC sensor would not interpret the left/right movement as flow. It is also true that the pen can be moved away and toward the subject as well as up and down and the results are that the DSC sensor naturally suppresses vibration noise in all three axis. Figure 6. Function of the Differential Switched Capacitor (DSC) sensor 4

5 tools available to properly size a vortex meter for an application so the lowest flows do not fall below 2000 Re (Reynolds number). It is important to note that although advances in the vortex measurement technology continue, like all flowmeter instrumentation, installation requirements must be observed in order to achieve the manufacturers specified accuracies (Figure 8). Figure 7. Function of a vortex flowmeter in either a gas or liquid application Applications The development of the DSC sensor has allowed for the expansion and use of vortex flowmeters in applications that were previously seen as challenging, or in applications where start-up or shut-down conditions were likely to damage the traditional non DSC sensor design. Today s vortex meters are not only expected to measure volumetric flow but mass and energy as well. Some of these examples are: Steam: The vortex meter has been applied in steam for many years whether in saturated or superheated service. With improvements in software steam tables and other gas related information has been added so that one instrument can compute many of the variables needed for precise flow and energy measurements. The DSC sensor design has improved the operation of the meter specifically under start-up and shut down conditions where water hammer effects can occur. The DSC sensor design also detects a wider range in flow because of it s immunity to noise and vibration. This translates into a larger turndown ratio where one flowmeter can typically handle the extremes of seasonal flow changes (Figure 7). using vortex technology are: CO2, Propane, Oxygen, Air, Nitrogen, Argon, Ammonia, as well as any combinations of mixtures. Liquids: The vortex meter once again can measure practically any fluid as long as it is compatible with the instrument body and is not corrosive or erosive to the metal and seals. Some examples of liquid applications are: Oil, crude, vegetable, LNG, LPG, potable water, demineralized water, solvent feed, refined hydrocarbons and many others. There are Conclusion The DSC sensor design has propelled the vortex meter into new applications and industries with vibration and temperature immunity. As we embark on future installations we are assured that the vortex technology is proven and accurate for a multitude of liquid or gas applications. With the current focus on energy conservation, vortex meters continue to play a significant role in mapping energy consumption in manufacturing plants, campuses, heating and cooling districts and process applications world wide. References: [1] Drag of Blunt Bodies and Streamlined Bodies Bicycle_web/blunt.html Gas: Much like steam applications, the vortex meter can handle practically any gas application that is not corrosive or erosive to the internal structure of the instrument. Today s meters commonly use internal calculations for natural gas such as AGA-NX19 as well as calculating the caloric values for given compositions. Some other gasses typically measured Figure 8. Typical piping requirements needed to achieve manufacturers specified flow accuracies 5

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