Optimizing flowmeter selection
With countless flow measuring instruments currently available, making an optimal flowmeter choice is often a daunting task. Users have traditionally based their flowmeter selection decision on price and published product specifications.
However, users have also discovered that:
- Flowmeter selection decisions based on the lowest price often result in the highest installed cost
- Selecting the highest accuracy meter based on product specifications often results in the least measurement accuracy in actual applications
- Relying on a comparison of published product specifications alone may actually result in costly process downtime.
After experiencing some of the risks in using these outdated selection techniques, many users are seeking a best-in-class approach to flowmeter selection that can consistently produce optimal decisions. To avoid surprises and reduce project risks, a systematic repeatable approach based on best-in-class methodology is required to identify the optimal flowmeter selection in virtually any flow application.
Flow measurement is simply a quantification of an amount of fluid or gas passing through a channel medium such as a pipe or a duct. A flowmeter is a measuring instrument used to quantify the flow rate of the fluid passing through the channel. Flow is typically quantified as a volumetric flow rate (e.g., gal/min) or as mass flow rate (e.g., lb/hr).
In a manufacturing environment, fluid flow measurement is often considered essential to operate a process plant that manufactures a product from raw materials or adds value to a product. The demand for more accurate flow measurement instruments is often driven by business demands or environmental restrictions, such as a need for tighter process controls leading to reduced emissions and increased efficiency.
Sensing a pressure loss to measure flow has been used for centuries. Over the years, better understanding of natural flow phenomenon such as vortex shedding, Coriolis effect, Doppler effect, and discoveries of principles such as electromagnetic induction have contributed to the development of additional flow sensing technologies and flowmeter products.
Currently, the following types of flow sensing technologies are widely used in various flowmeter devices available on the market:
- Differential pressure (DP) includes orifices, venturi tubes, flow nozzles, pitot tubes, averaging pitot tubes, and V-cone
- Positive displacement includes reciprocating piston, oval gear, nutating disk, rotary vane, and diaphragm
- Coriolis (or mass flow)
- Thermal dispersion
Many of the flow measurement instruments installed today are based on DP-sensing technologies. In the soon to be released ARC Advisory Group “Flowmeter Worldwide Market Outlook” report, analyst Lauren Clement estimates that differential pressure transmitters still account for nearly half of all flow measurement transmitters shipped each year. The improvements in measurement range, accuracy, repeatability, and the ability to make multiple measurements, provided by intelligent multivariable pressure transmitters have added new life to this mature but obviously still viable technology (see Figure 1).
Traditional flowmeter selection
Making the best flowmeter choice for a particular application is challenging for many users because the current proliferation of flow measuring instruments seems to be endless. Some users prepare large spreadsheets filled with selection criteria to evaluate potential flowmeters—many of which may not be relevant to the user’s current flowmeter application. Consequently, having too many choices and too little expertise in understanding the flow application or flowmeter capabilities often leads to a poor selection.
Many users may be generally aware that a particular flowmeter or a flow sensing technology may be best suited for measuring fluid flows that exhibit certain matching flow characteristics. For example, a Coriolis type mass flowmeter may be an ideal choice for meeting requirements of a custody transfer application that requires high accuracy, magnetic flowmeters are not suitable for measuring flow of nonconducting fluids commonly found in hydrocarbon processing industries, and DP-based flowmeters may not be suitable to measure laminar flows. However, users may not have the experience or willingness to acquire the knowledge to consider relevant factors that can affect flowmeter selection.
Table 1: Installed cost: traditional installation
Therefore, decisions are often made based on purchase price and a cursory comparison of published product specifications. Selecting flowmeters based on initial purchase price alone may not be ideal in the long run due to hidden costs. Depending on the selected flowmeter type, the total installed cost typically is significantly more than the initial purchase price (see Table 1).
Changes in processes and environmental conditions may significantly affect instrument accuracy. Selecting a flowmeter rated 0.065% of span accuracy, for example, may in fact provide an actual or installed accuracy three to four times less accurate because of changes in flow rate and variations because of temperature and pressure changes (see Figure 2). Thus, selecting a flowmeter based on purchase price and published accuracy specifications may increase operating costs and, in some cases, cause process downtime.
In many cases, a decision may be simply based on continuation of past practices—making a status quo selection—without reviewing the available options. While a status quo selection may be perceived to reduce project risks, it may potentially overlook advances in flowmeter capabilities. With microprocessors being used increasingly in recent flowmeter designs, many products currently offer improved accuracy and more functionality such as multivariable measurement for a nominal cost increase. For example, many multivariable flowmeters have built-in functionality to compensate flow measurements for changes in temperature and line pressure. As a result, a status quo selection could result in higher installed costs for the user because additional measurement devices and/or additional piping may be required to achieve comparable results.
Best-in-class selection methodology
After experiencing some of the conventional flowmeter selection risks, many users are seeking a best-in-class approach to flowmeter selection that consistently produces optimal decisions. To avoid surprises and reduce lifecycle costs, a systematic, repeatable approach based on a best-in-class methodology is required to identify the optimal flowmeter selection in virtually any flow application.
The methodology starts with a definition of the requirements to gain an understanding of the purpose behind making a process flow measurement (see Figure 3). Typical examples of “purpose of measurement” may include volumetric or mass flow measurement of fluids in process applications such as batching, continuous blending, control loop, custody transfer, filling, government regulations, inventory control, loading/unloading, mass balance, monitor/indicate, and safety.
By gaining an understanding of the primary purpose behind making a flow measurement, users may be able to focus on relevant selection criteria and prioritize among multiple selections. For example, in a flow measurement application that has local monitor/indicate capability as the primary purpose, accuracy-related selection criteria may be less relevant compared to installed cost. It is important to recognize that each flow sensing technology and each flowmeter has advantages (described as a “best fit” or “sweet spot” application) and disadvantages.
Next, it is desirable to define the flow characteristics such as pipe size; fluid characteristics such as liquid/gas/steam; whether the fluid is clean/dirty or has corrosive/erosive properties; whether the fluid’s viscosity may significantly affect flow measurement; whether operating temperature, pressure ranges, and maximum allowable pressure drop have been specified; whether the fluid has conductive/nonconductive properties; and whether flow is unidirectional/bidirectional or steady/pulsating. By gaining a better understanding of the flow characteristics, users can focus on determining whether particular flowmeters or flow sensing technologies under consideration are capable of supporting the requirements.
After gaining insight into the flow application requirements, an understanding of flow measurement device performance in an actual plant installation is required. This ensures that the instrument’s capabilities match the process flow measurement requirements. Installation- and commissioning-related evaluation criteria may include inline versus insertion mounting styles, local readout requirements, straight pipe length requirements, ease of engineering, configuration, installation, availability of power source, mounting in intrinsically safe areas, presence of vibrations, presence of corrosive fluids, presence of electromagnetic field, and frequency of calibration.
External issues such as environmental factors, vendor capabilities, and government regulation related to potential selection criteria may include availability of service, potential of fugitive emissions of hazardous fluids, and health- or safety-related considerations. For example, reliability and environmental safety of an installation can be increased by selecting a direct-mount DP flowmeter instead of a traditional instrument that requires impulse lines. By installing a direct mount DP flowmeter, the number of potential leak points can be reduced by 80%, which can reduce total maintenance costs significantly.
Because economic evaluation factors are included in virtually all purchasing decisions, they are considered to be critical in order to evaluate their impact on budgets and potentially eliminate nonaffordable selections. Economic comparisons can include quantifiable installed or lifetime cost considerations. Quantification of cost for each of the choices is done to make an objective decision by assessing an economic impact (e.g., by making a comparison of purchase price versus installed cost versus lifecycle cost of the flowmeter).
In the final step of the methodology, flow measurement products are compared with respect to the relevant selection criteria to construct an optimal product selection matrix. To simplify and better manage the product selection process, it may be desirable to apply sufficient filtering criteria to generate an optimal product selection matrix. Obviously, the selection process complexity will increase significantly as the number of relevant evaluation criteria and/or product selections increase.
An optimal flowmeter selection that provides the best value for the available budget is made by determining the relative importance of each relevant selection criteria by prioritizing and ranking the relevant selection criteria, and selecting a flowmeter that delivers the most value. In some applications, the list of relevant selection criteria may be weighted to arrive at an optimal selection.
Successful best-in-class methodology implementation requires understanding the purpose for making the flow measurement. For example, consider a custody transfer steam-flow measurement application (see Figure 4). The piping constraints at the plant site have limited the available straight pipe length for measuring the steam flow to about 15 x D, where D is the diameter of the pipe in inches. Flow measurements that do not provide accurate results with this piping constraint should be eliminated.
Because the purpose of measurement in this example is custody transfer, the device’s installed performance is an important consideration for selecting the flowmeter. Parameters to quantify installed performance include installed accuracy (not just published accuracy) and overall measurement error. For example, it is desirable to reduce the overall error by having built-in temperature and pressure compensation for accommodating changes in operating conditions.
With tighter budgets, overall installed costs and lifecycle costs (not just the initial purchase price) are also important considerations. For example, flowmeters that require installation of auxiliary equipment for providing temperature and pressure compensation will likely result in higher installed cost. Flowmeters that require frequent flow-sensing device replacement or require frequent calibration may also result in higher lifecycle costs.
To reduce energy usage and costs, the permanent pressure drop across the flowmeter should be minimized for this application. Thus, after applying the aforementioned requirements as filters to the potential evaluation criteria, the relevant selection criteria for this application are:
- Installed performance
- Installed cost
- Permanent pressure drop
- Straight pipe length requirement/constraint.
Selecting flowmeter products for comparison
Flow sensing technologies for measuring steam flow in a custody transfer application include DP, vortex shedding, turbine, variable area, and Coriolis. Considering user preferences for DP-measurement-based flowmeters and to simplify the product comparison process for illustrative purposes, the best-in-class product selection technique focuses on comparing various types of DP flowmeters. Although these flowmeter product selection techniques compare DP-based flow measurements, they apply to any type of flow measurement technology.
Table 2: DP flowmeter comparisons
Based on the relevant selection criteria for the custody transfer steam flow measurement example, four DP flowmeter types are selected for comparison (see Table 2).
Installed performance: Accuracy, discharge coefficient variation, and dynamic compensation are parameters that could affect flowmeter performance and must be specified and evaluated over the entire flow range. Transmitters specified as a “percent of span” are typically limited in flow turndown capability. Transmitters specified as “percent of reading” perform better over wide flow turndowns. Using dynamic compensation for variations in pressure and temperature typically improves performance and reduces error. In addition, full compensation is often required for a repeatable flow measurement.
Based on the system performance comparison, DP flowmeter 2 and DP flowmeter 3 are ranked best because they provide compensated flow. DP flowmeter 4 is ranked second and DP flowmeter 1 is ranked fourth because these two flowmeters do not provide flow compensation (see Figure 5).
Installed cost: Some installation techniques such as direct flowmeter mounting are more reliable and cost less by eliminating unneeded equipment, in this case, impulse lines. They also reduce costly leaks. Based on a relative installed cost comparison, DP flowmeter 1 is ranked best because it offers the lowest installed cost (see Figure 6). However, its flow measurements are less accurate because it is not compensated for temperature and pressure variations. DP flowmeter 3 and DP flowmeter 4 are ranked second because they are direct mounted. DP flowmeter 2 is ranked fourth because it has the highest installed cost.
Permanent pressure loss: Some flow sensing techniques, such as orifice plates, inherently have greater permanent pressure loss compared to others, such as averaging Pitot tubes. In many flow applications, increased pressure losses due to the presence of the flow sensor may not be acceptable because it increases pumping/energy costs; increases sizes/capacities of compressors, pumps, and boilers; and reduces throughput. A permanent pressure loss comparison of the four DP flowmeters reveals that DP flowmeter 4 provides the least pressure drop, especially when the flow rate is at the full scale value (see Table 2).
Straight pipe requirements: In most plants, re-piping is seldom justified to comply with straight pipe run requirements. Thus, the presence of physical constraints such as 15 x D straight pipe length, for example, may eliminate the use of certain flowmeter types.
The relative straight pipe requirements comparison listed in the product selection matrix reveals that DP flowmeter 3 is ranked best, DP flowmeter 4 is ranked second, DP flowmeter 2 is ranked third, and DP flowmeter 1 is ranked fourth for this criterion (see Table 3).
Table 3: Product selection matrix
Product comparison results: The final flowmeter selection from the product selection matrix depends on the purpose of measurement (see Table 3). In one application, the primary criteria for selecting the flowmeter device may include installed performance and straight pipe requirement. Secondary criteria include permanent pressure drop across the measurement device and overall cost. DP flowmeter 3 is the best product fit for this application.
If the primary criterion is permanent pressure drop across the measurement device, then the DP flowmeter 4 is the best product fit because it has the best rating for the selected criterion.
Each flow sensing technology and each flowmeter has advantages and disadvantages. Based on these advantages and disadvantages, the best-in-class flowmeter selection methodology prioritizes the selection criteria and matches flowmeter candidates that have the best fit.
Brian Fretschel is a global marketing engineer for the Rosemount Measurement Business Unit of Emerson Process Management. He is part of the Pressure Marketing organization with focus on various DP flow and multivariable products. Fretschel has been with Emerson for two years and has experience with all of Rosemount’s pressure products. He has a BS in Mechanical Engineering from the University of Minnesota, Duluth.
Amy Johnson is the director of pressure marketing for the Rosemount Measurement Business Unit of Emerson Process Management. She is part of the Pressure Marketing organization with focus on DP flow, DP level, and manifold products. Johnson has been with Emerson for 30 years and has experience with all of Rosemount’s flow products. She has a BS in Mechanical Engineering from the University of Wisconsin and an MBA from the University of Minnesota.