Common Mistakes in Choosing Flow Meters

Flow measurement is an important aspect of operation in today’s industrial plants. Despite there being a wide range of possible instruments for any one flow metering requirement, the achievable performance of each one varies considerably depending on the operating conditions under which they are required to work. Each meter type has considerably different measuring ranges, sensitivity to process conditions, installation demands and media-quality requirements. This article examines common mistakes in the selection of flow measurement devices. Experience has shown it is wise to consult with a knowledgeable instrumentation supplier prior to making a flow meter purchase. The effort spent learning about basic flow measurement techniques and available meter options will help ensure a successful application once the equipment is installed.

Introduction

In modern industrial plants, personnel need to make smarter decisions faster by capturing, managing and analyzing the right data at the right time. These facilities rely heavily on flow processes, and thus accurate and reliable measurement technology is vital to the efficiency and safety of their operations (see Image 1). Flow meters are an integral tool for measuring the flow of liquid, gas or a mixture of both in applications used in food and beverage processing, oil and gas plants, and chemical/pharmaceutical factories, among other industries. Inaccurate flow measurements lead to poor flow and related controls, poor quality products, and incorrect fiscal measurement, resulting in inadequate allocation of revenue among partners. In addition, an unsafe process due to wrong quantities could lead to facility damage and create an unsafe environment. When evaluating a flow measurement device, it is just as important to know what an instrument cannot do as it is to know what it can do before a final selection is made. Each flow meter has advantages and disadvantages, and the degree of performance satisfaction is directly related to how well its capabilities and shortcomings are matched to the application requirements. Often, users have expectations of a flow meter’s performance that are not consistent with what the supplier has provided. Experience has shown that a thorough evaluation should be done using a logical, step-by-step approach before making a decision on the type of flow meter. Because of the importance of meter selection, it is crucial to allocate sufficient time and resources for this evaluation.

Typical Evaluation Errors

With countless flow measuring instruments currently available, making an optimal meter choice is often a challenging task. Industrial organizations have traditionally based their selection decision on price and published product specifications. However, users have also discovered:

• Flow meter selection based on the lowest price often results in the highest installed cost.
• Choosing 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.

When users are sizing flow meters in general they tend to use legacy information, not the most recent data for making decisions. They might end up with the wrong size meter because they haven’t looked at the latest innovations. One of the biggest problems industry-wide is the lack of qualified instrumentation engineers. Few companies have enough people with adequate knowledge and time to do these tasks as they should be done. As companies move more of their workload out, they no longer have the people with process-specific know-how, so they rely on engineering firms to do this work for them. Typical technical errors in flow meter selection involve fluid characteristics (single or double phase, viscosity, turbidity, etc.), flow profile (laminar, transitional, turbulent, etc.), flow range and accuracy requirements. Mechanical restrictions and output-connectivity requirements are additional key factors. All too often, instrumentation purchasers fail to define flow meter suitability based on criteria such as: process parameters, desired accuracy and repeatability, unidirectional or multidirectional flow, upstream and downstream straight run requirements, piping geometry upstream of a flowmeter, filter requirements, continuous or totalized readings, analog or digital signals, possible impact of vibrations on measurement accuracy, data update frequency and meter access for possible maintenance. In addition, some flow meter users overlook important considerations ranging from historic failure rates and plant personnel familiarity and confidence level with specific flow meter types to calibration and maintenance ease, spares availability, etc. They may not consider both tangible and intangible factors along with total installed cost as part of an overall technology comparison.

Making the Right Choice

Companies purchasing flow meters must remember accurate instruments cost more based on their capabilities. It is better to locate the type of meter suited to a specific application before sacrificing features for cost savings. Users should closely evaluate their process conditions, including flow rates, pressure and temperature, and operating ranges. They should not be swayed by lower priced alternatives that would be applied outside of their capabilities. Users are wise to look at the “big picture” when making an instrument purchase and plan to assign costs to the installation, set-up, preventative maintenance, repair and running costs of a particular flow meter during the decision-making process. When considering installation costs, be aware that many flow meters require a straight length of pipe or channel upstream and downstream of the meter to ensure a stable and repeatable flow velocity. The infrastructure and/or pipes required in this situation could easily cost more than the meter itself. Each of the popular flow measurement technologies has pros and cons under real-world conditions. For example, positive displacement flow meters are generally the best option for the greatest certainty of measurement. Electromagnetic meters provide for the widest flow range, and turbine meters are usually the best choice for the highest short-term repeatability. Despite their high initial cost, Coriolis meters are ideal for measuring particularly viscous substances and anywhere that the measurement of mass rather than volume is required (see Image 3). Important questions to ask when evaluating available flow meter technologies include: 1. What exactly is the flow meter expected to accomplish? Identifying the place, or environment, where the flow meter will be used can help in determining the size and range of the device that will be necessary to achieve required accuracy. 2. What are the flow rates and characteristics of the liquids/gases? By profiling the flow of a fluid or gas through the process piping, it is possible to determine how it behaves and from there narrow down the choice of flow meters to those best able to cope with the conditions of the application. The flow profile of a fluid will vary according to whether it is Newtonian or non-Newtonian. Newtonian liquids have a directly proportional relationship between the pressure of the liquid flowing through and the resistance, or shear force, caused by the fluid sticking to the pipe walls. 3. What size flow meter will be sufficient for the service requirements, and does the meter’s material or construction have any impact on the process medium? A dimensionless unit called the Reynolds Number influences the performance of flow meters. It is defined as the ratio of the liquid’s inertial forces to its drag forces. Depending on the piping configuration and other installation conditions, the flow may be either turbulent or laminar. 4. What real-life conditions may contribute to equipment downtime? There are a host of environmental conditions that may affect everything from flow meter selection to reliability of operation. In addition, factors tied to the installation of the meter and the lifespan of the equipment might also weigh in on the final instrument selection. 5. What is the purchase price of the flow meter and related equipment, and what about installation, maintenance and operational costs? Regardless of their technology preference, flow meter users should examine long-term ownership costs. A flow meter with a low purchase price may be very expensive to maintain. Alternatively, a meter with a high purchase price may require very little service. Lower purchase price does not always represent the best value. Generally speaking, flow meters with few or no moving parts require less attention than more complex instruments. Meters incorporating multiple moving parts can malfunction due to dirt, grit or grime present in the process fluid. Meters with impulse lines can also plug or corrode, and units with flow dividers and pipe bends sometimes suffer from abrasive media wear and blockages. Changes in temperature also affect the internal dimensions of the meter and require compensation. The need to recalibrate a flow meter depends on how well the instrument fits a particular application. If the application is critical, meter accuracy should be checked at frequent intervals. Otherwise, recalibration may not be necessary for years because the application is non-critical, or nothing will change the meter’s performance. No matter the chosen flow measurement solution, overall system accuracy will not exceed that of equipment used to perform the meter calibration. The most precise flow calibration systems on the market employ a positive displacement design. This type of calibrator, directly traceable to the National Institute of Standards and Technology (NIST) via water draw validation, provides total accuracy of at least 0.05 percent. The best and easiest way to avoid making mistakes during flow meter selection is to use sizing and selection tools supplied by the meter manufacturer. Companies anticipating a flow meter purchase should also consult with a knowledgeable instrumentation supplier in the early stages of a project.

Conclusion

Selecting the right flow meter for an application from the various technologies and designs on the market can be rather daunting. By understanding the properties of the fluid being used, knowing the application’s flow rates and required measurement accuracy, and being aware of physical constraints and operating conditions, the user can narrow down the choices faster while avoiding selection mistakes that can significantly impact their operational and business performance.