Avoid these Flowmeter Sizing “Gotchas”

When setting up flowmeters, a common error is to use incorrect operating units such as gallons per hour when it should be gallons per minute.

When specifying flowmeters, accompanying software can help avoid common errors that could affect process performance.

By Chris Costlow, Yokogawa Corp.

Sizing is one of the top issues that affect all flowmeter technologies. Inaccurate readings from incorrectly sized flowmeters can cause unsafe conditions and result in out-of-spec products. Most flowmeter manufacturers will tell you that incorrect sizing is a common cause of the returns and replacements they process. Even the most experienced engineers often overlook the small details that determine selection of the correct model for an application. 

With a majority of industrial-automation instruments, there are a few flowmeter models or ranges that fit most process conditions, but one size does not fit all. Flowmeter manufacturers often assist users by offering a sizing tool. However, even the most advanced systems cannot prevent users from entering incorrect information or using process-design elements that lead to improper sizing. 

Forgiving & unforgiving flowmeters

The main flowmeters on the market are the coriolis-mass, magnetic, vortex, ultrasonic, and variable-area meters. Several other technologies are also available: turbine, thermal mass, positive-displacement, and a variety of differential-pressure (DP) solutions. This article focuses on the top flowmeter technologies and categorizes them as being forgiving (magnetic and ultrasonic) or unforgiving (coriolis-mass, vortex, and variable area).

Forgiving flowmeters do not require sizing software to select the correct unit for the process. Manufacturers of these flowmeters provide accuracy performance tables that help select the correct device. Typically, a table lists the available meter sizes and the flow velocity those size meters will have at certain accuracies. As an example, a magnetic flowmeter specification might say that a 1- to 16-in. meter with a flow velocity between 0.5 and 33 ft./sec. will have a +/- 0.3% rate of accuracy. If the process is within those boundaries, the size of the meter does not matter in terms of maintaining accuracy.

With forgiving flowmeters, even though they do not require sizing software, there are still some mistakes to avoid. Magnetic flowmeters are designed for liquids, not gases, and require that the process be conductive to be measured correctly. There are some capacitive magnetic flowmeters that measure low-conductive liquids but, typically, traditional magnetic flowmeters (magmeters) require the process to be 1 μS/cm or larger.

Ultrasonic meters, i.e., clamp-on Doppler versions, are limited to clean liquids and gases to guarantee the ultrasonic energy signal transits through the liquid between the transducers and delivers an accurate measurement. Additionally, magmeters and ultrasonic flowmeters require a full pipe, with upstream and downstream pipe diameter requirements for stable flow profiles. Before selecting one of these flowmeters, verify the general specifications and ensure this type of meter is best for your application.

Unforgiving flowmeters require appropriate sizing to correctly select the flowmeter for the application. Fortunately, most manufacturers offer sizing software that helps users ensure they are selecting the correct flowmeter. These programs identify critical fields as mandatory entries, allowing built-in AI to analyze all entries before listing possible selections for the user to consider. 

Some manufacturers’ sizing software generates an accuracy table, plotting the accuracy curve of the flowmeter’s performance over the flow range. This allows users to select the appropriate flowmeter for the flow conditions within their processes. Although many of these software programs are free and open for end-users, it is recommended that the manufacturer or a sales representative review the sizing sheet before placing an order for a flowmeter. 

Gotchas

When teams size flowmeters, they need to watch out for the gotchas—those factors that tend to catch a team off guard and cause issues in the near and long terms. Some are obvious, but some are a bit tricky.

Gotcha #1: Incorrect application condition data
Sizing meters for designs instead of actual conditions might be the number one issue across all flowmeter technologies. For example, the process may be designed to run at 1,000 gpm, but it never operates at that rate and often operates between 200 and 400 gpm. This is called over-sizing and leads to many inaccurate readings, especially for manufacturers that base accuracy on flow span instead of flat-spec readings. To avoid issues, use actual flow conditions.

Gotcha #2: Incorrect media viscosity and density
Take care to enter the correct media viscosity and density. Some manufacturers have a built-in table of popular liquids and gases. Make the appropriate selection and the software automatically populates the viscosity and density data. This is very helpful when entering the process temperature, as the predefined media compensates for the temperature change based upon the effects of the media’s relationship to temperature.

When a medium is not in the drop-down list, you must manually enter the viscosity and density in the sizing software as it relates to the actual conditions of the process. If entering data manually, remember that, as temperature changes, most media change. This is also a concern when a mixed medium is being measured. If the pre-defined list does not have the correct medium the end-user is using, then do not select something by guessing and hoping for the best. Confirm the media and data specifications to ensure the correct information is being entered into the sizing software.

Gotcha #3: Incorrect engineering units
Another gotcha is selecting incorrect engineering units. It’s quite surprising how many selections are easily overlooked. A common one is sizing a flowmeter for gph (gallons per hour) versus gpm (gallons per minute). This is like thinking you are drinking from a garden hose but finding out it is a fire hose. Double check the engineering units with the end user. Is the process in SI or metric units? Additionally, confirm the measuring rate is correct and the flowrate expected is suitable for the meter type selected.

Gotcha #4: Selecting the first option provided by sizing software
Sizing software provides users a list of particular meters based on the information entered. Sometimes slight adjustments make all the difference in finding the right meter for the process. In fact, some flowmeters offer similarly priced options based upon the process conditions entered in the sizing software. The vortex meter, for example, typically offers several minimum and maximum flowrates and multiple size selections. Often, increasing or decreasing the minimum flowrate will open additional selections, thus selecting the best meter for the cost.

Another consideration is pressure loss. For example, sizing software for the coriolis flowmeter will often provide a selection of several models, based upon the process data entered. A gotcha that is often missed is ignoring the pressure loss for the maximum flowrate of each selectable option. Keep in mind the process and whether you can afford the drop in pressure based on the maximum flowrate. It is not recoverable after the meter. A smaller maximum may be required to maintain a minimum acceptable pressure loss.

Gotcha #5: Lack of experience
In today’s working environment, there are fewer workers, and retiree knowledge might not have been transferred to newer workers. Digital transformation has helped ease this transition, but repetition and experience are golden.

Sizing software has come a long way. It can validate user entries, attempts to ensure the entered data is accurate, and might compensate, to some degree, for a lack of experience. To offer more assistance, software has warnings, cautions, and even help boxes to simplify correct flowmeter selection. Regardless, the best practice is for the manufacturer or sales representative to confirm that the sizing has been done correctly before ordering the product.

These five gotchas and additional background will guide teams as they size flowmeters for the real conditions of their processes. As teams learn the practice, their expertise grows. It always helps to have an experienced partner by your side to assist. EP

Chris Costlow is the Flow Product Manager at Yokogawa Corp. of America, with more than 30 years of Industrial Automation experience, including application engineering, product line management, marketing, and product development. He holds a degree in Business Management from the University of Phoenix. For more information about flowmeter selection, visit yokogawa.com.