Improving Accuracy in Older Flow Meters with Flow Conditioners
By Don Lundberg, Sr Member Technical Staff
The Vortab Company
Many plant managers are today being challenged to squeeze more product throughput at reduced operating costs from their aging facilities. One of the potentially simplest and often least expensive solutions is to measure air, gas and liquid flow more accurately. Plant upgrade projects that focus on the continuous improvement of flow measurement and control can trim overall production costs by eliminating waste and reducing maintenance costs through:
• Higher product through-put and yield
• Reduced use of consumables
• Lower energy costs
• Simplified maintenance (avoiding unplanned maintenance)
• Safe operations for employees
• Environmental compliance
This approach all goes back to that simple truism, which we’ve all heard so often that we sometimes overlook it: “You can’t control what you don’t measure.” To go one step further, let’s add “accurately measure” to the formula for process control cost reduction.
Analyzing flow measurement
The best way to start is by analyzing the different medias and flow instruments already in place, with an eye toward those that are affecting the most costly processes. You’ll want to prioritize your search by potential cost impact (not necessarily fluid volume) so that you achieve the most significant return-on-investment.
While every process and plant is somewhat different, there are some common critical measurement applications necessary for efficient operations:
• Pump protection
• Plant gases distribution
• Fluid additive/injection monitoring
• Compressed air consumption
• Boiler fuel gas and air optimization
• Tank blanketing
• Analyzer flow assurance
• Stack gas monitoring (CEMS,QAL1, MCERTS)
• Flare gas measurement
Low or poor measurement accuracy in these applications can be very expensive over time. The next step is to audit the efficiency of the flow instruments in place that are performing these critical application fluid measurements. You’re going to need to ask yourself a lot of questions about your plant’s flow meters and flow switches.
The next step is to determine which flow measurements are the most critical to efficient process throughput and the quality of the end product. Are you primarily measuring air, gas, liquids or steam? Which fluid is the most expensive to generate and move through the plant?
After you’ve identified the most critical processes and prioritized their costs, you’ll then want to look at the flow meters and flow switches already in place. If your plant moves liquids, gases and steam, then you’re likely to be looking at several different flow sensing technologies.
Why is understanding the various flow sensing technologies important? There are many different types of flow meter and flow switch sensing technologies. Some are exclusively for air or gas. Some are better at liquids, but don’t measure steam or gas. Do you have the best flow sensing technology in place for the media that you need to measure?
Now that you know the types of flow meters and flow switches that are in place around the plant, then ask that very impolite question: How old are they? Many flow instruments are designed to last for decades, but that doesn’t mean a newer technology won’t perform more accurately or consistently. Is it worth the cost to replace your older flow instruments with newer technology devices?
Be sure to ask yourself this question: Do I have flow measurement instruments everywhere I need them to optimize plant process control, ensure a safe operating environment and to meet environmental or other regulatory requirements? To optimize a process, sometimes multiple points of measurement are helpful to compare or confirm accurate flow rates or totalized flows.
But before you make any expensive decisions about upgrading flow instruments, you might want to think about implementing flow conditioners to enhance existing flow instruments. Improving the flow measurement accuracy in your process control loop doesn’t always require a new flow meter or flow switch (Fig 1).Flow conditioners are another cost-effective option to consider.
An Alternative Solution
One of the leading causes of inaccurate flow measurement is asymmetrical and/ or swirling flow conditions in the upstream and downstream pipe runs from the flow meter or flow switch (Fig 2). Asymmetry and swirl “confuse” many flow sensors because their accuracy depends on a steady-state uniform flow regime.
For example, several popular liquid flow meter meters, including differential pressure (dP), magnetic, ultrasonic and turbine, feature sensing technologies that can be significantly affected by swirl and asymmetrical flow conditions in the pipe. Depending on the particular technology, these meter sensing technologies require a minimum of 6 to 10 upstream and up to 5 downstream straight pipe diameters for accurate measurement.
Unfortunately, most process industry plants are rich with bulky equipment and other real estate limitations, which make it difficult to achieve the required pipe straight-run configuration necessary for accurate flow measurement with these popular technologies. Many times too, unrelated plant upgrades and retrofits require adding more flow meters in locations that are difficult to accommodate without flow conditioning devices because of their straight pipe run requirements.
About flow conditioners
Flow conditioners reduce straight pipe diameter requirements and can correct the flow profile distortions and swirl caused by upstream flow disturbances. These unpredictable flow profile variations are neutralized by a well-designed flow conditioner to present a consistent and predictable outlet flow profile to the flow meter that results in accurate and repeatable flow measurements.
The different types of low conditioner designs include: honeycomb vanes, perforated plates, tabs, tube bundles and vanes. They all have their advantages/disadvantages, depending on the viscosity of your particular process media, the metering technology, pipe layout, cost requirements, etc. Considering your process requirements will likely shorten the list of potential candidates. It’s especially important to think about the viscosity of the process media when selecting a flow conditioner. Some technologies are prone to clogging in slurries, for example, and/or contaminant build-up in dirty gas processes.
The installation of Vortab Flow Conditioners (Fig 3) with their tab type design offers extremely low pressure drop and provides significant energy cost savings over other flow conditioning technologies while helping to improve overall flow meter accuracy. They help maximize attainable plant product throughput by improving measurement, which also can reduce operating costs. There are four different Vortab flow conditioner designs to accommodate a variety of process requirements. The insertion sleeve style is the most cost effective version. It can be installed directly into existing pipe or specially designed to meet unique piping configurations.
The meter run style tab flow conditioner offers a complete, simple pipe section replacement for new and existing piping systems. It is a seven pipe diameter long spool piece comprised of three pipe diameters of flow conditioning internals and a built in downstream settling chamber. The short run style tab type flow conditioner offers a simple flow conditioning pipe section replacement for new and existing piping systems. It is a three pipe diameter long flow conditioning spool piece with just the tab flow conditioning internals. The elbow style tab type conditioner is a 90° long radius elbow. It offers a simple flow conditioning elbow replacement for new and existing piping systems. It not only has the ability to isolate irregular velocity profiles and swirl normally encountered in a single elbow, but it also has the ability to isolate any irregular velocity profiles entering the flow conditioner.
How they help
The process team responsible for an oil/gas production field needed to add new waste gas flow meters to a flare gas unit to meet environmental requirements. Unfortunately, the existing piping layout couldn’t accommodate their chosen flow meter’s minimum piping requirements. Adding a tab-type meter run style flow conditioner to the meter assembly eliminated the required straight run for accurate measurement. In a cramped wastewater treatment plant with a big expansion project, new air flow meters were required in the aeration process for efficient and cost-effective pumping of air into the treatment ponds. Adding a tab-type meter run style flow conditioner saved valuable plant real estate, reduced the piping costs and allowed room for additional equipment.
Aboard an offshore oil pumping station, the station required a new pump to increase capacity. There was no room for the pump’s required straight run and no way to expand the platform to accommodate the newly identified piping requirements. Placing a tab-type elbow conditioner in the elbow itself solved the space problem with a large cost savings and freed up real estate for other possible uses.
When you’re looking for ways to cut process costs, be sure to look at enhancing the accuracy of your flow instrumentation. That doesn’t always mean purchasing a new flow meter or a new switch, though that may be the best solution. Can you get the same results by adding a flow conditioner instead? Or if you must purchase a new flow instrument, can a flow conditioner reduce your required installation space, piping and total installed cost?