Mass Flow Meters

How do I select a gas flow meter that will provide the performance I need for my application?

Match your application to the appropriate measurement technology.  Accurate flow measurement starts with selecting the best flow meter technology for your application. Every application has a set of requirements that narrows the choice of technologies. For example, thermal dispersion might work best in a dirty process gas, like biogas, because this technology provides no-moving-parts reliability, direct mass flow measurement, and wide range ability. However, positive displacement might be the best technology choice for the custody transfer of natural gas.

An Instrument Specification Sheet is a good place to find information that will help select the most appropriate flow meter technology for an application. This sheet identifies the application's process temperature and pressure, gas composition, piping configuration, accuracy requirements, and more. Narrow your technology search by matching the application information against the suggestions shown in the accompanying Gas Flow meter Selection Chart.

Now forward your application information to vendors that offer the most appropriate flow meter technology. Be sure to include as much information about the application as possible and highlight your realistic performance expectations. Do not request 0.5 percent accuracy if the application needs only 5 percent accuracy. Ask these vendors to evaluate your application and provide a product recommendation. Use the information you receive to revise your specification (if necessary), finalize your preferred vendor list, and prepare your request-for-quote.

What can be done if I can't meet the flow meter's recommended installation requirements?

Use a flow conditioner.  Flow meter manufacturers usually provide "recommended installation" instructions with their products. These instructions specify the minimum amount of straight, unobstructed pipe that should be located upstream and downstream of the flow meter. It's often not possible to provide this required straight pipe due to space constraints and economic demands. Consequently, flow meter accuracy suffers as a direct result of an inadequate amount of straight pipe. If high flow meter accuracy is demanded, even with inadequate straight pipe, a flow conditioner should be used. There are many types of flow conditioners available (vanes, tube bundles, perforated plates); however, the new Vortab flow conditioner offers several advantages. The Vortab's unique tab design provides excellent isolation from disturbances, little pressure loss, and immunity to fouling.

I need to accurately measure gas flow in a very large duct.  What type of mass flow meter should I specify ?

Specify a multipoint, insertion flow element.  Single-point, insertion flow meters are a sensible choice when flow rate measurements must be made in large size pipes or ducts. However, because gross flow velocity profile distortions are likely in very large ducts, a single-point flow meter cannot accurately measure these highly skewed flow conditions . Selecting a technology that allows for the averaging of multiple measurements provides a reasonable solution to this problem.

When making multipoint air or gas flow measurements in large ducts, thermal dispersion technology offers significant benefits that place it far ahead of the other technologies. Thermal technology offers direct mass flow measurement (without separate temperature, pressure, or density measurement), fouling immunity, low flow sensitivity, and wide rangeability.

Why isn't this mass flow meter indicating flow rate, temperature or totalization correctly ?

Check the flow meter's functionality, specification, installation, and validate the comparison standard.  If you suspect that your newly installed flow meter isn't indicating correctly, start your troubleshooting procedure with a flow meter functionality test, but don't stop there. Review your application parameters against the Gas Flow Meter Selection Chart and seek advice from the product vendor if your flow meter appears to have been incorrectly specified. Next, confirm that the flow meter has been installed per the manufacturer's installation guidelines. Finally, check the measurement method or calculation that was used as the reference or comparison standard. Many "reference" methods are, at best, estimates (generic blower curves, valve positions, operator experience, stoichiometric calculations). As a last resort, make zero and span adjustments.