When it comes to mining and metals production, oil/gas refining, electric power generation and many other industries, engineers are always looking for ways to beat the heat by selecting plant instru mentation and controls that withstand rugged operating conditions.
Air/gas flow meters are no exception. While performance, ease of installation, maintenance and other criteria are all important, flow meters must always be evaluated according to their operating environment and process conditions, which often range from 260°C to 454°C in high temperature process industries. They must first be compatible with high temperatures if they will measure the flow of high temperature fluids or be located near process equipment that includes burners, boilers, furnaces, ovens, piping and stacks.
The measurement of hot air, gases and fluids is critical in many process plants to meet product quality standards, throughput rates, safety requirements, pollution monitoring regulations and more. If your flow meter can’t take the heat, then a whole range of nasty prob lems may occur that can include inaccu racy, poor repeatability, excess mainte nance/repair or a short life. The costs in terms of product quality and cost-of- ownership alone make it important to be sure that your flow meter is well matched to your process environment.
Not all flow meter technologies and devices are compatible or designed for high temperature operation. In some cases, high temperatures cause flow sensor measurement drift that leads to inaccuracy or repeatability issues. In other situations, the hardware may be fine but any changes to process tempera tures may result in reduced accuracy.
Choosing a flow meter
In selecting a flow meter for any applica tion, the first step is choosing the appro priate flow technology. There are multiple flow sensing technologies avail able, which include:
All these technologies have their advantages/disadvantages, depending on the media (air, liquid, gas or steam) and your application’s requirements. Some may be the only choice in certain media for your application. By looking at these factors, as well as your plant’s layout, environmental conditions, maintenance schedules, energy cost and Return On Investment, you will quickly be able to narrow the field to one or two best choices. When looking at any flow tech nology, after you consider the media, there are always several other factors to consider:
Accuracy and repeatability
Maintenance and Mean Time Between Failures (MTBF)
Accuracy and repeatability
You need to know the accuracy, repeata bility and process conditions for the flow meter that you plan to use. Most manu facturers provide the specifications for these parameters in water, air or a specific gas. Meter technologies have now evolved from mechanical devices, previously only capable of providing volumetric measurement. Newer flow technologies perform well in applica tions where changes in process tempera tures can greatly affect measurement accuracy.
Today, we have the choice of solid state meter technology that offers direct mass flow measurement, and gone are the days when we needed to install multiple devices with multi-variable transmitters to provide a calculated mass flow measurement at higher cost and risk. Solid State meters (including Thermal Dispersion) are able to operate at extreme process conditions, where other meter technologies fail, and provide the following benefits:
Operating to low flow rates of 0.25 SFPS
Wide range of flow to 1000:1 turn down, versus 10:1 to 100:1 maximum for other technologies
Very low pressure drop compared to other technologies
Wide range of process temperatures from -10°C to 454°C
Impervious to vibration and resistant to dirty gas flow
Thermal dispersion provides a gas flow measurement solution that is easy to install and virtually maintenance free.
With no moving parts or additional components it is inherently multivari able, measuring both flow and temperature. Thermal dispersion tech nology places two thermowell protected platinum RTD temperature sensors in the process stream.
One RTD is heated while the other senses the actual process temperature. The difference in temperature between the two sensors is measured and is directly proportional to the mass flow rate of the fluid.
Check that the accuracy, repeatability and flow range in the manufacturer’s specification aligns with your process media. Also, choose a supplier that can provide a NIST traceable meter calibration (at the factory) in the same media as required by your application to allow for quick delivery of a meter that requires no further field calibration.
Some flow meters are more straight forward than others when it comes to installation. For instance, a thermal dispersion flow meter can be hot-tapped directly into the process pipe versus other technologies where an inline configuration requires the piping to be cut and spliced in at least two places.
The more penetrations made into the pipeline or ductwork the greater the risk and increase in the complexity and overall cost of the installation.
Electric Power — Pulveriser Air Flow Monitoring: In coal-fired power genera tion plants, the measurement and control of mill air flow in pulveriser units is a constant challenge. A major factor in maximising plant efficiency involves accurately measuring air flow to control mill slugging, feeder run-back and reduction of coal spillage.
Typical applications involve high temperatures and dirty particulate laden air (coal dust). In addition, cramped plant layouts often lead to less than optimal pipe straight runs that result in irregular velocity air flow profiles. The process team at one major electric utility consulted Fluid Components International (FCI) about a solution. The recommendation was a high temperature multipoint thermal mass flow meter to replace the pitot tubes that had been in use.
With its sealed, no- moving parts thermal dispersion sensing element, FCI’s MT91 Flow Meter eliminated previous problems with fly ash that clogged and fouled the pitot tubes. The use of a multi-point sensing MT91 also provided the capability to average the air mass flow across several areas in the process piping for more accurate measurement.
Oil/Gas — Flare Gas Vent Monitoring: An oil company required a flow meter to provide an accurate gas flow measure ment on a vapour header line from its loading/unloading station a tanker port. Incorrect vapour flow rate measurements can cause an improper mix of gases or even non-combustion in the incinerator. If an improper mix occurs, the risk of releasing dangerous and toxic vapours into the environment increases. The release of these gases results in emer gency plant shutdowns, regulatory reviews and higher process costs.
The plant’s process team recognised there was considerable variation in the density of these mixed waste gases, and they needed to measure the gas mass flow rate (not volumetric). After contacting FCI, an FCI GF90 Flow Meter thermal mass flow meter was installed that was suitable for high temperature operations.
The flow meter accepts a process input signal from a densitometer and selects a flow calibra tion curve that matches the actual vapour composition, to maintain accu racy at all times under these difficult conditions. This configuration essentially provided a single meter, instead of three meters that would have been required using other technologies.
Your plant’s operating conditions will play a major role in accurate flow meas urement. When considering factors such as process conditions, climate (hot/cold extremes), and ease of installation you will find that some flow measurement technologies function well in extreme operating conditions including high temperatures and perform better over time. In addition, solid state, direct mass flow measurement is now available as a cost-effective solution for the most rugged applications.
FCI products are sold in Australia by AMS Instrumentation and Calibration.
[Stephen Cox is a senior member of the technical staff at Fluid Components International.]