Low-flow meters are designed to provide accurate monitoring and control of very low liquid flow rates that can be problematic for traditional flowmeters. Many industries rely on low-flow meters to ensure the proper flow rates of liquids in processes for batching, water separation, paint circulation, fuel measurement and chemical injection.
For example, in the oil and gas industry, a chemical injection system uses low-flow meters to calculate the correct amount of chemicals such as antifoaming scale inhibitor and methanol for injection into oil wells. Proper chemical dosing ensures the well system runs efficiently and increases productivity at minimal cost. Dosing too much chemical results in overexpenditures, while underdosing may require premature maintenance and underperformance of the well. Low-flow meters are critical to ensuring correct dosage for accurate and cost-effective process operations.
Meters for liquid applications
A variety of mechanical and electronic low-flow meters are available on the market for liquid measurement. Most commonly used are mechanical flowmeters that feature some type of moving part such as a gear, turbine or rotor to measure volumetric flow or velocity as liquid passes through it during a set interval of time. For instance, a gear flowmeter incorporates two gears that mesh with each other. The gears are manufactured to tight tolerances with small internal clearances to minimize any fluid slippage (internal leakage around the gears). As the fluid flows through the meter, the gears turn at a speed proportional to the volumetric flow rate.
For applications with caustic chemicals, dirty fluids or where flow measurement is a critical parameter of an application, electronic flowmeters operate without moving parts that can wear over time and shorten meter life. A Coriolis flowmeter is an example of an electronic flowmeter that can measure volume and mass flow for applications.
Resolving conditions that affect accuracy
Low-flow measurement presents different challenges because of the small, incremental assessment of liquid flow. Installers must be aware of possible conditions that can affect flowmeter performance and life and how to address them.
Noise — In most electronic meters, lower flow rates equate to smaller signals for flow rate determination. Low signal strength can affect repeatability in producing consistent measurement. Because noise can interfere with signals, installers should minimize potential noise interruptions by ensuring power supply lines are well-regulated and clean. Good cable-shielding also prevents the introduction of external electromagnetic/radio- frequency interference into the measuring circuit.
Vibration — In addition to noise, shocks and vibration caused by machinery can affect repeatability in mechanical and electrical meters and more quickly wear mechanical flowmeters, with bearings experiencing differences in load. Pulsating flows also can trigger false readings. While vibration dampeners can address the problem with vibration, pulsation dampeners can remove hydraulic shock to enhance the reliability of flow measurements and instrument life.
Resolution — In low-flow applications, mechanical meter gears must be small to obtain good resolution. The smaller the measurable flow rate, the smaller the gear size requirement. Too large a gear size can cause too much liquid to slip through. In mechanical gear meters, slippage of fluid past the gears can account for inaccuracies in measurements since fluid that slips past gears is not measured. Along the same lines, turbine blades will not spin if enough flow is not present to cause rotation. And if electronic meters are too oversized, low flows can cause “noise” in the flow signal. Selecting the right-sized meter becomes important to ensure accurate flow measurement.
Accuracy — Accuracy is proportional to the number of measurements collected for flow calculation (1 percent accuracy requires a capture of at least 100 readings). Therefore, low-flow meters must be able to process a certain number of signals to calculate flow accurately. For example, if a measurement accuracy of .05 percent is to be achieved, the meter must capture a minimum of 200 readings or pulses to determine such accuracy (1/200 to .05 percent).
Blockage — Blockage caused by solids and slurries can wear on the flowmeter and diminish accuracy. Frequent downtime caused by clogging and heavy maintenance also adds to costs and inaccurate flow measurement. Filters can prevent solids from entering, jamming and damaging gears.
Leakage — During low-flow conditions, the mass of a gear meter and the inherent friction interface of its rotating elements can negatively impact its operation, causing more fluid to leak past the gears rather than turning them. This phenomenon can occur when a meter starts from a zero-flow condition. To avoid slippage at low flow rates, installers may have to increase flow rate at startup, then back it off once the right momentum is achieved. This is an example of Newton’s first law: An object at rest tends to stay at rest and an object in motion stays in motion with the same speed unless acted upon by an unbalanced force. Installing gear meters so internal shafts are in a horizontal position further reduces rotational friction and improves low-flow measurements. Determine startup effect and make adjustments.
Air — Ensure all air is purged properly when first installing mechanical gear meters for low-flow measurement. Even a small air bubble can prevent small gears from rotating. Make sure initial startup flow is high enough to purge all air.
K factor — Should the flow profile of a pipeline transition from turbulent (high velocities) to a laminar flow regime (low profile), flow measurement technologies may experience a change in K factor that can result in low resolution. Always consult with the meter manufacturer if the Reynolds number will be between 2,100 (laminar) and 4,000 (turbulent flow).
Liquid composition — Lubricating properties and entrained solids will affect how easily fluid flows through a meter. Understanding the composition of the liquid is important when choosing the right meter for the job. While some gear flowmeters remain in operation for 40 or more years within calibration when metering lubricating oils, the same meter may only last days or hours with metallic flake paint. With these filled fluids, an electronic meter with no moving parts would be a better choice.
Pressure — A pump must provide sufficient pressure to move fluids through a pipeline at a required rate that maintains flowmeter operation. Select the right pump that supports the fluid delivery requirements of the application. In addition to fluid delivery, the choice between a piston, gear and vane pump can depend on the characteristics of the medium, dirt tolerance, noise, size, weight and pressure range. While piston pumps have the highest pressure capabilities, gear pumps are the lightest and vane types are the quietest. If in doubt, refer to the manufacturer to determine the best pump match for an application.
Because most users install their own meters, mistakes are not uncommon. These insights can help serve as installation guidelines in achieving better performance and avoiding operational challenges associated with low-flow meters. Always follow manufacturer recommendations to get the best results.
Chris Husson is senior design engineer at AW-Lake Company, responsible for quality systems. With more than 29 years in the flowmeter industry — all with AW-Lake — he has designed high-resolution, low-flow gear meters, gear meter electronic sensors and interface electronics. Scott Honsberger is senior mechanical engineer, Dave Hahn is R&D engineer and Mark Iverson is general manager for AW-Lake Company. Visit aw-lake.com for more information.