Turbine Flowmeters: Simple Elegance
Available in sizes to 12 in. (300 mm) and capable of handling process pressures to 10,000 psi (689 bar) and temperatures to 1,000 °F (538 °C), turbine flowmeters are versatile enough for many of the world's industrial, chemical, petrochemical, pharmaceutical, and food and beverage applications.
|
Available in sizes to 12 in. (300 mm) and capable of handling process pressures to 10,000 psi (689 bar) and temperatures to 1,000 °F (538 °C), turbine flowmeters are versatile enough for many of the world’s industrial, chemical, petrochemical, pharmaceutical, and food and beverage applications.
The basic construction of turbine flowmeters is a bladed rotor installed in a flow tube. (See “Typical turbine flowmeter design” diagram.) As the flowing media (gas or liquid) passes over the rotor blades, a low-pressure area is produced on the backside of each blade making it rotate.
Performance influences
Basic turbine flowmeter technology has remained unchanged for at least 10 years, thus a great deal of information is known about influences that can affect its performance.
It’s this accumulated knowledge that makes turbine flowmeters highly repeatable and applied as master meters (provers) more often than any other flowmeter technology.
When used in liquid measurement applications, key performance-related influences include:
-
Cavitation;
-
Specific gravity; and
-
Viscosity.
When a liquid’s operating pressure is too near its vapor pressure, bubbles can form and collapse. This physical phenomenon is called cavitation, and it can cause serious damage to piping and equipment, including turbine meter rotor blades.
| Turbine flowmeters install directly in the flow path; avoiding cavitation is critical to performance. |
When liquid passes the turbine’s rotor blades, its velocity increases and creates a low pressure area on the backside of the rotor blades. When this low-pressure area is less than the liquid’s vapor point, bubbles form. As the bubbles move downstream, pressure recovery allows the bubbles to collapse (return to a liquid state). Collapsing of the vapor bubbles releases energy and produces a noise similar to what would be expected if gravel were flowing through the pipe. When the energy is released in close proximity to solid surfaces, small amounts of material can be torn away leaving a rough, cinder-like surface.
Besides the wear and tear, cavitation usually causes the rotor to spin faster than it should for the liquid flowing conditions, thus producing an inaccurate flow measurement signal.
“Maintaining system backpressure of two times the flowmeter pressure drop plus 25 times the product vapor pressure is sufficient to prevent turbine flowmeter cavitation,” said Barry Ellison, senior sales and developmental engineer with Sponsler Co.
Specific gravity is another factor affecting differential pressure across rotor blades. As the specific gravity of the liquid decreases, the pressure differential also decreases. Fluids with very low specific gravity and a low flow rate produce very low differential pressure across the rotor blades leaving very little energy to turn the rotor.
Turbine flowmeter manufacturers compensate for low specific gravity/low flow conditions by increasing the angle of the rotor blades.
A third performance influence is flowing media viscosity. Because viscosity is a measure of a liquid’s resistance to flow, its performance influence occurs in the space between the stationary meter body and the rotor blades and in the meter’s rotor bearings. Turbine flowmeter manufacturers compensate for viscosity influences in several ways, including adjusting the rotor blades’ shape and length, and/or adding a shroud around the rotor.
| As each rotor blade passes near the pickup coil it causes a deflection in the magnetic flux field. This change in reluctance generates a voltage pulse within the pickup coil. |
Each performance influence is well understood by turbine flowmeter manufacturers, and compensation means can be applied using physical design adjustments and/or modification to a meter’s K-factor. (The K in K-factor stands for the limits of knowledge and is a generally accepted way of expressing unknowns or the difficult-to-express effects of a correction factor.) Continued on p.41.
Bearing selection
Many bearing options are available for turbine flowmeters, including metallic and ceramic ball bearings and sleeve-type constructions of tungsten carbide and ceramics.
Ball bearings generally offer the highest accuracy, lowest cost, and widest usable range, thus it remains the bearing of choice in many fluids. Tungsten carbide and ceramic bearings offer the greatest durability in compatible service fluids.
Because the flowing fluid is expected to provide bearing lubrication, fluids providing high natural lubrication tend to prolong bearing life.
For applications where the fluids provide little or no bearing lubrication, Hoffer Flow Control offers units with self-lubricating ceramic ball bearings.
Regardless of a fluid’s ability to lubricate turbine flowmeter bearings, manufacturers use varying techniques to minimize bearing wear. For example, ball-bearing units contain non-metallic bearing retainers, and the sheer hardness of tungsten carbide and ceramic bearings tends to extend bearing life.
Bearing life is approximately inversely proportional to the square of bearing speed. Therefore, to prolong turbine flowmeter bearing life, it’s best to operate the flowmeter at lower flow rates. For example, if a flowmeter is operated at 33% of its maximum flowrate, bearing life will be extended by a factor of ten.
For turbine flowmeters using ball bearings, it’s recommended the bearings be inspected every six months.
Installation influences
Like most flowmeters, the accuracy of turbine flowmeters is highly dependant on the ability of the installation to ensure non-swirling conditions.
Even at constant flow rates, swirl can change the angle of attack between the fluid and the rotor blades, causing varying rotor speeds and thus varying flow rate indications.
Effects of swirl can be reduced or eliminated by ensuring sufficient lengths of straight pipe—a combination of straight pipe and straightening vanes, or specialized devices, such as Vortab’s flow conditioner—are installed upstream and downstream of the turbine flowmeter. (See “Installation rules of thumb” table.) Continued on p.43
Turbine flowmeters for liquid applications perform equally well in horizontal and vertical orientations, while gas applications require horizontal flowmeter orientation to achieve accurate performance.
When installing turbine flowmeters in intermittent liquid applications, it’s recommended that the flowmeter be mounted at a low point in the piping.
Turbine flowmeters are designed for use in clean fluid applications. Where solids may be present, installation of a strainer/filter is recommended. Also, because the strainer/filter can introduce swirl, it needs to be located beyond the recommended upstream straight pipe lengths.
Signal output
The signal produced by turbine flowmeters is a peak-to-peak voltage pulse. Pulses are most often generated using the principle of reluctance. (See, “Turbine flowmeter principle-of-reluctance operation” diagram.)
In the principle-of-reluctance method, a pickup coil is wrapped around a permanent magnet and installed on the exterior of the flow tube immediately adjacent to the rotor. Each rotor blade, as it passes near the pickup coil, causes a deflection in the magnetic flux field. This change in reluctance generates a voltage pulse within the pickup coil.
Electronics have significantly advanced turbine flowmeters in the past several years. Capability to count thousands of pulses per second, for example, improves accuracy and rangeability.
When installed in electrically noisy environments, or if the distance between the turbine flowmeter and the electronics exceeds 200 feet, signal conditioning may be necessary. Continued on p.45
Application examples
Besides the more traditional industrial applications of liquid and gas turbine flowmeters, companies like Hoffer, Omega Engineering, and Sponsler are successfully applying turbine flowmeters to less traditional applications:
-
Viking Yachts ( www.viking yachts.com ) produces a wide variety of ocean-going yachts. Because running out of fuel while at sea is dangerous, Viking needs to provide its customers with accurate information about a yacht’s range. Working in conjunction with Sponsler, Viking accurately measures a yacht’s diesel engine fuel consumption under varying operational conditions, thus helping to ensure yacht captains reach safe harbor with fuel to spare.
-
Transferring milk products from the dairy to a tanker requires the accuracy and repeatability of custody transfer measurement, and also requires sanitary conditions. Omega’s FTB series and Sponsler’s SP714 are precision sanitary turbine flowmeters designed to allow thorough cleaning of internal parts without disassembly.
-
The largest reserves of oil and gas lay below the ocean’s floor, but tapping those reserves introduces unique challenges at depths of over 10,000 ft., internal operating pressures above 10,000 psi, external pressures greater than 5,000 psi, and saltwater temperatures below 32 °F.
Hoffer is meeting these subsea application challenges with specially designed 5/8 in. and 4-in. turbine flowmeters. Turbine flowmeters continue to be a popular way to measure flow; and why not? They provide:
-
Wide flow rangeability;
-
Outstanding accuracy at low cost;
-
Construction materials that permit use with many process fluids;
-
Simple, durable, field-repairable construction;
-
Flexibility in connecting to associated electronic readout devices for flow control and computer interface;
-
Wide variety of process connections; and
-
Operation over a wide range of temperatures and pressures.
Comments? Email: dharrold@reedbusiness.com
Paddlewheel flowmeters are a low-cost substitute
When accuracy is less important, paddlewheel flowmeters represent a low-cost substitute to more expensive flowmeter technologies.
Available as insertion designs, paddlewheel sensors can accommodate line sizes up to 36 inches (914 mm).
When using the insertion design, the rotor and blades remain perpendicular, not parallel, to the flow stream. (See, “Typical insertion paddlewheel flowmeter installation” diagram.)
| Because insertion paddlewheel flowmeters barely intrude into the flow stream, accurate performance depends on attention to installation detail. |
A major advantage of the insertion-design feature is it makes paddlewheel flowmeters very tolerant of entrained particulates, thus strainer/filters are seldom required.
Because the paddles contact only a small cross-section of the flow stream, insertion depth of the rotor and assurance of a proper flow profile is critical to accurate, repeatable performance. To ensure proper insertion depth and rotor orientation, most paddlewheel flowmeter manufactures offer specially designed installation fittings.
What to do when viscosity varies
Ideally, when a flowing liquid’s viscosity varies, a positive displacement flowmeter will be used. However, if a turbine flowmeter is used, most manufacturers can develop a special viscosity calibration curve (SVCC) specifically for the meter.
Development of the SVCC requires the turbine flowmeter be calibrated using a range of viscosities—typically three or four. Each viscosity curve characterizes the meter’s performance and is used to develop a polynomial equation installed in a computer-based control or monitoring system.
Additionally, a temperature sensor is installed in the fluid line and connected to the control or monitoring system.
Based on the temperature sensor input, the appropriate SVCC is applied to the turbine flowmeters output value to obtain the viscosity adjusted flow rate.
Do you have experience and expertise with the topics mentioned in this content? You should consider contributing to our CFE Media editorial team and getting the recognition you and your company deserve. Click here to start this process.
