This article is for turbine flowmeters; their general design, features, advantages, disadvantages, and precautions in their application. Main keywords for this article are Turbine Flowmeter Design Requirement, Turbine Flowmeter Principle,Turbine Flowmeter Application,Turbine Flowmeter Advantages,Turbine Flowmeter Disadvantages, Turbine Flowmeter Selection Precautions.

Turbine Flowmeter Design Requirement in Process Industry

Turbine Flowmeter Principle

• Turbine flowmeter is a velocity-type meter which measures flow by directing the fluid through an axial multibladed turbine rotor. The fluid stream exerts a torque on the rotor causing it to rotate at an angular velocity proportional to the fluid flow rate. A pickup device, located in turbine flowmeter housing, produces a pulse for each passing blade as the rotor rotates. The pulse rate from the pickup device is proportional to  the fluid flow rate and each pulse represents a discrete volume of fluid. Turbine flowmeters are designed to operate with fully developed turbulent flow patterns.
• Turbine flowmeter consists of a housing, rotor, bearings, flow conditioner, and pickup device. See Figure.

  

• A pickup device is used to sense the passing of the turbine blades.

1. Magnetic pickup is the most common device. It consists of a fine copper wire wrapped around a small cylindrical magnet mounted in the housing of turbine flowmeter. The magnetic field is changed each time a turbine blade cuts through it, inducing a current in the wire coil around the magnet. The magnetic pickup produces a magnetic drag on the rotor, which can become significant when operating small meters (less than 4 inches) at the low end of the designed operating range. Field preamplifiers shall be used where transmission distances exceed 200 feet. The preamplifier converts low-frequency pulses from the magnetic pickup to a voltage level square wave which is less susceptible to electrical noise.
2. An active, or radio frequency (RF), pickup can be used to eliminate magnetic drag on the rotor. The pickup has no magnet associated with it, only a coil and an oscillator. The passing of the turbine rotor
   blade through the radio frequency field changes the coil impedance. The amplitude of the carrier signal is modulated at a rate corresponding to the rotor speed. Field preamplifiers are not required since the output is a high frequency signal.

• Turbine flowmeters have only one moving part, the rotor. Proper rotor bearing selection is of vital  importance for the life, performance, and reliability of turbine flowmeter. These bearings minimize friction due both to rotational and thrust forces. There are two basic types of bearing construction, ball or journal. Design details and materials of construction vary widely. Generally, ball bearings provide better rangeability, while journal bearings provide longer life, but rangeability may be reduced.
• Performance Calculation
  Turbine flowmeter performance for liquids is calculated by mean calibration factor (Ki ):

Kl=60 f / Q
Where
Ki = Pulses per gallon
f = Pulses per second
Q = Flow rate, gpm

• • A typical calibration curve, plotting the calibration factor versus flow rate, is shown in Figure 2. Each turbine meter has its own curve.
  • Turbine flowmeter performance is expressed as a function of flow rate and not of full-scale flow. Performance is also a function of the kinematic properties of the liquid.
  • Turbine flowmeter performance for gases is also calculated by mean calibration factor (Kg):

Kg = 60f / C
Where
Kg= Pulses per cubic foot
f = Pulses per second

C = Flow rate, ft³ /min at a specified base pressure and temperature

•  A typical calibration curve is shown in Figure . Each turbine meter has its own curve. The gas pressure, temperature, and specific gravity shall be defined carefully when the turbine flowmeter is calibrated.

  

• Performance Criteria
   Accuracy. ±0.25 to 0.50 percent of actual flow rate over a specified flow range.
  Repeatability. ±0.05 to 0.10 percent of flow rate over a specified flow range.
  Linearity. ±0.25 to 0.5 percent of flow rate over a specified flow range. This standard is a function of fluid viscosity .
  Rangeability. Normally defined within the linearity specification, and is typically 10:1 to 30:1.
• Performance varies with the specific meter; associated signal conditioning and read-out fluid  properties; and piping installation. Performance approaching manufacturers’ specifications is possible only
  when viscosity and density are controlled; the flow rate is held within the most linear part of the calibration curve; and the piping arrangement does not introduce swirl in the fluid. Errors in gas service are usually greater than those in liquid service due to variations in density and temperature. For best performance, turbine flowmeter shall be calibrated by the manufacturer. Bearing life will decrease when turbine flowmeter is operated above rated flow rates for extended periods of time. Under extremes, the calibration factor is generally lower than the factor at rated flow; that is, turbine flowmeter reads low outside the normal range.
• Fluid properties can change turbine flowmeter performance. For best performance, turbine flowmeter shall be calibrated with the fluid to be metered. If this is not possible, another fluid with similar properties as defined in Precaution portion of this article”.

1.Specific Gravity. Since the turbine flowmeter is a velocity-type meter, variations in specific gravity can
cause significant errors when a mass flow rate is being inferred. Both fluid composition and temperature
shall be carefully controlled.
2.Viscosity. Turbine flowmeters operate best at low (1 centistoke), constant viscosity. As viscosity increases, the calibration factor shifts and the linear range decreases. An individual meter calibration factor
as a function of viscosity can be estimated using a universal viscosity curve as shown in Figure 3. This curve is not valid over a wide range of viscosities or for fluids of different lubricities. Fluid viscosity shall be controlled carefully to achieve good turbine flowmeter performance.

3.Lubricity. Significant differences in calibration factor and repeatability can be caused by liquid lubricity. Liquid lubricity is difficult to define and is not directly related to viscosity. Fluids with good lubricity,
for example jet fuel or light oils, will promote better performance. Water is considered to have low lubricity and is more difficult to meter.

•  In gas applications, the variations in viscosity will have an insignificant effect on meter performance; density will be the primary performance variable. Static pressures and temperatures shall be measured carefully to determine mass flows. The gas conditions from the manufacturer’s calibration charts shall also be reviewed carefully before correlating mass flows, and the necessary density correction shall beapplied. As gas specific gravity decreases, the rangeability of a turbine flowmeter will also decrease. Vapor metering requires special application engineering.

Turbine Flowmeter Application

• Turbine meters are selected where high accuracy is important and the fluids to be measured will not damage turbine flowmeters. Turbine flowmeters have excellent repeatability as compared to other flow metering devices. Rated full range flow can be from less than 1.0 gpm to 40,000 gpm. Special meters for temperatures from cryogenic to 300 °C; jacketed meters for high temperature applications; and body pressure ratings to 7,000 psig are available. The majority of turbine meters have a 300 series stainless steel body, and a 400 series stainless steel (magnetic) rotor. Turbine flowmeters in the smallest sizes often  have flare type tubing ends. Industrial sizes ( 1 /4inch to 24 inches) are available with suitable pipe threads or flanges.
• Turbine flowmeters are applicable to batch or continuous processes. In either case they shall be ‘run in’, that is, operated in the metered fluid for approximately 1 / 2 hour to approach the specified performance. Turbine flowmeters operate best at a constant flow rate. They can operate continuously at specified performance (± 0.25 percent) for long periods, however, their calibration factors often shift (up to 1 percent) during flow-rate changes. Practical rangeabilities are often 5:1 rather than the 10:1 specified.
  Turbine flowmeters will not only become nonlinear, but repeatability and accuracy will degrade as the operating range is decreased. Turbine flowmeter readings are not exactly linear with flow rate and this fact is often overlooked in batch applications. Turbine flowmeters rarely achieve performance specifications during start-up, so to minimize this effect it would be desirable in batching applications to use a small meter which could operate over a longer period of time rather than a large meter for a short period of time.
•  A variety of receiving instruments are available for turbine flowmeters. Flowmeter signal compatibility with the receiving instrument shall be checked carefully because there is wide variation in signal levels. Generally, these instruments are divided into the following three classes:

a. A totalizer accumulates pulses from the turbine flowmeter and displays the count. The pulses can be digitally scaled (multiplied or divided) to display them in engineering units. Digital comparators
can be combined with a totalizer to form a batcher.
b. A frequency-to-analog converter converts the frequency output of the turbine flowmeter to an electronic or pneumatic analog output. The power supply on this converter can often be used to
power a field-mounted preamplifier. This converter being an analog device is subject to zero and span drifts. Crystal stabilized converters are available with
0.05 percent repeatability. At low input
frequencies, the output may have a small amount of ripple. Complex input filters and threshold adjustments improve converter noise rejection capabilities.
c. A digital controller receives the pulse output from the turbine meter and modulates a valve to control the flow rate. These controllers use digital techniques and do not convert the input to an
analog signal. Their specifications are based upon flow rate, not full-scale span as with a typical analog controller. These controllers are used primarily in blending or ratio flow control applications.

Turbine Flowmeter Advantages

• Turbine flowmeter is an accurate way to measure flow rate when the fluid and environment permit. It is commonly used to prove other meters.
• For applicable fluids and environment, repeatabilities can approach ±0.05 percent over a 5:1 flow range.
•  It is usable, with reduced repeatabilities, over a 10:1 or greater flow range.
• Digital output provides for direct totalizing, batching, or digital blending without reducing accuracy.
• For certain batching operations it can offer the most economical installation.
• There is less tendency to read high in pulsating flow than in head or variable-area type meters.
• It responds in milliseconds to flow rate changes. Response is related to meter size (larger is slower).\

Turbine Flowmeter Disadvantages

•  It may not be usable in dirty streams or with corrosive materials. It is subject to fouling by foreign materials, for example fibers, tars, and materials which may polymerize.
• Bearings are subject to wear or damage. If bearings are replaced there may be a shift in calibration.
• Accurate calibration requires special facilities at the manufacturer’s shop or at the plant.
• Turbine flowmeter can be damaged by overspeeding (over 150 percent) or by hydraulic shock.
• Pressure loss at rated flow varies widely among models and can be high.
• Body sizes, fittings, and electrical signals are not standardized. Meters from different manufacturers may not be interchangeable.
• Delivery times may be long for all but the most common sizes and materials. Repairs may take equally  as long.
• Cost increases rapidly above 2 inch body size.

Turbine Flowmeter Selection Precautions

• Turbine flowmeter shall be calibrated in the same orientation it will have when installed. If possible, it should be calibrated with the fluid to be metered and under operating conditions. Where fluid varies significantly under normal operating conditions suitable temperature composition shall be incorporated. If a test fluid is used, it shall be within 10 percent of the kinematic viscosity and density of the metered fluid and have similar lubricity.
• Turbine flowmeter design shall be selected carefully. Body, rotor, bearings, and fittings shall be appropriate for the metered fluid.
• Liquid systems shall be kept filled with the metered fluid to achieve maximum accuracy.
•  A field-mounted preamplifier shall be installed to amplify the low frequency signal from the magnetic  pickup and avoid interference problems. Turbine flowmeter signals range from 20-2,000 Hz and industrial environments generate noise at these frequencies. Signal cables shall not be run in the same tray or conduit with power wiring. Preamplifiers are available in either two-wire or three-wire configurations with safe or explosion-proof ratings.
• Turbine flowmeter shall be installed with proper valving, to allow removal for service.
•  A screen or filter shall be installed to protect turbine flowmeter in accordance with manufacturer’s  recommendations. As a minimum, temporary screens shall be used during start-up to prevent meter
  damage. Screens shall be sized to stop entry of particles of sizes over one-half the clear area between rotor blades.
• In batch dispensing, the speed of the valve opening and closing, especially closing, affects system accuracy. A throttling-type valve shall be used to prevent damage to turbine flowmeter by hydraulic shock waves.
• Where high accuracy is required, a constant flow rate shall be used.
• Turbine flowmeters shall never be over ranged greater than 150 percent of rated capacity.
• If no other portion of the piping system limits flow rate, a flow limiting device shall be installed to prevent overspeeding and possible damage.
• Meters shall not be operated in reverse.
• If liquid meters are purged with gas, the rotor shall not be run overspeed. Liquid flowmeters shall not  be purged with steam, nor gas meters with liquids or steam.
• Piston-type, positive-displacement pumps shall not be used in series with turbine flowmeters.
• Turbine flowmeters shall be excluded from hydrostatic testing or line flushing during start-up.