The Turbine Flow Meter and its Calibration

The Turbine Flow Meter and its Calibration

Turbine Meters

A turbine meter consists of a practically friction-free rotor pivoted along the axis of the meter tube and designed in such a way that the rate of rotation of the rotor is proportional to the rate of flow of fluid through the meter. This rotational speed is sensed by means of an electric pick-off coil fitted to the outside of the meter housing.

The only moving component in the meter is the rotor, and the only component subject to wear is the rotor bearing assembly. However, with careful choice of materials (e.g., tungsten carbide for bearings) the meter should be capable of operating for up to five years without failure.

There are several characteristics of turbine flow meters that make them an excellent choice for some applications. The flow sensing element is very compact and light weight compared to various other technologies. This can be advantageous in applications where space is a premium

Primary Vs. Secondary Standards

A primary standard calibration is one that is based on measurements of natural physical parameters (i.e., mass, distance, and time). This calibration procedure assures the best possible precision error, and through traceability, minimizes bias or systematic error.

A secondary standard calibration is not based on natural, physical measurements. It often involves calibrating the user's flow meter against another flow meter, known as a "master meter," that has been calibrated itself on a primary standard.

Calibration

"To calibrate" means "to standardize (as a measuring instrument) by determining the deviation from a standard so as to determine the proper correction factors." There are two key elements to this definition: determining the deviation from a standard, and ascertaining the proper correction factors.

Flow meters need periodic calibration. This can be done by using another calibrated meter as a reference or by using a known flow rate. Accuracy can vary over the range of the instrument and with temperature and specific weight changes in the fluid, which may all have to be taken into account. Thus, the meter should be calibrated over temperature as well as range, so that the appropriate corrections can be made to the readings. A turbine meter should be calibrated at the samekinematic viscosity at which it will be operated in service. This is true for fluid states, liquid and gas.

Master Meter

A master meter is a flowmeter that has been calibrated to a very high degree of accuracy. Types of flowmeters used as master meters include turbine meters, positive displacement meters, venturi meters, and Coriolis meters. The meter to be calibrated and the master meter are connected in series and are therefore subject to the same flow regime. To ensure consistent accurate calibration, the master meter itself must be subject to periodic recalibration

Gravimetric Method

This is the weight method, where the flow of liquid through the meter being calibrated is diverted into a vessel that can be weighed either continuously or after a predetermined time. The weight is usually measured with the help of load cells. The weight of the liquid is then compared with the registered reading of the flow meter being calibrated

Volumetric Method

In this technique, flow of liquid through the meter being calibrated is diverted into a tank of known volume. The time to displace the known volume is recorded to get the volumetric flow rate eg gallons per minute. This flow rate can then be compared to the turbine flow meter readings

K-Factor.

“K” is a letter used to denote the pulses per gallon factor of a flowmeter.

Repeatability.

The maximum deviation from the corresponding data points taken from repeated tests under identical conditions.

Positive Displacement Calibrators:

Some of the most dramatic improvements in flow calibrator technology involve the evolution of Positive Displacement calibrators. PD systems are Primary Standard calibrators, which take into account the varying conditions under which flowmeters operate. These calibrators are able to compensate for temperature, density, viscosity and other variables that can shift a meter’s output.it utilizes a precision machined measurement chamber, or flow tube, that houses a piston. This piston acts as a moving barrier between the calibration fluid and the pressurizing media used to move the piston. Attached to the piston is a shaft that keeps the piston moving in a true path and provides the link between the piston and the translator. The translator converts the linear movement of the piston through the precision flow chamber into electrical pulses that are directly related to the displaced volume. Calibrators of this style can be directly traceable to the National Institute of Standards and Technology via water draw validation. Total accuracy of this type of calibrator is conservatively specified at 0.05%

Flow Transfer Standards:

Unlike primary flow standards, whose most important characteristics are their traceability to primary physical measurements (resulting in the minimization of absolute uncertainties, with less concern for usability or cost issues), the key criteria for secondary Flow Transfer Standards are portability, low cost and the ability to calibrate the flowmeter in the physical piping configuration it lives in.

Instead of removing flowmeters from service for recalibration, FTS devices allow users to “bring the calibrator to the flowmeter.” These portable, documenting field flow calibrators are intended for in-line calibration and validation of meters using the actual process conditions for gas or liquid. Advanced FTS systems incorporate hand-held electronics with built-in signal conditioners, thus eliminating bulky interface boxes and the need to carry a laptop computer into the field. High-quality Flow Transfer Standards also have the capability of measuring and correcting the influences of line pressure and temperature effects on flow.

Operation of a portable Flow Transfer Standard requires that a master meter be installed in series with the flowmeter under test. The readings from these instruments are compared at various flow rates or flow totals. A technician can install the master meter in the same system as the test meter, perform the calibration, and note any changes in performance. New calibration data might cause rescaling or new data points to be programmed into a flowmeter’s computer to align the measurement with the current flow calibration data.

Typical Calibration Techniques

Most flowmeter calibration service suppliers provide a choice of calibration techniques to accommodate different applications and flow measurement requirements. One of the most common techniques is the single-viscosity calibration, which consists of running 10 evenly spaced calibration points at a specified liquid viscosity. Single-viscosity calibrations are recommended when the viscosity of the liquid being measured is constant. If a higher degree of accuracy is needed, again, the more data points taken the better defined the meter calibration curve will be

Strouhal Number/Roshko Number

The best, and only completely correct way to present the data for a Turbine Meter is Strouhal Number as a function of Roshko Number, i.e., through the use of two dimensionless parameters. The St vs. Ro presentation takes into account all of the secondary effects to which the meter is sensitive. This presentation or correlation is correct for both liquids and gases. It is almost a must for gas calibrations since the density and kinematic viscosity are a function of both temperature and pressure

Your Calibrated Flowmeter

Once your flowmeter is calibrated, it may still read exactly the same under the same flow conditions as it did before it was calibrated. The difference is that you will know exactly how close those values are to the true values, and you will have a formula to use to calculate the true values from the actual values read by your flowmeter. You can have a correction factor obtained from calibratiob which you can apply to the flow meter readings to obtain the correct or true flowmeter readings. K-factor ignores the effects of changing temperature on the meter body since the meter will change diameter when the temperature changes. The use of Strouhal Number instead of simple K-Factor will account for this temperature effect.