Introduction to Flow Meter :
- A flow meter is a measuring instrument designed to measure fluid flow rate such as liquid or gas when it passes through a confined transmission system.
- Various types of flow meters such as orifice meters, venturi flow meters, Rotameters, flow nozzles, etc. are used for flow measurement.
- Every flow meter has different operating principles. For example, an orifice flow meter determines the fluid flow at the orifice which is the entrance and exit cross-section of a narrow, restricted opening.
- Likewise, a rota meter is a flow meter that measures the volumetric flow rate inside multiple tubes placed at different areas of the transmission pipe.
What is flow calibration?
- Flow calibration is the process or method of comparing of metering of a flow meter to a standard scale of measurement and adjusting to its metering standard.
- Calibration is the process of checking an instrument for accuracy.
- Generally, the calibration of a flow meter can be achieved in several methods that include comparing the flow rate of the meter against a reference standard of higher accuracy.
- In instrumentation and control, the term calibration of an instrument is essential to maintain a line of traceability.
- The performance of the flow meter can be easily understood by calibrating the flow meter on a found basis to estimate the meter drift analysis.
- Flow meter errors can be minimized by further adjustment of the flow meter reading based on customers’ agreement.
- Separate calibration is done for the flow meter having transmitter and indicator, wet calibration for the flow meter, and dry calibration for the transmitter and indicator.
- The flow meter is divided into the primary portion where the flow signal is generated and the secondary portion where the flow signal is processed.
- The accuracy of the overall system must be sufficiently maintained, to calculate overall system accuracy during calibration.
Describe the calibration methods and procedures of various flow meters associated with liquid flow?
- Liquid flow meters can be calibrated in several ways but includes the comparison with the standard reference.
- The standard reference may be a primary or secondary reference standard.
- Depending upon the accuracy requirement, the calibration methods, and procedures of various flow meters associated with liquid flow use three main principles
1. Gravimetric calibration,
2. Piston Prover Calibration
3. Volume or transfer standard calibration methods.
1. Gravimetric Calibration:
- For volumetric and mass flow meters, this gravimetric calibration method is considered the most accurate method.
- This calibration is classed as a primary standard calibration.
- This gravimetric method is ideal for calibrating liquid flow meters such as oil and water-based flow meters which they can be sent to an external calibration laboratory.
- This calibration procedure includes the deviation of the fluid onto a mass comparator over specified time duration.
- The basic mass flow rate measurement is derived by mass overtime calculation.
- Fluid density needs to be measured to obtain volume-based flow measurement.
- This process is simple, it is known as the bucket and stopwatch method.
- To achieve the highest accuracy a significant effort is required to realize and minimize the measurement uncertainty contributors.
2. Piston Prover Calibration:
- This Piston Prover flow meter calibration method provides high-accuracy results.
- Due to the easier portability of the piston provers, this is easily available to a wider customer base.
- This calibration method guides a known volume of fluid through the flow meter at a given rate.
- The volume flow rate is obtained from the volume of fluid expelled by the piston multiplied by the travel of piston length over the measured period of time.
3. Transfer Standard Master Meter Calibration:
- This transfer standard master meter calibration method consists of a reference grade called a master flow meter,
- This master flow meter will compare the reference flow against the flow meter under test at the same operating conditions.
- It is essential to note for any variation in the operating conditions due to the addition of another flow meter into the test line, and any variations in fluid temperature or pressure.
- The quality of reference meters used must be high and they should have a detailed history of performance.
- Master flow meters used must be calibrated to an accredited ISO 17025 standard via approved officials such as NIST, UKAS, DaKKs, COFRAC, etc.
List the various manufacturers of flow meters:
There are several manufacturers that offer quality flow transmitters such as
1. Rosemount flow transmitters.
2. Foxboro flow transmitters,
3. Endress Hauser Group
4. Emerson Electric Corporation
5. Honeywell International Inc.
6. KROHNE Messtechnik GmbH
7. Siemens AG
8. ABB Ltd
What are the types of flow calibration?
Flow calibration can be performed in three methods:
- Wet calibration using actual flow.
- Dry calibration using flow stimulation by electronic or mechanical means.
- Measurement check of physical dimensions and using emperical gables that relates flow rates.
Calibration of Differential Pressure (DP) Transmitter:
- These differential pressure (DP) transmitters are used to measure the flow of fluid in a pipe line.
- But the measurement in differential pressure transmitter is performed based on flow fluctuation.
- Note: The flow rate of the fluid is the square root of the pressure drop. Therefore, flow rate can be calculated by squaring values of differential pressure.
The calibration of differential pressure transmitters is as follows:
- Connect both operating differential pressure DP transmitter and master meter or standard calibrator in series with each other.
- Start both transmitters with equal pressure simultaneously on the test pipeline.
- The calibrations of DP transmitters must be performed for five points, 10%, 30%, 50%, 70%, and 90%.
- Inputs must be collected at each point for both standard and operating transmitter devices.
- Compare the readings of both devices at each point for any differences. If there is a difference found in readings of both devices then that difference is termed a correction factor.
- Tune the parameters of the operating DP transmitter till zero correction factors are obtained.
- Repeat the same procedure for the remaining checkpoints. Once you obtain zero correction factor for the remaining checkpoints, then the DP transmitter is considered fully calibrated.
- Hysteresis must be checked because electronic devices may produce fluctuating readings for the same checkpoints.
- At each point, the output of the DP transmitter will be achieved. To get the flow rate the differential pressure value needs to be squared.
- The final flow rate value must match the original manufacturing parameters, mentioned in the manual. This helps in cross-checking for final calibration.
Calibration of Magnetic Flow Transmitters:
- Magnetic flow transmitters’ uses a conductive fluid as a conductor,
- The magnetic flow transmitter is calibrated using a standard calibrator which acts as a master meter for calibration.
- This calibration involves five checkpoints.
- Calculate the input signal. The required values are indicated on the device.
- Input Signal = Upper Range Value x Calibration Factor x Phase band factor.
- Record the output values for five different checkpoints to calculate deviation and accuracy.
- Deviation= Theoretical Value- Actual Value.
- Accuracy= (Deviation/Span) x 100.
- Compare the accuracy with the manufacturer’s specifications and adjust till zero deviation is obtained.
Calibration of Vortex Flow Transmitters:
- The vortex flow transmitter operates according to vertex shedding frequency.
- In this calibration, both the input pulse signal and output vertex flow frequencies are compared with a master calibrator.
- To perform calibration of the vortex flow transmitter the following steps must be followed.
- Calculate Vortex Shedding Frequency:
- Frequency of Vortex Shedding = Reference factor x Conversion Factor x (Upper range value)
- Frequency of vortex shedding is represented in pulse per second (PPS). and
- The values of RF, CF, and URV, are mentioned in the device manual.
- After calculation of vortex shedding frequency,
- Set the span jumpers to five checkpoints.
- Connect the input terminal of the transmitter to a frequency generator. Connect the master calibrator output and flow transmitter in series with each other.
- Finally, set the fine span and make some adjustments by following steps shown below.
- Set frequency generator at upper range value,
- Adjust the fine span screw till 100% value is reached.
- Then adjust zero on the output value.
- Remove the frequency generator connection and connect the signal lead. Then lower range value is adjusted to zero.
- Adjust zero, till input and output match.
- Perform the same process for all checkpoints to achieve complete the calibration.
Why is calibration necessary?
- Every field instrument is subjected to aging, wear, and dirt.
- The aging of some mechanical and electronic components causes some minor deflection in measurement.
- Full service, repair, and new adjustments for the instrument are suggested if there is more deflection caused by dirt and wear.
- The calibration of the instrument is necessary to ensure the device provides accurate results.
- Periodic checking of the instrument is often recommended due to legislation, norms, or directives