Knowing how to accurately use a flow meter is one of the most important tasks for a measurement professional in liquids.
A flow meter is critical for supporting inventory, loss control, billing, safety, and third-party contracts. Knowing how to use a flow meter ensures the safe and reliable transport of liquid product from Point A to Point B to benefit your company and meet contractual obligations.
To perform these critical tasks in liquid measurement, it’s important to understand the guiding principles of flow meters.
The Difference Between Direct and Inferential Flow Meters
One of the key distinctions among flow meters is the difference between a direct flow meter and an inferential flow meter.
– Direct flow meters are meters that physically measure a displaced volume or mass. One class of direct flow meter is a positive displacement (PD). Another is turbine. Both types of meters correlate physical movement to a unit of volume.
– Inferential flow meters are those meters that infer volume or mass from a signal and a computation. Inferential meters include orifice, ultrasonic, and coriolis. Each type of meter has it own unique computation related to the signal type.
- Orifice – Differential Pressure
- Ultrasonic – Speed of Sound
- Coriolis – Tube Resonance (Vibration)
Another consideration in gas measurement is a linear meter.
– What are Linear Meters? Linear meters are those where the raw value used for calculation is a pulse. Before electronics, there were basically two type of meters: Differential and Linear.
Differential meters put out an analog value representing inches of water column and linear meters put out a mechanical pulse representing a unit of volume. This actually can cause confusion because modern ultrasonic and coriolis meters are both inferential and linear. These meters have electronic heads that translate the raw signal into a high-speed pulse out.
For example, an ultrasonic flow meter acts as an ultrasonic sensor to determine the speed of sound along multiple flow paths.
The meter head uses a speed of sound calculation to determine velocity, and then determines volume rate by using this equation: flow rate equals the flow area multiplied by the flow velocity. Volume is then translated into a pulse, which is used by the electronic flow meter to calculate standard volume and mass for reporting at contract conditions.
The Importance of Verifying the Accuracy of a Flow Meter
After understanding the different types of flow meters for measuring liquid product, the natural conclusion is that each type of meter must be installed, set up, and verified differently. That is correct.
Because accuracy is critical, measurement professionals need to be cognizant of performance verification techniques to ensure that each type of meter produces accurate readings.
This is important because liquid measurement should be carried out as meticulously as possible. What might seem like an innocuous error or a slightly-off reading could produce significant changes in the measurement of the product. The undesired result is a potential discrepancy in the recording of a measurement, the back-office report, and a third-party contract.
To avoid discrepancies, measurement professionals should focus on the front-end of verifying the accuracy of a flow meter through meter proving. Two techniques used in meter proving include:
– Density Prover: uses a device, pycnometer, to capture a sample of fluid of known dimension (both volume and weight) for comparison against calibration data for the purpose of verifying density readings produced through densitometry.
– Meter Prover: verifies flow meter uncertainty to determine the K-Factor (pulses per unit of volume), the meter factor of a meter, the linearity over a calibrated flow range for the meter, and the repeatability of the meter system. [Read More About the Fundamentals of Meter Provers from ASGMT.]
Additional Considerations in Performance Verification
Understanding how to use each type of flow meter and verify the accuracy of measurements from each meter is only part of the battle.
Because measurement takes place in the field and not in a vacuum, there are other considerations in performance verification:
- Understanding acceptable meter factor deviation limits
- Ensuring efficient proving operations
- Understanding why some meter types are harder to prove than others
- Estimating required proof volumes
- Accounting for variations in meter frequency due to turbulence
The key for a measurement professional is understanding where the boundaries are. For example, if a meter factor deviates from a previous reading by a significant amount, an operational check should be performed to determine the factor shift. If the cause of the deviation cannot be determined and corrected, the meter may need to be repaired or replaced.
Calling for a meter to be repaired or replaced is a major decision in terms of cost, potential downtime, and affect on operations. That is why it is critical for measurement professionals to have a firm grasp on the meters used in the field, their proving techniques, and their understanding of deviations to support performance verification.
Consider GCI for the Fundamentals of Liquid Measurement
Gas Certification Institute (GCI) provides a Fundamentals of Liquid Measurement course for measurement professionals. The course material covers critical topics on how to use a flow meter in the field and the technical details of how to support accurate measurement.
The course is designed for newer measurement professionals and more experienced measurement professionals who need a refresher on the technical aspects of liquid measurement to continue performing critical tasks in the field.
Register for our next liquid measurement course offered at our GCI training location in Houston. We also work with companies to bring our training class on-site to your location. Consider having your measurement supervisor contact our team directly to inquire about on-site training.