The most common type used these days is Ultrasonic flow meter for gasses.
Ultrasonic meters measure the speed of gas movement by measuring the speed at which sound travels in the gaseous medium within the pipe. American Gas Association Report No. 9 covers the proper usage and installation of these meters, and it specifies a standardised speed-of-sound calculation which predicts the speed of sound in a gas with a known pressure, temperature and gas composition.
The Ultrasonic flow meter are of multi-path design wetted type transducers and are generally based on the transit-time measurement technique. The measured time difference is weighted and or averaged to arrive at the time average velocity of the gas.
The meter are generally compensate for any loss of path or paths in order to maintain the custody accuracy and 100% path availability at all times.
A loss of a single path does not affect the accuracy of custody transfer meter. Path substitution feature shall be available in the meter to maintain flow meter accuracy and 100% path availability at all times.
The flow meter has proven swirl compensation and self-draining transducer installation.
The Ultrasonic flow sensor operates in high frequency and have the capability to eliminate ultrasonic noise from pressure reducing station present in the upstream / downstream of the meter, to provide trouble free operation.
Metering system consist of Z- configuration with two runs (2 x 100 %) of UFM configured in operation / check on demand configuration.
Ultrasonic sensors are retractable during the pipelines operating conditions and without any requirement of external adjustment and meter recalibration after transducer reinsertion or replacement.
Straight length requirements, inlet and outlet connections and/or the type and location of a low pressure-loss flow conditioner should be as per Manufacturer’s specifications.
Straightening vanes, if recommended by Vendor to suit the 10D requirement, shall be inserted ahead of the upstream spool piece to minimize flow pattern disturbance. For Flow straighteners standardized design (ISO 5167) are recommended.
Flow transmitters shall be SMART microprocessor based and with suitable range The flow transmitter shall have following output as a minimum.
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Two nos. of RS-485 serial outputs
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4-20mA analog output with superimposed HART or digital communication approved for the Project.
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Pulse output from Transmitter to flow computer
The flow and diagnostic information is passed from flow transmitter to flow computer. Flow calculation and diagnostics are in the electronics of the flow computer.Transmitter is mounted integral with the ultrasonic meter.
Meter runs
Upstream spool piece is minimum 10D straight run and the upstream of the spool piece shall comprise of vent and drain connections for allowing the depressurization before de-mounting the ultrasonic flow meter.
For unidirectional flow measurement, the downstream spool piece is minimum 5D straight run.
In general, the downstream section shall contain:
1 no. thermowell with RTD for temperature measurement.
1 no. sample point for gas chronograph.
Flow Computer
The flow computer and the associated counter should comply with independent authorities such as Weights & Measures, and as per the Sales gas agreement.
Generally one flow computer should be dedicated for meter run. Any flow computer associated with fiscal gas measurement should have no functions other than those associated with the flow measurement.
Care should be taken that Signal transmission from the gas flow meter and from secondary instruments should be free of interference and cause no systematic errors.
The flow computer should have adequate redundancy to ensure that failure of any one component will not affect the metering functionality.
The metering system is designed for Z-configuration are used for both service and proving. Individual flow computers are provided for each run and during normal operation the service meter UFM-1 and Flow computer-1 should online.
During meter proving UFM-2 and Flow computer-2 will be in series with the service meter. The flow computer for each ultrasonic flow meter should be capable of carrying out real time super compressibility calculations using online pressure, temperature and gas data to arrive at the gas volume and totals at standard conditions, as well as average and total energy for an accounting day.
The flow computer should compute the following:
· Gross heating value in BTU/SCF with an uncertainty better than or equal to ± 0.5%.
· Flow measurement uncertainty of the overall skid shall be ± 0.25% of flow rate or better.
The pressure and temperature transmitters on each run are repeated to the flow computer on the other rung using signal repeaters. The serial output from flow transmitters on each runs are also connected to both flow computers.
Flow computer shall be capable of calculating flow rate and accumulate standard volume & energy units. The calculations shall conform to the requirements of AGA report 9. The whole measurement and calculation cycle shall be performed within every second.
Generally flow computer are capable of performing calculations while the instruments are being calibrated. Flow computers are to be provided with a keypad to access data and other functions and generally have password and key lock protection for data security. Flow computer have diagnostics software for monitoring the healthiness of the flow computer.
The flow computer should compute the Speed of sound and this is transmitted to the Supervisory System for verification of Speed of sound as per AGA-10.
The flow computers is generally provided with a LCD display capable of displaying the following information minimum:
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Instantaneous Flow Rate.
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Totalized volume.
- Instantaneous Temperature.
- Static Pressure.
- Accumulated Heating Value.
- All factors used in the flow equations.
Minimum Input/ Output Signal Interface:
The flow computers should receive following input signals as a minimum:
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One Pulse I/P for flow rate measurement from flow transmitter.
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Four 4~20mA analog inputs for pressure and temperature transmitters from each run for flow compensation.
- RS-485 Serial link shall be provided between ultrasonic flow transmitter and flow computer for passing of flow rate, diagnostic information and for configuration.
- Manually entered data
Flow computer should provide the following output signal interface as a minimum:
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Interface with GC
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Interface with printer.
- The flow computer shall have minimum of four numbers of isolated 4~20mA output signals, which are independently configurable.
- Interface with Supervisory System for passing of flow and diagnostic parameters.