Flow Computer Solutions for Fiscal and Custody Transfer Metering

What Is a Flow Computer?

A flow computer is a purpose-built measurement and computation device that processes the signals arriving from a metering stream and calculates the true quantity of the fluid. A meter on its own reports only the volume or the pulse count it has measured, and that raw value cannot be used as a commercial quantity until the effects of pressure, temperature, composition and compressibility have been corrected. The flow computer applies those corrections with standard calculation methods, reduces the result to reference conditions and accumulates the quantity.

Alongside the calculation, the flow computer also carries the traceability of the measurement. Alarm and event logs, an audit trail of configuration changes, periodic reports and data logging together form the records needed to verify a commercial quantity after the event. It is therefore not merely a calculator but the recording and audit centre of the metering point.

Modern units go well beyond a single line. They can gather several metering streams in one chassis, produce stream totals, take part in station supervision through valve control and PID (proportional-integral-derivative) functions, and deliver data to host systems over serial or Ethernet links. The product group offered by TLY Enerji in this category consists of the device itself, the input/output and communication boards chosen to suit the application, and the accessories and spare parts that keep the installation running.

The Role of the Flow Computer in Fiscal and Custody Transfer Metering

Fiscal metering is the measurement on which invoicing, taxation and transfer of ownership depend. The calculation is produced in four steps:

1. Signal acquisition: Pulse, frequency or serial data is read from the meter; pressure, temperature and differential pressure from the transmitters; composition and density from a gas chromatograph or a density meter.

2. Correction and calculation: Compressibility, density and volume corrections are computed with the standard method that suits the fluid, and on gas streams an energy calculation can be carried out as well.

3. Totalising and reporting: Instantaneous flow rates are accumulated on an hourly, daily and batch basis; reports are generated and written to the logs.

4. Monitoring and intervention: Alarm conditions are evaluated, valve commands can be issued to bring a stream into or out of service, and the data is passed to the host system.

Primary element and meter types

The calculation follows the primary element the flow computer is wired to. Pulse-generating turbine, rotary and positive displacement meters, ultrasonic meters (by pulse counting or serial communication), differential pressure elements such as orifice, venturi, V-cone and nozzle, and Coriolis meters are the types in common use. Different meter types can coexist in the same chassis, which makes a single-device solution possible on stations with mixed streams.

Pressure, temperature and analysis inputs

Line pressure and temperature have to be measured directly for the correction calculation. Input/output boards read pressure, temperature and differential pressure transmitters, or multivariable transmitters, over HART (Highway Addressable Remote Transducer) loops, and a four-wire direct PRT (platinum resistance thermometer) input also accepts a resistance thermometer. Composition data can be taken from a gas chromatograph over a serial link.

Flow computation and station supervision

A single flow computer can carry both the stream calculation and the station functions. Stream summation, valve control and remote operation, maintenance functions, PID and sampler functions, batch loading and system diagram display all fall within this scope. On small and medium stations, measurement and supervision can therefore be brought together in the same hardware without a separate station supervisory computer.

Host system communication

Serial and Ethernet options are used to pass metering data to SCADA, DCS or RTU systems. Modbus master/slave communication can run over RS232/RS485 as well as over TCP/IP, and web server functions and network printing are supported too. The communication board should be selected to match the connection topology of the station.

When these four steps are built correctly the result is not only accurate but also auditable. Most commercial disputes arise from missing records and weak traceability rather than from a calculation error.

Key Features of the Product Group

·         Multi-stream architecture: Between 1 and 5 fiscal metering streams in a single chassis, with different meter types able to work side by side in the same chassis.

·         Modular board structure: Up to 6 user plug-in cards per chassis, in a combination of digital, analogue, switch and communication boards chosen for the application.

·         Broad standard library: International calculation methods for dry and wet natural gas, other gases, hydrocarbon liquids, water and LNG/LPG.

·         Double precision computation: 64-bit double precision floating point arithmetic throughout all calculations, with a half second cycle time.

·         Colour touch screen: 5.7 in colour VGA touch screen, QWERTY keyboard, 360° rotary navigation and high brightness indicator LEDs.

·         Audit trail and logging: Alarm, event and data logging, with recording capacity expandable through an optional 2 GB SD card.

·         Prover support: Bi-directional and uni-directional ball provers, small volume provers, master meter and dual chronometry methods.

·         Flexible communication: RS232/RS485 serial ports, 10/100 Ethernet, Modbus master/slave and Modbus over TCP/IP options.

·         On-site configuration: Local setup and data downloading through the front panel high speed USB port and Windows based configuration software.

·         Compact panel mount: Half width 3U chassis to the 19 in standard, for panel or chassis mounting.

Technical Summary of the Product Group

The values below summarise the flow computer platform offered in this category; the detailed figures appear on the product pages linked at the end of the page.

Key Advantages

·         Measurement and supervision in one unit: Because stream calculation and station supervision can sit in the same hardware, panel volume and field wiring are reduced.

·         Scales with the project: The board architecture lets stream count and input/output capacity be configured for the application without changing the device.

·         Auditable custody transfer: Alarm, event and audit trail records make it easier to verify an invoiced quantity after the event.

·         Suited to mixed-stream stations: Since different meter types run in the same chassis, ultrasonic, turbine and orifice streams can be managed together.

·         Maintenance-friendly in the field: The board-level architecture, front panel USB access and spare board options shorten downtime.

·         Local engineering support: Supply, configuration, commissioning and maintenance support are provided from a single point of contact in Türkiye.

Applications

Flow computers are used wherever the measured quantity has a commercial or legal consequence.

Natural gas metering and pressure regulating stations

Volume and energy calculation corrected for line pressure and temperature, with stream summation and standby stream management on multi-stream stations. Legal metrology approvals matter for the gas meter conversion function.

Fuel and chemical loading terminals

Batch measurement on tanker loading and unloading lines, multiple product operation, and interaction with sampling and loading arm control.

LNG and LPG facilities

Dedicated calculation methods for density and volume correction on liquefied gases, with custody transfer records on ship and tanker transfers.

Crude oil and product pipelines

Reduction of hydrocarbon liquids to standard volume, multi-stream measurement and regular meter proving with a prover.

Refinery and petrochemical internal transfers

Consistent accounting on inter-unit transfers and measurement of utility fluids such as steam and nitrogen.

Power and industrial plants

Monitoring of fuel consumption, support for CO2 emission calculations and data supply to production reporting.

Flow Computer Selection Criteria

The right flow computer follows from the metering purpose of the station and from its field connections.

Fluid and calculation method

·         Fluid type: natural gas, hydrocarbon liquid, LNG/LPG, steam, nitrogen or water

·         Purpose of the measurement: fiscal and custody transfer, internal transfer or process monitoring

·         Calculation standard to be applied and whether an energy calculation is needed

·         Source of the composition data: gas chromatograph, fixed composition or measured density

Stream count and input/output requirement

·         Number of fiscal streams today and the streams to be added later

·         Meter signal type: pulse, frequency, encoder or serial communication

·         Number of pressure, temperature and differential pressure inputs, and the HART loop requirement

·         Digital inputs and outputs needed for valve control, alarm output and proving

Communication and integration

·         Host system type: SCADA, DCS or RTU

·         Required protocol and physical interface: serial, Ethernet or both

·         Whether redundant communication is needed

·         Transfer of reports to a printer or over the network

Legal metrology and site conditions

·         Need for gas meter conversion or liquid metering approval

·         Temperature and humidity conditions inside the panel

·         Hazardous area classification and panel location

·         Spare part and maintenance plan

Selection should weigh not only today's stream count but also the growth plan of the station; if card slot and communication capacity are not planned at the outset, the cost of the next step rises.

Compliance and Legal Metrology

In custody transfer, legal compliance matters as much as calculation accuracy. The published compliance headings for the platform are as follows:

·         MID 2004/22/EC: Measuring instruments directive scope for gas meter conversion devices; applied together with EN 12405-1:2005.

·         MID WELMEC 8.8: Assessment for the measurement of liquids other than water, referencing OIML R117-1:2007.

·         EMC Directive 2004/104/EC: Electromagnetic compatibility scope.

·         LVD 2006/95/EC: Low voltage directive scope; applied together with EN 61010-1:2001 (2nd edition).

·         Environmental classes M2/E2: Mechanical and electromagnetic environment classification.

·         Calculation standards: ISO, AGA and EN series methods on the gas side; API, ASTM, GPA and OIML series methods on the liquid side.

TLY Enerji Flow Measurement Solutions

TLY Enerji treats the flow computer not as a single device purchase but as the whole metering point. Meter type, transmitter selection, calculation method, panel layout and host system connection are assessed together, which prevents the mismatches that otherwise surface during commissioning.

Starting from the process data and the purpose of the measurement, our team defines the device and board configuration, prepares the input/output list and the communication architecture, and produces the documentation needed for panel design and field connections. After installation, field support covers configuration, calculation verification, report templates and host system integration, and a spare part and maintenance plan is proposed for the operating period.

On the product side, TLY Enerji supplies the family and provides technical support as the distributor in Türkiye. For existing stations, improvement proposals are prepared for issues such as missing records, dropped communication links or inconsistent calculations.

·         Device and board selection according to the metering purpose

·         Preparation of the input/output list, panel and communication architecture

·         Configuration, calculation verification and report templates

·         Commissioning, training and field support

·         Spare part and maintenance planning

Frequently Asked Questions

What is a flow computer and what does it do?

A flow computer takes meter and transmitter signals and calculates the true quantity of the fluid with the effects of pressure, temperature and composition removed. Alongside the calculation it keeps alarm, event and data logs, so the measurement stays traceable; the quantity that invoicing is based on is the value this device produces.

What is the difference between a flow computer and a flow meter?

A flow meter senses the flow physically and produces a raw signal. The computing unit processes that signal, applies the pressure and temperature corrections with standard methods, reduces the quantity to reference conditions and totalises it. In custody transfer the two work together: the meter measures, the computer calculates and keeps the record.

Which calculation standards are used in fiscal metering?

On the gas side, methods such as ISO 5167, AGA 3, EN 12213-2 (AGA 8), AGA 10, ISO 6976, NX19 and EN 12213-3 (SGERG) apply; on the liquid side API 11.2.1, API 11.2.2, ASTM D1250, the GPA TR25 and TR27 series and OIML R022 are used. ISO 6578 comes into play for LNG and LPG. Which method applies is set by the fluid and by the commercial contract.

How many metering streams can one flow computer manage?

The flow computer platform in this category manages up to 5 fiscal metering streams in a single chassis and carries up to 6 user plug-in cards. In non-fiscal applications one board can serve two streams. Different meter types can coexist in the same chassis.

Why does prover and master meter support matter?

Whether a meter has drifted over time is checked by comparing it against a known volume. Support for bi-directional and uni-directional ball provers, small volume provers, master meter and dual chronometry means proving can be carried out on site without changing the installed equipment.

How is a flow computer connected to the host system?

The connection is made over serial ports using RS232/RS485 or over 10/100 Ethernet. Modbus master/slave communication can be used on serial lines as well as over TCP/IP, and the web server function also allows remote monitoring and configuration. The connection architecture is chosen to match the topology of the station.

What should be considered for use in hazardous areas?

The "Ex~i" element of the product name is part of the manufacturer's company name and does not by itself denote a certification; no ATEX or IECEx code is published for the device. In plants with a hazardous area classification the unit therefore has to be installed in a panel in the safe area, with suitable isolator solutions engineered for the field signals.