MASS FLOW METERS FOR FLARE/VENT GAS MEASUREMENT
FLARE/VENT GAS MEASUREMENT
Offshore Lease Operators are now required to account for flare and vent gas volumes from deepwater offshore facilities processing more than 2000 bpd. Federal Regulation- Title 30 Part 250, Subpart K defines these requirements and the time line for implementation. Accuracy is required to be 5%.
Accurate measurement of flare and vent gas flows is important from the perspective of regulatory compliance, energy conservation and emission accounting. Unlike conventional flow measurement requirements, flare/vent gas measurements present unique challenges in terms of rangeability and response to changing flow extremes. During normal platform operations, flows are usually low. Depending on header size, discharge velocities are usually under 1 f/s (60 SFPM). In upset conditions these flow rates can flare up to over 300 f/s (18,000 SFPM) Flow meters used in custody transfer or process control are not capable of handling these flow extremes and maintaining the accuracies required. The flare/vent measurements are associated with safety systems, and swings in flow rate are both unpredictable and extreme.
MEASUREMENT SOLUTIONS
The flow meter of choice to address this challenging measurement is the Sage Metering Thermal Mass Flow Meter. It utilizes technology that does not require external pressure and temperature compensation and only requires a single penetration of an insertable flow element.
Sage Metering’s PRIME Series NIST Traceable Digital Thermal Mass Flow Meter is unique in making flare/vent gas measurements. A single meter can handle the extremely wide flow ranges both in normal operations and upset conditions, while operating within the prescribed accuracy requirements.
Considering these meters are typically required to be retrofitted into existing piping galleries, installation costs can be a significant factor in choosing flow meter technologies. With Sage, there is no requirement to break the existing piping or to even take the lines out of service. Using an insertion measuring probe installed through a small isolation valve assembly makes this possible. The Sage meters have a high resistance to gas contaminants and virtually no annual maintenance, making them reliable and very easy to own and operate. In addition, the Flow Meter has negligible pressure drop which is an important feature in lower gas flows.
Sage Prime Mass Flow Meters
Sage Prime is a thermal dispersion type of flow meter, utilizing the fast respnding, constant temperature difference method of measuring gas mass flow rate. It contains two reference grade platinum RTD sensors clad in a protective 316 SS sheath. It features direct Mass Flow for gases, wide rangeability, low pressure drop and no moving parts.
The Sage Prime is microprocessor based, has digital drive circuit, and has Modbus RS485 RTU communications. It is powered by 24 VDC (12 VDC optional), or 115/230 VAC. The power dissipation is under 2.5 watts (e.g. under 100 ma at 24 VDC) for the DC version. The power and output terminals are in a separate sealed compartment for ease of installation.
The Sage Prime display is a high contrast photoemissive OLED display, and it displays mass flow rate, totalized flow, and temperature, as well as a bargraph representation of flow rate. In addition, the calibration milliwatts (mw) is continuously displayed, providing ongoing diagnostics. Outputs include a 4-20 mA signal proportional to mass flow rate, and pulsed outputs of totalized flow (24VDC solid state [sourcing] transistor drive), as well as Modbus compliant RS485 RTU communications.
Calibration is NIST traceable, and covers a wide variety of gas calibrations. Sage Prime can measure gas flow up to 500°F at pressures up to 1000 PSIG.The zero gas flow rate can be verified in real time by monitoring the mW value recorded during initial calibration with the mW value after installation. This provides continuing calibration verification eliminating frequent calibration required by other technologies.