The United States Environmental Protection Agency (EPA) release a rule, 40 CFR Part 63 – National Emission Standards for Hazardous Air Pollutants: Petroleum Refinery Sector, in 2015. Elements of the rule were reconsidered with a final rule made effective in February 2020.
For refiners, compliance means the monitoring, reporting, and control of flares. Measuring and reporting flow measurements are part of that requirement. At the 2020 4C Health, Safety, & Environmental conference here in Austin, Texas, Emerson’s Meha Jha joined another supplier to present in a Flares for Experts session. Her topic was Flare Flow Mass and Volume Measurement. Meha discussed flow measurement solutions used to comply to the Refinery Sector Rule requirements for flows other than flare vent gas.
The focus of Meha’s presentation was to help session participants understand the advantages and considerations for using different types of flow technologies for steam and purge or supplemental gas measurements. She noted that 40 CFR Part 63 Subpart CC at §63.670, §63.671 and Table 13 requirements for accuracy were relatively easy to meet. The challenge is the very high turndown ratio for the flow measurements, especially for steam as well as the biennial calibration required due to the expense of shutting down your flare systems every two years to inspect flow meters.
She compared the flow measurement choices for these applications:
- Differential Pressure (DP) measurements are cost effective but are very limited on turndown and are commonly used for steam
- Vortex measurements are also cost effective and have better turndown when compared to DP. Both are commonly used for steam
- Coriolis meters have a wide turndown range and ease of calibration, but are not applicable on steam applications
- Ultrasonic can be used but is more expensive so typically we see most refineries have used a combination of vortex, DP, and Coriolis
For steam applications, Meha noted that vortex or DP are typically used. DP measurements, due to limited turndown range, typically requires parallel streams and a smart control system transition, valving, and multiple meters to cover the entire flow range. Advanced filtering capabilities in vortex meters can extend the turndown beyond 30:1 in a single device. While Coriolis flow measurement technology is not used for steam flow measurements, it is a great technology for purge and supplemental gas applications. Coriolis meters provide ease of reporting and have the turndown and accuracy required.
Vortex meters operate on the Von Karman phenomena. Flow is obstructed by a buff body in the flow path. You can see this phenomenon in action naturally when wind blows a flag mounted on a flag pole. The pole acting like the blunt body causes the alternating vibrations/flapping of the flag.
In a Vortex meter, flow is obstructed by a buff body in the flow path known as a shedder bar. The shedder bar separates the flow into alternating differential pressure forces known as vorticities that form at the back of the shedder bar. These alternating forces case the flexure which is a portion of the shedder bar to move. The flexure movement causes the sensor which is separate from the shedder bar to vibrate. The alternating vibrations from the sensor are converted into an electrical signal. This frequency of the alternating vortices is linearly proportional to flow velocity.
The accuracy of Vortex meters in steam applications is not as sensitive to shedder bar wear as other technologies are to the primary element. There are no gaskets, leak points in these meters other than pipe connection flanges. From a maintenance perspective, technicians can change out the sensor while the meter is still in the line to avoid shutdown of the flare as well as all the units that relieve into it.
When using DP flow measurement for steam, typical transmitters are limited to 3:1 turndowns. The Rosemount 3051S Ultra for Flow increases the turndown ratio up to 14:1 turndown with 0.5% accuracy.
Coriolis meters work by having the flow split in half between two tubes. The sensor tubes are vibrated in opposition to each other by energizing a drive coil to oscillate at their natural frequency. Magnet and coil assemblies, called pick-offs, are mounted on the flow tubes. As each coil moves through the uniform magnetic field of the adjacent magnet it creates a voltage in the form of a sine wave. Density measurement is based on the natural frequency of the system, including the flow tubes and the process fluid.
The strengths of Coriolis measurements include low to no maintenance required, no flow conditioning or straight runs required, excellent repeatability and turndown, and simplified calibration on meters with Smart Meter Verification (SMV).
SMV makes reporting to the Environmental Protection Agency (EPA) easier. SMV can be automatically run and generate a report verifying that meter is within factory baseline performance. This methodology has been accepted as a method for validating meter performance in lieu of a biennial calibration. SMV helps identify meter damage causes a change in tube stiffness. If this change is detected, the meter is likely to require recalibration or replacement. For Vortex, Coriolis, or DP meters, the manufacturer can provide testing reports, calibration data, and manufacturer’s recommendation practices to extend the biennial calibration up to 7-10 years.
Meha wrapped up her presentation with a case study at a refinery with multiple flow meters to meet 100:1 range with ±5% of mass flow. Rosemount Vortex meters were selected for several of these steam injection measurements because of their reliability and fairly-high turndown.
For the high-flow atomizing steam range, a Rosemount 3051S DP flow meter is used with a conditioning orifice plate to provide measurements to calculate mass flow. In the low-flow atomizing steam range, a Rosemount 8800 Vortex meter with temperature compensation is used to measure mass flow. For the center steam to maintain flame shape and stability, an 8800 Vortex meter was used. Together these met the refinery’s requirements for this flare application.
Visit the Flow Measurement section on Emerson.com for more on these technologies and application best fits. You can also connect with other flow measurement experts in the Measurement Instrumentation group in the Emerson Exchange 365 community.