MONITORING FLARE GAS ACCURATELY TO MEET ENVIRONMENTAL REGULATIONS
The European Union, the U.S. Environmental Protection Agency (EPA) and many other global groups are working to reduce the greenhouse gases (GHGs) that are the main cause of the global warming phenomenon. The oil and gas industry in response to new regulations and their requirements has increased its focus on accurately measuring, monitoring and reporting the fl ared GHG gases that result from production, refi ning, storage and distribution activities.
Flare gas measurement and monitoring is an instrumentation challenge that requires accurate gas fl ow sensing and metering in demanding rugged environments. Flare gas fl ow meters are an essential tool to signal abnormal process changes, early leak detection and provide the fl ow measurement data that are used for environmental agency reporting.
Typical Issues
Flare gas applications present several challenges to plant, process and instrument engineers when selecting a fl ow meter solution, which can include any or all of the following dilemmas:
Low and High Flows - Sensitivity to low fl ow conditions is required to identify and measure leaking valves and the normal low fl ow associated in day to day operations. The capability to measure very high fl ows is needed during system upset conditions requiring a meter that needs to measure fl ow accurately over an extremely wide turndown range.
Calibration - This application requires the calibration of fl ow meters specifi cally for hydrocarbon composition gases and matching to actual process conditions is essential. Gas composition changes and wide turndown can lead to relatively poor measurement accuracy in fl are applications.
Large Line Sizes - As pipe sizes increase, the number of effective and suitable fl ow meter sensing technologies decreases and the costs increase too.
Lack of Straight-Run - All velocity based fl ow meter technologies have pipe straight- run requirements upstream and downstream from the meter in order to achieve repeatable fl ow measurement. These straight-run requirements are often diffi cult to achieve in crowded production sites and process plants.
Environmental Compliance - In nearly every application, the fl ow meter must meet performance and calibration requirements mandated within local regulations such as the EU Directives 2003/87/EC and 2007/589/EC; U.S. EPA’s 10 CFR 40; 40 CFR 98; U.S. MMR 30 CFR Part 250, Subpart K, Section 250 and others.
Figure 1: ST100 Series Thermal Air/Gas Flow Meter
Tight Access - Limited pipe access and re-access for
installation, maintenance or servicing is frequently diffi cult. For example, spool-piece fl ow meters can require prolonged process shut-downs and extensive on-site labor costs to install and continuously maintain the system as opposed to insertion style meters that can be easily inserted or retracted into or out of the process through a ball valve.
Agency Approvals - When installing meters in hazardous (Ex) locations, the entire fl ow metering instrument should carry agency approval credentials for installation in environments with potential hazardous gases; enclosure only ratings are inadequate.
FPSO/Offshore Platforms and LNG vessels - The ocean’s corrosive salt water environmental effects such as those encountered by offshore platforms, fl oating production vessels and LNG tankers can require the use of stainless steel on all exposed instrument materials including the enclosures.
Flare Gas Sensing Technologies
There are three fl ow meter sensing technologies regularly considered in fl are gas measurement applications: ultrasonic, optical and thermal. While all fl ow meter technologies have their advantages and disadvantages, some are generally better than others depending on the specifi c media (liquids [volume] and air/ gas [mass]) and the application environment.
Ultrasonic fl ow sensing technology relies on ultrasound and the Doppler Effect to measure volumetric fl ow rate. In ultrasonic fl ow meters, a transducer emits a beam of ultrasound to a receiving transducer. The transmitted frequency of the beam is altered linearly by particles or bubbles in the fl uid stream. The shift in frequencies between the transmitter and receiver can be used to generate a signal proportional to the fl ow rate.
Optical fl ow metering relies on laser technology and photo detectors. This technology requires the presence of tiny droplets or particles of condensation, lubricants, dust and other particles in the gas stream. These particles scatter the light beam and the time it takes for these particles to travel from one laser beam to the other laser beam can be used to calculate the gas velocity and volumetric fl ow rate.
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