Flow measurement and control for environmental monitoring

Types of environmental air monitoring applications

There are many kinds of environmental air monitoring, including the three types of air monitoring listed below. Each one poses unique challenges for flow measurement and control.

  • Ambient air monitoring examines the concentration of greenhouse gases, volatile organic compounds (VOCs) and particulate matter (PM) that is present in the outside air that we breathe.
  • Stationary source monitoring examines how much of the polluting materials listed above are being expelled into the atmosphere from stationary sources like refinery, power plant and cement kiln smokestacks, as well as boilers from universities and other large institutions.
  • Fugitive emissions monitoring focuses on detecting leakages of polluting material from unintended sources, including gas and oil wells, and minimizing their impact. This process is called leak detection and repair (LDAR).

Volumetric flow control for ambient and stationary source air sampling

The fundamental requirement of air monitoring programs is knowing how much pollution is in a sample of air so that the appropriate measures can be taken to eliminate or reduce it. Because the end goal is determining the concentration of pollutants within a known volume of air, many air sampling applications require active control of volumetric flow rates, not mass flow rates.

Alicat MCW “Whisper” Series mass flow controller

A challenge in these situations is ensuring that the measured volume of air is not altered by the pressure drop induced by the flow controller. Alicat’s Whisper gas flow controllers (MCW series) use an extremely sensitive differential pressure sensor coupled with minimally restrictive flow bodies to keep pressure drop to a minimum. For example, at the flow rate of 16.7 lpm, which is so critical to ambient air monitoring, a 20-lpm Whisper flow controller has just 0.2 psi (14 mbar) of pressure drop. Keeping pressure drop low ensures that volumetric flow measurement is as accurate as possible.

Flow totalization of leaks for fugitive emissions

In some air monitoring applications, active flow control is undesirable. When leakages are discovered through fugitive emissions monitoring, it is important to quantify how much gas is leaking out of the pipe under normal operating conditions. Adding a flow control valve would interfere with the actual flow rate, so in this case the volume of gas flowed can be quantified using a gas flow meter with flow totalization.

Alicat Whisper-series portable mass flow meter.

Alicat Whisper-series portable mass flow meter.

Again using Alicat’s Whisper series of low pressure drop instruments, a Whisper portable flow meter (MWB series) can be equipped with a precision flow totalizer to quantify the total volume of gas that has passed through the instrument during a period of time. The totalizer averaging feature can also convert the totalized volume into a flow rate that has been averaged over the sampling duration. When the concentration of the gas sample is analyzed at the lab, the information can be combined with totalizer information to determine the total amount of pollutant gases that have escaped from the leak, as well as the leak rate.

Instant flow calibration for environmental air monitoring

Because knowing the volume of air is so fundamental to determining concentrations of pollutants, calibration is a critical process. All instruments that use active flow control require regular flow checks and calibrations of flow, pressure and temperature to ensure that the data they produce is accurate and repeatable. For these applications, FP-25 Flow Calibrator feature the most precise calibration offered on any Alicat instrument. Even when ambient temperatures drop or humidity increases, these instruments provide better than 1% of reading accuracy all the way down to 1/200th of their full-scale ranges.

When gas concentrations are being analyzed, another kind of calibration is required to ensure that the gas analyzer is detecting the proper amount of pollutant gas within the air sample. Dynamic gas dilution calibrators use mass flow controllers to mix bottles of calibrated gas mixtures with zero air to feed a gas analyzer with a known concentration of a gas pollutant. Here, Alicat’s standard mass flow controllers (MC series) allow these dilution calibrators to achieve new flow rates very rapidly, even when flow rates go below 20 sccm. This keeps the gas mixture precise and reduces the required calibration time.