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Process Automation

As the EPA mandates accurate representation of the amount of gas that is being flared off, how do you ensure that you have measured the correct amount?

If you flare-off gas rather than use a vapor recovery unit (VRU), or even flare-off the exposed gas after using the VRU, then you may have already experienced your share of challenges – particularly when it comes to measuring the gas you’re flaring.

Challenges can arise from a number of external agents. In particular, I’ve seen a surge of inaccurate and unreliable measurements of flare gas applications running under low (and extremely low) pressures in steel pipes. Whether you are venting into the atmosphere or into a VRU, the biggest challenge in measuring low pressure flare gas in steel pipes is the conflict in their acoustic impedances that isn’t suitable for clamp-on ultrasonic flow meter measurement.

Gas measurement in steel pipes is complicated by the relatively low acoustic impedance of gas compared to steel. This differential causes a significant loss of signal incidence at the pipe wall/gas interface with the majority of the ultrasonic signal remaining in the pipe wall and not transferring through the gas. This conflict is typically resolved by increased pressure of the gas stream.

How do you fix the problem?

One of the best solutions that I’ve seen is the implementation of plastic spool sections. This results in a better acoustic “match” to the gas. Acoustic impedance is defined as sonic velocity x density. Gas has a very low impedance while steel has a very high impedance. However, plastic has both a low density and a low sonic velocity hence a low impedance, which makes it more compatible with the acoustic impedance of gas. As a result, with plastics, you’re able to measure very low pressures, even a slight vacuum or negative pressure.

Depending on your how you operate your system, you may or may not experience measurement issues. For instance, steel pipe applications operating at pressures of 100psi or greater will not likely be affected. On the other hand, if your applications involve low pressure, even atmospheric or vacuum conditions, you would not be able to properly measure the gas flow.

In these application situations, the inability to measure accurately stems from the improper use of the clamp-on ultrasonic meter. While clamp-on flow meters can be applied to a wide range of applications, some conditions are unacceptable due to the physics of ultrasound. In a situation such as the aforementioned, for low pressure flare gas, the most effective and cost-efficient solution is to install a plastic spool pieces into the line to facilitate the installation of the clamp-on ultrasonic flow meter. This relatively simple modification will produce proper acoustic impedance compatibility, ensuring accurate flow measurement at low pressure conditions

Still having trouble or unsure of the best solution for your set-up?