Accuracy In Adversity

It is a fact of the industry that oil and gas companies are required, by various rules and regulations, to routinely perform gas sampling to establish gas composition, flow rates and gas properties. In order to comply with regulations, gas sampling tools must achieve high accuracy at low flows with varying gas compositions, temperatures and pressures. For many technologies, these requirements can be difficult to meet.

This article reviews various gas sampling challenges and describes how several recent innovations in thermal mass flow sensor technology offer gas analyser manufacturers, and oil and gas companies, new flow metering options to solve the gas sampling challenges that vex the industry.

Naturjal gas sampling challenges

In order to comply with state and federal regulations, oil and gas companies need to take an adequate gas sample that is representative of the gas flow. This requirement to take a `representative sample` is challenging for engineers. They must first carefully consider where to take this `representative` gas sample from the source stream. The most accurate representative samples cannot be taken from a `dead leg` or an area of heavy flow disturbance. In addition, the sample`s chain of custody must be maintained in order to avoid contact with other contaminants. Engineers must also reduce and control the pressure to the analytical tool, stabilise and control the flow, all while protecting the analytical instrument from particulates, moisture and pressure/flow excursions. Engineers must take the gas sample in as real time as possible, so that it correlates with actual process flow. With these gas sampling application challenges, the goal for oil and gas engineers is to take the most accurate gas flow sample, as quickly as possible, and with the lowest incurred cost.

When the optimal gas sampling location has been determined, there are still other inherent flow challenges to consider:

Wide flow rate variations: turndowns of up to 1000:1 may be required.
Changes in gas composition - wide gas density variations: traditional flow meters cannot successfully manage changes in gas composition and still maintain accuracy.
Non-uniform flow profile: gas measurements generally have asymmetric and swirling flow.
Very low pressure with variable temperature: most lines operate near atmospheric conditions with gas temperatures that vary with the gas source.

A lack of solutions

There are various analytical tools on the market today that attempt to meet all of the above gas sampling and flow metering application requirements. Gas chromatographs are still the most common tool, while new micro-analyser systems are gaining wide acceptance. A common thread in all such analysers is that the gas sample flow must be precisely measured and controlled, remain independent of pressure and temperature variations, and measure over a fairly wide range of flows at various compositions. In reality, it is not possible to have the flow rate unaffected by pressure and temperature variations.

Common technologies, such as averaging pitot tubes and insertion turbine meters, demonstrate poor performance in gas sampling applications. These devices measure volumetric flow, not mass flow, where mass flow is the required measurement. They also require a clean gas with constant gas composition. Additionally, they often cannot measure down to the low flows some gas samplers require. As a result, these technologies do not effectively provide the precise `representative sampling` data required to meet government regulations.

There is. however, a new technology innovation based on the thermal dispersion principal that meets these challenges. This technology will be examined in detail below.