A key factor in the proper functioning and continued profitability of the natural gas industry is the ability to make precise measurements throughout the entire process of gas recovery. James Doorhy reports.
Whether you are doing household chores, regulating the temperature of a commercial building or operating a manufacturing plant, there is a good chance that natural gas is helping you to achieve your goal.
Today, natural gas comprises a significant portion of the global energy supply across the residential, commercial and industrial sectors. This is due not only to a global abundance of natural gas, but also to the fact that it is cleaner, more efficient and less harmful to the environment than almost any other existing source of energy.
A key factor in the proper functioning and continued profitability of the natural gas industry is the ability to make precise measurements throughout the entire process of gas recovery, beginning with the wells drilled deep into the ground. Without an accurate way to monitor the flow of natural gas and make appropriate adjustments as needed, well operators would find it difficult or even impossible to optimize the recovery process.
Unfortunately, natural gas extraction presents a number of unique challenges when it comes to measuring flow, which makes it critical for operators to employ a measurement technology capable of maintaining a high level of accuracy and robustness under even the harshest of conditions. As an Argentinean gas company discovered, one form of flow measurement technology is particularly well-suited to meeting these demands: clamp-on ultrasonic flow meters.[Page Break]
Valve noise
In the Patagonia region of Argentina, a major gas company was looking for a new flow meter to measure the flow of natural gas from several of its wells.
Complicating the application was the fact that construction practices within the natural gas industry require only a minimal amount of pipe to be exposed above the ground, which considerably decreases the risk of damaging the pipe.
To work within this limited range of available piping and still ensure the highest level of accuracy, the measurement would need to be made in close proximity to the choke valve - only 20 pipe diameters downstream.
This location was problematic for several reasons: excessive valve noise, extremely high flow velocities and large amounts of impurities (such as sand and water) mixed in with the gas. The primary difficulty was considered to be valve noise. When natural gas with high sand and water content passes through the choke valve, the valve erodes and generates a significant amount of noise.
Over time this noise level increases as the valve closes tighter to compensate for erosion and maintain a steady flow rate.[Page Break]
Tolerant technology
Most traditional ultrasonic flow meters do not operate well in noisy environments like the one in Patagonia. Valve noise often occurs within the same frequency range as the signal used by many ultrasonic meters to measure flow. Therefore, if too much noise is present, an ultrasonic meter can have trouble distinguishing between the signal and the noise, ultimately resulting in measurement errors due to reduced signal quality.
Given these circumstances, the gas company was faced with just two choices. The first was to modify their requirements, shifting the measurement point far enough downstream from the choke valve (180 to 200 pipe diameters) to allow for some attenuation of the noise generated by the valve. This approach would expose a large amount of pipe above ground, increasing the risk of damage to the exposed portion of the pipe. As this option was considered less than ideal from both a construction and safety standpoint, it was disregarded.
The alternative option was to find a flow meter capable of tolerating the noise despite being located so close to the choke valve. The company researched various technologies and soon discovered that clamp-on ultrasonic flow meters from Siemens offered a viable solution to their problem. These meters feature WideBeam transit-time technology, which utilizes the pipe wall as a waveguide to inject the signal into the flowing gas. This optimizes the signal-to-noise ratio and increases precision by making the meter immune to most pressure-reducing valve noises. Because the WideBeam technique produces such a strong and coherent signal, it is also much more resistant to anomalies in gas flow (eg sand and water) and less sensitive to changes in pressure or medium type. The result: these meters can perform exceptionally well in most natural, specialty and process gas industry applications, including gas extraction.
An added benefit is that clamp-on flow meters feature sensors that are mounted on the outside of the pipe. External sensors minimize maintenance expenses and make it unnecessary to cut the pipe or stop the flow, eliminating the potential for additional leakage points at flanges.
In the case of the Argentinean gas company, this meant that it would be able to install the new meter in a very short period of time without being forced to make costly interruptions to the extraction process.
Armed with this new information, the gas company selected the Sitrans FUG1010 clamp-on ultrasonic flow meter.
In addition to the benefits already discussed, this meter features built-in analytical software incorporating a speed-of-sound calculation in compliance with the American Gas Association's AGA-10 standard, as well as an internal AGA-8 compliant table for fixed gas composition.[Page Break]
Consider clamp-on
Accurate flow measurement is vital to the natural gas industry, which relies on precise calculations to set fair prices. This means that even the smallest of inaccuracies can cost suppliers, buyers, distributors and ultimately customers dearly. Measuring flow can prove particularly challenging during the natural gas extraction process due to valve noise that interferes with the ultrasonic signals produced by many traditional flow meters. Clamp-on ultrasonic flow measurement is one form of technology that has proven capable of withstanding valve noise and producing dependable results.
James Doorhy is with Siemens AG Industry Sector Hauppauge, New York, USA. www.siemens.com
