According to a report by the U.S. Environmental Protection Agency, more than 51,000 reciprocating compressors are operating in the U.S. natural gas industry, each with an average of four cylinders, representing over 200,000 piston rod packing systems in service. These systems contribute over 72.4 Bcf per year of methane emissions to the atmosphere, one of the largest sources of emissions at natural gas compressor stations.
All packing systems leak under normal conditions; the amount of which depends on cylinder pressure, fitting and alignment of the packing parts, and the amount of wear on the rings and rod shaft. A new packing system, properly aligned and fitted, may lose approximately 11 to 12 standard cubic feet per hour (scfh). As the system ages, however, leak rates will increase from wear on the packing rings and piston rod. One Natural Gas STAR Partner reported measuring emissions of 900 SCFH on one compressor rod.*
As the EPA works toward additional methane regulations, the emphasis on compressor emissions figures to increase. Methane is a greenhouse gas with over 20 times the global warming potential as carbon dioxide. Technologies must progress to ensure oil & gas companies are able to meet the growing requirements of regulatory committees and monitor their methane emissions accurately.
The following case study highlights how a major oil & gas company leveraged an insertion thermal mass flowmeter to monitor vent gas emissions from the packing systems of compressors as system components wore over time. This solution enabled the company to minimize emissions and determine the most efficient timing for replacement parts.
- Monitor the amount of gas being leaked from packing system of reciprocating compressors.
- ~10-1,000 SCFH leakage.
- Typically small line sizes (2”-4” / DN50-DN100).
- Low flowrates and vent pressures.
- As seals, packing rings, and piston rods wear, the flowrates increase.
- Insertion meter must be capable of being relocated and reconfigured for multiple line sizes at the unique compressor stations.
- Greenhouse gas measurement.
A leading oil & gas company required a gas flow measurement technology that could accurately measure natural gas flowrates at its compressor stations. One of the applications of interest was measuring vent gas emissions from the packing systems of compressors. This application not only required flow measurement at very low flows and low pressures, but the capability of achieving high turndown as well. Emissions can tend to increase over time as components wear, thus increasing the potential flowrates.
Turndown is the range within which a flowmeter can reliably provide accurate flow measurement. Most thermal mass flowmeters offer 100:1 turndown, which is a drastic improvement over other technologies, such as differential pressure. That said, the oil & gas application under consideration here required greater than 100:1 turndown.
Turndown is the range within which a flowmeter can reliably provide accurate flow measurement. Most thermal mass flowmeters offer 100:1 turndown, which is a drastic improvement over other technologies, such as differential pressure. That said, the oil & gas application under consideration here required greater than 100:1 turndown. This presented an improvement opportunity for the Magnetrol Thermatel Model TA2 thermal mass flowmeter.
Since the TA2 can hold multiple calibration curves, this capability helped it meet the customer requirement in this application. Magnetrol developed Auto Switching technology to fully utilize two distinct calibration curves. Auto Switching enabled the user in this case to automatically alternate between a low flow curve and a high flow curve to maximize data in the flow range and extend turndown. This ensured optimal performance at both low flows and the abrupt high flow (upset) condition, or an increase in flow over time due to degradation of components.
This content is sponsored by Magnetrol International. Sponsored content is authorized by the client and does not necessarily reflect the views of the Process Flow Network editorial team.
* “Reducing Methane Emissions From Compressor Rod Packing Systems,” U.S. Environmental Protection Agency, https://www.epa.gov/sites/production/files/2016-06/documents/ll_rodpack.pdf