Methane emissions

BY BENJAMIN HELLMIG AND GUILLAUME DE FOMBELLE

Methane accounts for the largest share of greenhouse gases after carbon dioxide. It is many times more potent but has a shorter lifespan in the atmosphere. Methane emissions are usually not entirely avoidable. Compressors in gas pipeline compressor stations leak due to their design as the mechanical dry gas seals currently used do not seal completely.

Pipeline operator GRTgaz from Western Europe decided to tackle the problem. Its infrastructure consists of several thousand kilometers of pipeline network with around 10,000 delivery, regulating, or sectioning stations as well as numerous compressor stations. Dry gas seals have been installed at new compressor stations for 25 years, and pipeline venting emissions (mainly for maintenance needs) are fed back into the pipeline with mobile gas compressors. Based on 2016 figures, the company has achieved a two-thirds reduction in methane emissions. The next target is to divide that number by five by 2024.

TANDEM SEALS

Conventional dry gas seals reduce emissions by up to 90% compared with oil-lubricated mechanical seals. Their tandem design meets safety requirements. If the main or primary seal fails, a secondary seal withstands full pressure. Other advantages include lower friction, longer service life, and efficiency.

If compressors do not remain in continuous operation, this can lead to seal contamination. There are several strategies to avoid this: A common practice is to depressurize the compressor housing, which results in methane escaping into the atmosphere. A better way is to keep the compressor housing pressurized during downtime. GRTgaz has implemented this strategy at several compressor stations using a rotating seal gas booster.

Another way to minimize leakage is to use dry gas seals in a double arrangement. They are supplied with nitrogen, which then enters between the seal surfaces as a sealing gas. This means that no impurities are created, and only small amounts of nitrogen enter the compressor. However, the reliability of the supply is crucial. Without nitrogen, double seals are damaged. In addition, their safety level is only sufficient for lower pressures.

Coaxial dry gas seals designs are similar to double seals. They can prevent leakage, are supplied with nitrogen, and are used at low pressures. The main advantages over double seals: their small footprint (comparable to a single seal) and resilience to nitrogen failure arising from a problem with the nitrogen generator.

This coaxial design has been adapted for use at higher pressures. This gives components more stability as well as additional secondary seal faces behind the coaxial seal to provide a safety level like that of tandem seals. If the nitrogen supply fails, the system can continue to operate with process gas. In such a case, only small amounts escape into the vent line.

This seal eliminates leakage during operation and shutdown. As it is permanently pressurized with nitrogen (N2), venting is no longer necessary. This also prevents contamination. The system operates at up to 160 bar operating pressure, even in the event of an N2 failure. Modifications to the compressor housing are unnecessary. It is suitable for new compressors and retrofits.

Although it can operate without nitrogen, an uninterrupted supply is important for environmental protection and to ensure clean gas. Instead of transporting it to the compressor station, the nitrogen had been successfully generated on site in the GRTgaz pilot project through pressure swing absorption. The nitrogen accumulator that secures the supply operates at 330 bar. Pressure is reduced downstream to the required seal supply level. It is estimated that this new seal design saves on average 230,000 cubic meters of process gas per year while reducing overall operating costs.