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Apr-2015

Stop gas leaks from compressor packings

A new piston rod sealing technology based on a pressurised oil barrier can eliminate a significant source of gas leaks from reciprocating compressors.

Christian Hold and Tino Lindner-Silwester
Hoerbiger

Viewed : 7551


Article Summary

Reciprocating compressors keep shale gas flowing. These workhorses are found throughout the industry: at wellheads, gathering stations and gas treatment plants, and along pipelines. Their ability to be driven directly from engines fuelled by natural gas makes them especially valuable in remote locations beyond the reach of grid power.

Today’s recips are durable, versatile, energy efficient and cost effective, especially where high pressures are needed. Taking advantage of modern materials, computer models, manufacturing techniques and control systems, they are fully competitive with turbocompressors in many applications.

Each piston rod in a double-
acting compressor requires a seal or ‘packing’ to contain the high-pressure gas on the crankcase side of the cylinder (see Figure 1). Modern rod packings are effective and long lasting, but there is always a degree of gas leakage.

Leaks cost money, through the value of the lost gas and the nitrogen purge that is sometimes required to ensure safe operation. Leaks also have environmental and regulatory consequences. Methane is a powerful greenhouse gas, and some natural gas contains toxic components.

Now, compressor operators can for the first time eliminate packing leaks completely with the new XperSEAL piston rod sealing system developed by Hoerbiger. The system surrounds the piston rod with a pressurised oil barrier; as long as the oil pressure exceeds the gas pressure, gas cannot leak out.

Simple in principle, the new sealing system nevertheless depends on an advanced understanding of hydrodynamics and lubrication theory. It is 100% effective at stopping leaks, works on both moving and stationary compressors, eliminates the need for nitrogen purging, and is fully fail safe.

After thorough laboratory testing, the system has been trialled successfully at three plants handling natural gas. In each case gas leakage was eliminated and oil consumption remained at or below the levels measured with conventional rod packings.

Reasons to reduce leakage

Modern sealing rings and rod packings are made from various combinations of PTFE, bronze, graphite and elastomers. These offer excellent performance, but over the long term there is always some gas leakage. This has consequences in terms of costs, safety and the environment.

A typical oil-lubricated packing on a large compressor leaks 300 litres/hour of gas; a total of 500-1200 litres/hour can be considered a reasonable total leakage rate for a large lubricated compressor. That means up to 24 tons/year of gas lost for a four cylinder unit, which at $3.6/MMBtu is worth $5000/year. For worn or damaged packings the figure could be several times greater, and as packings approach the end of their lives, leakage rates – and costs – can become unacceptable.

Another operating cost is the oil used for lubrication: typically 1.5 litres/day per packing. This oil must be bought, managed and disposed of after use.

Since safety is always an issue when handling natural gas, some compressor installations use a stream of nitrogen to carry leaked gas away to a flare stack. This adds both capital and operating costs, especially if the ‘distance piece’ between the cylinder and the crankcase must be purged as well as the packing box.

Older compressors of the single-compartment type (API 618, 6.12.1.3 type B) have a leakage path between cylinder and crankcase, allowing gas to dissolve in the crankcase oil. This lowers the flashpoint of the oil and increases the fire hazard. The crankcase can be pressurised to prevent this, again at extra cost.

Some compressors are required to remain pressurised when they are stationary. Standard rod seals are not designed for this task, so special – and expensive – pneumatically operated static seals are sometimes installed.

The market value of gas lost through packing leaks is tiny compared to that of the gas passing through the compressor, and in any case static gas leakage is orders of magnitude higher than that which occurs when the compressor is running. However, the global warming potential (GWP) of methane is 30–80 times greater than the same weight of carbon dioxide (CO2), depending on the timescale chosen. As a result, the value of methane calculated in terms of the ‘social cost of carbon’ (SCC) is 6–18 times greater than its value as a fuel.1 Future carbon taxes could therefore significantly alter the economics of methane leakage.

Finally, toxic components such as benzene in raw natural gas are of concern to some operators. Eliminating leaks at source avoids the need to monitor the performance of flares or other disposal systems.

Oil provides gas-leak-free sealing
Fortunately, a truly gas-leak-free rod sealing system is possible. The principle behind the new zero-emission packing is the use of oil, rather than a solid material, as the sealing medium. A volume of pressurised oil surrounds the piston rod and is kept in place by two specially designed oil seal rings (1 and 2 in Figure 2). As long as the pressure of this oil barrier is higher than the gas pressure, the gas cannot leak out. And, because the rod is always covered by a film of oil, the oil seal rings operate virtually without wear.

Even when the compressor shuts down, the oil volume maintains an effective static seal as long as the oil pressure is maintained. As a result, a compressor that must remain pressurised during shutdown needs no additional arrangements to ensure effective static sealing.

As well as the oil seal rings (3a, 3b and 3c in Figure 3), the complete gas-leak-free packing contains two or three conventional single-acting packing rings (1), a buffer volume (2), and a wiper ring (4). All the rings are floating; in other words, they are free to move with lateral movements of the piston rod.

The purpose of the buffer volume (2) is to stop the oil barrier from seeing the full discharge pressure of the cylinder. This allows the oil pressure to be set just above the suction pressure, rather than above the discharge pressure, as it would have to be if the buffer were not present. This pressure reduction in turn lowers the mechanical loading on the oil seal rings.

The buffer volume is able to remain at the suction pressure of the cylinder thanks to the conventional packing rings (1) upstream. Any leakage past these rings during the compression stroke will increase the pressure in the buffer somewhat, but because the rings are single acting, the pressure immediately falls again during the suction stroke. In practice the rings need only modest sealing performance, and even worn rings will hold the buffer at the suction pressure satisfactorily.


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