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Segmented sealing rings for piston rods

Reduced wear, long service life and low emissions
Segmented sealing rings for piston rods

About 70 % of all reciprocating compressor failures are the result of eight main causes, the most common one being valve failures at more than 35 %. Packing problems, which make up about 18 % of all failures, rank second. Other causes only account for a maximum of 8 %. What this means is that the failure rate can be reduced enormously simply by replacing the packing rings. BCD rings, which are designed specifically for the prevailing conditions in each case, have proven especially effective.

The author: Andreas Zeiff Freelance journalist, Stutensee

Reciprocating compressors are essential components for the compression and transportation of gases or mixtures. Reliability is therefore paramount and the sealing systems used play a central role. In particular, the piston rod seal often causes problems. One solution is to incorporate a special sealing ring that is tail-ored exactly to the particular conditions at the sealing point. Compared to conventional seals, the service life of Hoerbiger’s BCD rings is more than twice as long and gas leakages are dramatically reduced. This sealing element, which can be easily retrofitted, provides solutions for even the most complex challenges, thus eliminating the need for large investments in new plant components. Moreover, in new installations, the compact BCD packing seals also save valuable space in the compressor. BCD rings perform particularly well in natural gas applications, where more and more compressors are non-lubricated. With conventional so-lutions, these scenarios lead to increased wear. Numerous practical cases have confirmed a considerable improvement in the system’s life and reliability after replacing the valves and packing seals with modern components.
Application with triple benefit
BCD rings have been used on a NuovoPignone 2HA12 alkylation compressor at an Italian refinery since April 2009. As part of an upgrade project, the operator wanted to increase the service life of the existing packing rings and at the same time cut down gas emissions through leakage. Gas leakage is especially significant for dry, non-lubricated hydrogen compression. Hoerbiger provided customised BCD packing seals based on the compressor’s key parameters, the composition of the gas to be compressed and the required pressures, volumes and velocities. The compressor went straight back to full-load operation after the BCD rings had been installed. The results of the upgrade were altogether impressive. Even du-ring the first hours of operation, no gas emissions were detected. The leaks that had previously plagued this unit were reduced to zero. In addition, it was evident that the packing ring design could achieve a longer service life in continuous duty, as requested by the customer. The targeted 10,000 operating hours have already been exceeded – with neither mainte-nance nor compressor shutdowns. The upgrade has allowed the operator to extend the service intervals and synchronise them with other maintenance jobs.
Significantly extended service life
Another example from the process gas industry highlights the advantages of the modern ring design. One refinery, for instance, dramatically improved the operating times of its hydrogen compressor. Hydrogen with traces of methane is compressed in two stages to 66 bar at temperatures of approximately 100 °C. The old water-cooled, non-lubricated seals operated at a piston rod speed of approximately 2.6 m/s. The BCD rings were fitted into the existing, cooled packing case. The material of the rings was selected to meet the specific requirements associated with very dry operating conditions. In continuous duty with the new sealing rings it was possible to increase the service life to more than 22,000 hours (approx. 2.5 years). Despite the narrower, split design, the sealing performance and durability are very high whereas the rod temperature is now significantly lower. As easily retrofittable components, BCD rings are suitable for all applications where the rod seal formerly required short maintenance intervals.
Fundamental advantages
The case studies described above clearly illustrate the increased benefits of the sealing rings in terms of service life, wear and emissions. It is evident that the specific design of the BCD rings offers fundamental advantages as well as enhanced pressure balancing and sealing efficiency. As a direct result of the favourable pressure distribution between the rings and the piston rod, the contact pressure – and thus friction and heat generation at the ring – are reduced. This contributes towards substantially lower wear rates and therefore a longer lifetime combined with consistently high sealing efficiency. The durability of the BCD seal has been approximately doubled compared to previous designs. Problems due to overheating during off-load operation, multiple start-ups or static leakage are now a thing of the past.
The special design of the BCD rings leads to much higher reliability and sealing efficiency: the four ring segments are divided into primary and “cap” elements, each fulfilling different tasks. While the cap elements seal in the axial and radial directions, the primary segments seal against the piston rod and form a gap to compensate for wear. This ingenious design ensures correct pressure balancing and very efficient sealing. Besides the above reference installations, both manufacturers and operators of reciprocating compressors have testified to the benefits of the BCD ring in their own applications and trials. For example, independent test series conducted by Howden Thomassen Compression show up to 70 % lower leakage rates compared with conventional seals. The company carries out these tests as part of its compressor component selection procedure. Unlike traditional sealing solutions, the leakage rate remains minimal even when the pressure increases while the frictional heat to be dissipated is also well below the usual levels.
The narrow design of the BCD ring means it can be retrofitted in existing compressor units without difficulty. Thanks to its segmented structure, the ring can be inserted without having to dismantle the rod, which can be time-consuming. The special geometry renders the anti-rotation lock which was previously standard in segmented rings superfluous. In addition to reduced gas leakage during operation or shutdowns under pressurised conditions, the compressor’s environmental performance is likewise significantly improved.
Determining the footprint
Modern reciprocating compressors are designed to be robust, durable and efficient. However, depending on the actual design and environmental conditions, certain components wear out more rapidly. These represent the maintenance “hot spots”, which can have a negative impact on the overall eco-nomic balance. The rod seal is one of these critical components. Until not long ago, it was impossible to determine the exact thermophysical processes taking place at the sealing ring. The seals were developed purely on the basis of empirical experience, which was not neces-sarily transferable to new applications. For this reason, Hoerbiger came up with a mathematical model that is used to predict the gas pres-sure profiles between the sealing ring and rod for each type of compressor; this information is then used to calculate the “footprint” of the required ring design. The model is also able to simulate the breakdown pressure on the packing as well as the temperature profiles at the piston rod. The exact pressure curve, surface pressures and frictional heat generation can be computed in this way. Hoebiger’s experts succeeded in developing an easy-to-install, segmented sealing ring based on the theoretical findings. Its special geometry takes account of all pressure influences from static and dynamic loads. Comprehensive tests on the test rig confirmed the drawing board results with respect to sealing performance, pressure distribution and frictional behaviour.
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