In compressor applications, the sealing elements of piston rod packings are mostly manufactured out of optimized PTFE and PEEK compounds with additives for reinforcement and self-lubrication. The lifetime of the sealing elements is sometimes the bottleneck towards a sufficient service time between two routine maintenances and choosing the right material is a crucial task for the reliability of compressor systems. In industrial process applications the wear behavior of sealing materials can be strongly influenced by the characteristics of the application.
Even the most suitable sealing materials are sometimes failing due to variations of the process parameters as well as due to impurities in the gas that might suddenly occur. In some field cases the piston rod packings suddenly start to fail within short times while material and design of the sealing elements are the same that did run satisfactory for several years. Additionally, sometimes “A” and “B” machines in e.g. 2 x 100% concepts that are compressing the same gas at the same conditions with the same material and design are showing a significantly different lifetime performance.
In summary, even the best combination of sealing material and sealing ring design can only reduce the risk of unplanned downtime to a minimum but it never can ensure 100 % of reliability. In consequence, an online wear status of the sealing elements of a piston rod packing is needed to achieve the best reliability of the system and to utilize the maximum running hours of the sealing elements.
Background of all these methods is the assumption that the amount of leakage gas strongly corresponds to the wear status of the piston rod packing.
Compressor tests were performed to investigate this link. A fast wearing material (full wear after approx. 800 hours) was used in order to measure the leakage rate of the piston rod packing throughout the full operational duration of the internals.
It was found that the leakage rate shows high fluctuations during the whole test duration (see Figure 1). The flow rate exceeded 30 m³/h shortly after 100 % of the lifetime was exceeded and the test was finished.
Therefore, the wear status of piston rod packing internals cannot be estimated by measuring the leakage rate at the vent line and a direct measurement of the packing internals is needed.
Already today, the industry tries to estimate the wear status of the piston rod packing internals by
a) measuring the temperature at the vent line,
b) measuring the leakage gas flow or
c) measuring the temperature within the pressure packing.
The concept starts with the question about the dimensional changes of the sealing elements during wear. The wear takes place at the inner diameter of the sealing ring and the elements are moving towards the piston rod. Thus, the outer diameter of the sealing ring decreases, and this can be measured by using an adequate sensor arrangement.
As suitable measurement system, an Eddy Current Sensor was chosen because of its following benefits:
The central idea behind Smart Packing is to transfer the movement of a single sealing ring towards the piston rod by using an indicator pin (illustrated by Figure 2). The indicator pin will follow the movement of the sealing element and an Eddy Current Sensor will measure the movement of the indicator pin. From the measurement of the sensor, the wear status of the sealing ring can be calculated.
The sensor signals of the different sealing rings within the pressure packing are collected and processed by controllers that are located at the outside of the distance piece. The sensors as well as the controller are ATEX certified for maximum security. All signals from the controllers are leading into the “STASSKOL smart box” for analysis of the current wear status at the different packings and their sealing rings. The result of this data analysis can then be transferred at a certain frequency (e.g. one update per hour) to the customers monitoring system.
An unexpected compressor failure can produce high down times and therefore high financial losses for the end user. In order to be on the safe side, the lifetime of sealing elements – especially in non-lube applications – are not utilized to 100 %, meaning, that valuable running-hours are unused in exchange to increased reliability. The Smart Packing System (see Figure 3) measures the wear status of the internals of a pressure packing of a reciprocating compressor to deliver a reliable forecast of the remaining lifetime. This enables a utilization of the maximum lifetime of the sealing elements to close to 100 % and ensures, that new parts and technicians are available right in time for a planned overhaul. Furthermore, with the clear wear status of the packing known, maintenance work on other components like rings and valves can be planned better. This saves both costs and unnecessary downtime.
Thus, Smart Packing is really a new milestone for the condition monitoring and predictive maintenance at reciprocating compressors, giving the customer full information about the wear status of the pressure packings as key wear parts of the whole compressor system.