Since their development in the 1980s, Dry Gas Seals have become the de-facto standard for rotating shaft sealing applications in centrifugal and screw compressors across the oil and gas sector. However, with gas compressors having operating lives running into several decades, there are still many running with wet lubricated (oil) seals. OEMs have almost universally switched to dry gas seals for new compressors. Should operators of older compressors be actively considering the retrofit of new, dry gas seals to their installed compressor base?
In principle, the answer is yes! Dry gas seals typically offer more tolerant operation with reduced (sometimes significantly) maintenance costs and environmental impact. However, in the real world, the answer is not so simple. Like so often, it depends!
Dry gas seals became common in the 1980s and 90s. They represented a major advance in shaft sealing technology over the established oil seals used up until then. Oil seals required extensive infrastructure to deliver and maintain a clean oil supply demanding, in turn, frequent maintenance. Oil seals also produced significant emissions.
It almost seemed the dry gas seal was the perfect solution to these issues. However, almost as soon as dry gas seals started to be introduced, oil seal manufacturers responded by starting to address these issues. Consequently, over the past 35 years, oil seal technology has progressed significantly. Therefore, modern oil seals, including upgrades from older seals, still have an important role to play.
When deciding to use oil seals or retrofit dry gas seals to a compressor, as always, the key is cost-efficiency.
Let’s consider 4 key areas:
Swapping out existing oil seals and retrofitting dry gas seals to a compressor is far from simple. As well as the high cost of the dry gas seals, the work to remove and eliminate the existing oil supply system from the compressor is costly and must be factored in. There is also the expense of a replacement gas panel. Moreover, where it isn’t practical to use process gas for sealing, an inert buffer gas, together with the additional cost, must be considered. Additionally, will retrofitting dry gas seals on selected compressors bring maintenance complications? A site with established maintenance routines and procedures for oil seals will need to retain these for legacy systems while introducing new maintenance processes for the dry gas seals.
Of course, dry gas seals also offer real financial benefits. Significantly simplified support systems mean lower maintenance requirements. Furthermore, the improved sealing efficiency of dry gas seals means reduced gas loss and improved overall compressor efficiencies.
A wide range of compressor designs has been developed and installed over many years. Some of these designs incorporate specific oil lubricated seals. Consequently, it may not always be practical to retrofit dry gas seals into certain compressors. The physical dimensions of the compressor shaft and/or space available for the original oil seals may make a dry gas seal retrofit functionally impossible. Also, it may not be feasible to use or modify the existing oil ports for dry gas seals.
On the other hand, it has long been established that dry gas seals are less able to handle dirty process gas than wet seals. However, even here the picture is becoming more blurred. Modern, multi-ring, circumferential dry gas seals are proving more tolerant of contaminated process gas, albeit only for low-pressure applications.
There is no question that dry gas seals operate with significantly lower environmental impact than oil seals. Oil seals produce several gallons of sour oil each day. Significant gas losses to flare from the sour oil pots can occur unless a flare gas recovery unit is available or the gas is re-introduced at compressor suction. Managing and preventing seal oil emissions can also be challenging and expensive, particularly when seals are unreliable and/or failing.
With some older oil seal and system designs, it may not be possible to upgrade them to a level that meets the necessary environmental standards. In these cases, retrofitting dry gas seals may be the only realistic option.
New, engineered systems are typically more efficient than older systems. This general statement is equally true when it comes to compressor seals. Retrofitting dry gas seals in place of existing oil seals coming to the end of their service life will undoubtedly improve compressor performance and reliability going forward. However, there are numerous reasons, as we discuss above, why retrofitting to dry gas seals is challenging, or even impractical.
In these, and indeed other cases, it may be possible to improve/upgrade existing oil seals and systems to achieve many of the same benefits – without the need to go to the extreme of a complete switch to a dry gas system.
Modern, improved oil seal designs address many of the issues (engineering and environmental) presented by older seal designs. Making the decision to use oil seals or dry gas seals in your compressor is now even more challenging.
Expert advice is the answer
While there is clearly no ‘one-size-fits-all’ solution as to whether oil seals or dry gas seals offer the best approach for your compressors, the answer lies in taking a wide range of factors into consideration. One thing is clear – the best answer does not lie in THIS brand of seal or THAT brand of seal. The best solution lies in having a thorough and expert understanding of seal engineering and performance for both oil seals and dry gas seals, coupled with the technologies and procedures to ensure that your solution is driven first and foremost by real-world seal performance in your application.