Myth: New turbomachinery is always better than older machines

Turbomachinery Magazine, September/October 2021,

The phrase "if it ain't broke, don't fix it" has served us well in the turbomachinery industry.


As engineers, we usually operate under the mantra “if it ain’t broke, don’t fix it.” If an old piece of plant equipment operates effectively, we maintain it with tender loving care. Otherwise we don’t touch it. This approach has served us well in the turbomachinery industry, with many legacy machines worldwide functioning well after more than 50 years of service. Some “dinosaur” refinery compressors actually exceed 80 years and still work properly.

But there are reasons — economic, performance, and environmental — that drive us to replace these machines with newer models. They may break too often or cost too much to maintain. Spare parts may no longer be available. A degraded machine’s efficiency may be too low, or it may not otherwise meet design flow, pressure, or power requirements of a growing plant. It may be incapable of handling process changes like higher pressures or pressure ratios and the resulting rotordynamic challenges. Finally, environmental concerns and regulatory requirements may force the retirement of old equipment when it cannot meet acceptable emissions or other standards.

Industrial machinery like compressors, turbines, and expanders has come a long way over the last 100 years. Efficiency has improved, emissions have been reduced, and reliability/availability generally has increased. But there are cases where the newest equipment is not always better than what has been operating for a long time, particularly if operating conditions have remained the same.

Admittedly, older machines were sometimes engineered and manufactured “heavier” than modern machines. Older design tools were not as accurate as today’s advanced FEA and CFD methods. More conservative design margins were required. Increasing the energy density of the machine reduces capital cost per power installed. This has led to cost savings, smaller footprints, less weight, often better aerodynamics with better range and efficiencies, and more head per compressor casing — but not necessarily a more rugged machine.

Modern aerodynamic blades have been refined and optimized and typically are very efficient. They have complex 3D shapes and some-times operate on tighter margins of aerodynamic stability. We have learned a lot over the last 50 years about rotordynamics, blade dynamics, and bearing/seal design. Our design tools have gone through many generations of improvements and validation using test data. Although modern rotor and blade designs may not always look as rugged as previous generations, if properly designed, they are as reliable and have similar life expectancies.

Does it make more sense to uprate/overhaul/refurbish/repair an old machine or replace it with a newer model?

But there are exceptions: By cramming more head into a compression stage, sometimes newer machines may have lower efficiencies. Since they run faster with higher tip speeds, compressible effects can lead to larger aero losses.

The question thus becomes: Does it make more sense to uprate/overhaul/refurbish/repair an old machine or replace it with a newer model? The answer depends on many factors, including maintenance cost, service parts availability, performance and the machine’s ability to meet current safety, environmental or operational requirements. Another factor is whether the OEM still supports the machine and has the capability to bring the old machine to required new standards.

This discussion is too long to cover in a single article. But the following examples provide some insights into a few technical advances that did not necessarily result in more reliable machines:

Example 1: Most very old reciprocating compressors installed on U.S. pipelines are low-speed (up to 350 rpm) integral machines. These large, heavy machines manufactured prior to the 1960s have proven reliable and efficient. Nonetheless, for the past 25 years, pipeline operators have mostly installed high-speed, (up to 2000 rpm) separable compressors. They’re not nearly as reliable as old integral machines, and their efficiency is substantially lower because of higher flow, pulsation, and valve losses. But operators prefer the much lower initial cost (on a per-horsepower basis) over efficiency and availability and have been slowly replacing their integral machines with high-speed, separable reciprocating compressors.

Example 2: Dry gas seals were introduced about 40 years ago. After some infancy issues, they have proven reliable and effective in avoiding the introduction of oil into the process stream. Dry gas seals have completely replaced wet seals in the market, and for the last 15 years, few wet seals have been utilized in new compressors. However, in some dirty compression applications with significant particulate matter in the gas stream or other solid contamination, dry gas seals are sometimes less reliable than older wet seals, which are better at handling particulate matter. Yet it is nearly impossible to purchase compressors with wet seals anymore since very few OEMs offer them as an option

Example 3: To meet ever stricter NOx air emissions requirements, gas turbine OEMs have developed dry low NOx (lean premixed) combustion systems. In parallel, to increase output power and efficiencies, firing temperatures have drastically increased over the last 30 years, requiring advanced super alloys and complex but very sensitive cooling schemes. Conventional low-temperature diffusion flame combustors used between 30 and 60 years ago are generally more forgiving. But they simply do not meet modern emissions standards and today’s expectations for total plant efficiencies.

Many turbomachines still operate well after 30 and sometimes 50 years of service. If a machine is well maintained, restaged, refurbished, over-hauled, and repaired as necessary, it can operate well beyond its original 25-year API target design life. There are even cases where older machines are more efficient and reliable than newer machines.

When considering the replacement of older machines with newer models, take care to evaluate the impact that doing so may have on all aspects of operation, performance, and emissions of a plant. Legacy equipment that has been in operation for decades may not necessarily be inferior to modern equipment when all factors are considered. It is often possible to update, upgrade, and overhaul equipment for much less money than it costs to buy new. When considering replacing an old machine versus refurbishing or overhauling it, take into account not just efficiency but also reliability, operability, range and maintenance costs.