Paying Special Attention to Rotors and Coolers Can Yield Performance Improvements

Paying special attention to rotors and coolers is pivotal to achieving high performance for centrifugal compressors. This is one example of some of the maintenance practices that will move machine performance from good to great. Leveraging Ingersoll Rand’s global expertise, this article summarizes maintenance practices that can boost system performance, yield fewer breakdowns, reduce operating costs and improve productivity. The first section of this article offers best practices in five key maintenance areas. The second section focuses on rotor assembly maintenance, and the advantages of working with an expert to maintain compressors. The final section discusses rebuilds, reconditioning, and rerates.

1. Conduct preventative maintenance.

Centrifugal compressors are used widely across many industries, as they require less maintenance while providing higher airflow than similar-sized positive-displacement compressors. With few moving parts and an energy efficient design, they are simpler to maintain and also offer lower comparative lifecycle costs. As with any high performance equipment, environmental conditions, wear or neglect can affect centrifugal performance. Conducting practical maintenance can extend the operating life and system performance levels.

2. Customize maintenance programs.

It’s worth noting that regardless of how similar two facilities may be, their maintenance needs may differ dramatically. Centrifugal compressor operators should customize maintenance programs to specific equipment by taking into consideration unique operating conditions, hours of operation, the volume of starts and stops, seasonal shutdowns, and equipment age. By following good-practice basics, including regular inspection, cleaning, adjustment and repair, compressor owners can realize greater system reliability and performance. They also can improve environmental performance, worker safety and cost-efficiency of their operation. 

3. Conduct daily inspections.

Daily or shift change inspections provide operators with a baseline understanding of normal operating conditions. They are also excellent ways to catch early signs of trouble, which can include atypical noises, discolored oil, leaks, atypical vibrations, temperature variations or condensate drain clogs, before they impact the performance of the machine. A single deviation alone might not be enough to cause system failure, but when several deviations combine, the risk of failure increases dramatically. Recognizing minor changes in operation and responding appropriately can help reduce risk and keep compressors running reliably. Routine items that fall into daily inspections include recording your system’s operating parameters, wiping external surfaces, checking for leaks and checking condensate drain function. 

4. Schedule routine maintenance.

As with any type of rotating industrial machinery, regular maintenance is key to identifying and reducing repair costs. Major centrifugal compressor breakdowns can result in damaged impellers, gears and motors, and an increase in repair costs well beyond the cost of scheduled preventative maintenance. Emergency repairs can run as much as three times the cost of standard maintenance work and replacing a complete rotor assembly can run many times more.

Preventative maintenance involving professional inspection and timely replacement of common components is not only best practice, but it is also particularly important to avoid a protracted shutdown that could seriously affect production. A typical preventative service checklist should cover a thorough inspection of the drive motor, gear case operation, cooler and lubrication systems, control panel and valve operation, and all filter elements.

Advantages of Preventative Centrifugal Compressor Maintenance

• Routine non-invasive analysis of compressor component condition is significantly more economical than invasive, unplanned maintenance
• Controlled maintenance when needed can prevent catastrophic failure and reduces overall costs
• Maintenance planning is simplified when equipment performance is confidently understood
• Reduced unplanned downtime for increased production and revenue

5. Pay special attention to coolers and rotor assemblies.

Rotors and cooler components are central to high-performing aerodynamics of today’s turbo machines, so regular inspection and proper maintenance are key to maintaining system reliability and performance.  Many of today’s modern centrifugal compressor designs allow operators greater access to perform routine inspections and cleaning tasks.

Inspect for inorganic debris and potentially corrosive acids. Lack of inspection and maintenance can create numerous problems with air and oil coolers including elevated temperatures that can impact overall compressor performance, affect bearing life and downstream processes. Organic and inorganic debris as well as potentially corrosive acids may flow through the coolers and foul the cooler tubes. In addition to monitoring cooling water quality to minimize fouling, it is important to conduct periodic inspections and cleanings to maintain the mechanical performance of the compressor.

Routinely check cooler performance. The condition and life of any cooler can be positively affected with proper maintenance. With proper cleaning and maintenance, a heat exchanger can perform well for years. Operators should regularly check cooler performance by measuring the cold temperature difference (CTD). CTD is the difference between water entering and the air leaving the cooler and should register at 15 degrees Fahrenheit or better. Scheduled cleaning will depend on the quality of the water source and how the equipment is being used.

Both the water and air-sides must be cleaned and the condensate drain should also be inspected to ensure moisture isn’t compromising the quality of the compressed air and downstream performance.

For waterside cleaning, use an air gun or tube-cleaning kit with a range of brush sizes. Begin by probing the length of the tube with a diameter rod to check for any obstructions. If standard U-tubes are part of the cooler design, do not force the rod into the U-bend.  With the tube sheets up, fill the tubes with a commercial descaling agent and allow to stand for two hours. During this process, pay special attention to keep chemical cleaners away from the aluminum fins.

Once complete, carefully drain the descaling solution from the tubes and flush thoroughly with water. Create a simple rod-and-brush assembly by attaching a soft bristle nylon or brass brush to a rod. Connected to a drill motor, this simple mechanism can be used to remove build-up from tube walls while flushing with clean water. Drain the water and refill the tubes with the descaler allowing to process for one hour before flushing again with clean water. This time with a brush sized accordingly for the diameter of the tube, use the rod-and-brush assembly to further clean tubes while simultaneously flushing with clean water.

For air-side cleaning, compressed air, pressurized steam or chemicals can be used when cleaning the intercooler fins.  Removing the upper and lower baffle plates will provide access to the complete finned surface. Fins can be cleaned with low-pressure steam or water and if debris has accumulated, use an aluminum-friendly chemical cleaning agent. Be sure to inspect and carefully straighten any bent fins before replacing the baffle plates.

Inspecting and cleaning the condensate removal system following cooler cleaning is essential to avoiding contamination that can damage intercooler fins, inlets, diffusers and impellers. Often overlooked as part of preventative maintenance, condensate drains should be inspected daily and cleaned on a quarterly basis or sooner, if needed.

Modern compressors are built with service and maintenance in mind. Maintenance on newer models such as the Ingersoll Rand Centac C800 and C1000 or TURBO-AIR NX 12000 air compressors is made easier with an open-cooler casing and design that offers easy access to critical package components.

Focus on rotor assembly maintenance

One of the greatest advantages of a centrifugal compressor is that it has very few moving parts, which minimizes mechanical problems, maintenance and energy losses due to friction. Other than the bullgear and driver, the only other moving part in a centrifugal compressor is the rotor, making it one of the most critical components of the system.

Even with routine maintenance and normal operation, the rotating assembly will attract a buildup of solids on the induction side of the impeller and can cause increased vibration on the individual stages of the compressor as well as decreased efficiency. The amount of acceptable vibration varies among models, however, as the pinion vibration increases, the likelihood of nuisance trips or even premature failure of the pinion and/or bearings increases. Failures of this type can be costly if replacement of the part is required.

Routine cleaning and balancing performed by either an original equipment manufacturer (OEM) or an expert repair service will prevent premature wear, keep the compressor running at peak performance and extend the use of your original rotor assembles for an indefinite period.

After acquiring vibration and phase measurements, rotor balancing may be required. The cleaning process begins with glass-bead blasting to remove all dirt and debris from the impellers. Pinions are polished on a high-speed lathe to ready them for inspection including mechanical and electrical run-out analysis which should be done prior to and after any repair work.