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Advanced condition monitoring methods have been used to operate rotating machines reliably and efficiently. We can relate the process parameters with vibration data to provide a comprehensive solution to the issues related to your Turbo Machineries & rest of the assets. Practical knowledge of availability improvement plans, condition monitoring techniques, predictive maintenance methods, troubleshooting techniques, and asset life extension plans are important to ensure that your assets are safe to operate.
With many years of experience, we have attained knowledge to serve you better in troubleshooting your assets.
For effective reliability analysis as well as failure analysis and troubleshooting, all facts particularly process condition changes, piping and foundation changes, and ambient condition changes need to be considered. The rotating machine should be considered as a complete system including the driver, transmission system, and coupling, all involving auxiliaries such as gear unit (if applicable), lube oil system, cooling system (if any), seal system, etc. The rotating machine packages regardless of type always become customized because of their environment (its process, site conditions, its unique battery limits, unique piping arrangement, specific foundation, etc). Each machine has its unique signature.
Condition monitoring is based on trending. It requires that suitable sensors are used, dedicated parameters are monitored, a baseline (normal condition) is defined and a trend of data is captured to identify condition changes. Effective condition monitoring requires that all abnormal conditions be identified (compare to normal or baseline conditions). But it is not possible for all and every component of a complex rotating machine.
Major machine components considered in common-in-use condition monitoring systems are:
Regardless of the type of rotating machine, monitoring of the four above-mentioned categories determines the condition of the machine.
It is important to obtain baseline information as soon as possible after the startup of the rotating machine. Baseline conditions are usually ignored in the project and it badly affects condition monitoring. Without a baseline, there is no reference for comparing and interpreting data.
Rotating machines do not fail randomly. There are root causes for each failure. Usually, the condition of failed part is changed and leads to failure. To stop failure, it is necessary to know why failure occurs. Based on failure analysis knowledge, critical components should be selected for monitoring. Proper parameters, sensors, and set points need to be defined for condition monitoring. By being aware of the major reason for failure and by observing the condition of necessary components, a high level of reliability can be achieved. Most failures in predictive maintenance and trouble-shooting exercises occur because the entire rotating machine system is not considered. Defining a complete rotating machine system (all components, systems and parts involved, in upstream, downstream, driver or auxiliaries) is a very important step.
All rotating machines react to the process system requirements. They do what the process requires. It shows why process condition changes are so important.
For positive displacement rotating machines (such as reciprocating compressors, screw compressors, screw pumps, etc), the flow rate is not significantly affected by the processing system. Flow is a very good monitoring parameter to measure positive displacement machine health and determine if there is any problem or wear-out.
Dynamic machines (such as centrifugal compressors, axial compressors, centrifugal pumps, etc) using high-speed rotating parts (such as blades, impellers, etc) to increase the velocity of the fluid and then reduce the velocity of fluid mainly in volute to increase the fluid pressure. They offer variable flow. In other words, flow varies with operating conditions such as differential pressure or fluid density. The reliability of dynamic rotating machines (as well as the reliability of their driver and auxiliaries) is considerably affected by the process and operating conditions. Since the flow is determined by the process requirements, the machine loading, transmitted torques, driver power and auxiliary functioning is affected by the process. As an example, process requirements for a higher flow may result in driver overload (in the case of an electric motor driver, a trip may occur). The reliability of machine components (bearing, seal, etc) is directly related to the reliability of the auxiliary systems. In many cases, the root cause of the component failure is found in the supporting auxiliary system. As an example, changes in auxiliary system supply temperature, resulting from cooling water temperature change (for water-cooled systems) or ambient air temperature change (for air-coolers) can be the root cause of component failure (such as bearing failure in case of extra-hot oil). Process changes can have similar effects. Usually failure of a machine or component occurs because the equipment is subjected to conditions that exceed the design values.
Most machinery damage and wear occur during transient conditions such as start-up or shutdown conditions. During this time, the equipment is subject to rapid temperature, pressure and speed changes.
In many cases, the root cause of dynamic rotating machine mechanical damage is that the head required by the processing system exceeded the capability of the machine. For a given impeller vane slope, the head produced by a dynamic compressor or pump is a function of impeller diameter and impeller speed. Once the impeller is designed and fabricated, it will produce only one value of head for a given shaft speed and flow rate. The only factor that causes a lower value of head is if the machine experienced mechanical damage or if it is fouled. Pressure (particularly differential pressure) is a very good monitoring parameter to monitor dynamic machine health.
Based on experience, the main failure root cause is process/operating condition change. The second famous reason is the installation and commissioning issue. Design or manufacturing problems (including engineering errors, material problems, manufacturing defects, etc) have a third ranking. But design problem usually shows up shortly after startup. There are rare cases when design problem manifests after extended operation time. The main cause of the design problem is that the component is not designed for specified operating conditions. Component wear-out is often the effect and not the root cause. Wear-out of bearing, seal, wear-ring, etc is usually due to process condition changes. Various bearings often suffer from assembly or installation problems.
Trouble-shooting is to discover and eliminate the root cause of trouble.
Alarm and trip set points are very important for efficient and reliable operation. These set-points should be properly selected to avoid unnecessary alarms or shutdown (trip) in transient but safe situations. On the other hand, malfunctions and problems must be identified in the early stages to avoid catastrophic damages. Set points must be selected case by case considering all involved factors such as process, machine, shop-test results, performance-test data, baseline, etc but some rules of thumb and practical recommendations are noted as follows.
Regarding anti-friction bearings, housing vibration (peak) and bearing housing temperature limits are recommended around 10 mm/s and 85oC respectively. For hydro-dynamic bearings, housing vibration (peak to peak) and bearing housing temperature limits are recommended around 60 microns and 110oC respectively (limits are higher for hydro-dynamic bearings compared to anti-friction). For anti-friction and hydro-dynamic thrust bearings, axial displacement limits are around 1 mm/s and 0.5 mm respectively. Lube oil supply and return temperatures are usually around 60oC and 90oC respectively. Lube oil analysis is a very effective tool to evaluate bearing health. Lube oil viscosity reduction to less than 50% (compare to lube oil producer specification), particle size larger than 25 microns or lube oil water content above 200 ppm need careful attention (can be considered as alarm limits).
As an indication, for dynamic machines, if for a given flow rate and shaft speed, the head produced falls below the value predicted (based on certified and tested curve), by greater than 10%, the dynamic machine should be inspected at the first opportunity.
