Advanced condition monitoring methods have been used to operate rotating machines reliably and efficiently. Practical notes on availability improvement plans, condition monitoring techniques, predictive maintenance methods, trouble-shooting techniques and smart operation are discussed in the article.
Modern plants and production facilities depend heavily on rotating machines. To maximize plant profit, rotating machines should be operated with maximum reliability, maximum capacity, maximum efficiency and minimum operating and maintenance costs. Effective maintenance starts with basic-design phase, particularly with rotating machine specification and machine basic design. Inadequate specification impacts extensively rotating machine reliability. Proper bidding phase clarification particularly extensive bid evaluation prior vendor selection and purchase order placement establish machine reliability basis. Proper development strategy needs to be selected for successful condition monitoring and predictive maintenance. Operation companies maximize profits by operating the un-spared (critical) rotating machines without shutdown. They are investing heavily on identifying and eliminating potential reliability issues through effective condition monitoring and predictive maintenance to meet continuous and efficient operation. Rotating machine manufacturers maximize profit by manufacturing the machinery to meet project specifications and applicable codes at the lowest cost to assure that the equipment will be reliable for the warrantee period. Rotating machine manufacturers usually do not initiate improvements to extend reliability and trouble-free operation for beyond manufacturer warrantee period. Many manufacturers believe they cannot stay competitive and in business if they design and produce equipment beyond code and client specification requirements for a long-term (let say 20 years) trouble-free operation.
For effective reliability analysis as well as failure analysis and trouble-shooting, all facts particularly process condition changes, piping and foundation changes, and ambient condition changes need to be considered. Rotating machine should be considered as a complete system including driver, transmission system, 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 its environment (its process, site conditions, its unique battery limits, unique piping arrangement, specific foundation, etc). Each machine has its own unique signature.
Condition monitoring is based on trending. It requires that suitable sensors are used, dedicated parameters are monitored, baseline (normal condition) is defined and trend of data to be 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 component considered in common-in-use condition monitoring systems are:
1- Bearings: Including radial and thrust bearings.
2- Seal and packing.
3- Rotor (shaft or crankshaft mechanism).
4- Auxiliaries: such as lube oil system, seal system, cooling system, etc.
Regardless of type of rotating machine, monitoring of four above mentioned categories determines condition of machine.
It is important to obtain baseline information as soon as possible after startup of rotating machine. Baseline conditions are usually ignored in project and it badly affects condition monitoring. Without baseline, there is no reference for comparing and interpretation of data.
Failure analysis and troubleshooting
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, 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 complete rotating machine system (all components, systems and parts involved, in upstream, downstream, driver or auxiliaries) is a very important step.
Major failure categories are summarized as follows:
1- Process condition changes: including process and operating condition changes as well as changes in operating procedure. It is most important reason for rotating machine failure.
2- Installation and commissioning issues.
3- Design, fabrication and assembly problems.
4- Machine wear-out.
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), flow-rate is not significantly affected by process 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 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. Reliability of dynamic rotating machine (as well as reliability of its 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 are affected by process. As an example, process requirement for a higher flow may result in driver overload (in case of electric motor driver, 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 machine or component occurs because 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 process system exceeded the capability of the machine. For a given impeller vane slope, 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, main failure root-cause is process/operating condition change. Second famous reason is installation and commissioning issue. Design or manufacturing problems (including engineering errors, material problems, manufacturing defects, etc) have 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 design problem is that the component is not designed for specified operating condition. Component wear-out is often the effect and not 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 root cause of trouble. Incomplete facts and insufficient information are famous reasons for failed trouble-shooting exercises. Usually in the rush to define what the problem is, many trouble-shooting engineers do not take sufficient time to obtain all of the facts. All changes should be properly identified for all components in rotating machine system (consider all parts such as auxiliaries, etc). Equipment functions particularly all component and sub-system functions need to be clearly identified. It is necessary to include all groups such as operators, maintenance people, manufacturers, sub-vendors, related contractors, etc in exercise. It is also very important to find all baselines related to the major parameters involved. Consider following guides for trouble-shooting investigation:
1- Carefully observe failed component(s), its conditions and mode of failure. Failed component inspection is critical. Vendor opinion and consultant advices should be considered.
2- Find unit history, particularly operation time before failure and history of previous failures especially similar failures.
3- Identify unit parameters, particularly parameters related to failed component (prior to failure). Base-line conditions should be obtained or established. Operator's logs and reliability data are very useful sources of information. Special attention is required for parameters exceeding normal value. Trends are always important.
4- Collect data of failed component supply-source (and manufacturing sources), design, materials, manufacturing details as well as assembly data and tolerances.
5- Identify all changes particularly changes in process conditions, operation, and any new thing happen in process or unit. Investigate unit piping, foundation and all surrounding facilities.
New modeling methods, advanced simulation techniques and numerical calculations play important roles in modern trouble-shooting and root-cause analysis. For example steam turbine rotor rub to the casing are often reported. Realistic dynamic and thermal expansion simulations of rotor and inner casing are required for root-cause analysis of such cases. For many reliability issues, accurate finite element analysis (FEA) of machine is necessary to find real root-cause and correct trouble-shooting. Short cuts or simplified models may result in wrong conclusion.
The first case study is presented for a large instrument air 450 rpm reciprocating compressor for a large process plant. It supplies instrument air to an air-receiver vessel (down stream of compressor) which feeds instrument air for various continuous and intermittent requirements of plant. Compressor train is started and stopped by pressure (instrument air volume remaining) of air-receiver. It is a classic design used for many units. Compressor uses oil-free technology, piston and packing work without any lubrication. Machine crankshaft system still needs lube oil. It requires pressurized lubrication system, using shaft driven oil pump. Compressor uses sleeve type bearings. Compressor suffers from bearing failure. Bearings are totally black. Babbitt is not found on bearing shells. High vibration was not reported. It was confirmed that bearings were correctly selected and installed. Extensive investigation shows that actual instrument air consumption of plant increases five times compare to design value. This increase is mainly due to purge flow increase of major electrical machines. It results in compressor start and stop five times more than specified value (compressor design condition). In other words, since the flow produced by the compressor is constant, the increased consumption flow now reduces the pressure in air-receiver more rapidly and results in more frequent stops and starts. It puts bearing under more transient stresses, results in more duration of transient lack of lubrication and excessive load (during transient situations). It leads to bearing failure.
This case study is inline with majority of previous root-cause studies that point process and operation changes as main reason for machine failure. Simply rotating machine operated in condition different than its design conditions.
Compressor vendor confirmed that this compressor model was originally designed for continuous operation. It can be operated in long durations of intermittent operation, as proposed for specified condition in biding stage and purchase order (based on calculation and also available successful references). But this compressor cannot afford five times more starts/stops. Proposed solution is changing compressor control philosophy by eliminating stop/start, operating compressor continuously and using a bypass control valve to suction to maintain air-receiver pressure.
The second case study is presented for oil contamination in the bearing brackets of a steam turbine. Water in the oil causes several bearing failure incidences. The source of water is from the carbon ring seal leakage of steam into the bearing bracket. Since the main reasons of problems are usually process condition/operation changes or assembly/installation issues, first, these potential causes are checked. A detailed analysis is confirmed that it is a design problem. The carbon ring seal system (carbon ring seals and bearing bracket isolator) is not designed to prevent oil contamination in the bearing bracket. For this case, two solutions can be proposed:
1) A bearing isolator to positively prevent steam condensate from entering the bearing bracket (such as "Impro" type).
2) Install an educator system to positively prevent leakage from the seal assembly.
Generally first option is preferred. In this case study first option is selected and implemented.
Alarm and trip set points are very important for efficient and reliable operation. These set-points should be properly selected to avoid unnecessary alarm or shutdown (trip) in transient but safe situations. On the other hand, malfunctions and problems must be identified in 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 rule 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 compare 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 very effective tool to evaluate bearing health. Lube oil viscosity reduction to less than a 50% (compare to lube oil producer specification), particle size larger than 25 micron 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.
(Amin Almasi is a rotating machine consultant based in Brisbane, Australia)