tive maintenance program must be developed with clear goals and objectives that permit maximum utilization of the technologies. The program must be able to cross organizational boundaries and not be limited to the maintenance function. Every func- tion within the plant affects equipment reliability and performance, and the predictive maintenance program must address all of these influences. Vibration monitoring and analysis is the most common of the predictive maintenance technologies. It is also the most underutilized of these tools. Most vibration-based predictive maintenance programs use less than 1 percent of the power this technology provides. The primary deficiencies of traditional predictive maintenance are: • Technology limitations • Limitation to maintenance issues • Influence of process variables • Training limitations • Interpreting operating dynamics 13.1.1 Technology Limitations Most predictive maintenance programs are severely restricted to a small population of plant equipment and systems. For example, vibration-based programs are generally restricted to simple, rotating machinery, such as fans, pumps, or compressors. Ther- mography is typically restricted to electrical switchgear and related electrical equip- ment. These restrictions are thought to be physical limitations of the predictive technologies. In truth, they are not. Predictive instrumentation has the ability to effectively acquire accurate data from almost any manufacturing or process system. Restrictions, such as low speed, are purely artificial. Not only can many of the vibration meters record data at low speeds, but they can also be used to acquire most process variables, such as temperature, pressure, or flow. Because most have the ability to convert any propor- tional electrical signal into user-selected engineering units, they are in fact multime- ters that can be used as part of a comprehensive process performance analysis program. 13.1.2 Limitation to Maintenance Issues From its inception, predictive maintenance has been perceived as a maintenance improvement tool. Its sole purpose was, and is, to prevent catastrophic failure of plant equipment. Although it is capable of providing the diagnostic data required to meet this goal, limiting these technologies solely to this task will not improve overall plant performance. When predictive programs are limited to the traditional maintenance function, they must ignore those issues or contributors that directly affect equipment reliability. Outside factors, such as poor operating practices, are totally ignored. 268 An Introduction to Predictive Maintenance Many predictive maintenance programs are limited to simple trending of vibration, infrared, or lubricating oil data. The perception that a radical change in the relative values indicates a corresponding change in equipment condition is valid; however, this logic does not go far enough. The predictive analyst must understand the true meaning of a change in one or more of these relative values. If a compressor’s vibra- tion level doubles, what does the change really mean? It may mean that serious mechanical damage has occurred, but it could simply mean that the compressor’s load was reduced. A machine or process system is much like the human body. It generates a variety of signals, like a heartbeat, that define its physical condition. In a traditional predictive maintenance program, the analyst evaluates one or a few of these signals as part of his or her determination of condition. For example, the analyst may examine the vibra- tion profile or heartbeat of the machine. Although this approach has some merit, it cannot provide a complete understanding of the machine or the system’s true operat- ing condition. When a doctor evaluates a patient, he or she uses all of the body’s signals to diagnose an illness. Instead of relying on the patient’s heartbeat, the doctor also uses a variety of blood tests, temperature, urine composition, brainwave patterns, and a variety of other measurements of the body’s condition. In other words, the doctor uses all of the measurable indices of the patient’s condition. These data are then compared to the benchmark or normal profile for the human body. Operating dynamics is much like the physician’s approach. It uses all of the indices that quantify the operating condition of a machine-train or process system and eval- uates them using a design benchmark that defines normal for the system. 13.1.3 Influence of Process Variables In many cases, the vibration-monitoring program isolates each machine-train or a component of a machine-train and ignores its system. This approach results in two major limitations: it ignores (1) the efficiency or effectiveness of the machine-train and (2) the influence of variations in the process. When the diagnostic logic is limited to common failure modes, such as imbalance, misalignment, and so on, the benefits derived from vibration analyses are severely restricted. Diagnostic logic should include the total operating effectiveness and effi- ciency of each machine-train as a part of its total system. For example, a centrifugal pump is installed as part of a larger system. Its function is to reliably deliver, with the lowest operating costs, a specific volume of liquid and a specific pressure to the larger system. Few programs consider this fundamental requirement of the pump. Instead, their total focus is on the mechanical condition of the pump and its driver. The second limitation to many vibration programs is that the analyst ignores the influence of the system on a machine-train’s vibration profile. All machine-trains are Operating Dynamics Analysis 269 affected by system variations, no matter how simple or complex. For example, a com- parison of vibration profiles acquired from a centrifugal compressor operating at 100 percent load and at 50 percent load will clearly be different. The amplitude of all rota- tional frequency components will increase by as much as four times at 50 percent load. Why? Simply because more freedom of movement occurs at the lower load. As part of the compressor design, load was used to stabilize the rotor. The designer balanced the centrifugal and centripetal forces within the compressor based on the design load (100 percent). When the compressor is operated at reduced or excessive loads, the rotor becomes unbalanced because the internal forces are no longer equal. In addition, the spring constant of the rotor-bearing support structure also changes with load: It becomes weaker as load is reduced and stronger as it is increased. In more complex systems, such as paper mills other continuous process lines, the impact of the production process is much more severe. The variation in incoming product, line speeds, tensions, and a variety of other variables directly impacts the operating dynamics of the system and all of its components. The vibration profiles generated by these system components also vary with the change in the production variables. The vibration analyst must adjust for these changes before the technology can be truly beneficial as either a maintenance scheduling or plant improvement tool. Because most predictive maintenance programs are established as maintenance tools, they ignore the impact of operating procedures and practices on the dynamics of system components. Variables such as ramp rate, startup and shutdown practices, and an infinite variety of other operator-controlled variables have a direct impact on both reliability and the vibration profiles generated by system components. It is difficult, if not impossible, to accurately detect, isolate, and identify incipient problems without clearly understanding these influences. The predictive maintenance program should evaluate existing operating practices; quantify their impact on equipment reliability, effectiveness, and costs; and provide recommended modifications to these practices that will improve overall performance of the production system. 13.1.4 Training Limitations In general, predictive maintenance analysts receive between 5 and 25 days of train- ing as part of the initial startup cost. This training is limited to three to five days of predictive system training by the system vendor and about five days of vibration or infrared technology training. In too many cases, little additional training is provided. Analysts are expected to teach themselves or network with other analysts to master their trade. This level of training is not enough to gain even minimal benefits from predictive maintenance. Vendor training is usually limited to use of the system and provides little, if any, prac- tical technology training. The technology courses that are currently available are of limited value. Most are limited to common failure modes and do not include any train- ing in machine design or machine dynamics. Instead, analysts are taught to identify simple failure modes of generic machine-trains. 270 An Introduction to Predictive Maintenance To be effective, predictive analysts must have a thorough knowledge of machine/ system design and machine dynamics. This knowledge provides the minimum base required to effectively use predictive maintenance technologies. Typically, a graduate mechanical engineer can master this basic knowledge of machine design, machine dynamics, and proper use of predictive tools in about 13 weeks of classroom training. Nonengineers, with good mechanical aptitude, will need 26 or more weeks of formal training. 13.1.5 Understanding Machine Dynamics It Starts with the Design Every machine or process system is designed to perform a specific function or range of functions. To use operating dynamics analysis, one must first fully understand how machines and process systems perform their work. This understanding must start with a thorough design review that identifies the criteria that were used to design a machine and its installed system. In addition, the analyst must also understand the inherent weaknesses and potential failure modes of these systems. For example, consider the centrifugal pump. Centrifugal pumps are highly susceptible to variations in process parameters, such as suction pressure, specific gravity of the pumped liquid, back-pressure induced by control valves, and changes in demand volume. Therefore, the dominant reasons for centrifugal pump failures are usually process related. Several factors dominate pump performance and reliability: internal configuration, suction condition, total dynamic pressure or head, hydraulic curve, brake horsepower, installation, and operating methods. These factors must be understood and used to evaluate any centrifugal pump-related problem or event. All centrifugal pumps are not alike. Variations in the internal configuration occur in the impeller type and orientation. These variations have a direct impact on a pump’s stability, useful life, and performance characteristics. There are a variety of impeller types used in centrifugal pumps. They range from simple radial-flow, open designs to complex variable-pitch, high-volume enclosed designs. Each of these types is designed to perform a specific function and should be selected with care. In relatively small, general-purpose pumps, the impellers are nor- mally designed to provide radial flow, and the choices are limited to either enclosed or open design. Enclosed impellers are cast with the vanes fully encased between two disks. This type of impeller is generally used for clean, solid-free liquids. It has a much higher effi- ciency than the open design. Open impellers have only one disk, and the opposite side of the vanes is open to the liquid. Because of its lower efficiency, this design is limited to applications where slurries or solids are an integral part of the liquid. Operating Dynamics Analysis 271 In single-stage centrifugal pumps, impeller orientation is fixed and is not a factor in pump performance; however, it must be carefully considered in multistage pumps, which are available in two configurations: inline and opposed. Inline configurations (see Figure 13–1) have all impellers facing in the same direc- tion. As a result, the total differential pressure between the discharge and inlet is axially applied to the rotating element toward the outboard bearing. Because of this configuration, inline pumps are highly susceptible to changes in the operating envelope. Because of the tremendous axial pressures that are created by the inline design, these pumps must have a positive means of limiting endplay, or axial movement, of the rotating element. Normally, one of two methods is used to fix or limit axial move- ment: (1) a large thrust bearing is installed at the outboard end of the pump to restrict movement, or (2) discharge pressure is vented to a piston mounted on the outboard end of the shaft. 272 An Introduction to Predictive Maintenance INLINE CONFIGURATION 100 PSID 100 PSID 300 PSI 100 PSI 100 PSI 100 PSID 100 PSID 100 PSID OPPOSED CONFIGURATION Figure 13–1 Impeller orientation. Multistage pumps that use opposed impellers are much more stable and can tolerate a broader range of process variables than those with an inline configuration. In the opposed-impeller design, sets of impellers are mounted back-to-back on the shaft. As a result, the other cancels the thrust or axial force generated by one of the pairs. This design approach virtually eliminates axial forces. As a result, the pump does not require a massive thrust-bearing or balancing piston to fix the axial position of the shaft and rotating element. Because the axial forces are balanced, this type of pump is much more tolerant of changes in flow and differential pressure than the inline design; however, it is not immune to process instability or to the transient forces caused by frequent radical changes in the operating envelope. Factors that Determine Performance Centrifugal pump performance is primarily controlled by two variables: suction con- ditions and total system pressure or head requirement. Total system pressure consist of the total vertical lift or elevation change, friction losses in the piping, and flow restrictions caused by the process. Other variables affecting performance include the pump’s hydraulic curve and brake horsepower. Suction Conditions. Factors affecting suction conditions are the net positive suction head, suction volume, and entrained air or gas. Suction pressure, called net positive suction head (NPSH), is one of the major factors governing pump performance. The variables affecting suction head are shown in Figure 13–2. Centrifugal pumps must have a minimum amount of consistent and constant positive pressure at the eye of the impeller. If this suction pressure is not available, the pump will be unable to transfer liquid. The suction supply can be open and below the pump’s centerline, but the atmospheric pressure must be greater than the pressure required to lift the liquid to the impeller eye and to provide the minimum NPSH required for proper pump operation. At sea level, atmospheric pressure generates a pressure of 14.7 pounds per square inch (psi) to the surface of the supply liquid. This pressure minus vapor pressure, friction loss, velocity head, and static lift must be enough to provide the minimum NPSH requirements of the pump. These requirements vary with the volume of liquid trans- ferred by the pump. Most pump curves provide the minimum NPSH required for various flow conditions. This information, which is usually labeled NPSH R , is generally presented as a rising curve located near the bottom of the hydraulic curve. The data are usually expressed in “feet of head” rather than psi. The pump’s supply system must provide a consistent volume of single-phase liquid equal to or greater than the volume delivered by the pump. To accomplish this, the Operating Dynamics Analysis 273 suction supply should have relatively constant volume and properties (e.g., pressure, temperature, specific gravity). Special attention must be paid to applications where the liquid has variable physical properties (e.g., specific gravity, density, viscosity). As the suction supply’s properties vary, effective pump performance and reliability will be adversely affected. In applications where two or more pumps operate within the same system, special attention must be given to the suction flow requirements. Generally, these applications can be divided into two classifications: pumps in series and pumps in parallel. Most pumps are designed to handle single-phase liquids within a limited range of spe- cific gravity or viscosity. Entrainment of gases, such as air or steam, has an adverse effect on both the pump’s efficiency and its useful operating life. This is one form of cavitation, which is a common failure mode of centrifugal pumps. The typical causes of cavitation are leaks in suction piping and valves or a change of phase induced by liquid temperature or suction pressure deviations. For example, a one-pound suction pressure change in a boiler-feed application may permit the deaerator-supplied water to flash into steam. The introduction of a two-phase mixture of hot water and steam into the pump causes accelerated wear, instability, loss of pump performance, and chronic failure problems. Total System Head. Centrifugal pump performance is controlled by the total system head (TSH) requirement, unlike positive-displacement pumps. TSH is defined as the 274 An Introduction to Predictive Maintenance (H vp ) VAPOR PRESSURE (Hf) FRICTION LOSS IN SUCTION VELOCITY HEAD LOSS AT IMPELLER USEFUL PRESSURE AVAILABLE N.P.S.H. LOSS DUE TO USEFUL PRESSURE AT SURFACE OF LIQUID ATMOSPHERIC PRESSURE AT SURFACE OF LIQUID STATIC LIFT Figure 13–2 Net positive suction head requirements. total pressure required to overcome all resistance at a given flow. This value includes all vertical lift, friction loss, and back-pressure generated by the entire system. It deter- mines the efficiency, discharge volume, and stability of the pump. Total Dynamic Head. Total dynamic head (TDH) is the difference between the dis- charge and suction pressure of a centrifugal pump. Pump manufacturers that generate hydraulic curves, such as those shown in Figures 13–3, 13–4, and 13–5, use this value. These curves represent the performance that can be expected for a particular pump Operating Dynamics Analysis 275 200 150 50 100 100 200 300 400 500 600 700 800 1000 FLOW in gallons per minute (GPM) Total Dynamc Head (Feet) 65% 70% 80% 80% 70% 75% 65% 75% Best Efficiency Point (BEP) Figure 13–3 Simple hydraulic curve for centrifugal pump. 200 100 100 200 300 400 500 600 700 800 1000 150 50 65% 70% 80% 80% 75% 75% 65% 70% Best Efficiency Point (BEP) FLOW in gallons per minute (GPM) Total Dynamc Head (Feet) Figure 13–4 Actual centrifugal pump performance depends on total system head. under specific operating conditions. For example, a pump with a discharge pressure of 100psig and a positive pressure of 10psig at the suction will have a TDH of 90psig. Most pump hydraulic curves define pressure to be TDH rather than actual discharge pressure. This consideration is important when evaluating pump problems. For example, a variation in suction pressure has a measurable impact on both discharge pressure and volume. Figure 13–3 is a simplified hydraulic curve for a single-stage centrifugal pump. The vertical axis is TDH, and the horizontal axis is discharge volume or flow. The best operating point for any centrifugal pump is called the best efficiency point (BEP). This is the point on the curve where the pump delivers the best combination of pressure and flow. In addition, the BEP defines the point that provides the most stable pump operation with the lowest power consumption and longest maintenance- free service life. In any installation, the pump will always operate at the point where its TDH equals the TSH. When selecting a pump, it is hoped that the BEP is near the required flow where the TDH equals TSH on the curve. If it is not, some operating-cost penalty will result from the pump’s inefficiency. This is often unavoidable because pump selection is determined by choosing from what is available commercially as opposed to select- ing one that would provide the best theoretical performance. 276 An Introduction to Predictive Maintenance 200 100 100 200 300 400 500 600 700 800 1000 150 50 65% 70% 75% 80% 80% 75% 70% 65% BEP 15 HP 15 HP 20 HP 20 HP Total Dynamc Head (Feet) Figure 13–5 Brake horsepower needs to change with process parameters. For the centrifugal pump illustrated in Figure 13–3, the BEP occurs at a flow of 500 gallons per minute with 150 feet TDH. If the TSH were increased to 175 feet, however, the pump’s output would decrease to 350 gallons per minute. Conversely, a decrease in TSH would increase the pump’s output. For example, a TSH of 100 feet would result in a discharge flow of almost 670 gallons per minute. From an operating dynamic standpoint, a centrifugal pump becomes more and more unstable as the hydraulic point moves away from the BEP. As a result, the normal service life decreases and the potential for premature failure of the pump or its com- ponents increases. A centrifugal pump should not be operated outside the efficiency range shown by the bands on its hydraulic curve, or 65 percent for the example shown in Figure 13–3. If the pump is operated to the left of the minimum recommended efficiency point, it may not discharge enough liquid to dissipate the heat generated by the pumping oper- ation. This can result in a heat buildup within the pump that can result in catastrophic failure. This operating condition, which is called shut-off, is a leading cause of pre- mature pump failure. When the pump operates to the right of the last recommended efficiency point, it tends to overspeed and become extremely unstable. This operating condition, which is called run-out, can also result in accelerated wear and premature failure. Brake horsepower (BHP) refers to the amount of motor horsepower required for proper pump operation. The hydraulic curve for each type of centrifugal pump reflects its performance (i.e., flow and head) at various BHPs. Figure 13–5 is an example of a simplified hydraulic curve that includes the BHP parameter. Note the diagonal lines that indicate the BHP required for various process conditions. For example, the pump illustrated in Figure 13–2 requires 22.3 horsepower at its BEP. If the TSH required by the application increases from 150 feet to 175 feet, the horse- power required by the pump increases to 24.6. Conversely, when the TSH decreases, the required horsepower also decreases. The brake horsepower required by a centrifugal pump can be easily calculated by: With two exceptions, the certified hydraulic curve for any centrifugal pump provides the data required by calculating the actual brake horsepower. Those exceptions are specific gravity and TDH. Specific gravity must be determined for the specific liquid being pumped. For example, water has a specific gravity of 1.0. Most other clear liquids have a specific gravity of less than 1.0. Slurries and other liquids that contain solids or are highly Brake Horsepower Flow GPM Specific Gravity Total Dynamic Head Feet 3960 Efficiency = () ¥¥ () ¥ Operating Dynamics Analysis 277 [...]... difficult to quantify If the pumped liquid is delivered to an intermediate storage tank, the configuration of the tank’s inlet determines 280 An Introduction to Predictive Maintenance if it adds to the system pressure If the inlet is on or near the top, the tank will add no back-pressure; however, if the inlet is below the normal liquid level, the total height of liquid above the inlet must be added to the total... plane has a greater freedom of movement and, therefore, contains higher amplitudes at 1¥ than the vertical plane Failure-Mode Analysis 289 Figure 14–1 Single-plane imbalance Multiplane Multiplane mechanical imbalance generates multiple harmonics of running speed The actual number of harmonics depends on the number of imbalance points, the severity of imbalance, and the phase angle between imbalance... or vaned machinery such as fans and pumps, process instability creates an unbalanced condition within the machine In most cases, it excites the fundamental (1¥) and blade-pass/vane-pass frequency components Unlike Failure-Mode Analysis 2 97 true mechanical imbalance, the blade-pass and vane-pass frequency components are broader and have more energy in the form of sideband frequencies In most cases, this... rotor-support stiffness (X-axis) and critical rotor speed (Y-axis) Rotor-support stiffness depends on the geometry of the rotating element (i.e., shaft and rotor) and the bearing-support structure These two dominant factors determine the response characteristics of the rotor assembly 14.2 FAILURE MODES BY MACHINE-TRAIN COMPONENT In addition to identifying general failure modes that are common to many... instability, and process loading The process loading of most 290 An Introduction to Predictive Maintenance Figure 14–2 Multiplane imbalance generates multiple harmonics machine-trains varies, at least slightly, during normal operations These vibration components appear at the 1¥ frequency 14.1.3 Mechanical Looseness Looseness, which can be present in both the vertical and horizontal planes, can create... mechanical rotor imbalance is relatively small Single-Plane Single-plane mechanical imbalance excites the fundamental (1¥) frequency component, which is typically the dominant amplitude in a signature Because there is only one point of imbalance, only one high spot occurs as the rotor completes each revolution The vibration signature may also contain lower-level frequencies reflecting bearing defects and... Centrifugal pumps cannot absorb constant, rapid changes in operating environment For example, frequent cycling between full-flow and no-flow ensures premature failure of any centrifugal pump The radical surge of back-pressure generated by rapidly closing a discharge valve, referred to as hydraulic hammer, generates an instantaneous shock load that can literally tear the pump from its piping and foundation... classifications can be further broken into single-stage and multistage Each of these classifications has common monitoring parameters, but each also has unique features that alter its forcing functions and the resultant vibration profile The common monitoring parameters for all centrifugal pumps include axial thrusting, vane-pass, and running speed End-suction and multistage pumps with inline impellers are prone to. .. “constant-speed” machine-trains, such as electricmotor–driven centrifugal pumps Generally, the change is relatively minor (between 5 to 15 percent), but it is enough to affect diagnostic accuracy This variation in speed is enough to distort vibration signatures, which can lead to improper diagnosis With constant-speed machines, an analyst’s normal tendency is to normalize speed to the default speed used in the database... Figure 14–9, a resonance peak represents a large amount of energy This energy is the result of both the amplitude of the peak and the broad area under the peak This combination of high peak amplitude and broad-based energy content is typical of most resonance problems The damping system associated with a reso- 298 An Introduction to Predictive Maintenance Figure 14–9 Resonance response nance frequency is . theoretical performance. 276 An Introduction to Predictive Maintenance 200 100 100 200 300 400 500 600 70 0 800 1000 150 50 65% 70 % 75 % 80% 80% 75 % 70 % 65% BEP 15 HP 15 HP 20 HP 20 HP Total Dynamc Head. dynamics. Instead, analysts are taught to identify simple failure modes of generic machine-trains. 270 An Introduction to Predictive Maintenance To be effective, predictive analysts must have. contributors that directly affect equipment reliability. Outside factors, such as poor operating practices, are totally ignored. 268 An Introduction to Predictive Maintenance Many predictive maintenance