Maintenance Fundamentals 2011 Part 13 pptx

pressure for the fluid being pumped. Any vapor bubbles formed by the pressure drop at the eye of the impeller are swept along the impeller vanes by the flow of the fluid. When the bubbles enter a region in which local pressure is greater than saturation pressure farther out the impeller vane, the vapor bubbles abruptly collapse. This process of the formation and subsequent collapse of vapor bubbles in a pump is called cavitation. Cavitation in a centrifugal pump has a significant effect on performance. It degrades the performance of a pump, resulting in a degraded, fluctuating flow rate and discharge pressure. Cavitation can also be destructive to pump internals. The formation and collapse of the vapor bubble can create small pits on the impeller vanes. Each individual pit is microscopic in size, but the cumulative effect of millions of these pits formed over a period of hours or days can literally destroy a pump impeller. Cavitation can also cause excessive pump vibration, which could damage pump bearings, wearing rings, and seals. A small number of centrifugal pumps are designed to operate under conditions in which cavitation is unavoidable. These pumps must be specially designed and maintained to withstand the small amount of cavitation that occurs during their operation. Noise is one of the indications that a centrifugal pump is cavitating. A cavitating pump can sound like a can of marbles being shaken. Other indications that can be observed from a remote operating station are fluctuating discharge pressure, flow rate, and pump motor current. RECIRCULATION When the discharge flow of a centrifugal pump is throttled by closing the discharge valve slightly, or by installing an orifice plate, the fluid flow through the pump is altered from its original design. This reduces the fluid’s velocity as it exits the tips of the impeller vanes; therefore the fluid does not flow as smoothly into the volute and discharge nozzle. This causes the fluid to impinge on the ‘‘cutwater’’ and creates a vibration at a frequency equal to the vane pass x rpm. The resulting amplitude quite often exceeds alert setpoint values, particularly when accompanied by resonance. Random low-amplitude, wide-frequency vibration is often associated with vane pass frequency, resulting in vibrations similar to cavitation and turbulence, but is usually found at lower frequencies. This can lead to misdiagnosis. Many pump impellers show metal reduction and pitting on the general area at the exit tips of the vanes. This has often been misdiagnosed as cavitation. Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 6:12pm page 354 354 Maintenance Fundamentals It is very important to note that recirculation is found to happen on the discharge side of the pump, whereas cavitation is found to happen on the suction side of the pump. To prevent recirculation in pumps, pumps should be operated close to their operational rated capacity, and excessive throttling should be avoided. When a permanent reduction in capacity is desired, the outside diameter of the pump impeller can be reduced slightly to increase the gap between the impeller tips and the cutwater. NET POSITIVE SUCTION HEAD To avoid cavitation in centrifugal pumps, the pressure of the fluid at all points within the pump must remain above saturation pressure. The quantity used to determine whether the pressure of the liquid being pumped is adequate to avoid cavitation is the net positive suction head (NPSH). The net positive suction head available (NPSH A ) is the difference between the pressure at the suction of the pump and the saturation pressure for the liquid being pumped. The net positive suction head required (NPSH R ) is the minimum net positive suction head necessary to avoid cavitation. The condition that must exist to avoid cavitation is that the net positive suction head available must be greater than or equal to the net positive suction head required. This requirement can be stated mathematically as shown below. NPSH A ! NPSH R Figure 17.22 Vane pass frequency. Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 6:12pm page 355 Pumps 355 A formula for NPSH A can be stated as the following equation: NPSH A ¼ P suction À P saturation When a centrifugal pump is taking suction from a tank or other reservoir, the pressure at the suction of the pump is the sum of the absolute pressure at the surface of the liquid in the tank plus the pressure caused by the elevation difference between the surface of liquid in the tank and the pump suction, less the head losses caused by friction in the suction line from the tank to the pump. NPSH A ¼ P a ¼ P st À h f À P sat where NPSH A ¼ Net positive suction head available P a ¼ Absolute pressure on the surface of the liquid P st ¼ Pressure caused by elevation between liquid surface and pump suction h f ¼ Head losses in the pump suction piping P sat ¼ Saturation pressure of the liquid being pumped PREVENTING CAVITATION If a centrifugal pump is cavitating, several changes in the system design or operation may be necessary to increase the NPSH A above the NPSH R and stop the cavitation. One method for increasing the NPSH A is to increase the pressure at the suction of the pump. If a pump is taking suction from an enclosed tank, either raising the level of the liquid in the tank or increasing the pressure in the gas space above the liquid increases suction pressure. It is also possible to increase the NPSH A by decreasing the temperature of the liquid being pumped. Decreasing the temperature of the liquid decreases the saturation pressure, causing NPSH A to increase. If the head losses in the pump suction piping can be reduced, the NPSH A will be increased. Various methods for reducing head losses include increasing the pipe diameter; reducing the number of elbows, valves, and fittings in the pipe; and decreasing the length of the pipe. It may also be possible to stop cavitation by reducing the NPSH R for the pump. The NPSH R is not a constant for a given pump under all conditions but depends Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 6:12pm page 356 356 Maintenance Fundamentals on certain factors. Typically, the NPSH R of a pump increases significantly as flow rate through the pump increases. Therefore, reducing the flow rate through a pump by throttling a discharge valve decreases NPSH R . NPSH R is also dependent on pump speed. The faster the impeller of a pump rotates, the greater the NPSH R . Therefore, if the speed of a variable-speed centrifugal pump is reduced, the NPSH R of the pump decreases. The net positive suction head required to prevent cavitation is determined through testing by the pump manufacturer and depends on factors including type of impeller inlet, impeller design, pump flow rate, impeller rotational speed, and the type of liquid being pumped. The manufacturer typically supplies curves of NPSH R as a function of pump flow rate for a particular liquid (usually water) in the vendor manual for the pump. TROUBLESHOOTING Design, installation, and operation are the dominant factors that affect a pump’s mode of failure. This section identifies common failures for centrifugal and positive-displacement pumps. CENTRIFUGAL Centrifugal pumps are especially sensitive to (1) variations in liquid condition (i.e., viscosity, specific gravity, and temperature); (2) suction variations, such as pressure and availability of a continuous volume of fluid; and (3) variations in demand. Table 17.1 lists common failure modes for centrifugal pumps and their causes. Mechanical failures may occur for a number of reasons. Some are induced by cavitation, hydraulic instability, or other system-related problems. Others are the direct result of improper maintenance. Maintenance-related problems include improper lubrication, misalignment, imbalance, seal leakage, and a variety of others that periodically affect machine reliability. Cavitation Cavitation in a centrifugal pump, which has a significant, negative effect on performance, is the most common failure mode. Cavitation not only degrades a pump’s performance, it also greatly accelerates the wear rate of its internal com- ponents. Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 6:12pm page 357 Pumps 357 Table 17.1 Common Failure Modes of Centrifugal Pumps Source: Integrated Systems, Inc. Causes There are three causes of cavitation in centrifugal pumps: change of phase, entrained air or gas, and turbulent flow. Change of Phase The formation or collapse of vapor bubbles in either the suction piping or inside the pump is one cause of cavitation. This failure mode Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 6:12pm page 358 358 Maintenance Fundamentals normally occurs in applications such as boiler feed in which the incoming liquid is at a temperature near its saturation point. In this situation, a slight change in suction pressure can cause the liquid to flash into its gaseous state. In the boiler- feed example, the water flashes into steam. The reverse process also can occur. A slight increase in suction pressure can force the entrained vapor to change phase to a liquid. Cavitation caused by phase change seriously damages the pump’s internal com- ponents. Visual evidence of operation with phase-change cavitation is an impel- ler surface finish like an orange peel. Prolonged operation causes small pits or holes on both the impeller shroud and vanes. Entrained Air/Gas Pumps are designed to handle gas-free liquids. If a centrifugal pump’s suction supply contains any appreciable quantity of gas, the pump will cavitate. In the example of cavitation caused by entrainment, the liquid is reasonably stable, unlike with the change of phase described in the preceding section. Nevertheless, the entrained gas has a negative effect on pump perform- ance. While this form of cavitation does not seriously affect the pump’s internal components, it severely restricts its output and efficiency. The primary causes of cavitation that is due to entrained gas include two-phase suction supply, inadequate available net positive suction head (NPSH A ), and leakage in the suction-supply system. In some applications, the incoming liquid may contain moderate to high concentrations of air or gas. This may result from aeration or mixing of the liquid prior to reaching the pump or inadequate liquid levels in the supply reservoir. Regardless of the reason, the pump is forced to handle two-phase flow, which was not intended in its design. Turbulent Flow The effects of turbulent flow (not a true form of cavitation) on pump performance are almost identical to those described for entrained air or gas in the preceding section. Pumps are not designed to handle incoming liquids that do not have stable, laminar flow patterns. Therefore, if the flow is unstable or turbulent, the symptoms are the same as for cavitation. Symptoms Noise (e.g., like a can of marbles being shaken) is one indication that a centrifugal pump is cavitating. Other indications are fluctuations of the pres- sure gauges, flow rate, and motor current, as well as changes in the vibration profile. How to Eliminate Several design or operational changes may be necessary to stop centrifugal-pump cavitation. Increasing the available net positive suction head (NPSH A ) above that required (NPSH R ) is one way to stop it. The NPSH required to prevent cavitation is determined through testing by the pump manu- facturer. It depends on several factors, including type of impeller inlet, impeller Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 6:12pm page 359 Pumps 359 design, impeller rotational speed, pump flow rate, and the type of liquid being pumped. The manufacturer typically supplies curves of NPSH R as a function of flow rate for a particular liquid (usually water) in the pump’s manual. One way to increase the NPSH A is to increase the pump’s suction pressure. If a pump is fed from an enclosed tank, either raising the level of the liquid in the tank or increasing the pressure in the gas space above the liquid can increase suction pressure. It also is possible to increase the NPSH A by decreasing the temperature of the liquid being pumped. This decreases the saturation pressure, which increases NPSH A . If the head losses in the suction piping can be reduced, the NPSH A will be increased. Methods for reducing head losses include increasing the pipe diam- eter; reducing the number of elbows, valves, and fittings in the pipe; and decreas- ing the pipe length. It also may be possible to stop cavitation by reducing the pump’s NPSH R , which is not a constant for a given pump under all conditions. Typically, the NPSH R increases significantly as the pump’s flow rate increases. Therefore, reducing the flow rate by throttling a discharge valve decreases NPSH R . In addition to flow rate, NPSH R depends on pump speed. The faster the pump’s impeller rotates, the greater the NPSH R . Therefore, if the speed of a variable-speed centrifugal pump is reduced, the NPSH R of the pump is decreased. Variations in Total System Head Centrifugal-pump performance follows its hydraulic curve (i.e., head versus flow rate). Therefore any variation in the total backpressure of the system causes a change in the pump’s flow or output. Because pumps are designed to operate at their BEP, they become more and more unstable as they are forced to operate at any other point because of changes in total system pressure, or head (TSH). This instability has a direct impact on centrifugal-pump performance, reliability, operating costs, and required maintenance. Symptoms of Changed Conditions The symptoms of failure caused by variations in TSH include changes in motor speed and flow rate. Motor Speed The brake horsepower of the motor that drives a pump is load dependent. As the pump’s operating point deviates from BEP, the amount of Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 6:12pm page 360 360 Maintenance Fundamentals horsepower required also changes. This causes a change in the pump’s rotating speed, which either increases or decreases depending on the amount of work that the pump must perform. Flow Rate The volume of liquid delivered by the pump varies with changes in TSH. An increase in the total system backpressure results in decreased flow, while a backpressure reduction increases the pump’s output. Correcting Problems The best solution to problems caused by TSH variations is to prevent the variations. While it is not possible to completely eliminate them, the operating practices for centrifugal pumps should limit operation to an acceptable range of system demand for flow and pressure. If system demand exceeds the pump’s capabilities, it may be necessary to change the pump, the system requirements, or both. In many applications, the pump is either too small or too large. In these instances, it is necessary to replace the pump with one that is properly sized. For the application in which the TSH is too low and the pump is operating in run-out condition (i.e., maximum flow and minimum discharge pressure), the system demand can be corrected by restricting the discharge flow of the pump. This approach, called false head, changes the system’s head by partially closing a discharge valve to increase the backpressure on the pump. Because the pump must follow its hydraulic curve, this forces the pump’s performance back to- wards its BEP. When the TSH is too great, there are two options: replace the pump or lower the system’s backpressure by eliminating line resistance caused by elbows, extra valves, etc. POSITIVE DISPLACEMENT Positive-displacement pumps are more tolerant of variations in system demands and pressures than centrifugal pumps. However, they are still subject to a variety of common failure modes caused directly or indirectly by the process. Rotary Type Rotary-type, positive-displacement pumps share many common failure modes with centrifugal pumps. Both types of pumps are subject to process-induced failures caused by demands that exceed the pump’s capabilities. Process- induced failures also are caused by operating methods that either result in radical changes in their operating envelope or instability in the process system. Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 6:12pm page 361 Pumps 361 Table 17.2 lists common failure modes for rotary-type, positive-displacement pumps. The most common failure modes of these pumps are generally attributed to problems with the suction supply. They must have a constant volume of clean liquid to function properly. RECIPROCATING Table 17.3 lists the common failure modes for reciprocating-type, positive-dis- placement pumps. Reciprocating pumps can generally withstand more abuse and variations in system demand than any other type. However, they must have a consistent supply of relatively clean liquid to function properly. Table 17.2 Common Failure Modes of Rotary-Type, Positive-Displacement Pumps Source: Integrated Systems, Inc. Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 6:12pm page 362 362 Maintenance Fundamentals The weak links in the reciprocating pump’s design are the inlet and discharge valves used to control pumping action. These valves are the most frequent source of failure. In most cases, valve failure is caused by fatigue. The only positive way Table 17.3 Common Failure Modes of Reciprocating Positive-Displacement Pumps Source: Integrated Systems, Inc. Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 6:12pm page 363 Pumps 363 [...]... cost  100 ¼ % Total maintenance cost 12 Percent Supervision Cost of Total Maintenance Costs Total cost of supervision  100 ¼ Total maintenance cost 13 Maintenance Cost Percentage of Total Manufacturing Cost Cost of maintenance  100 Manufacturing cost 14 Maintenance Cost Percentage of Sales Total maintenance cost  100 Dollar value of sales 15 Cost of Maintenance Hour Total cost of maintenance  100... examples of reports 376 Maintenance Fundamentals Broad Indicators This group includes the ratio of maintenance costs to sales ratio of maintenance costs to value of assets maintenance expenditures by cost centers Work Load Indicators This group includes current backlog, total backlog ratio of preventive maintenance to total maintenance, ratio of daily maintenance to total maintenance, ratio of work... $1,000 invested in maintenance In summary, planned maintenance increases productivity and decreases costs for maintenance Concurrent with the increase in planned maintenance and OEE, the maintenance cost is gradually decreasing by 33% Performance Measurement and Management 379 Other findings are that planned maintenance increases the technical life of the equipment At 10% planned maintenance, the average... of wasted time, maintenance labor costs versus maintenance material cost, and maintenance cost per unit of production Cost Indicators This group includes work class or type percentages, actual maintenance cost as compared with budgeted costs, and percentage of maintenance administration cost to total maintenance cost These reports will be developed from information provided by all maintenance personnel... Value of Low Quality Maintenance Breakdowns Direct main cost and lost production cost  100 % Total number of breakdowns 9 Maintenance Dollar Percentage Mill Book Investment Total maintenance cost  100 ¼ % Plant investment book 10 Percent Labor Costs to Material Costs Total maintenance labor cost  100 ¼ Total maintenance material cost 11 Percent Clerical Manpower Costs of Total Maintenance Cost Total... thus focus too much attention on cutting the maintenance cost We all know that we can easily do this for a short period of time, but we have to pay back later The measurement goal for a maintenance organization should be: PQV=M FACTOR long term maintenance effectiveness measure Prime Quality Volume À X Maintenance Cost Maintenance Efficiency Percent Unplanned Maintenance Jobs $1000 ¼ PQV=M Percent Waiting... maintenance, ratio of daily maintenance to total maintenance, ratio of work performed under blanket work orders (or charge numbers) to total maintenance, ratio of capital work to total maintenance, ratio of shutdown work to total maintenance, ratio of area maintenance to total maintenance, and ratio of craft equipment backlog Planning Indicators This group includes jobs completed versus jobs planned, jobs completed... and maintenance and implementation of continuous improvement processes Improvement efforts in maintenance performance alone can often affect more than half of the improvement potential, and increased integration between operations and maintenance improvement efforts will give you the full effect of your improvement efforts Maintenance efforts to increase OEE will almost always result in savings in maintenance. .. always result in savings in maintenance costs in the range of 5–40% Our experience data show that invest- 378 Maintenance Fundamentals ments required to improve maintenance performance are in the range of 0.5–5% of the maintenance budget during the duration of an improvement project As a whole, maintenance improvement projects have a potential to pay back 5–15 times investments annually PRODUCTIVITY INDICATORS... is that planned maintenance is a key success factor and that planned maintenance cannot be achieved unless you have condition monitoring practices implemented to feed your planning procedures, which we refer to as conditionbased maintenance (CBM) The PQV/M Factor indicates how much Prime Quality Volume is being produced per $1,000 invested in maintenance This is a Results Oriented Maintenance productivity . has often been misdiagnosed as cavitation. Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 6:12pm page 354 354 Maintenance Fundamentals It is very important to note that recirculation. given pump under all conditions but depends Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 6:12pm page 356 356 Maintenance Fundamentals on certain factors. Typically, the NPSH R of. one cause of cavitation. This failure mode Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 6:12pm page 358 358 Maintenance Fundamentals normally occurs in applications such as boiler