Maintenance Fundamentals Episode 2 part 9 pps

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 [...]...364 Maintenance Fundamentals to prevent or minimize these failures is to ensure that proper maintenance is performed regularly on these components It is important to follow the manufacturer’s recommendations for valve maintenance and replacement Because of the close tolerances between the pistons and the cylinder... radiates out through the cap, thus diminishing the pressure over the disk, opening the trap to discharge condensate 368 Maintenance Fundamentals Figure 18.3 Thermodynamic steam trap Wear and dirt are particular problems with a disk-type trap Because of the large, flat seating surfaces, any particulate contamination such as dirt or sand will lodge between the disk and the valve seat This prevents the valve... accumulated liquids OPERATING METHODS Steam traps are designed for a relatively constant volume, pressure, and condensate load Operating practices should attempt to maintain these parameters as 3 72 Maintenance Fundamentals Table 18.1 Common Failure Modes of Steam Traps Source: Integrated Systems, Inc Steam Traps 373 much as possible Actual operating practices are determined by the process system rather... through a mechanical linkage The bucket sinks when condensate fills the steam trap, which opens the outlet valve and drains the bucket It floats when steam enters the trap and closes the valve 365 366 Maintenance Fundamentals Figure 18.1 Inverted-bucket trap As a group, inverted-bucket traps can handle a wide range of steam pressures and condensate capacities They are an economical solution for low- to mediumpressure... opens when condensate, air, and other gases are present Two key advantages of bimetallic traps are (1) compact size relative to their condensate load-handling capabilities and (2) immunity to hydraulic-hammer damage Steam Traps 3 69 Figure 18.4 Bimetal trap Their biggest disadvantage is the need for constant adjustment or calibration, which is usually done at the factory for the intended steam operating... operate at any steam temperature In steam-tracing applications, it may be desirable to allow controlled amounts of condensate to back up in the lines to extract more heat from the condensate 370 Maintenance Fundamentals Figure 18.5 Thermostatic trap In other applications, any hint of condensate in the system is undesirable The thermostatic trap can handle either of these conditions, but the thermal... condensate to be handled by the steam trap In addition to an increased volume of condensate, poor steam quality may increase the amount of particulate matter present in the condensate High concentrations of solids directly affect the performance of steam traps If particulate matter is trapped between the purge valve and its seat, the steam trap may not properly shut off the discharge port This will... inverted-bucket trap designed for 125 -psi service will operate at lower pressures, but its capacity is so diminished that it may back up the system with unvented condensate Therefore it is critical to select a steam trap designed to handle the application’s pressure, capacity, and size requirements Float-and-Thermostatic The float-and-thermostatic trap shown in Figure 18 .2 is a hybrid A float similar to... Inverted-bucket and float-type steam traps are highly susceptible to mechanical damage If the level arms or mechanical linkages are damaged or distorted, the trap cannot operate properly Regular inspection and maintenance of these types of traps are essential Calibration Steam traps, such as the bimetallic type, must be periodically recalibrated to ensure proper operation All steam traps should be adjusted on... thermostatic element purges non-condensable gases, it closes tightly when steam enters the trap The advantage of this type of trap is that it drains condensate continuously Steam Traps 367 Figure 18 .2 Float-and-thermostatic trap Like the inverted-bucket trap, float-and-thermostatic traps as a group handle a wide range of steam pressures and condensate loads However, each individual trap has a very . 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 .20 04 6:12pm page 3 62 3 62 Maintenance. capabilities and (2) immunity to hydraulic-hammer damage. Figure 18.3 Thermodynamic steam trap. Keith Mobley /Maintenance Fundamentals Final Proof 15.6 .20 04 7:39pm page 368 368 Maintenance Fundamentals Their. given pump under all conditions but depends Keith Mobley /Maintenance Fundamentals Final Proof 15.6 .20 04 6:12pm page 356 356 Maintenance Fundamentals on certain factors. Typically, the NPSH R of