failure mechanism of impeller of centrifugal pumps
Failure Mechanisms of Centrifugal Pump Impeller
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Pumps are the prime movers in different process industries. Being a backbone of successful plant operations, centrifugal pumps are subjected to some potential risks that can result in impeller failures. Centrifugal pump impellers are highly susceptible to different failures. The core failures discussed includes cavitation, erosion, and corrosion. Each of these major classes of failures is further sub-divided based on their probable causes along with the degree of severity. Cavitation arises by reduction of suction pressure, low inlet fluid flowrate and Net Positive Suction Head Available (NPSHA) less than Net Positive Suction Head Required (NPSHR). Issues related to corrosion are highly diversified and is because of the different mechanism leading to impeller failure. Apart from that, erosion failures are also quite serious and cause serious damage to pumps. Analysis of these core aspects is done through different methodologies that encompass preventive and corrective actions. As part of the preventive action of impeller failure, FMEA (Failure Mode and Effect Analysis) is conducted.
Failure Mechanisms of Centrifugal Pump Impeller
Pumps are one of the prime movers used in different industries based on their applications. The core purpose is to transport fluids (mainly liquids) that can either be slurries or solutions. Centrifugal pumps are categorized by their capability of dealing with higher flowrate and moderate-to-high pressure. However, centrifugal pumps are subjected to some potential anomalies during operations that can ultimately result in complete failure of the impeller. Impeller, being the core part of pumps, has the capability to generate required pressure and desired flowrate for different users in any industry. From small to large scale pumps, problems like cavitation, microbial growth, vibrations, corrosions, erosion, shaft imbalance and excessive power consumption usually arise due to the impeller. For each of these core problems, there exists a separate cause and effect analysis that is also a part of this research paper. Moreover, troubleshooting of such problems, cause analysis, FMEA (Failure Mode Equipment Analysis) will also be discussed in detail. The frequency of such failures also plays a significant role in determining the nature and severity of problem faced by maintenance engineers.
Failures Mechanism of Centrifugal Pumps
Bloch Has generated a framework of different commonly occurring failures and characterized them by following one or more root causes. It includes material defects, faulty design, installation or assembly defects, processing or fabrication defects, unintended service condition, maintenance deficiencies and most importantly, improper operations of a centrifugal pump. Details of these root causes will be discussed in ample detail in upcoming sections of the research paper.
Most of the hydraulic failures are the result of alteration in pressure that can either be in the inlet/outlet pipelines or volute casing. It leads to significant damage to centrifugal pumps and more specifically, its impellers. However, there are some factors that can include changes in temperature, velocity and volumetric flowrate of the flowing fluid. Following are some common failures that occur as part of the above changes:
Cavitation is the first and most dangerous condition for the pumps. It is caused when the pressure of liquid falls below that of its vapor pressure. The phenomenon causes the formation of vapor bubbles that strike impeller, and the damage is dependent on volumetric flowrate of fluid and rpm of the impeller. The condition arises due to different reasons. It can be due to the reduction of suction fluid pressure, an increase in inlet temperature or increase in inlet flowrate above designed conditions. One condition or a combination of above conditions gives rise to cavitation. From designers’ point of view, the pumps are designed for not operating at peak (efficiency) conditions and for that, operating range of pumps is defined. The centrifugal pumps usually operate in the range of around 85-110% BEP (Best Efficiency Point). Nonetheless, most of the pumps usually operate beyond that range. For that designers tend to make the bubbles collide within the pipelines rather than the pump’s impeller vanes to avoid damage.
Enhanced volumetric flowrate is one of the causes of cavitation. For analyzing this problem, there are different indicators that are linked to it. With a high volumetric flowrate, the pump would face a sharp decrease in the volume of the system along with abnormal temperature increase. The operator would surely face an abrupt increase in suction temperature of the fluid as one of the indicators of cavitation. This condition can also accelerate towards higher fluid vapor pressure at pump suction along with flow instability at the inlet. In extreme cases, flow closer to zero will also result in rapid pump casing heat-up thereby leading to vapor locking of the system.
For characterizing cavitation in pumps, a term called Net Positive Suction Head (NPSH) is usually used during plant operations. Following figure clearly shows the vivid representation of NPSH for predicting cavitation behavior in pumps:
Figure SEQ Figure * ARABIC 1: NPSH vs. Flowrate for Centrifugal Pumps
From the figure, it is clearly evident that there is a higher risk of cavitation and impeller failure whenever there is a small different between the NPSHR (Net Positive Suction Head Required) and NPSHA (Net Positive Suction Head Actual). The small difference is commonly referred to as NPSH Margin, is an indication of reduced pressure at suction due to a head loss at suction side. It can also indicate that the pump is operating above its design or nominated conditions. The NPSHA is calculated using different inlet characteristics of fluid; however, NPSHR is the amount that is required to avoid cavitation. It is independent of inlet properties and is same under any circumstances or conditions in which the pump is operating.
Cavitation damages, which occur from low pressure, have following four symptoms:
The collapsing of fluid vapor bubbles in the area of higher pressure results in extreme local stresses on impeller’s inlet vane. Signs of erosion will commonly appear as pitting as part of fluid or water hammering upon collapsing vapors. Damage is mainly occurring because of the collapse of cavities thereby resulting in the release of the jet of water (or fluid) on hitting the surface of the impeller with local speed as that of the speed of sound. It creates a higher surface pressure that is usually greater than the ultimate tensile strength of the material.
Figure SEQ Figure * ARABIC 2: Factors associated in Head Capacity Curve.
From experimentation, it has been found that the rate of cavitation rapidly multiplies by the factor of 4 with increasing volumetric capacity from 100-120% of shocks flow. The point on the NPSH vs. flowrate curve is generally around 1.1-1.3 times the capacity of BEP flow. However, if the flow continues to increase the shocks flow, the NPSHA will become lesser than the NPSHR thereby resulting in cavitation.
Another Important symptom of cavitation is the massive sound of cavities collapsing as part of the higher pressure. The intensity of noise is a clear measure of severity of cavitation in pumps. The sound usually originates from the inlet or in the area near pump’s suction. However, if the crackling sound is random with high intensity of knocks, it is an indication that the pump is facing cavitation in suction recirculation. Based on the acoustic radiation, cavitation is divided into three types with the difference in location of cavities formation and location of an implosion of vapor bubbles.
Sheet Cavitation is the type in which the cavities are formed all around the vanes surface having pump operations near design flow but reduced pressure. The noise produced is in the range of 2-40 kHz with low amplitude.
It is another type of cavitation in which the cavities are formulated downstream of the very cavity sheet under the condition of low suction pressure pump operation and quite closer to the designed flow rate. Considering cloud cavitation, it is considered to be the loudest of all forms of cavitations and has frequencies as high as 20-40 kHz. The sound resembles quite closely with pump gravels under high fluid flow rates.
Vortex is the third type that is witnessed as part of impeller’s failure. It is the most unstable and high detrimental cavitation form that deals with the operations of a pump having inlet backflow and low inlet flow rates. The phenomenon that is associated with it is quite similar to collapsing of bubbles that is relatively lesser damaging than its counterparts. The core reason behind it is that the vapors collapsing phenomenon occur quite away from the solid surface of the impeller. Vortex Cavitation is quite commonly characterized by its randomizing noise bursts that are followed by typical cavitation noise. During the conditions having NPSHA approaching 3% head decay line with the pump operating in backflow regime, these conditions generate a low-frequency noise in the range of 1-4 Hz. Moreover, this phenomenon is also called cavitation surge.
Vibrations in pumps are usually characterized by low-frequency and high amplitude that is usually found within the range of 0-10 Hz.
Pump Efficiency Reduction
The creation of vapor bubbles in the vicinity of impeller strongly impedes the current of fluid flow thereby resulting a sudden drop in pump’s efficiency. This drop of efficiency is also one of the most prominent sign or troubleshooting tool for detecting cavitation within pumps. However, noise and vibrations are not that much prominent unless the intensity of cavitation is massive and damaging to the impeller. Also, in some rare circumstances, it has been observed that the pump efficiency would face some slight increase in efficiency of the pump from time to time before the beginning of cavitation phenomenon. It can be due to the reason of reduction in efficiency because of friction during the start of separation within flow just before the implosion of cavities. There are also different means for detecting cavitation in pumps rather than indicators like noise, vibration, and periodic efficiency randomness. For that, a suction gauge or a pressure sensor at the inlet of the pump is installed for determining whether NPSHA is less than or equal to that of NPSHR. It can also be detected via specialized microphones that have associated features of capturing acoustic radiation emerging from cavitation process.
Pumps usually face radial thrusts that are towards the center of rotation of the impeller. Pump’s impeller is faced with a dynamic cyclic load that is superimposed by a steady state load. The core issue arises when the-the dynamic component of load keeps on increasing with the pump is in low flow conditions. On the other hand, the static load is a function of high as well as low flow rate operations with its value quite near to BEP. This phenomenon is quite commonly observed in single volute casing design as compared to double volute design. However, it is quite rare in pumps with diffuser designs. Based on these core understandings, it can be deduced that the radial thrust is also dependent on volute geometry, and it is an important factor for reducing it.
Excessive radial thrust is quite commonly a result of one or more different conditions. It can include enhanced shaft deflection that can lead towards mechanical seals and packing problems and in extreme circumstances, can also result in impeller shaft failure. Impeller shaft failure is quite usually in multistage and double suction pumps. The critical location of the failure is usually in the mid of shaft span. Pumps having end-suctions face impeller shaft failure at shaft’s shoulder where the shaft sleeve and impeller are joined as part of a single assembly. This location bears significantly high-stress concentration. An excessive rise in bearing temperature can also result from radial thrust thereby leading to the `reduction in bearings lifespan.
Detection of radial thrust issues is quite difficult in operating pumps. The rise of bearing temperature could be a symptom for excessive radial thrust on impeller shaft. High bearing temperature can also be due to improper lubrication, misalignment, and axial loading thrust. Troubleshooting these aspects of failure would help in pinpointing the failure due to radial thrust. Most of the issues arising from radial thrust are because of low fluid flow rate. For mitigating this risk, the pumps are operated at high flow rates and more importantly, by installing a small diameter discharge line to pump suction for making up the suction pressure.
Axial thrust is also a superimposed dynamic cyclic load that is either inward or outward of impeller’s shaft. Increasing the axial thrust would result in enhanced stresses thereby leading towards metal fatigue. These stresses would also result in excessive temperature increase and shorter life expectancy of bearings. Most of the axial thrust failure in bearings arises from severe fatigue rising from dynamic axial and cyclic loads.
Damage to the bearing is because of dynamic as well as axial thrust. Significant static axial thrust would lead towards cracking of rollers and ball bearing. On the other hand, excessive dynamic loads would result in high level of fatigue on ball or rollers along with raceways of rolling elements in bearings.
Impeller shaft failure can also be caused due to cyclic loading that is induced on the shaft having pump operations at partial recirculation. In this specific scenario, cyclic stresses in an axial direction are greatly reduced by enhancing the pump throughput along with an installation of pump recirculation bypass line. However, if these conditions cannot be allowed during plant operations, it can be compensated by replacing impeller shaft material of construction from another more endurable alternative.
For detection of axial thrust, different types of equipment are used for determining the magnitude and severity of the issue. Among those, proximity-based sensors are employed for the determination of shaft’s axial movement on bearing housing.
Erosion and Wear Ring Damage
Erosion can occur in different forms in pumps. Cavitation erosion was discussed in the previous sections. However, additional erosion types include abrasive wear, fretting, adhesive wear and erosion through solid particles impingement.
Adhesive wear is quite commonly caused as part of the material to material contact. It is one of the most common reasons for material loss while handling fluids having suspended solids and particulate matter. Different surfaces inside the pump including pump impeller are subjected to such material to material contact thereby giving rise to material grooving, surface disruptions and deposition of solid foreign particles on impeller blades.
Fretting is a specialized case that is a part of adhesive wear. It occurs when two components of the pump are subjected to small cyclic amplitudes. It takes place on surfaces having a close clearance that includes the area between the impeller and a shaft. The pumps have the potential for fretting because of the small cyclic amplitude of loosely fitted impellers along with areas under loose bearings and within the space between the impeller hub and wear rings of impellers. Based on small amplitudes, it is quite impossible for detecting fretting. It can only be detected through the introduction of red powdered oxide on the fretted area. During the pump operations, this surface is usually washed out because of the current of fluid; however, some of it sticks with the surface of fretted part thereby making it possible to detect. For avoiding such catastrophic results, these parts should have closer clearance, and it can also be shrinking fitted for preventing such small but disruptive movements. However, under the operating conditions that make fretting unavoidable. Lubricating and coating the exposed surfaced is usually done. Common coatings of silver plating, flame-sprayed high-nickel alloys are done on either one or both ends of it.
Another important type is abrasive wear, and it is characterized by the interactions of solids with different internal components. It can either be two or three-body wear. Among them, three body wear is the primary mechanism of different damages in pumps. It usually happens whenever the solid particles come in contact between impeller keyway and ring fit areas. For minimizing these damaging effects, the wear ring clearance plays an important part. It is the clearance between the impeller ring and casing ring. If the particles coming in contact with it are either too large or too small, they will get stuck within the clearance area. However, if the particles have the same fit as that of wear ring clearance, they would become lodged between those two rings thereby resulting in damage.
Another category is about solid particle impingement. Most of the pumps are also used for transportation of slurries from one place to another as part of plant operations. These solid particles have a strong impact on pressure heads and fluid’s velocity. These particles can be a part of natural systems; like, water from the lakes and ponds would have a significant amount of foreign particles. Also, they can be a part of the system that can be the pipe burn or weld slag. These damaging potential of these particles is strongly related to the fluid velocity and can do great damage to pump’s internals and impeller vanes.
Corrosion is a common process and is the degradation of material through chemical action. Corrosion can be general, galvanic, dealloying, hydrogen embrittlement, microbiologically induced corrosion, stress corrosion cracking and intergranular corrosion.
General corrosion is a generalized form of corrosion that takes place without any localized attack. This type of corrosion is quite limited for metals and their alloys that do have the ability for forming a passive film that inhibits further corrosion. It usually occurs as part of the oxidation process and metal oxide is formed. The pumps have a copper base, cast iron and carbon steel alloys face this type of corrosion. The rate of corrosion is dependent on different factors including the velocity of fluid, pH, oxygen content, temperature and fluid chemistry. Hence, for most pumps, a protective coating is placed on exposed area of steel for inhibiting corrosion.
Another type of corrosion that occurs in pumps’ impellers is dealloying. It is a result from the removal of one phase of an alloy from multiphase alloy. It can also be the removal of one element of the alloying mixture. One of the most widely known cases is that of graphitic corrosion of gray cast iron. Grey cast iron is used because of its feasibility to fabricate and low cost. However, this type of corrosion is quite commonly evident in saltwater and freshwater. The higher the conductivity of salt, the greater would be the tendency of water to corrode pump inner casing and impeller. The mechanism of dealloying corrosion can be decreased by having high mineral content in the fluid.
Microbiologically induced corrosion (MIC) is another case of corrosion that is commonly encountered in stagnant water. This type of corrosion occurs whenever the pump has been on shutdown condition for a prolonged amount of time. Most of the sulfates producing bacteria form tubercles which are a slimy and reddish colored colonies are formed on cast iron and carbon steel. Scrapping this type of corrosion would leave a saucer looking pit that is usually wet and black deposit. This specific pitting is because of formation of sulfuric acid from bacteria. Nevertheless, it does not cause imminent and premature failure of equipment. For counteracting with this issue, different biocides are used for mitigating this issue. However, the biological organism decay can also cause biological deposition on inner parts of pumps. This biological activity also has the capability of impeding with corrosion susceptibility of different bronzes.
Galvanic corrosion is another type of corrosion that occurs whenever the alloys are electricity connected with another alloy and is also exposed to the conductive environment. Different factors play an important role in it. It includes a ratio of exposed coupled area of alloys, the conductivity of the fluid and negative potential of different metallic alloys.
SCC (Stress Control Cracking) is another damaging form of corrosion and cannot be detected unless it has advanced to the extreme stage of propagation thereby resulting in catastrophic failure. It is not that much common in pumps’ impellers; however, it usually takes place in different classes of materials. The factors that support it include a highly susceptible material, residual or applied tensile stress, time of contact and corrosive environment.
Hydrogen embrittlement arises by the combination of applied or residual stress with hydrogen. The damage can be in the form of blistering, cracking and significant loss of ductile nature of metal. It is also quite rarely found in the pump and is usually an outcome of plating process and is used for rebuilding of shafts of pump’s impeller.
Intergranular corrosion is caused due to differences in local chemicals. One of the examples of it includes austenitic SS (Stainless Steel) plate having chrome deprived regions. Bronze alloys are also quite susceptible towards intergranular corrosion. It leads towards the formulation of fatigue cracks that are corrosion assisted having cyclic loading applied to the metal. These classes of corrosion are also quite detrimental for pump’s inner casing and impeller.
Failure Mechanisms – Methodology and FMEA
Different methodologies are taken to analyze possible and existing failure mechanisms in centrifugal pumps. Existing methods are used for performing structural failure analysis. The core aim of these methods is to prevent the failures specifically failures with detrimental consequences. This category can be further divided into steps that are taken in designing phase and steps that are taken in operations phase. During designing phase, FMECA (Failure Mode, Effects, and Criticality Analysis) and their corresponding FTA (Fault Tree Analysis) are conducted. For the design phase, if the risk of occurrence of failure is high, the modified design is presented, and different maintenance activities were also set forward.
On the other hand, the methods that are used after the failure have taken place. It includes finding preventive methods so that the failures would not reoccur in future. It can either be done by RCA (Cause Analysis) or by selection of failure based on the priority that includes degrader and Pareto analysis. Detailed discussion on detecting different failures of pump’s impeller have been discussed in ample detail in previous sections about their cause and mitigating steps.
Failure Mode and Effects Analysis (FMEA)
In this mode of analysis, all the possible failures for pump’s impeller are identified along with the detailed effects of these failures regarding safety, functional, and financial consequences. It is a bottom-up approach because the analysis is started with the possible causes of failures of components along with the consequences. For pump’s impeller failure, the probable causes can be cavitation, erosion, and corrosion.
The FMEA is conducted by a separate group of people having the background of operations, design, finance and maintenance. Also, FMEA is a qualitative analysis that deals with describing the modes of failure along with their effects. The analysis is also made by incorporating additional factor of criticality analysis. This method is quite common referred to as Failure Mode, Effects and Criticality Analysis (FMECA). By following this approach, each failure mode is quantified using the RPN (Risk Priority Number) that is mathematically defined as:
RPNi=SiOiDi As it is evident, the RPN is the product of occurrence (O), severity (S) and detection (D). The severity defined the nature of consequence of the failure mode. Values are usually obtained by predefined tables that include numbers from 1-10 or 1-5 reflecting different grades of severity. The occurrence variable defines the probability of failure occurrences. It can range from failures that are extremely unlikely to failures that are frequent. The detection term defines the probability considering the detection of the failure. These values are also obtained from predefined tables. Hence, the RPN number returns a higher risk factor if the severity of failure is high with frequent occurrence and inability to detection.
Different standards are available for conducting FMEA, which include Society of Automotive Engineers (J1739), British Standard (BS 5760), and MIL-STD-1629A. The general procedure for conducting FMEA includes FMEA group formulation, system analysis, FMEA, FMECA and correction action planning.
All in all, impellers in centrifugal pumps are the core equipment that can generate head and induce required flowrate of fluid. Furthermore, it is also subjected to different potential failure risks that can cause severe consequences to process facility as well as personnel. Periodic maintenance of pumps and its internal equipment should also be done to prevent such failures from occurring on the frequent basis. Different modes of failures including cavitation, erosion, and corrosion are the most common failure means that can reduce plant efficiency and can also become a risk for financial, safety and operational well-being of an organization.
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