11 centrifugal pumps trouble-shooting eleven

22/04/23 Prof. Maher Higazy 1

Centrifugal Pumps Trouble-Shooting

Prof. Dr. Eng. Maher Higazy


Two Basic Requirements for Trouble-Free Operation of Centrifugal Pumps:In general there are two basic requirements that have to be met at all the times for a trouble free operation and longer service life of centrifugal pumps. The first requirement is that no cavitation of the pump occurs throughout the broad operating range and the second requirement is that a certain minimum continuous flow is always maintained during operation. A clear understanding of the concept of cavitation, its symptoms, its causes, and its consequences is very much essential in effective analyses and troubleshooting of the cavitation problem.


(cont’d)Just like there are many forms of cavitation, each demanding a unique solution, there are a number of unfavourable conditions which may occur separately or simultaneously when the pump is operated at reduced flows. Some include:Cases of heavy leakages from the casing, seal, and stuffing boxDeflection and shearing of shaftsSeizure of pump internalsClose tolerances erosionProduct quality degradationExcessive hydraulic thrustPremature bearing failures


(cont’d)Each condition may dictate a different minimum flow low requirement. The final decision on recommended minimum flow is taken after careful “techno-economical” analysis by both the pump user and the manufacturer. The consequences of prolonged conditions of cavitation and low flow operation can be disastrous for both the pump and the process. Such failures in hydrocarbon services have often caused damaging fires resulting in loss of machine, production, and worst of all, human life. Thus, such situations must be avoided at all cost whether involving modifications in the pump and its piping or altering the operating conditions.


(cont’d)Proper selection and sizing of pump and its associated piping can not only eliminate the chances of cavitation and low flow operation but also significantly decrease their harmful effects.


IntroductionTroubleshooting is the art of applying engineering principles to pump maintenance. Many engineers consider maintenance to be based on industrial mythology or folklore. Therefore good engineering guidelines for determining the causes of many problems are not available. The basic questions of where to start and how to conduct the problem analysis must be addressed.


Components of a Pumping SystemMethodical investigation will help to identify the sources of pump problems. Before you start to troubleshoot the system, take time to review maintenance records. Go to the pump and have the operator explain what was seen. If possible, run the pump and demonstrate the problem. A typical pumping system can be divided into eight component areas for investigation:The foundation : Poor foundations, grouting, and

flexible base plate designs can cause many problems.The driver: Excitations from the vibrations of the

driver (motor, steam turbine, gearing) can be transmitted to other components.


(cont’d)Mechanical power transmission: Excitations from the

coupling area, especially due to misalignment of the driver or eccentrically bored coupling hubs, can be transmitted. Beware of incorrect positioning of the driver and pump such that the distance between shaft ends (DBSE) exceeds the axial flexing limits of the coupling.The driven pump: The design of the pump can greatly

influence the hydraulic interaction between the rotor and the casing and thus the problems encountered. Pump thermal-growth misconceptions can create problems.The suction piping and valves: Unfavourable incoming

flow conditions such as cavitation, intake vortex, or suction recirculation due to poor design and layout of suction piping and valves can cause flow disturbances.


(cont’d)The discharge piping and valves: Unfavourable

dynamic behaviour of piping because of loads from dynamic, static, or thermal causes, including resonance excitation, can cause trouble.Instrumentation for control of pump flow: Control

system-pump interaction during start-ups or other periods of low flow can produce pressure pulsations. High-pressure pulsations can result from the hydraulic instability of the entire pumping system.Failure to maintain the alignment: Once the

alignment is established, dowels into the baseplate must hold the pump alignment.


Vibration-Monitoring BasicsPredictive maintenance in the form of vibration-monitoring trend analysis is important in the hydrocarbon processing industry. The technique is not widely used for determining many potential pump and motor problems. Many technical papers have been published on vibration monitoring to evaluate performance problems on steam turbines and turbocompressors. There are few similar guidelines for pumps. The smaller size and the less critical nature of pumps are factors that cause this disparity. Another factor is that turbines and turbocompressors are pneumatic. The fluid flowing through the machine has relatively little mass compared to the rotor weight. The compressible fluid has only a small impact on the vibration patterns of the rotor.


(cont’d)Most of the vibration problems in pneumatic machines are mechanical. A pump, however, is a hydraulic machine. The fluid handled has considerable mass and transmits any pressure pulsations throughout the system undiminished. The interaction between the rotor and casing becomes very important and can be a point of confusion about the vibration patterns of the rotor. Interpretation of vibration data is very difficult.

22/04/23 Prof. Maher Higazy 12Figure Rotor vibration frequency/Machine speed


(cont’d)In every pump, dynamic forces of mechanical and hydraulic origin are present and must be separated and evaluated before solutions can be developed. There are many potential causes of vibration. Some of the causes are shown on the vibration spectrum of above figure. The hydraulic forces are as numerous and as great in magnitude as the mechanical forces. In any analysis program, a distinction between vibrations that are mechanical or hydraulic must be made. A detailed knowledge of the pump's design features as well as its present mechanical condition is necessary to accurately evaluate any vibration data.


Power Transmission VibrationsA major source of externally induced vibration is misalignment between the pump and its driver. This misalignment can result in an axial vibration reading as much as 1.5 times the vertical or horizontal readings. Vibration generally occurs at the running speed of the pump, although it may also occur at multiples of the running speed. Vibration caused by misalignment can be distinguished from a resonance disturbance by monitoring the pump during a coast-down period. Misalignment vibration will shift in frequency directly with the rotational speed. Shaft critical speeds or resonances will not shift.


(cont’d)Machines that normally operate at elevated temperatures must tolerate vibration during a temporary cold misalignment until the normal operating temperature is reached.

Flexible couplings will accommodate sizable amounts of misalignment without the life of the coupling itself being affected. That same amount of misalignment may cause damage to pump or driver bearings and mechanical seals. A gear-type coupling will transmit axial thrust imposed upon it and normally give twice the running speed.


(cont’d)A membrane or disk coupling gives little likelihood of the vibration caused by misalignment being twice the running frequency due to the axial softness of the coupling.Bearing Half-Frequency Vibration: Oil whirl may occur in lightly loaded (under 90 to 100 Ib/in2) journal bearings at a frequency of about 40 to 49 percent of running speed. It is a self-sustaining type of rotary motion and is highly destructive.


Internally Induced VibrationsVibrations can also be produced by mechanical and fluid-related problems within the pump itself. Typical vibration problems internally induced by the pump include the following:Unfavourable dynamic behaviour of the rotor due to

excessive wear ring, bushing, or other leakage clearances.Poor support of the rotor because of loose fits on the

shaft or housing in the case of ball bearings. Excessive bore clearance or lack of "clamp" on the shell OD of sleeve bearings causes the same effects.Mechanical imbalance of the rotating parts due to poor

balancing or careless assembly. Operational influences including cavitation, erosion, deposits, corrosion, damaged impellers, galled parts, and abrasion can also cause imbalance.


(cont’d)Increased axial and radial hydraulic forces when the

pump is operated outside of the design flow range. Some increase in vibration is normal when departing from the best efficiency flow rate due to suction recirculation.Pump manufacturer casting and/or machining defects.Radial hydraulic interaction between the rotating-

impeller liquid channels and the stationary channels in the casing. This is called the vane passing frequency and is strongly influenced by gap "B.“

The following is a table of common vane interactions from a paper by James Corley, presented at the Fourth Texas A & M Pump Symposium in 1987.


Vane passing frequencies for impeller-vane combinations

Number of diffuser vanes

Number of impeller vanes4567

8*15979810428106*621111210122112*25*35131225121414615631516*614


Vane Passing FrequencyThe vane passing frequency is a hydraulically induced vibration at a frequency determined by the number of impeller vanes, number of stationary vanes, and pump rotational speed. The vibration is created by the momentary disturbance of the wake of the liquid exiting the impeller liquid channels by the stationary diffuser or volute vane tips. gap "B" has been discussed in detail in Chaps. 4 and 8. In summary, the larger the gap, the more the flow can smooth out before it contacts a diffuser vane or volute tongue. Determination of the vane passing frequency sounds easy but can be confusing. The most common pump design has an impeller with an odd number of impeller vanes and a double volute in the casing. The vane passing frequency is the rotational speed times the number of vanes.


(cont’d)For diffuser-type pumps, the larger number of diffuser vanes coupled with the closer gap "B" than hat of a volute pump causes a different interaction of the rotor and casing. The match-up of stationary and rotating vanes has a different frequency. This will give a vane passing frequency that does not correspond to the number of vanes in either the impeller or the casing.


Random Positioning of ImpellersIn the diffuser style pump, a complete set of hydraulics can be created specifically for each pump application because the diffusers- are cast separately from the case and the vane angle and location can be readily changed. In the volute design, the volutes can be relocated only by a very expensive pattern change. In multistage volute pumps the degree of positioning of the volutes is severely limited by case design. It is necessary to randomly cut the keyways in the impellers to ensure that vanes on adjacent impellers are not aligned and do not pass volute tongues simultaneously. Frequently, this random keyway positioning is not done when manufacturing the impellers and the vanes on the impellers line up, producing a high vane passing frequency vibration.


(cont’d)The alignment of impellers and volutes in each stage should be carefully observed during witness testing, and reassembly on a new pump or when replacing the impellers during maintenance.


Correction of Vane Tip ShapeImpellers manufactured with blunt vane tips can cause trouble by generating hydraulic disturbances in the impeller exit wake area even when the impeller is the correct distance, gap "B" from the cutwater. This disturbance can be greatly reduced by sharpening the impeller vanes on the underside or trailing edge of the vane.


Piping Vibration LimitsHow much vibration can be permitted in the piping system of a pump? One rule of thumb states that permissible unfiltered velocity readings taken on the piping at the mid-span of its supports can be 3 times the permissible readings taken on the pump bearing caps. Bearing cap readings in the range of 0.5 to 0.6 in/s are the concern level for a pump, and 1.0 in/s is the emergency shutdown level. These pump vibration guidelines then give 1.5 to 3.0 in/s as the limit for piping.


Mechanical Seal FailuresTroubleshooting mechanical seal failures is a complex task. Vibration analysis does not identify potential seal failures which often can be the deciding factor leading to a pump outage. The number of variables that can affect the seal environment is almost infinite. Many people feel that any failure analysis must be done in a shop situation with the seal dismantled. While this approach is valuable in circumstances where corrosion is present, it cannot cover many factors present in the field. Most efforts to increase the mean time between failures (MTBF) have involved installing a more highly engineered seal. Many factors other than seal design, both internal and external to the seal's environment, lead to shorter seal life.


(cont’d)The seal must operate in an environment that can be variable and influenced by many factors, some controllable and others uncontrollable. Many are the same factors that influence the hydraulic performance of the pump. Vibration effects caused by poor coupling alignment, shaft run-out, bearing condition, and dynamic balance and other mechanical problems are not in the following list.


(cont’d)Changes in pump flow rate caused by:1.Automatic controllers responding to process demand.

2.Manual adjusting of valves by the pump operators at the pump site.

Changes in NPSH caused by:1.Pressure upsets in the suction vessel.2.Liquid level changes in towers or drums.3.Pressure or temperature changes in the process stream.


(cont’d)Changes in the characteristic of the fluid being sealed due to:

1.the Batch processing.2.System fluid temperature change due to process or atmospheric changes that alter fluid behaviour.

3.Pressure changes which affect behaviour of the fluid.

External seal flush changes caused by:1.Changes in demand on the seal flush system due to bringing other units on- or off-line.

2.Improper manual adjustment of valves in flush lines.3.Failure of the seal flush system pressure, cooling or heating.


(cont’d)Changes in flush circulated from the pump to the seal cavity caused by:

1.Throttle orifice washout or plugging.2.Flush-fluid heat-exchanger failure.3.Flush-fluid filters plugging.4.Cyclone separator flow reversals due to system differential pressure changes.

5.Increased clearance on stuffing box throttle bushings.


(cont’d)Fluid pressure pulsations (up to 10 percent of headdeveloped) caused by the impeller vane passing frequency and/or suction recirculation at the impeller eye.

The effects of utility piping for the seal due to:

1.Changes in the cooling water flow rate to the stuffing box water jackets.

2.Changes in the steam quenching, or steam tracing, of the piping and/or pump


(cont’d)These seven conditions are not always predictable and, in many cases, are just not known. A historical record of operating conditions can be used to correlate mechanical seal performance to increase the mean time between failure. In some applications even the most extreme changes in operating conditions would not be harmful to the mechanical seal life. Other critical applications may require analyzing and controlling these variables in a very precise manner. These factors must be identified and solutions developed for these environmental problems to improve the seal life.


(cont’d)These recommended solutions may include a seal design change, pump and process operation changes, or seal fluid environmental changes. The pump and seal characteristics that have led to repeated seal failures must be understood, and the problems minimized or corrected, so that trouble-free operation can be achieved.


Evaluation of Pump OperationThe pump operator's descriptions of a problem are rather cryptic: "The pump is not pumping," "the pump is noisy," or "the motor just kicked out." Translating these comments into engineering concepts and values must be done first. Approaching the pump operation problem from a component viewpoint, as outlined in Sec. 12.2, can help in the solution of the problem. Unfortunately, some cavitation and performance problems can be solved only by detailed system calculations. Most troubleshooting guides are oriented more to technical system analysis. The following paragraphs are mental checklists to use at the pump site to observe operating conditions. The list is not complete by any means, but it is a start.


(cont’d)Capacity-type problems: Problem: Pump does not have enough capacity. Background: This is no significant noise from the pump. Experience indicates that the majority of the so-called capacity problems turn out to be a head-limiting problem.


Conclusions: Pump troubleshooting basically consists of paying attention to a lot of details. These details fit together to form an overall picture of the operating problems


Wish You Wish You Best of LuckBest of Luck

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11 centrifugal pumps trouble-shooting eleven

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