Crane Troubleshooting White Paper

7/27/2019 Crane Troubleshooting White Paper

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Crane Engineering:

TroubleshootingPossible causes and solutions to most common problemsfound in pump installation and operation

General

In most pumping systems, pumps are likely to be the most vulnerable components. Thesymptoms frequently show the pump to be at fault regardless of what may be wrong. Theproblem is usually caused by inadequate control of the pumped fluid or a change inoperating requirements of which the system or pump is not capable of handling or acomponent malfunction.

Before starting to correctly identify the problem it is important to gather as muchinformation relating to the process as follows:

Reconfirm original duty conditions.

What has changed in the process since operation was last satisfactory? i.e. pressure,temperature, fluid viscosity etc. Was the system undergoing routine maintenance? Were any new or repaired components omitted to be fitted? When was the pump last serviced? What was the appearance and condition of the pump internal components? How long did the pump operate before the problem? Any changes in pump noise or vibration?

The most common problems found are generally as follows:

Loss of flow. Loss of suction. Low discharge pressure. Excessive noise or vibration.

Excessive power usage. Rapid pump wear. Seal leakage.

Common Problems

Loss of FlowA simple cause of this could be incorrect direction of shaft rotation, which althoughobvious is often overlooked. Loss of flow can be caused by excessive discharge pressureand/or by a change in fluid viscosity.

In general terms: For a rotary lobe pump if the viscosity is significantly reduced, the pumps rated flow willbe reduced, more so for higher-pressure operation. For a centrifugal pump if the viscosity is increased, the pumps rated flow will bedecreased.

Diagnosis of problemswill be greatly assistedby having pressuregauges fitted to both

pump inlet and outlet.


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Loss of SuctionLoss of suction can be minor, causing little short-term damage or sufficiently major tocause catastrophic damage. Loss of suction means fluid is not reaching the pumpingelements or not reaching them at a sufficiently high pressure to keep the fluid beingpumped in a fluid state. Loss of suction can be interpreted as the inability to prime,

cavitation or a gas content problem.

The rotary lobe pump can be classed as self-priming. This means that within limits, it iscapable of evacuating (pumping) a modest amount of air from the suction side of thepump to the discharge side of the pump. Filling the inlet system with fluid or at least fillingthe pump (wetted pumping elements) will make a major improvement in the pumpspriming capability.

The liquid ring pump can also be classed as self-priming when the pump casing is halffilled with fluid and the LKHSP centrifugal pump range is specially designed to be self-priming.

Cavitation is caused by insufficient system inlet pressure to the pump. This can be causedby an inlet system restriction, excessive fluid viscosity or excessive pump speed. Inletrestrictions can include dirty or clogged inlet strainers, debris floating in the fluid supply

that covers the inlet piping intake, or rags. If the fluid is cooler than design temperature, itsviscosity may be too high causing excessive friction (pressure loss) in the inlet pipingsystem. Cavitation is frequently accompanied by noise, vibration and significant increasein discharge pressure pulsation. If a pump is allowed to cavitate over long periods this willcause damage to the pumphead components. The surface of these components aretypically perforated and pitted.

Gas in the inlet pipework has the same impact on pump operation and creates the samesymptoms as cavitation. This can occur under other circumstances such as a pumpoperating at an inlet pressure below local atmospheric pressure. In this instance it is quitelikely that air is being drawn into the pipework through a loose pipe connection or pumpcasing joint, leaking inlet valve stem, defective or otherwise damaged joint gasket in thepipework system. In recirculating systems, such as a lubrication system where the fluidpumped is continuously returned to a supply source or tank, if the tank and return lines are

not adequately designed, located and sized, air is easily entrained in the oil andimmediately picked up by the pump inlet system. Be sure fluid level at its source is at orabove minimum operating levels. Lines returning flow to a supply tank should terminatebelow minimum fluid level.

Low Discharge PressureLow discharge pressure can only be caused by loss of flow. Pump discharge pressure iscaused only by the systems resistance to the flow provided by the pump. Either the pumpis not providing the flow expected or the system is not offering the expected resistance tothat flow. It is possible that flow is being restricted into the pump (cavitation), usuallyaccompanied by noise and vibration, the pump is not producing its rated flow (pump wornor damaged), or the pump flow is bypassing rather than being delivered into the system asintended.


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Excessive Noise or VibrationExcessive noise and/or vibration can be a symptom of cavitation, mechanical damage topump assembly, misalignment of drive or harmonics with other elements of the system.Cavitation is especially true if the discharge pressure is fluctuating or pulsating.Mechanical causes of noise and vibration include shaft misalignment, loose couplings,loose pump and/or driver mountings, loose pump and/or driver guards, worn or damaged

driver or pump bearings or valve noise that seems to be coming from the pump. Valves,especially on the discharge side of the pump can sometimes go into a hydraulic vibrationmode caused by operating pressure, flow rate and the valve design. Resetting or achange in an internal valve component is usually sufficient to solve the problem.

Excessive PowerExcessive power consumption can be caused by either mechanical or hydraulic problems.Mechanical causes include imminent bearing failure, pumping elements rubbing which canlead to a pump seizure and poor shaft alignments. Too high viscosity can result in themotor overloading. For a rotary lobe pump too high discharge pressure can cause the motor to overload. For a centrifugal pump too high capacity (too low discharge pressure) can cause themotor to overload.

Rapid Pump WearRapid wear of pumphead components is either caused by abrasives being present in thefluid, chemical corrosion, loss of shaft support (bearing failure), or operation at a conditionfor which the pump is not suitable i.e. cavitation, excessively high pressure or hightemperature. To avoid any abrasive foreign material entering the pump, strainers or filtersshould be employed wherever possible and practical. Rapid wear is sometimes not wearin the sense of a non-durable pump, but really a catastrophic pump failure that occurredvery quickly. Looking at the pump internal parts alone may not provide much help inidentifying the cause, thus the importance of knowing what was occurring in the timeperiod immediately preceding detection of the problem.

Seal LeakageMechanical seals fitted to centrifugal, rotary lobe and liquid ring pumps can be seen as theweakest point for any pump leakage and special care should be taken to ensure thecorrect seal for the application is installed i.e. mounting attitude, seal face combination

and elastomer selection.

Apart from mis-selection and poor servicing, seal leakage can be due to pump cavitation,too high discharge pressure, being allowed to run dry and unexpected solids in the fluid.


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Problem Solving Table

The table shown offers probable causes and solutions to the most common problemsencountered.

In ( ) next to the particular solution given you will find annotation relating to what pumptype the solution is for.

i.e. ce = Centrifugal Pumpliq = Liquid Ring Pumprlp = Rotary Lobe Pump

See table on the following pages:

Note: Reprinted with permission from Alfa-Laval Pump Handbook


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