Maintenance Fundamentals Episode 2 part 9 pps

It degrades the performance of a pump, resulting in a degraded, fluctuating flow rate and discharge pressure.. NET POSITIVESUCTIONHEAD To avoid cavitation in centrifugal pumps, the press

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

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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 POSITIVESUCTIONHEAD

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 (NPSHA) 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 (NPSHR) 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

NPSHA NPSHR

Figure 17.22 Vane pass frequency

A formula for NPSHAcan be stated as the following equation:

NPSHA¼ Psuction Psaturation

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

NPSHA¼ Pa¼ Pst hf Psat

where

NPSHA¼ Net positive suction head available

Pa¼ Absolute pressure on the surface of the liquid

Pst¼ Pressure caused by elevation between liquid surface and pump suction

hf ¼ Head losses in the pump suction piping

Psat¼ Saturation pressure of the liquid being pumped

PREVENTINGCAVITATION

If a centrifugal pump is cavitating, several changes in the system design or operation may be necessary to increase the NPSHA above the NPSHR and stop the cavitation One method for increasing the NPSHA 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 NPSHA by decreasing the temperature of the liquid being pumped Decreasing the temperature of the liquid decreases the saturation pressure, causing NPSHAto increase

If the head losses in the pump suction piping can be reduced, the NPSHAwill 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 NPSHRfor the pump The NPSHRis not a constant for a given pump under all conditions but depends

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on certain factors Typically, the NPSHR 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 NPSHR NPSHR is also dependent on pump speed The faster the impeller of a pump rotates, the greater the NPSHR Therefore, if the speed of a variable-speed centrifugal pump is reduced, the NPSHRof 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 NPSHRas 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

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

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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 (NPSHA), 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 (NPSHA) above that required (NPSHR) 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

design, impeller rotational speed, pump flow rate, and the type of liquid being pumped The manufacturer typically supplies curves of NPSHRas a function of flow rate for a particular liquid (usually water) in the pump’s manual

One way to increase the NPSHAis 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 NPSHA by decreasing the temperature of the liquid being pumped This decreases the saturation pressure, which increases NPSHA

If the head losses in the suction piping can be reduced, the NPSHA 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 NPSHR, which

is not a constant for a given pump under all conditions Typically, the NPSHR

increases significantly as the pump’s flow rate increases Therefore, reducing the flow rate by throttling a discharge valve decreases NPSHR In addition to flow rate, NPSHRdepends on pump speed The faster the pump’s impeller rotates, the greater the NPSHR Therefore, if the speed of a variable-speed centrifugal pump

is reduced, the NPSHRof 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

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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

POSITIVEDISPLACEMENT

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

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.

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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.