Maintenance Fundamentals 2011 Part 13 pptx

A small number of centrifugal pumps are designed to operate under conditions in which cavitation is unavoidable.. NET POSITIVESUCTIONHEAD To avoid cavitation in centrifugal pumps, the pr

pressure for the fluid being pumped Any vapor bubbles formed by the pressuredrop at the eye of the impeller are swept along the impeller vanes by the flow ofthe fluid When the bubbles enter a region in which local pressure is greater thansaturation pressure farther out the impeller vane, the vapor bubbles abruptlycollapse 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 Itdegrades the performance of a pump, resulting in a degraded, fluctuating flowrate and discharge pressure Cavitation can also be destructive to pump internals.The formation and collapse of the vapor bubble can create small pits on theimpeller 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 apump impeller Cavitation can also cause excessive pump vibration, which coulddamage 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 andmaintained to withstand the small amount of cavitation that occurs during theiroperation

Noise is one of the indications that a centrifugal pump is cavitating A cavitatingpump 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 thedischarge valve slightly, or by installing an orifice plate, the fluid flow throughthe pump is altered from its original design This reduces the fluid’s velocity as itexits the tips of the impeller vanes; therefore the fluid does not flow as smoothlyinto 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, particularlywhen accompanied by resonance

Random low-amplitude, wide-frequency vibration is often associated with vanepass frequency, resulting in vibrations similar to cavitation and turbulence, but isusually found at lower frequencies This can lead to misdiagnosis Many pumpimpellers show metal reduction and pitting on the general area at the exit tips ofthe vanes This has often been misdiagnosed as cavitation

354 Maintenance Fundamentals

It is very important to note that recirculation is found to happen on thedischarge side of the pump, whereas cavitation is found to happen on the suctionside of the pump.

To prevent recirculation in pumps, pumps should be operated close to theiroperational rated capacity, and excessive throttling should be avoided

When a permanent reduction in capacity is desired, the outside diameter of thepump impeller can be reduced slightly to increase the gap between the impellertips and the cutwater

NET POSITIVESUCTIONHEAD

To avoid cavitation in centrifugal pumps, the pressure of the fluid at all pointswithin the pump must remain above saturation pressure The quantity used todetermine whether the pressure of the liquid being pumped is adequate to avoidcavitation is the net positive suction head (NPSH) The net positive suction headavailable (NPSHA) is the difference between the pressure at the suction of thepump and the saturation pressure for the liquid being pumped The net positivesuction head required (NPSHR) is the minimum net positive suction headnecessary to avoid cavitation

The condition that must exist to avoid cavitation is that the net positive suctionhead available must be greater than or equal to the net positive suction headrequired This requirement can be stated mathematically as shown below

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, thepressure at the suction of the pump is the sum of the absolute pressure atthe surface of the liquid in the tank plus the pressure caused by the elevationdifference between the surface of liquid in the tank and the pump suction, less thehead 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 andpump 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 oroperation may be necessary to increase the NPSHA above the NPSHR andstop the cavitation One method for increasing the NPSHA is to increase thepressure at the suction of the pump If a pump is taking suction from an enclosedtank, either raising the level of the liquid in the tank or increasing the pressure inthe gas space above the liquid increases suction pressure

It is also possible to increase the NPSHA by decreasing the temperature of theliquid being pumped Decreasing the temperature of the liquid decreases thesaturation pressure, causing NPSHAto increase

If the head losses in the pump suction piping can be reduced, the NPSHAwill beincreased Various methods for reducing head losses include increasing the pipediameter; reducing the number of elbows, valves, and fittings in the pipe; anddecreasing the length of the pipe

It may also be possible to stop cavitation by reducing the NPSHRfor the pump.The NPSH is not a constant for a given pump under all conditions but depends

356 Maintenance Fundamentals

on certain factors Typically, the NPSHR of a pump increases significantly asflow rate through the pump increases Therefore, reducing the flow rate through

a pump by throttling a discharge valve decreases NPSHR NPSHR is alsodependent on pump speed The faster the impeller of a pump rotates, the greaterthe NPSHR Therefore, if the speed of a variable-speed centrifugal pump isreduced, the NPSHRof the pump decreases

The net positive suction head required to prevent cavitation is determined throughtesting by the pump manufacturer and depends on factors including type ofimpeller inlet, impeller design, pump flow rate, impeller rotational speed, and thetype of liquid being pumped The manufacturer typically supplies curves ofNPSHRas a function of pump flow rate for a particular liquid (usually water) inthe vendor manual for the pump

TROUBLESHOOTING

Design, installation, and operation are the dominant factors that affect a pump’smode of failure This section identifies common failures for centrifugal andpositive-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 aspressure and availability of a continuous volume of fluid; and (3) variations indemand Table 17.1 lists common failure modes for centrifugal pumps and theircauses

Mechanical failures may occur for a number of reasons Some are induced bycavitation, hydraulic instability, or other system-related problems Others are thedirect result of improper maintenance Maintenance-related problems includeimproper lubrication, misalignment, imbalance, seal leakage, and a variety ofothers that periodically affect machine reliability

Cavitation

Cavitation in a centrifugal pump, which has a significant, negative effect onperformance, is the most common failure mode Cavitation not only degrades apump’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.

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 insuction 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 Aslight 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 ponents Visual evidence of operation with phase-change cavitation is an impel-ler surface finish like an orange peel Prolonged operation causes small pits orholes on both the impeller shroud and vanes

com-Entrained Air/Gas Pumps are designed to handle gas-free liquids If a centrifugalpump’s suction supply contains any appreciable quantity of gas, the pump willcavitate In the example of cavitation caused by entrainment, the liquid isreasonably stable, unlike with the change of phase described in the precedingsection Nevertheless, the entrained gas has a negative effect on pump perform-ance While this form of cavitation does not seriously affect the pump’s internalcomponents, it severely restricts its output and efficiency

The primary causes of cavitation that is due to entrained gas include two-phasesuction supply, inadequate available net positive suction head (NPSHA), andleakage in the suction-supply system In some applications, the incoming liquidmay contain moderate to high concentrations of air or gas This may result fromaeration or mixing of the liquid prior to reaching the pump or inadequate liquidlevels in the supply reservoir Regardless of the reason, the pump is forced tohandle two-phase flow, which was not intended in its design

Turbulent Flow The effects of turbulent flow (not a true form of cavitation) onpump performance are almost identical to those described for entrained air orgas in the preceding section Pumps are not designed to handle incoming liquidsthat 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 sure gauges, flow rate, and motor current, as well as changes in the vibrationprofile

pres-How to Eliminate Several design or operational changes may be necessary tostop centrifugal-pump cavitation Increasing the available net positive suctionhead (NPSHA) above that required (NPSHR) is one way to stop it The NPSHrequired 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 beingpumped The manufacturer typically supplies curves of NPSHRas a function offlow 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 apump is fed from an enclosed tank, either raising the level of the liquid in thetank or increasing the pressure in the gas space above the liquid can increasesuction pressure

It also is possible to increase the NPSHA by decreasing the temperature of theliquid being pumped This decreases the saturation pressure, which increasesNPSHA

If the head losses in the suction piping can be reduced, the NPSHA will beincreased 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 theflow rate by throttling a discharge valve decreases NPSHR In addition to flowrate, NPSHRdepends on pump speed The faster the pump’s impeller rotates, thegreater 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 flowrate) Therefore any variation in the total backpressure of the system causes achange in the pump’s flow or output Because pumps are designed to operate attheir 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 motorspeed and flow rate

Motor Speed The brake horsepower of the motor that drives a pump is loaddependent As the pump’s operating point deviates from BEP, the amount of

360 Maintenance Fundamentals

horsepower required also changes This causes a change in the pump’s rotatingspeed, which either increases or decreases depending on the amount of work thatthe pump must perform.

Flow Rate The volume of liquid delivered by the pump varies with changes inTSH 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 anacceptable range of system demand for flow and pressure If system demandexceeds the pump’s capabilities, it may be necessary to change the pump, thesystem 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 inrun-out condition (i.e., maximum flow and minimum discharge pressure), thesystem 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 adischarge valve to increase the backpressure on the pump Because the pumpmust 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 thesystem’s backpressure by eliminating line resistance caused by elbows, extravalves, etc

Table 17.2 lists common failure modes for rotary-type, positive-displacementpumps The most common failure modes of these pumps are generally attributed

to problems with the suction supply They must have a constant volume of cleanliquid to function properly

RECIPROCATING

Table 17.3 lists the common failure modes for reciprocating-type, placement pumps Reciprocating pumps can generally withstand more abuse andvariations in system demand than any other type However, they must have aconsistent supply of relatively clean liquid to function properly

positive-dis-Table 17.2 Common Failure Modes of Rotary-Type, Positive-Displacement Pumps

362 Maintenance Fundamentals

The weak links in the reciprocating pump’s design are the inlet and dischargevalves 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.

to prevent or minimize these failures is to ensure that proper maintenance isperformed regularly on these components It is important to follow the manu-facturer’s recommendations for valve maintenance and replacement.

Because of the close tolerances between the pistons and the cylinder walls,reciprocating pumps cannot tolerate contaminated liquid in their suction-supplysystem Many of the failure modes associated with this type of pump are caused

by contamination (e.g., dirt, grit, and other solids) that enters the suction side ofthe pump This problem can be prevented by the use of well-maintained inletstrainers or filters

364 Maintenance Fundamentals

Five major types of steam traps are commonly used in industrial applications:inverted bucket, float and thermostatic, thermodynamic, bimetallic, and thermo-static Each of the five major types of steam trap uses a different method todetermine when and how to purge the system As a result, each has a differentconfiguration

Inverted Bucket

The inverted-bucket trap, which is shown in Figure 18.1, is a mechanicallyactuated steam trap that uses an upside down, or inverted, bucket as a float.The bucket is connected to the outlet valve through a mechanical linkage Thebucket sinks when condensate fills the steam trap, which opens the outlet valveand drains the bucket It floats when steam enters the trap and closes the valve

365

As a group, inverted-bucket traps can handle a wide range of steam pressures andcondensate capacities They are an economical solution for low- to medium-pressure and medium-capacity applications such as plant heating and light pro-cesses When used for higher-pressure and higher-capacity applications, thesetraps become large, expensive, and difficult to handle.

Each specific steam trap has a finite, relatively narrow range that it can handleeffectively For example, an inverted-bucket trap designed for up to 15-psiservice will fail to operate at pressures above that value An inverted-buckettrap 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’spressure, capacity, and size requirements

Float-and-Thermostatic

The float-and-thermostatic trap shown in Figure 18.2 is a hybrid A float similar

to that found in a toilet tank operates the valve As condensate collects in thetrap, it lifts the float and opens the discharge or purge valve This design opensthe discharge only as much as necessary Once the built-in thermostatic elementpurges non-condensable gases, it closes tightly when steam enters the trap Theadvantage of this type of trap is that it drains condensate continuously.Figure 18.1 Inverted-bucket trap

366 Maintenance Fundamentals

Like the inverted-bucket trap, float-and-thermostatic traps as a group handle awide range of steam pressures and condensate loads However, each individualtrap has a very narrow range of pressures and capacities This makes it critical toselect a trap that can handle the specific pressure, capacity, and size requirements

of the system

The key advantage of float-and-thermostatic traps is their ability for quicksteam-system startup because they continuously purge the system of air andother non-condensable gases One disadvantage is the sensitivity of the floatball to damage by hydraulic hammer

Float-and-thermostatic traps provide an economical solution for lighter sate loads and lower pressures However, when the pressure and capacityrequirements increase, the physical size of the unit increases and its cost rises

conden-It also becomes more difficult to handle

Thermodynamic, or Disk-Type

Thermodynamic, or disk-type, steam traps use a flat disk that moves between acap and seat (see Figure 18.3) On startup, condensate flow raises the disk andopens the discharge port Steam or very hot condensate entering the trap seatsthe disk It remains seated, closing the discharge port, as long as pressure ismaintained above it Heat radiates out through the cap, thus diminishing thepressure over the disk, opening the trap to discharge condensate

Figure 18.2 Float-and-thermostatic 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 willlodge between the disk and the valve seat This prevents the valve from sealingand permits live steam to flow through the discharge port If pressure is notmaintained above the disk, the trap will cycle frequently This wastes steam andcan cause the device to fail prematurely.

The key advantage of these traps is that one trap can handle a complete range ofpressures In addition, they are relatively compact for the amount of condensatethey discharge The chief disadvantage is difficulty in handling air and other non-condensable gases

Bimetallic

A bimetallic steam trap, which is shown in Figure 18.4, operates on the sameprinciple as a residential-heating thermostat A bimetallic strip, or wafer, con-nected to a valve disk bends or distorts when subjected to a change in tempera-ture When properly calibrated, the disk closes tightly against a seat when steam

is present and opens when condensate, air, and other gases are present

Two key advantages of bimetallic traps are (1) compact size relative to theircondensate load-handling capabilities and (2) immunity to hydraulic-hammerdamage

Figure 18.3 Thermodynamic steam trap

368 Maintenance Fundamentals