revitalizing vertical lineshaft turbines

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Revitalizing Vertical Lineshaft TurbinesLearn the causes of wear, the ways to scout them out, and how to bring your vertical pump back up

to speed. by David LaCombe, American Turbine

There are three major reasons to revitalize a vertical turbine pump. One of the most obvious is toprevent an unexpected and complete pump failure, thereby disrupting critical system or plantoperations. Secondly, increased clearances between the bowls and enclosed impeller skirts, increasedclearances between the bowl shaft and bearings, and breakdown of the fluid passages reduce thehydraulic efficiency of the unit. Over time, running a badly worn pump can be just as costly as somesystem shutdowns. Finally, revitalization may be required where operational conditions have changedfor example, alterations in pressure and capacity requirements due to fluid level variations.

Revitalizing a vertical lineshaft turbine pump can involve avariety of processes, from adjusting the impeller positionon a semi-open impeller pump to replacing the entire bowlassembly. In between these extremes, replacing bearingsor shafts, flame-spraying coatings on shafts, applyingcoatings to flow surfaces, wear ringing close toleranceparts and replacing shaft seals are some of the morecommon refurbishing processes. Often, these intermediaterepairs are more expensive than simply installing a newbowl assembly. Individual parts are costly, and such repairoperations normally require considerably longer downtimethan rebowling. While some companies offer completerepair facilities, any lineshaft turbine manufacturer will atleast have some form of a rebowling program.

Causes of Vertical Turbine Pump Wear

Barring any operational problems and normal wear, thereare two primary factors that cause a vertical turbine todeteriorate in performance: axial misalignment andpumping a fluid that the system was not designed tohandle.

Axial Misalignment

Lineshaft turbine pumps are relatively simple and ruggedpieces of rotating equipment. Their proper operation andperformance, however, depends on delicate axialalignment. Referencing the rotational axis of the unit, allbutted, machined surfaces must be perpendicular, and allregisters, bushings and bushing housings must beconcentric. The consequences of misalignment are

vibration, undue wear and compromised performance.

Fluid Properties and Suspended Solids

Figure 1. A typical vertical turbine pump configuration

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The longest equipment operational life for a vertical turbine is generally obtained when pumping cool,clean water. Often is the case, though, that turbines are adapted for other fluids or have contaminantsintroduced into the fluid media that will degrade the machinery and reduce its useful life. Materialerosion is expected in areas where rotating parts have a minimal clearance with stationary parts, suchas an impeller rotating inside a bowl or a shaft rotating inside a bearing. However, certain constituentssuspended in the fluid also contribute to erosion. Gases such as air, carbon dioxide or water vaporfrom cavitation can physically destroy the pump when they are entrained back into a liquid state.Suspended solids such as sand can cause the same results as a sand blaster. Finally, corrosion broughton by aggressive fluids deteriorates the wetted parts of the unit, creating additional turbulent flowchannels.

Monitoring Performance

The only way to know that a turbine pump is deteriorating is to monitor its performance and comparethe production and power usage to previous levels. A gradual decrease in production, increasedvibration levels and increased power usage are all indicators that the pump may be wearing.

Operational Problems

The loss of production can have several causes, depending on the application. In a well pump, it couldmean the water level is dropping. In any application, it can indicate that the bowl assembly isbeginning to wear (i.e. the bowl and impeller clearance and/or the bowl shaft and bearing clearance isincreasing). If the performance reduction is sudden and dramatic, the causes could range from areduction in operational speed due to faulty controllers, power supply irregularities or other driverfailures. Other causes include an impeller becoming dislodged from the shaft, or holes developing insome part of the flow channel. If the pump never met the expected performance to begin with and theunit uses a semi-open impeller, the lateral may not be properly adjusted. Also, check the application tomake sure sufficient NPSH is available to prevent cavitation. The gases produced from cavitationdecrease liquid flow rates simply because they take up the space where the liquid should be.Cavitation can eventually destroy the pump.

Vibration

Excessive vibration when the unit is first put into operation likely suggests an alignment or driverproblem. Vibration that progresses gradually indicates that close tolerance parts are wearing due tonormal operation, the pumping of unclean fluids or slight misalignments. In the case where vibrationbegins at startup, the first component to check is the driver. Disconnect it from the pump and operateit by itself. If the vibration level is the same, the problem is in the driver. If it operates much smoother,there is a problem in the pump itself. Check the straightness of the shaft extending through the driverand the shaft connections. Any rust inhibitors applied to the top of the discharge head must also beremoved. These are some of the most common problem areas, but every point of connection has thepotential for misalignment.

Power Usage

Abnormal power usage indicates problems as well. The lateral may not be set properly to prevent theimpellers from dragging in the bowls, misaligned shafts may be forced into contact with the bearings(Photo 1), or the clearances around rotating parts may be too tight because of scale buildup (Photo2). These situations will add to the driver load and cause it to consume more energy to produce thesame amount of work.

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Photo 1. A misaligned shaft caused this bearingbore to be "whipped out."

Photo 2. Scale buildup in a bowl

Scale and other types of surface degradation can also build up in fluid passages. Unless other systemchanges occur, it is likely that a loss in production and reduced power consumption will be noticed.Debris around the suction screen can have the same effect.

If the power consumption stays constant, but production levels drop, there is probably a holesomewhere in the fluid passages. The pump is doing the work; it just doesn’t show up where it’suseful. The energy is being expended pumping some of the fluid back to the source.

Making the Diagnosis

Once a pump is disassembled and inspected and the pump components appear to be degradedabnormally, it’s time to decide whether the pump is repairable or if it should be replaced. But beforemaking any repairs, pinpoint the cause so that the right problem is corrected. Visual clues oftenindicate exactly what the problem is. If you cannot determine the cause, contact the pumpmanufacturer for assistance.

Visual Clues

Wear patterns do give clues to their causes. Bowl shafts that come out hourglass shaped at thebearing locations usually indicate that the pump has been handling sand (Photo 3). Scoop marks in thebowls similar to a horse’s hoof print in the sand are signs that gas is in the fluid (Photo 4). Impellersthat are coated black and cast iron bowls that are soft enough to cut with a knife are evidence ofgraphitization. Graphitization is reaction of the metals to a catalyst, such as carbonic acid, thatremoves the iron from the bowls and plates it onto the impeller.

Photo 3. Sand wear on a bowl shaftPhoto 4. The scoop marks in the bowl are sign that gas was in the fluid.

If a bowl bearing or the impeller bore at the top of a bowl with enclosed impellers is worneccentrically and the corresponding wear on the rotating part is concentric, this indicates that there is

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a problem in a stationary part. Debris, such as small pebbles, can work themselves between the twosurfaces. In some cases the faces of mating parts, such as the bowls or pipe, are not assembled ormachined parallel. During assembly all parts should be clean and smooth so that they mate properly.Foreign material between the mating faces produces the same wear signs as faces not being machinedparallel or bearing bores and/or registers that are not concentric with the rotational axis.

If the wear on stationary parts is concentric and on rotating parts is eccentric, there is a problem in therotating assembly. The shaft faces must be parallel to each other and the shafts should be straight towithin .005" (TIR). The shaft coupling may have been installed into the tapered portion of the threads.The coupling must stay on the straight portion of the threads or it will tilt the shaft. Similarly, the endsof the shaft must be smooth and clear of debris. In both cases, improper assembly will simulate a bendin the shaft. The ends should be faced parallel after being straightened. If the ends are machined priorto straightening and then faced, they will no longer be parallel.

Troubleshooting Information

Above are just a few of the things that can happen. The variety of problems seen in one applicationwill be few compared to all the applications that the manufacturer sees. Pump manufacturers can helpyou troubleshoot. When contacting them for assistance, give them all the data you have available. If aflow meter and pressure gauge are available, record flow and head readings at several points,including shutoff if possible. Take amperage and a voltage reading at those points also. Give somehistory on the pump’s operation. Did the problem develop suddenly or gradually? Have there beenany changes in the application, such as temperature fluctuations, fluid level variations, flow or headrequirement changes, recent system maintenance operations or other developments that could possiblyaffect performance? The more information you give, the more likely it is the manufacturer can helpdetermine the problem and suggest a solution.

Rebowling

Just the cost of downtime can easily outweigh the cost of rebowling versus repair. If minimizingdowntime is critical to operational processes, a spare bowl assembly may be in order. In that instancea bowl assembly can be removed and replaced in one procedure, and the original unit can be repairedfor future use. Taking it one step further, if the unit is a short-coupled pump a complete standbyreplacement may be advisable.

When ordering a replacement bowl assembly, all design parameters need to be given to the pumpsupplier. In some cases the information may not affect the cost, but it will help the manufacturerdetermine what types of special adaptations will be required. That information can affect productiontime estimates. The minimum information that should be provided includes, but is not limited to, thefollowing: head, capacity, operating speed, driver rating, fluid temperature, NPSH available, lengthlimitations, width or diameter limitations, materials of construction, and bowl to pipe and shaftconnection information.

Column connections are the most difficult obstacle in rebowling. Threaded column connections arethe simplest to match, because normally they are machined to a pipe thread standard. Flanged andbolted connections, which do not have standards with respect to mating to a bowl assembly, requireexact measurements so that adapters can be made. Threaded enclosing tube connections are closebehind the flanged pipe in the variations that can exist. Finally, the thread standards on the end of thebowl shaft must be matched. Although industry-wide thread standards exist for the shafts, exactdimensional information is still required to match it to the proper standard. If at all possible, at leastthe parts to be mated (enclosing tube bearings, shaft couplings, and column adapters) should be sentto the pump supplier to ensure that all pieces fit together properly.

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Typical Bowl Assembly Repair Operations

If it is not the case that rebowling is less expensive than refurbishing, you will find that there aretypically four areas within a bowl assembly that are economically repairable: the bowl bearings, thebowl shaft, the bowls (diffusers) and the impellers. Some manufacturers offer both repair andreplacement services.

Bowl Bearings

Our first consideration is the bowl bearings. Most often they are bronze, rubber or a combination ofthe two. Other materials used include bronze backed rubber bearings (commonly called "marine"bearings), Teflon and carbon composites. The typical shaft/bearing clearance is .010", except whenrubber bearings are used. In that case the clearance can run up to .030", which makes it a much moreforgiving material in applications where abrasive solids are present.

Bowl Shafts

The bowl shaft is typically made of some grade of stainless steel rather than a carbon steel because ofthe resistance of stainless to abrasion/corrosion. If abrasive solids are pumped, chrome or other hardcoatings are sometimes applied to the shaft. Unless the shaft is an abnormal size or material (making itexpensive to replace) a new shaft is usually installed rather than refurbishing or coating the old one.The shafting is the heart of the pump. Make sure all shafts are straight and properly machined beforeassembly.

Bowls and Impellers

The third and fourth parts, which are limited to enclosed impeller pumps, are the bowls and impellers.Specifically, the areas around the impeller skirt and the adjacent area in the bowl are closely examined.The clearance between these two parts is typically around .015". If there is sufficient materialavailable to bore the bowl and/or turn the impellers, wear rings can replace the worn material andbring the parts back into tolerance. In some cases, both parts require wear rings. In others, however,it may be possible to bore or turn one part, while the other accepts a wear ring specifically sized tomeet the machined part. Wear rings are sometimes installed as part of the original assembly. Unlessthe fluid contains some particularly abrasive solids and the wear rings are made of materials tominimize the abrasive erosion, forego the expense of wear rings until the parts are worn. Then, duringmaintenance or repair operations, the parts can be machined to accept the wear rings. If wear ringsare ordered with the original unit for easy replacement later, both bowl and impeller pieces arenecessary to return the bowl assembly to its original tolerances.

Column Pipe, Enclosing Tubes and Shaft Supports

The next section of the pump to consider is the column assembly. The pipe may be pitted or encrustedwith scale, or the coating may be damaged. The degree of roughness will be the determining factor asto whether the pipe should be reused, recoated, cleaned or replaced. The exterior of enclosing tubes,when used, take the same consideration as the pipe. The faces of the tubes should be inspected forleakage or damage, and the tubes themselves should be replaced when necessary (Photo 5). Enclosingtube bearings should have approximately the same tolerances as the bowl bearings (.010"). Open lineshaft bearings should be replaced when they are visibly damaged. Most often they are rubber and canbe purchased directly from the pump manufacturer. The condition of these bearings does notnecessarily affect efficiency, but it is vital to supporting the shaft.

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Shafts

This brings us to shafts, which are commonly available intwo materials: stainless steel or carbon steel with analternative wear surface. Some grade of stainless steel isoften used when aggressive fluids are handled. In openlineshaft pumps, if the wear is not excessive enough toweaken the shaft and it is threaded the same on both ends, itcan be turned over and reused. This places a new wear

surface in contact with the column bearings. Care must be taken, though, in examining the faces of theused shafts. Recesses in the ends of the shafts may form ridges in the opposing faces that will need tobe removed before the unit is reassembled. Under the same conditions, similar methods can be usedon the carbon steel shafts, such as placing new sleeves on the opposite end of the shaft or renewingthe hardened coatings. Much like the bowl assembly, consideration needs to be given to the cost ofsimply replacing the parts instead of repairing them.

Discharge Heads

The most visible part of the pump, the discharge head, generally requires the least maintenance. Mostof the repairs are performed on the shaft seal, whether it is a stuffing box or a mechanical seal.Packing in the stuffing box should be replaced to control leakage. Stuffing box or seal housingbearings need to be checked to make sure the shaft is properly supported. Send mechanical seals tothe original manufacturer or a local repair expert for inspection.

Drivers

Last but not least, the condition of the driver should be reviewed. The thrust bearings should bechecked to ensure that they are not excessively worn. Motors upgraded to premium efficiency or atleast replaced or rewound can significantly increase overall pump performance. Motors, gear drivesand engines should be inspected by specialists to determine any maintenance procedures that willoptimize the system’s performance.

Other Repair Considerations

The fluid passages of the bowl and impeller are often overlooked by maintenance personnel. If theyare damaged or pitted or have a particle buildup or eroded coatings, there can be a significantdecrease in efficiency. Coatings are normally applied to the interior of the bowl assembly, mainly toenhance efficiency. Although they are applied for protection in some cases, the velocity of the fluidsand suspended solids being pumped often makes it difficult to keep the coating intact. Cleaning,recoating or replacing the affected parts are all options, depending on the severity of the problem.

Repair Specialists

Who should inspect the unit and perform repairs? Make sure it’s someone who is experienced andqualified to work specifically on vertical turbine pumps. Some problems are specific to verticalturbines and require a trained eye to identify them. Whenever possible, send the complete pump to therepair shop. When it is torn down and ultimately reassembled, there will be no question that the partsfit. If the fits are not checked until they’re in the field, the cost of having personnel and equipmentwaiting while adjustments are made can be high.

Photo 5. Worn enclosing tube bearing

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Why Revitalize?

Assume a 12" bowl assembly is in operation and the bowl/impeller clearances change from .015" to.064"(1/16"). The loss in capacity would be in the neighborhood of 10%, and the loss in bowlefficiency would be approximately 2.5 points. Unless the pump has some type of speed controller,such as a variable frequency drive or an engine that can be accelerated, that loss in capacity cannot beregained. In the case that other parts of the system, such as the discharge or column pipe, becomeirregular from damage or buildup, some capacity can be regained by opening valves. However, morepower will be expended to meet the same production rate. A 1000 gpm pump operating at a totaldynamic pressure of 100 psi will cost between $200 and $300 more per year to operate per point dropin overall efficiency. That is assuming operations of eight hours per day, every day of the year, with apower cost of $0.10 per KW-h.

The bottom line is that the user pays more in operational costs when operating a vertical turbine pumpthat has deteriorated beyond its original design tolerances. The challenge maintenance personnel arefaced with when the pump’s performance is no longer acceptable is to determine the mostcost-effective option either eliminating the cause of the deterioration or minimizing its effects. Don’twait for a catastrophic failure. Perform regular maintenance repairs when downtimes are acceptable.Otherwise, the pump system will inevitably break down when it is least desirable.

David LaCombe is the Engineering and Operations Manager for American Turbine Pump Company.He has been with the firm for 10 years. He is also a graduate petroleum engineer from Texas A&M

University.

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revitalizing vertical lineshaft turbines

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