crv applications

8/6/2019 CRV Applications

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CRV Applications

A Cure For Pump Cavitation

Cavitation means that cavities or bubbles are forming in the liquid that is being pumped. These cavities form on thelow pressure or suction side of the pump.Most pump piping systems have an elbow within ten pipe diameters of the

pumps suction inlet. The elbow causes a flow turbulence which results in pump cavitation. While the CRV itself hasno moving parts, it imparts a rotational flow to the liquid to exactly cancel the rotational effect of the elbow so that thefluid exits the elbow with no turbulence.

To operate properly Centrifugal Pumps require ten pipe diameters or more of straight pipe at their inlet, to dissipatethe elbow induced non-uniform flow. When pipe diameters become large, this requirement becomes hard to meet. Asa result, non uniform flow enters a pump resulting in a flow imbalance in the impeller. In the case of a double-suctionpump, induced axial unbalanced vibration, head loss, and impeller cavitation occur.

Figure 1a and 1b -Top view of

double-suctionpump showing howa plain elbow (1a)creates cavitationwhile a CRV (1b)promotes an evenflow distribution.

In Figure 1b, abalanced, uniformflow is seen as aresult of using aCRV . The CRVis a proven flow

conditioner which has solved thousands of industrial pumping installation problems.

The Cheng Rotation Vane (CRV ) consists of a set of stationary vanes in a cylindrical body which is placedimmediately upstream of an elbow or tee in a piping system. The device addresses the following flow problems

described sections below:

A. Pump CavitationB. Flow Measurement AccuracyC. Elbow ErrosionD. CompressorsE. Check ValvesF. Water HammerG. Specifying CRV

The CRV eliminates elbow induced turbulence whichnegatively impacts the performance of pumps, compressors,


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control valves, flow meters, and other equipment. The CRV inputs to the flow a counteracting gyroscopic motion tothe resultant elbow induced gyroscopic motion, and enables the fluid to negotiate the turn through the elbow and thenexit the elbow with a flat velocity profile. This results in an even distribution of process fluid through any cross-sectionof the elbow and transforms the elbow into the equivalent of a straight length of pipe, and there is no additionalpressure drop with the use of a CRV.

See Customer Testimonials regarding how Dupont saved money and how Bechtel improved their flowmetermeasurement accuracies below.

A. Pump Cavitation

Problems related to pump cavitation:

1. Pump cavitation, vibration and noise2. Frequent seal, bearing, or impeller replacement3. Non-uniform suction flow creating reduced flow and head4. Lack of space for proper pump installation

Solution: The CRV

Benefits:

1. Reduced maintenance intervals, less downtime, and higher reliability2. Reduced cavitation, vibration, and noise3. Improved Net Positive Suction Head (NPSH)

Customer Testimonial: Dupont reports savings of 50X the installation costs

Case History:

Figure 2 - CRV installation just upstream of a shortradius elbow and overhung centrifugal pump.

Figure 2 shows the feed piping configuration of a3,500 rpm pump utilizing an eight inch CRV. Prior toinstalling the CRV , the pumps operation had beennoisy and the unit required an overhaul every 4months. The vibration spectra of the pump before andafter the installation of a CRV are shown in Figures3a and 3b. The dramatic reduction in measuredcavitation (6-14 range on the frequency scale) alsoresulted in a reduction in impeller imbalance (1 on thefrequency scale). This reduced the noise; extendedseal, bearing, and impeller life; and increaseddelivered head and flow. Prior to the CRV sinstallation, three identical pumps in parallel operationwere required to deliver the required flow rate. AfterCRV s were installed before the elbow of eachpump, two pumps met the flow rate requirements, and

the third pump served as an installed spare.


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Figure 3a & 3b - Comparison vibration spectra of 3,500 rpm double-suction pump (a) without CRV and (b) with CRV

B. Flow Measurement AccuracyProblems related to flow measurement accuracy

1. Inaccurate flow measurements2. Lack of space for proper installation3. Trapped fluids in dangerous chemical concentrations

Solution: The CRV

Benefits:

1. Accurate flow measurements2. More compact pipe layout3. Eliminates trapping of chemical species

The CRV is a technology breakthrough that provides a uniform elbow exit velocity profile, without swirl, allowing the close couplingof pipe elbows to meters. Distortion and separated flow always accompany the fluid exiting a pipe elbow. Long meter-runs arerequired to allow the viscous forces to re-establish a steady equilibrium flow distribution before they encounter the measuringinstrument. The CRV is a set of stationary vanes, in a cylindrical body, which is positioned upstream of the elbow. The CRV willprecisely rotate the flow about its axis to compensate for the rotation caused by the elbow. The CRV produces a uniform and non-turbulent flow at the exit of the elbow, and allows the length of the meter-run to be dramatically reduced. Elimination of theseparated pulsating flow also increases the accuracy of the meter system.


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Customer Testimonial: Bechtel reports improved flowmeter accuracy in tight spaces

The conventional need for pipe meter-runs

Three examples of meter-run requirements:

1. For a plain elbow, standard meter-runs are up to 30 diameters long, depending upon the type of fluid being measured (seeexample A below).2. Tubular flow straighteners, used to homogenize the separated, non-uniform flow from elbows, still require at least 20 pipe

diameters in length for settling velocity distortion (see example B below).3. The CRV prevents the formation of separation and cross flow in the elbow. Now the meter-run can be as short as 1

diameter (see example C below).

It is not always convenient to provide a long meter-run, especially when metering instruments are added to an existing pipingsystem. The use of aCRV eliminates the need for long meter-runs, and provides a steady, uniform flow which results in consistentaccurate flow measurements for all types of meters.

Usage example for flow meters:

Continuous Emission Monitoring (CEM) is an EPA requirement for all power plants. A poorstack piping design will resultin

fluctuations, inaccurate pollutant readings, and burstphenomena (see Figure 1 below). Thisisa resultofthe separated flow causedby the ducting elbowsinthe stacks.


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Figure 1 - InatypicalinstallationofaCEM unitthe figure below illustrates flow trapping and attendant emission fluctuations.

With the properinstallationofa CRV intothe piping, the flow upthrough the stack becomes more uniform. This configurationavoidsthe burstphenomena (see Figure 2 below).

Figure 2 - With proper choice ofCRV, flow measurementsare more accurate.

C. Elbow ErosionProblems:

1. Frequent elbow erosion can be caused by particulate ortwo-phase flow2. Unsafe elbow erosion conditions

Solution: The CRV

Benefits:

1. Extended life of elbow/less downtime, more reliability2. Saferoperating conditions (nuclearplant)3. Elbow erosionis eliminated by preventing turbulence, internalpressure gradients, and cavitation, through the use ofan

internally hardened CRV. The particulates remaininthe flow stream, instead of hitting the outer wall, thus cavitationinduced erosionis eliminated.4. Reduced pump vibrationand noise.


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Above isatypical cavitationinduced elbow erosionthat developed inapowerplantoveraspanofonly two months.

Applications:

1. Coalslurry, particulate, and catalyst-blow downsystems2. Saturated liquid flashing3. Variousinternal coatings forthe CRV 'sare available to meetparticular customerapplications

Results:

1. Steam-waterpunch through was every two months. Three yearsafteraCRV wasinstalled, x-raysshow nosignsofextraordinary erosion.

2. Catalyst blow-downpipe waspenetrating the elbow wall every four months. Sixteen monthsafteraCRV wasinstalled, x-raysshow nosignsofany preferential wear.

D. CompressorsProblems:

1. Inlet distortions cause the compressortooperate atlessthanthe factory rated flow, head and efficiency

2. Interstage hunting caused by non-uniform flowSolution: The CRV

Benefits:

1. Operates closertothe factory rated flow, head, and efficiency2. Lowersuctionpiping pressure drop3. Reduced energy operating costs

Figure 1 - CRV in feed suctionpiping ofa multistage compressor

Centrifugaland Fiaxial compressoroperationand performance aresensitive to velocity and mass distributionatthe inlet. The ASMEPower TestCode for compressors (PTC-10) requiresa fully developeduniform velocity profile entering the compressor ORa minimumstraight runofthree pipe diametersis required atthe straightpipe inlettoproduce a flat velocity profile. In field operations, however,compressors donotalways have thisidealsuctionpipingconfiguration.

Poor field suctionpiping includessingle, double and triple elbowsimmediately upstream ofthe compressor, which create a considerableamountof distortion. For example, large multistage axialor centrifugalcompressors (i.e. a refrigeration compressor) thatsupply highpressure systems, asshownin Figure 1, can have upstream turnsin


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the suctionpiping which resultinnon-uniform massand flow profilesapproaching the splitterinthe compressor casing.

The CRV , whenplaced onthe inletside ofan elbow, producesa flat velocity profile and an even distributionofprocess gasattheelbow exit. Thisallowsthe compressorto more closely approach its factory testinlet conditionsand performance curves. TypicalCRV locationsin compressor feed piping systemsare shownin Figure 1 for multistage compressorsand Figure 2 forinterstage-cooled compressors.

Figure 2 - CRV'sinthe feed suctionpiping ofaninterstage-cooled compressor

E. Check ValvesProblems:

1. Chatter from check valves close coupled toan upstream elbow2. Discpin wearand breakage3. Poorsealing due to discpinand valve seat wear

Solution: The CRV

Benefits:

1. Eliminates elbow induced flow turbulence in check valves2. Extends discpinand valve seatlife3. Improvessealing reliability

Check valves, by the very nature oftheir design, respond to flow and pressure disturbancessuch asturbulence inthe upstreampiping system. This can resultinthe discoscillating back and forth onthe pinsupport. Whena check valve is close-coupled toan

upstream elbow, the turbulence becomessevere and ifthe oscillationsare ofalarge enough amplitude, the disc may bang againstthe stop. Eventually, the pin fails, leakage though the pinstarts, and the valve seat willnotseal.Resultsoftime history measurementsofthe complex fluid pressure waves resulting from the interactionofthe elbow generatedturbulence and the oscillating check valve generated turbulence are presented inthe figures below. Withoutthe CRV asshowninFigure a, it may be seenthatthe interactionofthese twoturbulant flows, resultsinperiodically large amplitude pressure bursts.These burstsare damaging tothe check valve because they resultina more severe check valve oscillation.


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In check valve systems with upstream close-coupled elbows, harmful high amplitude pressure bursts can be eliminated by installingaCRV upstream ofthe elbow. With aCRV mounted in frontofthe elbow-check valve combination, turbulence generated by theelbow is eliminated. Figure b showsanabsence of high amplitude pressure bursts. This resultsin minimized wearonthe checkvalve pins; extended life; reduced vibrationand noise; and bettersealing reliability.


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The discharge of many pumpsare equipped with check valvesinordertoprevent backflow whenthe pumpis being shut down. TheillustrationshowsatypicalCRV installation. WithoutaCRV , the oscillating check valve generatesapulsating back pressure onthe pump which causes excessive vibration, headbox breakage, bearing wearand evenshaft breakage. InstallationofaCRVprevents check valve oscillationand largely reducespump vibrationproblems. The CRV 'sinstallationmay also resultina higherflow rate and improved pump efficiency. Many utility boiler feed pumpsand cooling waterpumps have CRV sinstalled.

F. Water HammerProblems:

1. Traveling pressure waves reflect back and forth and can resonate inapiping system, resulting in high pressure shockwaves, which can break through pipe, joints, and elbows.

Solution: The CRV

Benefits:

1. The CRV canactasapassive one way dampener, which will dissipate the pressure wave energy into heat, along thepiping system.

Water hammerisactually atravelling pressure wave. Itisinitiated by the rapid stoppage ofanincompressible flowing liquid inapiping system. For example, the pounding heard inprocesspiping usually occurs due toa rapid valve closure or whenlarge steambubblesare introduced into waterand the water rapidly collapsesthe steam bubbles. These pressure waves reflect back-and-forthbetweenthe interior wallofapiping system, reinforcing themselvesassucceeding waves encounter each other. These wavesbecome so energeticthat catastrophicstructural damage canoccur.

The CRV s geometric featuresprove extremely usefulin controlling water hammer. When fluid is flowing inthe forward direction,the CRV ispassive and offersnopressure dropand keepsthe flow laminar. But whenthe fluid travels backward, it will exhibitahigh pressure dropand it willnot remainattached tothe CRV s vanes, asshownin Figure a. This high drag footprint (asshowninFigure b) fora backward flowing fluid and travelling pressure wave, actsasapassive dampener controlling the water hammer.


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Figure aand b - The CRV actsasapassive dampener, forthe reverse flow pressure waves, and thus controls water hammer

Dramaticimprovements were seeninapiping circuit with four elbows betweenthe supply line and the shut-off valve where the wallstaticpressure, with and withoutCRV s, was measured. The resultsindicate that with CRV sinplace upstream of each elbow,the amplitude ofthe peak pressure pulse was 49% ofthat withoutCRV s.

G. Specifiying CRVForthe convenience of engineers, architectsand designersin writing CRV specifications, Cheng Fluid Systemssuggeststhefollowing: The Cheng RotationVane (CRV ) must be located rightat butno furtherthanone pipe diameteraway from the elbow'sinlet. With thisarrangementthere will be a uniform velocity profile with no downstream residual rotation.

Depending onthe application, the CRV is designed toaddressspecific flow requirementssuch as velocity distribution, separation(which hasaccelerated and reverse flows), noise, vibration, erosion, eliminationofturbulence, and pressure drop reductions.

Cheng RotationVane (CRV ) Specification Sheet

MaterialsofConstructionThe CRV can be manufactured from most commercially available materials, pipe diameters, pipe schedulesand end connections.N-stamped unitsare available fornuclearapplications. Specialinternal coatingsand finishesare alsoavailable.

ApplicationsForthe convenience of engineers, architectsand contractorsin writing CRV specifications, please reference the applicationdiagramsprovided below.


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DimensionsCRV - standard unit dimensionin 2" to 12" sizesare shownin Table 1 below. Speciallength or designCRV s can be quoted onrequest.

Pipe Fitting GeometryWhenspecifying aCRV , itis criticalthatthe CRV matchesthe geometry ofthe fitting to which it will be attached. The fittingschedule (wallthickness) and type of fitting must be specified. We must know ifthe CRV will be used with along radiusorshortradius elbow. Incorrect fitting specifications will resultinpoorperformance because the angle ofthe CRV is very sensitive tothefluid path traveled inthe elbow. Toassistinthe properspecificationofaCRV , standard common fitting types, theirnomenclature,and dimensionsare shownin Figures 11-15 (Table 2) for both shortand long radiusapplications.


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Table 1

Table 2


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crv applications

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