Pump Inspection Handbook

Chapter 11

Fluid systems

CENTRIFUGAL PUMPSExperience shows that pumps fail more frequently, and generally causemore problems, than other components in a fluid circuit. Pump tests andinspections are therefore an important part of a good inspectionstrategy. There are several hundred identifiable types of pump designtailored for varying volume throughputs and delivery heads, andincluding many specialized designs for specific fluid applications. Themost common type, accounting for perhaps 80 percent of fluid transferapplications, is the broad centrifugal pump category. We willconcentrate on this type.

Fitness-for-purpose criteriaThe fitness for purpose of a pump is predominantly to do with its abilityto move quantities of fluid. There are many pump performanceparameters, some of which are complex and may be presented in anon-dimensional format. For works inspection purposes, however, youonly need to consider those which normally form part of the pump'acceptance guarantees'.

Volume flowrate (q)Flowrate is the first parameter specified by the process designer, whobases the pump requirement on the flowrate that the process needs inorder to function. This 'rated' flowrate is normally expressed in volumeterms and it is represented by the symbol q, with units of metres3/second.

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Head (H)Once the rated flowrate has been determined, the designer then specifiesa total head (H) required at this flowrate. This is expressed in metres andrepresents the usable mechanical work transmitted to the fluid by thepump. Together q and H define the duty point, the core FFP criterion.

Net positive suction head(NPSH)NPSH is slightly more difficult to understand. Essentially, it is a measureof the pump's ability to avoid cavitation in its inlet (suction) region. Thisis done by maintaining a pressure excess above the relevant vapourpressure in this inlet region. This pressure excess keeps the pressureabove that at which cavitation will occur. Acceptance guaranteesnormally specify a maximum NPSH required. The unit is metres.

Other FFP criteria Pump efficiency (n percent): the efficiency with which the pump

transfers mechanical work to the fluid. Power (P) in watts, consumed by the pump. Noise and vibration characteristics.

It is normal practice for the above FFP criteria to be expressed in theform of acceptance guarantees for the pump. The objective of theperformance testing programme is to demonstrate compliance withthese guarantees.

Basic technical informationA large number of pump designs fall under the general categorization ofcentrifugal pumps. These include radial, mixed-flow, and axial pumpsand they can be tested using similar methods. In most plantspecifications, you can expect to see a similar format of acceptanceguarantees for the various pump designs within this wide category. Thefirst step is to understand the set of curves that are used to describepump performance. These are commonly known as 'characteristics', orsimply 'curves'. Figure 11.1 shows a typical set.

The q/H curveFor most centrifugal pump designs the q/H characteristic looks like thatshown in Fig. 11.1. The test is carried out at a nominally constant speed

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Fig 11.1 A set of centrifugal pump characteristics

and the head (H) decreases as flowrate (q) increases, giving a negativeslope to the curve. Note how the required duty point is represented andhow the required pump power and efficiency change as flowrate varies.

The NPSH (required) curveOne reason why NPSH can be confusing is because it needs twodifferent sets of axes to describe it fully. The lower curve in Fig. 11.1shows how the NPSH required to maintain full head performance riseswith increasing flowrate, but note that this curve is not obtained directlyfrom the qH test - it is made up of three or four points, each pointbeing obtained from a separate NPSH test at a different constant q (seeFig. 11.4). This is normally carried out after the q/H test. In the NPSH

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test you will be looking for the pump to maintain full head performanceat an NPSH equal to or less than a maximum guarantee value.

Typical acceptance guarantee schedulePump acceptance guarantees are expressed in quite precise terms - if youlook at a good specification you will see something like this (I haveshown indicative values for a large circulating water pump to give youan idea of magnitude):Rated speed (n) 740 rpmRated flowrate (q) 0.9 m3/second together, these defineRated total head (H) 60 m the duty pointRated efficiency 80 percent at duty pointAbsorbed power 660 kW at duty pointNPSH Maximum 6 m at impeller eye for 3 percent

total head drop.Vibration Vibration measured at the pump bearing

shall not exceed 2.8 mm/sec rms at the dutypoint

Noise Maximum allowable level of 90 dB(A) atduty point (at agreed measuring locations)

Now the specification states:

Tolerances should be 1.5 percent on head (H) and +2 percent onflow (q) (these are typical, but can be higher or lower, depending onwhat the designer wants) but + 0 on NPSH.

The acceptance test standard, e.g. ISO 3555, is important - it tellsyou a lot about test conditions and which measurement tolerances totake into account when you interpret the curves.

Later we will see how to check whether the pump has complied with therequirements.

Specification and standardsWe are fortunate in that pump performance testing is well covered by atried and tested set of standards which relate specifically to the radial,mixed and axial flow category. These standards relate only to the pump

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itself. Pumps are only rarely subject to performance tests in the processsystem for which they are intended, normally they are tested in a specificperformance test rig. The main standards are listed below.

ISO 2548 (identical to BS 5316 Part 1) (1) is for Class C levels ofaccuracy. This is the least accurate class and has the largest allowablemeasurement tolerances which are applied when drawing the test curves,and hence the largest acceptance tolerances on q and H.

ISO 3555 (identical to BS 5316 Part 2) (2) is for Class B levels ofaccuracy, with tighter test tolerances than for Class C.

ISO 5198 (identical to BS 5316 Part 3) (3) is for Class A (or precision)levels of accuracy. This is the most stringent test with the tightesttolerances.

DIN 1944 'Acceptance tests for centrifugal pumps' (4). This isstructured similarly to BS 5316 and has three accuracy classes, in thiscase denoted Class I, II or III.

API 610 'Centrifugal pumps for general refinery service' (5). This is amore general design-based standard.

ISO 1940/1 (identical to BS 6861 Part 1) (6) is comonly used to definedynamic balance levels for pump impellers.

VDI 2056 (7) is commonly used to define bearing housing or pumpcasing vibration. A more complex method, measuring shaft vibration, iscovered by ISO 7919-1 (similar to BS 6749 Part 1).

DIN 1952 and VDI 2040 are currently withdrawn standards but arestill in common use to specify methods of flowrate (q) measurement.

In some inspection situations the test standard will not be quoted.This shouldn't happen. If it does I normally apply the following simpleguidelines:

If there is no guidance to the contrary use ISO 3555. If high test accuracies are necessary, i.e. if the pump guarantees are

very closely specified on a reasonably 'flat' q/H curve, use DIN 1944(Class I).

If the pump is NPSH-critical or the NPSH available from the fluidsystem is in any way uncertain (check with the system designer), or ifit is an experimental pump, then it is perhaps best to use ISO 5198. Itsdefinition of NPSH testing is quite comprehensive and it specifiesclear measurement tolerances of 3 percent NPSH or 0.15 metres.

There are other standards that you may meet, although frankly they arerarely used during a works inspection.


Inspection and test plans (ITPs)Check the ITP for the pump manufacturing sequence. It should addressas a minimum the following items:

Pump casingMaterial test certificates (to EN 10 204 type 2.2).Material identification records.NDT results.Record of casting defects, MPI and repairs.Hydrostatic test (normally at a maximum of 2 x working pressure).

Pump shaft and impeller Material test certificates (to EN 10 204 type 3.1B). Heat treatment verification. NDT tests as specified. Dynamic balance certificate (a common level is ISO 1940 grade

G6.3).Assembled pump Completed technical data sheet. Guarantee acceptance test results and report. Painting records and report. Pre-shipping documentation review.

Some manufacturers will exceed these minimum requirements, otherswill not.

Test procedures and techniquesThe pump acceptance test is carried out in a purpose-built test circuit inthe manufacturer's works. In practice the layout of the circuit may bedifficult to see as some of it is often underneath the test bay floor plates.Luckily most test circuits follow a similar pattern. Figure 11.2 showswhat to look for. Note that there are effectively two different parts, thebasic circuit for the q/H test and an auxiliary suction control loop whichis connected for the purposes of the NPSH test. The circuit should havesuitable instrumentation to obtain the performance data; many pumpmanufacturers have a fully computerized datalogging system to processthe data and display the results.

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It makes good sense to start every pump test with a series of circuitchecks. The act of checking the circuit will show that you are adopting alogical, professional approach - and will give you guidance on the likelyaccuracy of the test results.

Flowmeter

Fig 11.2 The pump test circuit

Some circuit checks Normally a shop motor (i.e. not the contract motor) is used to drive

the pump. Make sure it has the correct, or higher, power rating. Check the pipe arrangements either side of the flowrate measuring

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device, there should be sufficient straight run in order not tointroduce inaccuracies. Refer to ISO 5167 if in doubt.

Check the suction and discharge arrangements on either side of thepump. The pressure gauge or manometer connections should be atleast two pipe diameters from the pump or readings will beinaccurate.

Watch for flow straighteners fitted before the pump. These aresometimes fitted to produce the required inlet flow characteristics butthey can produce pressure losses and distort the results.

Ask the manufacturer to explain any variation of vertical levelsthroughout the circuit. These are particularly relevant to the NPSHtest.

Ensure that the volume of fluid in the circuit is sufficient to avoidtemperature rise during the q/H test. If pump input power is high inrelation to the volume, then additional cooling may be required.

Calibration. Check calibration records for all measuring andrecording equipment. Don't forget the transducers.

Empirical factors. The pump manufacturer may have factors built into his calculation routines that have an empirical basis. Fluid densitycorrections and level corrections are two common examples. Checkwhat they are.

These checks will only take a few minutes but are an essential part ofthe test. Make clear notes of what you have found. The pump testroutine will, with a few exceptions, follow a well-defined format. Oncestarted, the steps can follow in quite quick succession.

Step 1Don't just start, without any preparation. Check the circuit.Step 2The pump is started and the circuit allowed to attain steady-stateconditions by running for at least 30 minutes. Use this period to makean initial check of the measuring equipment readings to ensureeverything is working. Watch for any early indications of vibration ornoise.Step 3; The q/H testThe q/H characteristic is determined as follows. The first set ofmeasurements is taken at duty point (100 percent q). The valve is opened

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to give a flowrate greater than the duty flow (normally 120 or 130percent q) and further readings taken. The valve is then closed in a seriesof steps, progressively decreasing the flow (note that we are movingfrom right to left on the q/H characteristic). With some pumps, the finalreading can be taken with the valve closed, i.e. the q = 0 or shut-offcondition. This is not always the case, however - for high power pumps,or those with a particularly high generated head, it is undesirable tooperate with a closed discharge valve. During the test, it is useful to payparticular care to the spacing of readings around the duty point,particularly for Class A pumps where greater accuracy levels will beapplied. Close spacing around the duty point will help the accuracy ofthe results by better defining the shape of the curve in the duty region.

Once the test points are obtained, you can now check against theguarantee requirements. There are several discrete steps required here(refer to Fig. 11.3) Draw in the test points on the q/H axes. Using the measurement accuracy levels given for the class of pump,

draw in the q/H measured band as shown.Steps

, 1 Draw in the test points, 2 Draw in the band using the

measurement tolerances (from the standard)3 Add the rectangle, representing

acceptable limits (from the specification)4 This area meets the guarantee

Fig. 11.3 How to check compliance with the q/H guarantee

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Now add the rectangle, which describes the tolerances allowed by theacceptance guarantee on total head (H) and flowrate (q). ISO 3555indicates tolerances of 2 percent H and 4 percent q should beapplied, if nothing is stated in the specification.

If the q/H band intersects or touches the rectangle then the guaranteehas been met (this is the situation in Fig. 11.3). Note that therectangle does not have to lie fully within the q/H band to beacceptable.

It is not uncommon to find different interpretations placed on the wayin which ISO 3555 specifies acceptance tolerances. The standard clearlyspecifies measurement accuracy levels (2 percent q, 1.5 percent H)but later incorporates these into a rigorous method of verifying whetherthe test curve meets the guarantee by using the formula for an ellipse(effectively allowing an elliptical tolerance 'envelope' around eachmeasured point), specifying values of 2 percent H and 4 percent q to beused as the major axis lengths of the ellipse. Strictly, this is the correctway to do it - but I have always found the simplified method shown inFig. 11.3 easier to use.

Step 4: The efficiency testThe efficiency guarantee is checked using the same set of testmeasurements as the q/H test. Pump efficiency is shown plotted againstq as in Fig. 11.1. In most cases, the efficiency guarantee will be specifiedat the rated flowrate (q), the same one that you used for checking thehead guarantee. The principle of checking compliance is similar, i.e.draw in the characteristic bandwidth using the applicable measurementtolerances, followed by the rectangle representing any tolerancesallowed by the acceptance guarantees.

Step 5: Noise and vibration measurementsVibration levels for pumps are normally specified at the duty (100percent q) point. The most common method of assessment is to measurethe vibration level at the bearing housings using the methodologyproposed by VDI 2056. This approximates vibration at multiplefrequencies to a single velocity (rms) reading. It is common for pumpsto be specified to comply with VDI 2056 group T vibration levels - so alevel of up to around 2.8 mm/second is acceptable. Some manufacturersscan individual vibration frequencies, normally multiples of therotational frequency, to gain a better picture of vibration perfor-

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mance. This does help with diagnosis, if excessive vibration isexperienced during a test.

Pump noise is also measured at the duty point. It is commonlyspecified as an 'A-weighted sound pressure level' measured in dB(A) atthe standard distance of 1 metre from the pump surface (the principlesare explained in Chapter 7 covering gas turbines). It is sometimesdifficult to obtain accurate noise readings during pump tests owing tothe considerable background noise which can come from turbulence inthe rest of the fluid circuit. Any pump which has noise levels close to itsacceptance noise level should be checked very carefully for excessivevibration levels, then particular attention paid to bearings and wearrings during the subsequent stripdown.

Step6:The NPSH testThere are two common ways of doing the NPSH test. The first is simplyused for checking that the pump performance is not impaired bycavitation at the specified q/H duty point with the installed NPSH of thetest rig. This is a simple go/no-go test applicable only for values ofspecified NPSH that can be built in to the test rig. It does not give anindication of any NPSH margin that exists, hence is of limited accuracy.The more comprehensive and useful test technique is to explore NPSHperformance more fully by varying the NPSH over a range andwatching the effects. The most common method is the '3 percent HeadDrop' method shown in Fig. 11.4.

Using the test rig as used for the q/H test but with the suction pressurecontrol circuit switched in (see Fig. 11.2), the suction pressure is reducedin a series of steps. For each step, the pump outlet valve is adjusted tokeep the flowrate (q) at a constant value. The final reading is taken at thepoint where the pump head has decayed by at least 3 percent. Thisshows that a detrimental level of cavitation is occurring and defines theattained NPSH value, as shown in Fig. 11.4. In order to be acceptable,this reading must be less than, or equal to, the maximum guaranteevalue specified. Strictly, unless specified otherwise, there is noacceptance tolerance on NPSH, although note that ISO 3555 gives ameasurement tolerance of +3 percent or 0.15 metres NPSH. Sometimesyou will see this considered as being the acceptance tolerance - I haveused this interpretation in the specimen inspection report shown inChapter 15 of this book.

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Steps1 The curve represents q = 100% flow2 Suction pressure is reduced until 3% H drop3 NPSH measured is at point (X)4 The guarantee point is at (g) so this test

result is acceptable

Cavitation starts here

Adjustment of suction pressuregoes this way

Sufficient cavitation to give 3% H drop occurs here

NPSH (required)Guarantee point

Fig 11.4 Measuring NPSH: the '3% head drop' method

CorrectionsThere are a few commonly used correction factors that you need to useif the test speed of the pump does not match the rated speed. This oftenhappens. The following factors will give sufficiently accurate results.Remember to apply them to q, H, P and NPSH.

Flow q (corrected) = q (measured) X (Nsp/n) Head H (corrected) = H (measured) X (Nsp/n)2 Power P (corrected) = P (measured) X (Nsp/n)3 NPSH (corrected) = NPSH (measured) X (Nsp/n)2

n = speed during the testNsp = rated speed

Step 7: The stripdown inspectionAlways try to witness a stripdown inspection after the performance test.This may seem to be an almost incidental part of the test procedure, but

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it is the inspector's best opportunity to check and report on someimportant design and manufacturing features of the pump. Byinspecting carefully and reporting accurately you add value to whatyou do - a good example of effective works inspection.

Let us look at the best way to do this. First obtain some goodbackground technical information (remember the concept of 'qualifyinginformation' I introduced in Chapter 2?). The pump standard API 610 isuseful and will give you clear guidance on desirable mechanical designfeatures, irrespective of how much you know already. Then do somepreparation - make a list of the points to check, including any specificrequirements of the pump purchase order. Your list should look similarto the following.

Stripdown checks Watch the pump run-down - it should be smooth without any undue

noise or unbalance. Check how the casing sections come apart - they should be a firm

press fit but separate without needing excessive force. Check the casing joint faces for flatness, there should be no warping. Spin the shaft bearings by hand to check for any tightness or radial

wear. Check the bearing surfaces - there should be no evidence of

lubrication breakdown or overheating Check the mechanical seals - any chipping or wear indicates incorrect

assembly. Check the impeller fixing - it should be secured to the shaft with a

cap nut so the spindle threads are not exposed to the pumped fluid.The impeller should have an acceptable fixing to the shaft (somespecifications require a keyed drive, others do not).

Check the wear rings for excessive wear (get the limits from themanufacturer's drawings). You can use this opportunity to have alook at the wear ring fixings, they should be locked against rotationby a threaded dowel - not tack welded.

Surface finish is important. If in any doubt, you can use acomparator gauge - check for a finish of 0.4 mm Ra or better onshaft and seal surfaces. Pump casings should have a finish better than25 mm Ra on outside surfaces and 12.5 mm Ra on internal surfaces.

Visually inspect the impeller water passages - smooth surfaces(12.5 mm Ra) indicate good finishing during manufacture. Look alsofor any evidence of the impeller having been trimmed, or 'underfilled'

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on the trailing edges to make it meet its q/H requirements. These areacceptable but only within limits.

Then carefully record the findings for your inspection report.

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Common non-conformances and corrective actions: pumpsNon-conformance Corrective actionThe q/H characteristic is aboveand to the right of theguarantee point (i.e. too high).

The q/H characteristic is 'too low'- the pump does not fulfilits guarantee requirement for qor H.

NPSH is well above theacceptance guaranteerequirements

NPSH result is marginal

For radial and mixed-flow designs, this isrectified by trimming the impeller(s). The q/H curve is moved down and to the left.Watch for resultant changes in dynamicbalance. Repeat the test.Often, up to 5 percent head increase can beachieved by fitting a larger diameterimpeller. If this does not rectify thesituation there is a hydraulic design fault,probably requiring a revised impeller de-sign. Interim solutions can sometimes beachieved by: installing flow-control or pre-rotation

devices installing upstream throttles.This is most likely a design problem, theonly real solution being to redesign. Thenrepeat the test.

This can sometimes be a problem ofstability. The right thing to do is to try thetest again and see if you get an exactlyreproducible result, paying particular atten-tion to the measurement of the 3 percenthead decay (watch and listen for evidence ofcavitation). It is sometimes possible toaccept marginal NPSH performance underconcession - to do this properly you need tocheck the system NPSH available, to seewhether a satisfactory pressure margin(about 1 metre) still exists.


Non-conformance Corrective actionExcessive vibration over thespeed range

Excessive vibration at rated speed

Noise levels above theacceptance guarantee levels

The pump must be disassembled. Firstcheck the impeller dynamic balance (youcan use ISO 2373/BS4999 part 142/IEC.42or ISO 1940 for guidance).

Next check all the pump comp-onents for'marring' and burrs - these are a primecause of inacc-urate assembly. During re-assembly, check concentricities by measur-ing total indicated runout (TIR) with a dialgauge. Check for compliance with thedrawings.Then repeat the test.

Check the manufacturer's critical speedcalculations. The first critical speed shouldbe a minimum 15-20 percent above the ratedspeed. Then do all the checks shown above.It is important to describe carefully thevibration that you see. High vibration levelsat discrete, rotational frequency is a causefor concern. A random vibration signatureis more likely to be due to the effects of fluidturbulence.

Pump noise is difficult to measure because itis masked by fluid flow noise from the testrig. Take this into account. If high noiselevels are accompanied by vibration, astripdown and retest is necessary.

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Handbook of Mechanical Works Inspection_Clifford Matthews.pdfContentsRelated Titles of InterestChapter 1: How to use this bookChapter 2: Objectives and tacticsChapter 3: Specifications, Standards, and plansChapter 4: Materials of constructionChapter 5: Welding and NDTChapter 6: Boiler and pressure vesselsChapter 7: Gas turbinesChapter 8: Steam turbinesChapter 9: Diesel enginesChapter 10: Power transmissionChapter 11: Fluid systemsChapter 12: CranesChapter 13: LiningsChapter 14: PaintingChapter 15: Inspection reportsAppendix

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Pump Inspection Handbook