Tài liệu Pracrical Machinery Management for Process Plants P2 ppt

Excessive shaft axial end play: Excessive shaft movement can result in pitting, fretting, or wear at points of contact in shaft packing and mechanical seal areas.. Defective bearings in

26 Major Process Equipment Maintenance and Repair

If failed bearings are suspected in pump or motor:

Check radial clearance and end float in motor

Run motor and check for abnormal noise, vibration

If motor is bad, remove and repair

Diagnosing Pump and Seal Problems In the Shop

While the pump is being repaired it is advisable to carefully examine every component A recommended procedure is to match mark all parts prior to disassembly and to make the following checks while dismantling the pump:

1 Visually check impeller and nut for wear, erosion, corrosion and

2 Remove seal flange nuts and check seal tension

3 Record impeller position in relation to pump frame

4 Remove impeller nut and impeller

5 Jnspect wear rings inboard, if any

6 Check and record throttle bushing clearance

7 Check body gasket faces

8 Remove stuffing box body from pump frame

9 Check stuffing box gasket face, bore, and pilots

other deterioration

10 Remove and inspect all shaft keys

11 Remove sleeve, seal, sleeve gasket and sleeve flange If neces- sary, determine the cause of seal failure and inspect condition of parts

12 Check pump bearings for roughness Record shaft end float, check shaft for wear, erosion, corrosion and straightness

13 Excessive shaft axial end play:

Excessive shaft movement can result in pitting, fretting, or wear at

points of contact in shaft packing and mechanical seal areas It can cause over or under-loading on springs resulting in high wear rates and leakage It can also cause excessive strain and wear on pump

bearings Defective bearings in turn can cause excessive shaft end

To check for this condition a dial indicator should be installed so

that its stem bears against the shoulder on the shaft (Figure 1-6)

Play

RADIAL BEARING

THRUST BEARING

RADIAL BEARING

Figure 1-6 Checking for end play Figure 1-7 Checking for bent shaft

A soft hammer should be used to lightly tap the shaft on one end and then the other Total indicated end play should be between .001 in and .004 in for proper assembly

14 Bent shaft:

When a pump shaft is bent or out of alignment, bearing life, seal life, and performance are impaired Bent shafts also cause vibra- tion and coupling failures To check for this condition, install a dial indicator to the pump housing and adjust so that the stem bears

on shaft outside diameter Rotate shaft and check for run-out If run-out is greater than .002 in the shaft should be straightened (Figure 1-7)

The shaft should be checked in several different locations

15 Check all pilot fits for concentricity Also check for excessive shaft radial movement:

Excessive radial shaft movement allows shaft and seal to whip, deflect, and vibrate This type of movement is caused by improper bearing fit in pump bearing housings or possibly an undersized shaft If the bearing bore is oversized, determine if it was caused

by corrosion, wear or improper machining To check for this con- dition, a dial indicator should be placed on the shaft OD as close to the bearings as possible The shaft should be lifted, or light pres- sure applied to shaft If the total movement exceeds .003 in maxi- mum, bearings and bearing fits should be checked and necessary repairs made (Figure 1-8)

28 Major Process Equipment Maintenance and Repair

RADIAL BEARING

Figure 1-8 Checking for whip or deflection Figure 1-9 Checking for stuffing box

squareness

16 Stuffing box squareness:

If the face of the pump stuffing box is not perpendicular to the shaft axis, the mechanical seal gland will tilt when installed This may cause the seal to wobble and could lead to seal failure

To check for this condition, clamp a dial indicator to the shaft with the stem against the face of the stuffing box, after the cover has been bolted in place Total indicator measurement should not ex- ceed .002 in If face measurement should exceed this tolerance, the cover should be placed in a lathe and machined square Stuff- ing box faces should always be checked for pitting, nicks, burrs, and possible erosion before installing the seal (Figure 1-9)

17 Check for bore concentricity:

The concentricity of a stuffing box bore and shaft can be difficult

to measure because of rust or corrosion due to leaking gaskets Concentricity is critical and may have to be reestablished by weld- ing and remachining On large double-ended pumps where there is

a large separation between stuffing boxes it is very important that the concentricity be held to design tolerances

To check for concentricity, attach a dial indicator to the shaft and sweep as shown in Figure 1-10

Stuffing boxes should be concentric to the shaft axis within .005

in total indicator reading If readings are in excess of this, the pump may have to be realigned and redowelled

"U Figure 1-1 0 Checking for bore concentricity

18 If bearings are found to be rough or the end float is excessive: Remove pump shaft and bearing from housing

Remove bearings from shaft

Check shaft fits, coupling, bearings

Check shaft straightness and polish lightly

Clean and check bearing fits in housing

Repair or replace all faulty and worn parts prior to reassembly

Detailed Inspection Procedures

There are several basic rules that should be observed when inspecting and repairing process pumps Some of these are:

1 Have a good understanding what clearances and fits should be met

2 Record all data and measurements on suitable inspection forms (See Appendixes A and B at the end of this Chapter.) Record all unusual deterioration found while dismantling the pump

3 Use new gaskets and O-rings when reassembling the pump

4 Keep the work place clean

Inspection of Parts Shafts

1 Check for straightness: Runout is not to exceed .002 in Bearing

2 Inspect threads, keyways, and shoulders on shaft Repair if dam- seats must be in good condition

aged

30 Major Process Equipment Maintenance and Repair

3 Measure and record all shaft fits Undersized or damaged fits should be repaired by the procedures outlined in Volume 3 of this series

Case End Wall and Cover

1 Measure and record all fits between pump casing and mating parts

2 Remove all plugs and fittings to inspect threads Reinstall all plugs

3 Inspect and indicate mounting pads to ensure they are flat and par- and fittings

allel with pump centerline Machine, if out of alignment

Bearing Housing and Bearings

1 Observe good anti-friction bearing mounting procedures (see Vol- ume 3 for details)

2 Ball bearings: Replace if worn, loose, or rough and noisy when ro- tated If dirty, clean with solvent, dry and coat with a good lubri- cant New bearings should not be unwrapped until ready for use Whenever in doubt about the condition of a bearing, scrap it But if the bearing is still relatively new, and feels and looks good, don’t discard it

3 Sleeve bearings: Check surfaces of bearing and shaft for imperfec- tion, babbitt build-up, and hot spots Small imperfections do not harm the bearing A typical diametral clearance is .0015 in per in

of shaft diameter For proper operation, clearances should never ex- ceed .003 in per in of shaft diameter on typical pumps

Mechanical Seals

Refer to Chapter 8 in Volume 3 for maintenance and repair of mechani- cal seals

Impellers

1 Replace if excessively worn or corroded The impeller should have been statically and dynamically balanced at the factory, and static and dynamic balance must be maintained for proper operation of your equipment

2 Inspect and measure impeller bore and if worn or deteriorated, ma- chine true Recondition the shaft to fit revised impeller bore size Refer to Volume 3 for guidance

3 Measure outside diameter of impeller wear rings and record size Refer to Table 1-2 for diametral clearances

Table 1-2

Required Diametral Clearances-Process Pumps Wear Rlngs’

Diametral Clearance Wear Ring Diameter Under 500°F Over 500°F

* An additional diametral clearance of 005 in is provided ifboth wear rings are made of austenitic stainless steel, Monel or other materials with high galling tendencies

Casing and impeller wear rings are provided at both sides of the impel- ler on API-type pumps These rings allow a small clearance to be main- tained between the rotating impeller and stationary casing rings For proper hydraulic performance these clearances should approximate the experience values indicated in Table 1-2 Rings should be replaced when clearances have increased to a point where hydraulic requirements cannot

be met or where inefficient operation would prove wasteful For API val- ues refer to Table 1-3

Why do wear ring clearances deserve our attention? The following sec- tion will provide the answer

Keep Pumps Operatlno Efficiently* *

In centrifugal pumps, it is essential to pump operability and hydraulic performance that excessive internal leakage (or recirculation) be pre- vented This is accomplished by establishing and maintaining close run- ning clearances b e e n stationary and rotating wear rings which restrict fluid flow to seal between the inlet and outlet of each impeller and be- tween stationary and rotating interstage bushings These bushings effect sealing between the stages of a multistage pump Certain types of pumps contain hydraulic thrust balancing devices, another source of internal pump leakage

** From “Keep Pumps Operating Efficiently,” by J Lightle and J Hohman, Dresser

Industries, Pacific Pump Division, in Hydrocarbon Processing, Sept 1979 By per- mission of Dresser Industries, Pacific Pump Division

32 Major Process Equipment Maintenance and Repair

As the close clearances become larger through wear, corrosion, ero- sion or perhaps questionable maintenance practices, internal leakage rates increase The increased leakage must be pumped and repumped continuously by the impeller, requiring additional input horsepower The amount of added power to continuously recirculate excessive in- ternal leakage is a function of the pump specific speed* In low specific speed pumps (low capacity-high head) excessive running clearances re- sult in larger percentage changes in power requirements than occur in high specific speed pumps (high capacity-low head) This is reflected in the empirical data plotted in Figure 1-1 1

* For an explanation of pump specific speed refer to Figure 1-13

35

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30

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P

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5

E 20

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.- C

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m

C

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8

& 10

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C

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0

0 20 40 60 80 100 120 140 160 180 200

/ / /

/

Percentage increase in wear ring clearance Figure 1-1 1 Added power resulting from excessive wear ring clearance for different cific speeds

spe-

140

6 130-

D

f 110-

U

- 120-

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2100-

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90

70-

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Capacity -% of design

Figure 1-12 Pump performance curves

The data in Figure 1-1 1 are somewhat misleading since it may be easy

to conclude that high specific speed pumps do not cause excessive costs resulting from worn clearances Beware, however, that small percentage changes of large horsepowers result in large annual costs Also, as noted

in the following example, mechanical operation may be adversely af- fected by excessive clearances in pumps of various specific speed ranges

A typical example: Consider a single stage, overhung process pump-one designed to produce a total head of 725 ft at 1,550 gpm when operating at 3,550 rpm Such a unit can be considered a typical process pump Figure 1-12 shows the characteristic performance curves for an example pump; all scales are shown as a percentage of the design conditions The solid curves indicate performance of the pump in new condition

At the design operating capacity, the unit is 67 percent efficient, re- quiring 424 bhp* input horsepower (assuming the pumpage has a specific

gravity of 1 .O)

Referring to the specific speed nomogram (Figure 1-13), it is deter- mined that our example pump has a specific speed of 1,OOO

Now, going back to Figure 1-1 1, we see that if the wear rings have

worn to the point where running clearances have doubled (increased by

100 percent), a pump having a specific speed of 1,OOO will suffer an in-

* Brake horsepower

34 Major Process Equipment Maintenance and Repair

Figure 1-13 Specific speed nomogram

crease in required horsepower input of approximately 4.8 percent; in our example, this amounts to approximately 20 brake horsepower The .038

in wear performance curve on Figure 1-12 shows the worn-condition performance characteristics of the example pump

Figure 1-14 shows the annual power cost this extra 20 brake horse- power will represent to you, based on 300 days per year operation

If your power cost is 6C/kWh, your annual power cost resulting from

internal wear in this pump would be $6,440 If yours is a cctypical” 100,OOO bbl/day refinery using 25,000 pump horsepower, an overall in- crease of 5 percent in your pump horsepower requirements could repre- sent additional costs of WO0,OOO per year

Maintenance practices Normal operational wear is not the only cause of excessive part clearances in pumps, nor are wasted dollars and fuel the only adverse effects

Intentional opening up of wear ring or other wearing part clearances is used by some maintenance people to solve certain pump operating prob- lems Unfortunately, such practices sometimes appear to be effective- over the short run Over a period of time, however, such practices can create other problems The resulting increased internal leakages within

the pump (and the accompanying increased power required to pump the additional flow) seem to many to be a small price to pay, if in fact such criteria are considered at all But, from a purely mechanical standpoint, the stability of the rotor is perhaps safeguarded only as long as normal running clearances are maintained Typical consequences of liberally open clearances are likely to include excessive vibration, overheating and ultimately pump or driver bearing failure, shaft breakage, driver over- loading, and possible total pump destruction Ultimate maintenance costs can be very high and unit operation can be compromised through prema- ture and repeated outages

If two or more pumps are designed for parallel operation and share to- tal capacity, then unequal running clearances can cause unequal load sharing by the pumps One or more of the units can be forced to operate

at significantly more or less than its design flow rate Efficiency falls off and brake horsepower requirements increase even beyond those caused

by excessive running clearances

Running clearances Greater than normal wear ring clearances at the im- peller inlet eye increase the flow rate through the impeller (not out the discharge nozzle of the pump) , increase the effective inlet fluid tempera-

Increased power consumption, bhp

Figure 1-14 Annual costs based on 300 days per year continuous operation