Improving Machinery Reliability 3 Episode 9 pdf

One important factor often overlooked by engineers in the installation of a bellows expansion joint is the pressure thrust force inside the pipe.. Chapter 8 Startup Responsibilities Sum

excited by small, disturbing fluid forces I n addition, the piping loops enhance the internal fluid disturbance by creating cavities and other flow discontinuities associat-

ed with excessive pressure drops A system similar to that shown in Figure 7-5 expe- rienced very severe vibrations in one petrochemical plant The responsible engineer had to install a large cross beam anchoring all the loops in efforts to reduce vibration

to a manageable level The function of the original loops was lost by the anchoring system Moreover, the piping still experienced larger than normal vibrations due to flow disturbance caused by a loop which was now structurally fixed, but hydraulical-

ly still open to many changes in the direction of flow

Theoretical Restraints

A properly designed piping system generally includes restraints to control the movements and to protect sensitive equipment However, there are also restraints that are placed in desperation by piping engineers trying to meet the allowable load

of the equipment These so-called computer restraints give very good computer analysis results on paper, but are often very ineffective and sometimes even harmful Figure 7-6 shows some typical situations that work on the computer, but do not work

on a real piping system These pitfalls are caused by the differences between the real

system and the computer model Here are some of the more important discrepancies: Friction is important in the design of the restraint system near the equipment Fig- ure 7-6 (a) shows a typical stop placed against a long Z-direction line to protect the

Figure 7-6 Problems with theoretical restraints

equipment If friction is ignored in the design calculations, the calculated reaction

at the equipment is often very small However, in reality, friction acting at the stop surface will prevent the pipe from expanding in the positive X-direction This fric- tion effect can cause a high X-direction reaction at the equipment Calculations including the friction will predict this problem beforehand A proper type of restraint such as a low-friction plate or a strut would then be used

* An ineffective support member is another problem often encountered in the protec- tive restraints Figure 7-6 (b) shows a popular arrangement to protect the equip- ment The engineer’s instinct is to always put the fix at the problem location For instance, if the computer shows that the Z-direction reaction is too high, the natural fix is to place a Z-direction stop near the nozzle connection This may be all right

on the computer, but in reality it is very ineffective For the support to be effective, the stiffness of support member A has to be at least one order of magnitude higher

than the stiffness of the pipe Here, the pipe stiffness is very high due to the rela-

tively short distance from the nozzle to the support

A gap is generally required in the actual installation of a stop Therefore, if a stop

is placed too close to the nozzle connection, its effectiveness is questionable due to the inherent gap As shown in Figure 7-6 (c), the pipe has to be bent or moved a distance equal to the gap before the stop becomes active Due to the closeness of the stop to the equipment, nozzle stresses will often reach severe levels even before the pipe reaches the stop This configuration is not acceptable because the equipment generally can only tolerate a much smaller deformation than the con- struction gap of the stop

e Choking is another problem relating to the gap at the stop Some engineers are aware of the consequences of the gap at the stop mentioned above and try to solve

it by specifying that no gap be allowed at the stop This gives the appearance of solving the problem, but another problem is actually waiting to occur As shown in Figure 7-6 (d), when the gap is not provided, the pipe will be choked by the stop as soon as the pipe temperature starts to rise We generally remember to pay attention

TO the longitudinal or axial expansion of a pipe, but we often forget that the pipe expands radially as well When the temperature rises to a point when the radial expansion is completely choked by the support, the pipe can no longer slide along the stop surface The axial expansion will then move upward, pushing the entire machine upward

Expansion Joints

An alternative solution to keeping allowable nozzle loads in check involves the

use of bellows expansion joints Bellows expansion joints are popular in the exhaust

systems of steam turbine drives which typically have extremely low allowable pipe loads for pipes 8 inches and above Bellows joints are also often used for fitting units

coming off a common header, as shown in Figure 7-7 (b) A properly installed and

maintained bellows expansion joint should have the same reliability as other compo- nents, such as flanges and valves However, in real applications, expansion joints are often considered undesirable due to anticipated maintenance problems For instance, when covered with insulation, the expansion joint looks just like thickly insulated

piping Nobody knows exactly what is going on inside the mixed layers of covering Due to “blindness anxiety,” many installers have resorted to an uninsulated arrange- ment This not only creates an occupational safety hazard, but can also lead to cracks due to thermal shock from the environment andlor weather changes In refineries, fires around bellows-type expansion joints have often led to disaster

One important factor often overlooked by engineers in the installation of a bellows expansion joint is the pressure thrust force inside the pipe The bellows is flexible axially Therefore, the bellows is not able to transmit or absorb the axial internal

pressure end force This pressure end force has to be resisted either by the anchor at

the equipment or by the tie-rod straddling the bellows With the exception of very

low pressure applicators, such as the pipe connected to a storage tank, most equip- ment items are not strong enough to resist a pressure end force equal to the pressure times the bellows cross-sectional area The pressure thrust force has to be taken by the tie-rod These facts are not obvious to everyone and may result in some opera- tional difficulties Figure 7-7 illustrates two actual problems Figure 7-7 (a) shows

one of many steam turbine exhaust configurations installed in petrochemical plants, The expansion joint layout scheme appears to be sound, but the construction may not

be done properly When the base elbow is anchored, the tie-rod loses its function as soon as the pipe starts to expand In this case, the pipe expands from the anchor toward the bellow joint, making the tie-rod loose and ineffective The large pressure thrust force pushes the turbine, often causing shaft misalignment and severe vibra- tions Figure 7-7 (b) depicts a similar situation In one plant, the bellow expansion joints were used solely for fitting up the connections The tie-rods were supposed to

be locked; however, before start-up, an engineer had loosened the tie-rod nuts, apparently thinking the tie-rods defeated the purpose of the expansion joint The tur- bine encountered serious vibration and it took quite a while before it was discovered that the problem was caused by the loose tie-rods When the nuts are loose, the pres- sure end force simply pushes the machine out of alignment

Other Practical Considerations

As can be seen, pipe stress reductions are not always easy to achieve Especially when dealing with the low allowable nozzle loads specified for some equipment, the technique can become tricky and very often works only on paper Other practical approaches may have to be explored to further improve overall reliability One very important resource not to be overlooked is the experience found in operating plants

We have seen good, simple working layouts changed to complicated and question- able layouts only because a computer liked it that way Undoubtedly, computers are important tools, but they are only as good as the information we give them Since there are parameters such as friction, anchor flexibility, etc., that cannot be given accurately, computer results need to be interpreted carefully It is time to realize that

if something works well in a plant day in and day out, it should be considered good,

regardless of whether or not the computer predicted it to be good The process of examining and incorporating field experience is very important in designing a good,

reliable plant

Other solutions such as the use of sliding supports, spring supports, and more compact in-line arrangements as shown in Figure 7-8 also merit serious considera- tion It is understood that engineers do not feel too confident about movable assem- blies, but it is important to understand the difference between the movement of the whole assembly and the movement of only the pump or turbine When the whole assembly moves, shaft alignment can still be maintained, provided the distortion of the equipment is not excessive This pre-supposes that the piping load is still within

the allowable range It should be noted, however, that movable assemblies are just

potential alternatives One should not be oversold on the idea and blindly use sliding

or spring-supported schemes in a plant To make the sliding base or the spring-sup- port scheme work, an extra strong baseplate is required Then again, if we have that strong of a baseplate in the first place, it may well be possible to substantially increase the allowable piping load

ASME B73.1M-1991, “Specification for Horizontal End Suction Centrifugal Pumps

for Chemical Process,” American Society of Mechanical Engineers, New York

Chapter 8

Startup Responsibilities

Summary of Startup Preparations for Process Plant Machinery

A number of key elements are indispensable to ensuring successful commission- ing of rotating machinery in process plants These include the following mix of mandatory and contingency actions

1 Develop equipment lists for general reference and progress tracking

2 Assign mechanics during the erection period to observe or execute:

a Machine assembly:

Lifting and handling procedures

Cleaning, inspection, and installation of bearings and seals

Checking and recording critical clearances, and pre-operation settings and Full inspection of most centrifugal pumps

Machinery-related instrument installation and adjustments

adjustments

b Correct shaft alignment

c Parallelism and gaskets for major flange connections

d External flush systems cleanup

e Piping system cleaning for:

Compressors

Turbines

3 Set up training program for special machinery operation and repair:

a Consider vendor service representatives, as available

b Use classroom instruction together with practical demonstrations

c Use contract or on-loan startup advisors as time permits

d Verify soundness of auxiliary systems, starting interlocks, and shut-down

protection alarms, and provide routine checking and trouble-shooting during machinery operations

e Review vendor’s instruction books, cross-sectional drawings, and inspection procedures Verify completeness of records

f Expose mechanics to the same training as operators in such key subjects as: Steam-turbine operation

Centrifugal-compressor shaft seal system

e Recognizing machinery distress

339

4 Assign the plant’s machinery engineers to participate in startup activities full-

Assures continuity upon departure of temporary startup advisors

Plant machinery engineers should continue in full-time startup assignment

5 Train the plant’s electricians and instrument mechanics on machinery acces-

c Starting interlocks, alarms, and safety shutdowns

a Machinery auxiliary systems, including interlocks, alarms, and shutdown fea-

b Testing of auxiliaries during operation

c Machinery conditions requiring emergency shutdown

d Avoiding the kinds of operating errors that can damage machinery

a Prepare before run-in, revise before plant startup, finalize after successful startup is completed

b Integrate vendor’s instructions for driver and compressor into process startup instructions

c Prepare specific startup and shutdown procedures; checklists and data lists for monitoring: data on interlocks, alarms, and shutdown features; process factors; etc

6 Train operators in machinery areas:

tures

7 Prepare specific operating instructions for major machinery units:

8 Ascertain availability of test equipment for run-in and operation:

a S troboscope-high-speed coverage

b Hand-held vibration-measuring equipment

c Real-time vibration analyzer

d Computerized data-acquisition systems

a Investigate nearby facilities for emergency service such as:

9 Identify outside sources for special testing or balancing:

Vibration analysis

Dynamic balancing of rotors

Capacity to handle largest rotor

Balance quality achievable with available machines per recent experience

b Dynamic trim balancing in place:

Computerized techniques available

Special equipment required

Skilled technicians required

c Metallurgical testing laboratory

10 Investigate the availability of expert consultants:

c Make advance contacts

12 Investigate local repair facilities:

a Larger machine tools than available in plant shop; special shop facilities or

b Special casting repair techniques (“Metalock@” and “Metalstitch@”)

c Welding and metallurgy

a Establish procedures for obtaining services of vendor representatives for

11 Investigate plan and facilities for repair of remote vendors’ equipment:

tools

13 Identify machinery vendors’ service personnel:

run-in and startup operations

0 Determine official contact and responsible management

e Also make advance contacts for equipment where service representative is

to be on an “on call” only status, such as for governors on steam turbines, materials-handling equipment, gearing, centrifuges, etc

b Assess qualifications of assigned representatives quickly and obtain replace-

c If the erection advisor continues on as the startup advisor, verify that he is

d Utilize vendor representatives for training plant personnel as time permits

e Assign qualified plant mechanics to work supervised by vendor representa-

ments if qualifications are unsatisfactory

qualified in this area

tives

14 Arrange preventive maintenance details:

a Prepare equipment:

* Portable shelter

0 Rotor lifting and supporting rig

Verify access path and lifting position of mobile crane

b Develop pre-planned inspections of critical equipment for execution during

c Develop overall plan for preventive maintenance services on all equipment

d Establish records of inspections, repairs, and part replacements:

unplanned, brief plant operation interruptions

items

Separate records for each major machinery item

Use special forms, with sketches for recording vibration and other critical

0 Use vendors’ instruction books and startup advisors for developing forms Plan overhaul technique of most critical machines in detail (e.& instru- operating parameters

ment air compressor)

15 Understand spare parts situation:

a Status of deliveries

b Warehouses properly organized for pre-startup period

c Spare rotor storage and protection

d Review stocking for completeness and adequacy

a Determine requirements for each machine

b Verify appropriate product equivalent of vendor-recommended lubricants

c Procure ample quantity for startup Stock quantity to replenish possible seal leakage Allow for discard of initial charge

d Periodically sample test during operations

16 Outline lubrication requirements:

Machinery Startup Review Tasks

The preceding section outlined startup preparation in broad terms These prepara- tory tasks can be further broken down into completeness reviews, quality assurance tasks, cleanliness checks, etc

The following man-hour percentages could be considered representative for exe- cuting the field construction completeness review tasks associated with the startup of

a world-size steam cracker (ethylene plant):

Review completeness of installation (“but-list”)-21%

Lube supply lines

Review maintainability of equipment-7%

Spool pieces

Shims

Auxiliary line interference

Alignment devices

Ensure long-term, troublefree o p e r a t i o n 4 %

Determine offset values needed to accommodate thermal growth

Verify stress-free installation of machinery piping

Gear-tooth contact checks

Driver solo runs and rotation checks-9%

Steam-turbine overspeed trip settings

Vibration measurements

Cleanliness checks-5%

0 Lube and seal oil systems

Steam-turbine inlet piping

Pump mechanical seals

Autostart simulations (joint mechanical/instrument technician effort)-lS%

0 On lube and seal oil auxiliaries

0 On other autostart drivers

Documentation, ix., assembling the following data-3.5%

Acceptance forms

Equipment record folders

0 Computerized record input data form

API data sheets

0 Component (material) modifications

Cleaning, reassembling, and general repairs

Miscellaneous I 1 %

Executing the review tasks described on the preceding pages will be facilitated by

a series of checklists These must be developed to help personnel conduct the reviews and execute the various tasks in reasonably uniform fashion First and most important is a checklist restating every construction- and installation-related item contained in the job specification documents A typical checklist should look similar

to the one represented in Figure 1-36 Machinery startup review personnel must use

these checklists to verify that the installation complies with the job specification and that it is ready for imminent startup Having observed compliance or deviations, the engineer, technician, or millwright should note his comments on the appropriate completeness summary

Figures 8- 1 and 8-2 represent completeness summary forms for centrifugal pumps and general-purpose steam turbines Similar summaries should be used for all other categories of rotating machinery

Assembling a Mechanical Procedures Manual

While performing the various startup review tasks, the review team can assemble

a large number of documents which may be helpful later to the plant maintenance work forces

Construction contractors executing large petrochemical projects are using a multi- tude of written guidelines dealing with the installation of rotating equipment Many

of these procedures describe future plant maintenance tasks and should, therefore, be assembled as soon as possible Machinery startup engineers should be aware of this requirement and should assist the plant maintenance supervisor in identifying docu- mentation to be collected and retained for future use Table 10-4 represents a typical cross section of documents originating from various sources: design contractors, field contractors, startup engineers, manufacturer’s field representatives, etc At the termination of a major plant startup, these and other documents should be placed in a so-called “Mechanical Procedures Manual” and given to plant maintenance person- nel for future use and reference

Machinery Startup Reporting Structure

The startup responsibilities outlined earlier in this chapter can be effectively dis- charged by two organizations, one a mechanical startup section and the other a tech- nical startup section The technical startup section is manned by engineers and tech-

nicians with prior startup experience Their task is to define all necessary steps

leading to successful commissioning and satisfactory long-term operation of rotating machinery The mechanical startup section is supervised by a machinery engineer

Block #-System ,+- Centrifugal Pumps Equipment #

I Strainer

Case

Packing or Seal Gland

Packing or Seal Piping

Suction Pioina

I I

Small-Bore Piping Lubrication Guards

’

Figure 8-1 Completeness summary form for centrifugal pumps This summary should only be filled in by the reviewer after a careful on-site review has verified compliance with a “detailed checklist for rotating machinery.” The reviewer will normally compile such a detailed checklist from the purchaser’s original specification

Small-Bore Piping Lubrication Guards

Rev./Date NOTE: This summary should

and is staffed by mechanical supervisors, foremen, and machinists whose principal

task is to execute all necessary steps leading to the same results

As can be seen from the organization chart in Figure 8-3, the two section supervi-

sors are preferably reporting to the same startup leader or manager The reporting

route indicated in Figure 8-4 has been employed and can also be made to work

However, the latter’s effectiveness can only he assured if the two section supervisors

share mutual background and trust and respect each other If their relationship is

Area Supervisors Records Clerk Lubrication Specialist Millwrights

Mechanics

-

Figure 8-3 Machinery startup sections reporting to the same startup leader

Head, Technical Department

Head Maintenance (or Maintenance/

Construclion) Department

I

Instrumen1

(Definition Machinery Technical

(Definition) Group:

Machinery Engineers:

-

Mechanical (Definition)

Instrument Machinery

(Execution) I Mechanical

(Implementation Group:

I

Mechanical (Execution)

Supervisors Records Clerk Lubrication Specialist

Millwrights

Mechanics Figure 8-4 Machinery startup sections reporting to separate branches of organization

lacking in these elements, a game of “one-upmanship” and finger pointing may result Startup progress and effective utilization of available resources could suffer from this kind of relationship

Documentation for Effective Tracking of Progress

Machinery commissioning targets and accomplishments must be documented every step of the way if the startup is to progress in an orderly fashion

Once a basic set of equipment listings has been prepared, it can be readily modi- fied in the word processorkomputer or by spreadsheets This allows the document to

be readily adapted to a variety of documentation needs Figure 8-5 serves as an example

To begin with, Figure 8-5, represents the basic tabulation of pertinent pump data

in a given area of the plant The basic, or master tabulation contains 20 columns of information:

Figure 8-5 Basic tabulation of pertinent pump data

Plot plan or item number

Block or plant area

Manufacturer

Model designation

Manufacturer's serial number

Train type It is highly advantageous to produce isometric sketch-

es identifying the various train types, as shown in Figure 8-6 Supervisory personnel can rapidly visualize the relative size and complexity of machinery trains by referring to these train isomet- rics A second variant of these isometrics should be marked up to define where to install vibration transducers or to show operating technicians where to take vibration readings

Pumping service designation

Fluid pumped (pumpage)

Specific gravity This information is important when mining whether the pump is suitable for run-in of water

Pumping temperature

Pump suction pressure

Pump discharge pressure

Type of mechanical seal (T = tandem, P = packing, S = single,

SG = single gas-type, D = double, DG = double gas-type)

Flow rate, g p d o r cubic meters per hour

Shaft speed, rpm

Driver type (M = motor, ST = steam turbine)

Driver power output, HP or kW

Number of transducers for incipient failure detection on pump

Same as column 17, on driver

Total number of instrument wires available

In-plant inventory identification (yard number)

Figure 8-7 shows the basic equipment tabulation modified to track major checkout

segments as the startup progresses The form is now essentially used for scheduling

purposes Columns 1 , 2 , 3 , 4 , and 6 have been retained Subsequent columns identify

anticipated and actual checkout dates, partial completion etc

i i I I i i i I I I I I I I I I

I I I I I I I I I f I I I I I I

PLI-W- OLIE-

Figure 8-8 Equipment tabulation emphasizing coupling and mechanical seal data

Figure 8-8 represents another modification of the basic equipment tabulation This

time, the emphasis is on coupling and mechanical-seal data

Pump outages experienced during pre-commissioning activities are logged in to keep track of both duration and primary cause Figure 8-9 represents a typical outage

log sheet with entries following a time sequence of events

A different set of startup documentation is shown in Figures 8-10 and 8-1 1 These

figures represent an event log for major compressors and a trip/shutdown log for a given machine, respectively Note especially how Figure 8- 1 1, representative of dozens of sheets generated during a major startup, affords an overview of several important entries: purpose of run, discipline responsible for shutdown, duration of shutdown, cumulative operating time, and downtime

Figure 8-12 shows a checklist used for centrifugal-pump field installation and initial

operation Used in conjunction with field-posted startup instructions (refer back to Fig- ure 1-26), a similar checklist containing work items reflecting a given project philose

phy will go a long way toward reducing installation and commissioning oversights

a 1 odw

trouble with vacuum 3

Turbine Solo three times; running time approximately TC reading Hi 214”F, trip set at E

$

os

CT-0 1 7130198 Turbine solo run: checked mechanical bolt

Turbine Solo trip three times; running approximately six Outboard bearing thermocouple reading C

hours Hi (215”F), trip set at 4275 rpm, had s

CT-0 1 Turbine solo: checked electronic trip

four hours

Turbine solo: checked bolt trip and electronic trip each three times; running time approximately six hours

Vibrations good Outboard bearing

4170 rpm, vacuum problems

Vibrations good, bearing temps good, trip set at 6600

mechanical bolt trip, 6540

electronic trip, had trouble with vacuum Running time during solo runs for C-01: 10 hours, for C-02: 6.0 hours

Vibrations all good-high temperature turbine outboard bearing, some

“stall” noticeable on double flow inboard side at 1500 rpm- none above critical speed, had troubles with condenser, ejectors and seal oil AP

s

Sr

CT-02

Turbine Solo

Air run-in: First air run-in of compressor train:

low pressure-frst three stages of

compression; running time approximately five hours

Figure 8-10 Event log for charge gas trains

8/04/98

Downtime

(8130) 8/28 1715 1419

8/30 1445 1640

(911) 8/30 1650 1042

9/ 1 1930 1940

(914) 9/2 0000 0100

(915)

914 1430 1510

915 1520 2040

Air dry-out Air dry-out

Air dry-out Air dry-out

Air dry-out

Air dry-out

Ai dry-out

Instruments Instruments

Instruments Machinery Mechanical

Utilities

Machinery

Machinery

Problems with RCQ 0.4 241.95 236.80 RCQ logic

Problems with RCQ 0.25 243.85 237.05 RCQ logic

Problems with RCQ 8.8 261.75 245.85 RCQ logic

Gasket in Manual 4.3 261.90 250.15 1500# system

blown Loss of boilers

Manual 13.5 310.90 263.65

Woodward peo- Manual 0.15 335.55 263.80 ple worked on

governor Governor Manual 0.65 340.9 264.45 vroblem

Figure 8-1 1 Triplshutdown log