Code of Practice - Centrifugal Pump Operation and Maintenance

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Engineering Design Guide: GBHE-EDG-0501

Code of Practice: Centrifugal Pump Operation and Maintenance Information contained in this publication or as otherwise supplied to Users is believed to be accurate and correct at time of going to press, and is given in good faith, but it is for the User to satisfy itself of the suitability of the information for its own particular purpose. GBHE gives no warranty as to the fitness of this information for any particular purpose and any implied warranty or condition (statutory or otherwise) is excluded except to the extent that exclusion is prevented by law. GBHE accepts no liability resulting from reliance on this information. Freedom under Patent, Copyright and Designs cannot be assumed.



Engineering Design Guide: Code of Practice: Centrifugal Pump Operation and Maintenance

CONTENTS SECTION 1 SCOPE 1 2 SETTING UP A MAINTENANCE INSPECTION SYSTEM 2

2.1 Design and Purchase

2.2 Maintenance Data

2.2.1 Data File

2.2.2 Maintenance Data Card

2.2.3 Plant Pump Schedule

2.2.4 Maintenance Record Card

2.2.5 Pump Maintenance Report 2.2.6 Lubrication Schedule

3 CONTROL OF MODIFICATIONS 3 4 PUMP COMPONENT FAILURE MODES 4

4.1 Bearings 4.4.1 Adequacy of oil or grease supply

4.1.2 Mechanical Breakdown

4.1.3 External Factors



4.2 Seals

4.2.1 Seat Wear

4.2.2 Shaft or Sleeve Fretting

4.2.3 Multiple Spring or Bellows Breakages 4.2.4 Double Seal Arrangements (Back-to-Back)

4.2.5 O-Rings

4.3 Couplings

4.3.1 Gear Type Coupling (grease lubricated)

4.3.2 Flexible (Elastomer) Element

4.3.3 Multi-Disc Pack or Membrane Coupling

5 HAZARDOUS AND CRITICAL DUTY PUMPS

5.1 Quench Systems

5.2 Cooling Systems

5.2.1 Bearing Cooling 5.2.2 Pedestal Cooling 5.2.3 Stuffing Box Cooling 5.2.4 Throttle Bushes



6 PROCESS PRIORITIES 6

6.1 Immediate Attention (Priority 1) 6.2 Daily Attention (Priority 2)

6.3 Weekly Attention (Priority 3)

7 PROCESS OPERATION AND FAILURE DIAGNOSIS 7

7.1 Process Changes

7.2 Failure Diagnosis 8 B I BLI OGRAPHY 8



APPENDICES A AN ENGINEERS CASEBOOK - GUIDE TO FAULT ANALYSIS - DIRECT

DRIVE CENTRIFUGAL PUMPS

B AN ENGINEERS CASEBOOK - OIL LUBRICATED BEARINGS. C AN ENGINEERS CASEBOOK - TYPICAL SHAFT & HOUSING

TOLERANCES - ENG DATA FOR PUMPS WITH ROLLING ELEMENT BEARINGS.

D AN ENGINEERS CASEBOOK - THROTTLE BUSHES. E AN ENGINEERS CASEBOOK - HIGH SPEED CENTRIFUGAL PUMPS

AND COMPRESSORS. F AN ENGINEERS CASEBOOK - GLANDLESS PUMPS. G AN ENGINEERS CASEBOOK - RUNNING POLICY FOR SPARE

AND MULTIPLE UNITS. DOCUMENTS REFERRED TO IN THIS GBHE ENGINEERING DESIGN GUIDE



1 SCOPE This code of practice sets out to define systems for the satisfactory operation and maintenance of all types of centrifugal pumps. The code lays down the action required to set up and operate a maintenance system, in order to reduce the incidence of pump failures and minimize the outage to plant operation. The code defines operational criteria and diagnosis for the analysis of pump performance and gathers together information helpful to the plant maintenance organization. 2 SETTING UP A MAINTENANCE INSPECTION SYSTEM This section defines the minimum requirements for setting up information records from design to operation. 2.1 Design and Purchase The ordering authority should supply the following information: (a) Data Sheet - Standard centrifugal pump data sheet (See Appendix A). (b) Pump performance (test) curves together with test certificates. (c) Operating and maintenance manuals/instructions. (d) Installation drawings for pump and cross sectional details of the pump. (e) Interchangeability lists for spares. (f) Lubrication schedule 2.2 Maintenance Data This section covers the relevant Data files, record forms, schedules and reports required for a Pump maintenance system.



2.2.1 Data File A master data file should be kept for each process duty. This should comprise design data sheets, performance/test curves, modification details, manufacturers operating and instruction manual, and drawings. This should be kept by the local Maintenance Supervisor but duplicate data files may be held by the Plant Technical or Maintenance Manager, where appropriate. 2.2.2 Maintenance Data Sheet A standard form of maintenance data sheet should be completed for each installed location, Appendix C. 2.2.3 Plant Pump Schedule From the pump data sheets and interchangeability lists compile a plant pump schedule. Where data sheets are not available, this has to be produced from available information e.g. manufacturers data. This schedule should provide the engineering data required for quick reference and day to day maintenance. (See Appendix B). 2.2.4 Maintenance Record Card Together with the maintenance data sheet, a pump maintenance/ failure data sheet should be compiled for each individual rotating assembly. (See Appendix D). This forms the basis for any failure/change/modification carried out to each individual assembly and provides a continuous history base. The record should detail the condition of components which are replaced and the primary cause of failure. Evidence should be kept by component retention and/or by photography. (a) Interchangeability and Spare Units

When rotating assemblies are interchanged it is necessary to identify each assembly with individual identity numbers. Each movement should be recorded in order to trace the unit history hours run and different duty locations.



2.2.5 Pump Maintenance Report A yearly report should be compiled by the Technical/Maintenance Supervisor or other responsible person, listing all failures and reasons for failure, plus any modifications or other aspects which have caused pump downtime. This report is to be issued to the Technical Engineer and Maintenance Manager with a copy to the Site Machines Engineer.

2.2.6 Lubrication Schedule All pumps should be listed on the Plant Lubrication Schedule which should be available to the local Maintenance Supervisor. 3 CONTROL OF MODIFICATIONS This section deals with the necessary requirements to maintain control on all modifications to pump units. All modifications to pump units e.g. impeller size, bearing and seal type, quench or flushing arrangement, materials, lubrication etc. should be recorded in the respective works (site) Control of Modifications procedure. Where modified pumps are under trial, no change of original records should be made until the trial has been concluded. When a modification becomes a permanent feature then all relevant records and spares should be updated by the appropriate Engineer responsible. A reference of the permanent modification should be forwarded to the Site Engineering Machines Section. 4 PUMP COMPONENT FAILURE MODES This section gives a general guide to follow when analyzing pump component failure modes. A good reference for 'Fault Analysis' is given in Appendix E.



4.1 Bearings When bearing failure is diagnosed the following points should be checked: 4. 1.1 Adequacy of Oil or Grease Supply The following factors should be considered: (a) Correct lubricant is being used. (b) Oil leveler, correctly positioned and kept full refer to Appendix F - (c) Oil loss from leaking seals. (d) Frequency of greasing - Lubrication schedule. (e) Oil holes and grooves in housing clear of debris. (f) Signs of water ingress - emulsification. Excessive lubrication causes

overheating. (g) Grease relief. (h) Oil gauge breather hole blocked. 4.1.2 Mechanical Breakdown (a) Bearing internal clearances - is the designated bearing being used (C3

normally used - if not, why not?). (b) Housing bores - are they the correct size and aligned? Condition of bores

i.e. spinning of outer ring. (c) Shaft condition - diameters and shoulder locations correctly sized, bent

shaft ((Max TIR 0.0015 in (O.04mm), for mech. seals) damage at oil or grease seal locations.

(d) Many bearing failures occur due to the inner race spinning on the shaft. It

is important to check bearing and shaft fit tolerances when overhauled - see Appendix G for shaft and housing fit tolerances.



4.1.3 External Factors (a) Excessive vibration in the pump area. This may have the effect of

brinelling a bearing in a non-working pump. (b) Incorrect bearing is supplied from stores - check with Plant Technical

Engineer or Maintenance Manager. (c) Leaks in the vicinity of the pump. (d) Change of process duty - particularly where batch production is carried out

- check with Plant Technical Engineer or Maintenance Manager. (e) Vibration - can be caused by impeller out-of-balance due to corrosion,

solids build up or trapped debris. (f) Cavitation - caused by too low NPSH or operating too far from Design

point - consult with Plant Technical Engineer or Maintenance Manager if pump exhibits 'tinkling' or 'cracking' noises.

4.2 Seals Most seal failures are caused by external factors. It is important to check each seal component to determine which parts have failed causing the pump to be removed for overhaul. For each pump unit it is important to know the exact seal installation detail in order that an accurate diagnosis of cause of failure can be determined. Owing to the considerable variation of seal types, configurations, and systems it is not possible to list all the failure modes but the following may be taken as a general guide. 4.2.1 Seat Wear Rapid seat wear-possible causes could be: (a) Excessive compression loading.



(b) Heat generation (flashing) across the faces – usually caused by high temperature flushing medium, inadequate flush supply, high facial contact load or a combination of these.

Improved materials with higher wear factors (PV value) may need to be considered - refer to Plant Technical Engineer or Maintenance Manager for advice.

4.2.2 Shaft or Sleeve Fretting Fretting between the stationary sealing device and the sleeve is a common cause of failure. It is a form of crevice corrosion exacerbated by the fractional movement (axial) of the seal O-Ring or wedge ring on the shaft. It can be a particular problem on pumps which handle solids or slurries when it is made worse by trapped particles. If it causes frequent pump downtime and part replacements, engineering design section should be asked for advice on alternative seal designs e.g. bellows seals. 4.2.3 Multiple Spring or Bellows Breakages Examination of the broken part (s) should determine if it has failed in fatigue or been worn thin by contact with the seal housing - both are symptomatic of a non-aligned seal housing (tilted) or seal face. Check shaft/sleeve O/D and seal housing I/D for correct fit. Check tightening sequence for locating screws, clock the seal faces after installation. 4.2.4 Double Seal Arrangements (Back-to-Back) Displacement of the inner seal or seat is a common occurrence which often leads to inner seal leakage of the flushing medium into the product - the primary cause of this is over pressurization of the seal stuffing box (cavity). This can occur if a pump is suddenly 'dead-headed', and a high pressure surge is transmitted back to the pump suction. It is also common on plants where pressurized flushing or cleaning fluids are introduced into the suction side of pumps. This higher pressure medium can create a higher stuffing box pressure than the seal buffer pressure and lift the seal. With clean liquors the seal will most likely reseat but with solids/slurry pumps the solids fill the seal chamber and jam the seat and seal components. Spring breakages often result when this happens.



4.2.5 O-Rings These may exhibit symptoms of aging, swelling or damage. Materials used should be checked for suitability with the fluid being pumped. On low temperature duties special O-Rings are required. 4.3 Couplings Reference should be made to GBHE-EDS-MAC-1806 for suitable couplings. 4.3.1 Gear Type coupling (grease lubricated) Generally this type should be avoided or if possible designed out. Where they are employed, they should be inspected at least yearly and the gear parts cleaned and the grease replenished (refer to GBHE-EDG-MAC-1101for guidance on the frequency of greasing). Alignment is necessary to avoid gear teeth 'fretting' e.g. 0.004" (0.1 mm) TIR (Total Indicated Reading) or better. On hot duty pumps, alignment can change with pump operating temperatures as the grease or oil becomes less viscous and loses its load bearing properties. GBH Enterprises should be consulted as for the suitability of this type of coupling. Failure of gear couplings is usually preceded by increased noise levels and vibration. For further information on oil lubricated gear couplings refer to GBHE-EDG-MAC-1101. 4.3.2 Flexible (Elastomer) Element These types, e.g. tire, rubber pin and bush, are used generally on lower speed centrifugal pumps drives < 3000 RPM. These allow a greater degree of misalignment but will still suffer flexible member failure after a period of time due to repeated flexing or 'chafing' of the rubber(s}. Keep alignment to within + 005" TIR for good life. Connecting bolts and pins should be checked for wear and replaced as sets.



4.3.3 Multi-Disc Pack or Membrane Coupling These types, e.g. 'Metastream', 'Falk', 'Thomas', have flexible members comprising metal discs. Alignment tolerances need to be much tighter on these designs of coupling which are mainly used on > 3000 RPM or higher power drives; ±0.002" TIR is recommended. The axial gap is vital and should be checked (typically within + 1/32" (0.8 mm)). Pre-stretch or compression may be required to compensate for expansion or contraction at the operating conditions - check the manufacturers installation procedures. Failures on these couplings are rare and fall into three categories: (a) Seizure, e.g. foreign object locking the pump impeller or bearing failure. (b) Fatigue failure of membrane, discs or bolts caused by alignment problems

or incorrect axial gap. (c) Corrosion of membranes due to environment (occasionally exotic

materials are used e.g. monel). All driving parts should be replaced after a failure e.g. bolts, pins, discs or membrane units. Hubs should be examined to check that bolt holes are in good condition. Any damage, elongation, thread distortion or loss of shaft/ hub interference will require replacement hubs to be fitted, or a shaft replacement if this is found to be below tolerance.

5 HAZARDOUS AND CRITICAL DUTY PUMPS For the purposes of this document, all pumps on flammable, toxic or corrosive duties should be deemed to be in this category. 5.1 Quench Systems Pumps installed on these duties with only single seals are normally supplied with a quench fluid to prevent small leaks from creating hazardous situations: (a) This barrier fluid/gas is to be operational. (b) The barrier fluid should in no way cause premature failure of the barrier

seal e.g. dirty water or steam supplies.



(c) Any such barrier fluid should be clean or filtered and the operation and condition of the filters checked regularly.

5.2 Cooling Systems Hot pumping duties are often fitted with cooling systems to bearings, pedestals and stuffing boxes. 5.2.1 Bearing Cooling Cooling systems invite moisture condensation which produce emulsification leading to premature bearing failure. High temperature gradients adversely affect bearing clearances which lead to rapid failure. Examine the pump duty with GBH Enterprises Machinery Section and eliminate cooling wherever possible. A heating medium may be more desirable. 5.2.2 Pedestal Cooling Pedestal cooling is not required on any centrifugal pump generally found in chemical service. Pumping services with fluid temperatures as high as 393°C require nothing more than alignment verification between pump and driver. Eliminate pedestal cooling and verify with GBH Enterprises Machinery Section the hot and cold alignment tolerances. 5.2.3 Stuffing Box Cooling Stuffing box jacket cooling should not serve as an effective means of lowering the temperature of the seal environment. Where stuffing box jacket cooling is used on pumps the following alternatives should be examined: (a) Cooled seat design using clean water, steam condensate or cool flushing

oil through the seal seat. Crane 'K' or 'L' ('SE or 'CE') type are typical, distortion can be a problem.



(b) A high temperature bellows seal may be considered. Steel wedge or Grafoil types removes this limit.

5.2.4 Throttle Bushes Refer to Appendix H. The operation, inspection and maintenance of the throttle bush is critical to the safe operation of pumps on hazardous duties. 6 PROCESS PRIORITIES This section defines that each operating plant should assess its pumps in order of priority for maintenance. These lists should be available to all process and maintenance (day and shift) supervision when defining work scheduling and planning. 6.1 Immediate Attention (Priority 1)

List of pumps which involve process downtime when a failure occurs e.g. continuous process pumps with no installed spare.

6.2 Daily Attention (Priority 2)

List of pumps which have a history of failure but do have an available spare installed.

6.3 Weekly Attention (Priority 3)

Remaining pumps where installed spares are available and possible 3rd back up rotating assemblies are available. All pumps where no process outage would occur e.g. campaign pumps, dosing pumps (where not critical to quality).



7 PROCESS OPERATION AND FAILURE DIAGNOSIS This section defines the current operational checks required for Process operation and gives guidelines to follow when examining failure modes from the processing side. Also pump commissioning guidelines are set out for the start-up of new pumps. 7.1 Process Changes

Many plants have been affected by uprating of design duties, changes in Process operation or operation of pumps on several duties.

On older plants, modifications to the process may have occurred without a full check being carried out on the pump duties and some of the original design margins may have been eroded away. It is important to update the process design data to: (a) Determine that the pumps can meet the new duty or duties. (b) Determine which pumps are limiting for the duties and/or different

campaigns. (c) Provide information to Engineering of changes to pump operation in order

for a check to be made on the pump suitability. 7.2 Failure Diagnosis (a) The reason for removal of a pump may not be the cause of failure.

Maintenance diagnosis when the pump is stripped down should lead to probing the process operating conditions to determine the failure cause e.g. process problems (chokes, flushing, pressure etc), foreign objects, quench failure, dirty quench systems causing secondary seal failures, alignment, motor condition etc.

(b) Pump corrosion, erosion or cavitation may cause process problems with

inability to provide the correct flow or pressure - interchange of information between the process and maintenance organizations is important.

(c) Pump start up - has the correct procedure for starting the pumps been

followed e.g. open the suction valves, start the pump, open the delivery valve slowly.



It is important that the pump is primed correctly and that all air/gas is vented from the pump prior to start-up. This can be critical with low temperature pumps. It is necessary to vent the seal cavity behind the impeller on vertical pumps.

(d) If the electrical phase balance on the motor windings is not correct a false

pump speed will give low~ performance. (e) Corrosion and mesh failures of strainers in suction lines are common

causes of failure or poor pump performance. If strainers are required ensure that the construction is suitable for the fluid being pumped. Are suction strainers choked causing pump cavitation and loss of flow/pressure?

(f) When commissioning a centrifugal pump for the first time, record the

pump/motor at minimum and maximum pumping rates for pressure and power, to check that the pump is operating to the performance curve. From this check, the operating procedures can then be verified to ensure that the pump should operate to its designed duty.

(g) At the commissioning of any NEW pump installation the appropriate

engineer or his delegate should be present to endorse the handover completion, the start-up procedure and finally check the pump operation.



8 BIBLIOGRAPHY GBHE ENGINEERING DOCUMENTS GBHE-EDS-MAC-1803 Machinery Guarding. GBHE-EDS-MAC-1806 Bearing Arrangements for Machines. GBHE-EDS-MAC-2102 Limiting Noise Levels of Manufactured Items of

Equipment. GBHE-EDG-MAC-4506 Vibration Monitoring of Machines. GBHE-EDG-MAC-5100 Reliability Analysis - the Weibull Method. GBHE-EDG-MAC-5701 Lubricants. GBHE-EDG-MAC-5702 Mechanical Seals.



OTHER DOCUMENTS Running Policy for Spare & -An Engineers Casebook No.2 Multiple Units (referred to in Appendix L) A AN ENGINEERS CASEBOOK - GUIDE TO FAULT ANALYSIS - DIRECT

DRIVE CENTRIFUGAL PUMPS

B AN ENGINEERS CASEBOOK - OIL LUBRICATED BEARINGS. C AN ENGINEERS CASEBOOK - TYPICAL SHAFT & HOUSING

TOLERANCES - ENG DATA FOR PUMPS WITH ROLLING ELEMENT BEARINGS.

D AN ENGINEERS CASEBOOK - THROTTLE BUSHES. E AN ENGINEERS CASEBOOK - HIGH SPEED CENTRIFUGAL PUMPS

AND COMPRESSORS. F AN ENGINEERS CASEBOOK - GLANDLESS PUMPS. G AN ENGINEERS CASEBOOK - RUNNING POLICY FOR SPARE

AND MULTIPLE UNITS.



A AN ENGINEERS CASEBOOK - GUIDE TO FAULT ANALYSIS - DIRECT DRIVE CENTRIFUGAL PUMPS

The centrifugal pump is a common piece of equipment on a modern chemical plant. so common it is often taken for granted. In recent years. the centrifugal pump has undergone developments which have improved its performance. Pump manufacturers would have us believe that there pumps seldom fail and when they do, it is our fault because we did not read in full the small print in the operating Instructions. However, pumps do fail and those failures cost the “Company” hundreds of thousands of dollars per year in labor, materials and lost production. Failure can often be attributed to one or more of the following causes: (1) Incorrect specification of duty and materials: This is not always the fault of

the design engineer. Sometimes process conditions are changed slightly, for example, a slight change in operating pressure or temperature can alter the corrosion rate of some pump or seal materials. Note: some seal '0' ring materials are limited to an operating temperature of 150oC ·200oC.

(2) Incorrect installation or maintenance: Common faults are Incorrect fitting

of bearings, seats or rotating units or badly fitted pipework, giving rise to large distortion forces: also it is worth checking that the pump is rotating in the correct direction.

(3) Maloperation: For example, starting the pump without bleeding all the air

out and priming, or starting with the delivery valve open or the suction valve closed. It is quite common to find a suction strainer or an oil filter chocked or missing. This results In bearing or seal failures.

(4) Manufacturing faults: It is wise to inspect a new pump thoroughly before

Installation, making sure that the pump received is the pump ordered, particularly With respect to material composition.

Before the failure rate of a pump can be improved, the particular cause of failure needs to be established. It is hoped that the following chart may help in this. It is not comprehensive and is intended as a guide for new engineers and supervisors.



B AN ENGINEERS CASEBOOK - OIL LUBRICATED BEARINGS. Oil lubricated bearings consume oil whilst in use through spillage, vaporization, leakage from reservoir etc., thus creating a need to replenish the reservoir where bearings are designed for continuous lubrication. A simple and convenient way of doing this on small reservoirs, such as those on most pumps and fan bearings is to use a constant level oiler. A number of different proprietary oilers are in use and all use the same principle which is that of a bottle type chicken feeder. An inverted bottle has a tube secured to the centre of the bottle cap so that oil from the bottle can run down the tube. The tube, which may be from 1/4 inch to 1/2 inch in diameter and 1/4 inch to 3 inches long. depending on the design of the oiler, has its end chamfered at 45'. The inverted bottle is mounted, usually adjacent to the reservoir at a level such that the top of the chamfer on the feed tube is at the oil level to be maintained in the reservoir. As the oil level falls the top of the chamfered feed tube is exposed and air enters the tube rising up into the bottle. An equal amount of oil is released, the process continuing until the pre-set oil level is restored when the chamfered tube is sealed off and oil feeding ceases. In some cases the chamfered oil feed tube dips directly into the level In the reservoir or sees the same level through a partially full side branch. In others the surface fed by the bottle is entirely separate from that in the reservoir being one leg of a 'U' tube which connects the bottle to the reservoir. In such cases it is necessary to ensure that the surface fed by the bottle is open to the atmosphere and a vent hole(s) is provided for this purpose.



A recent pump bearing failure may have been due to the vent becoming blocked In a rather unusual way. The all bottle was of 'Denco' manufacture and has a split brass sleeve surrounding the separate oil and air feed pipes which protrude from the cap. This sleeve IS soft soldered Into the cap and two vent holes, each about 1/16 inch diameter, are drilled through the cap and sleeve after they have been sweated together. The sleeve is quite a tight push fit in the body of the adaptor and it is likely that operators twist the bottle when inserting or withdrawing It. The sweated joint between the cap and sleeve has sheared allowing one to rotate with respect to the other thus blanking off the two small vent holes. The holes can be easily checked with paper clip and when this should be done on all Denco bottles - one further case of a rotated sleeve was found and another case where the vent holes had been painted over. Some other potential causes of malfunction which may apply are: Adjusting sleeve set too Iowan Denco bottles arranged for side feeding, chamfered tube set at wrong height, bottle fitting or adaptor loose where screwed into reservoir (should be positively locked against rotation), leaks on screwed fittings, faulty bottle to cap joints, occluded bottle preventing sight of true oil level etc. A careful check round all constant oiler installations might prevent unnecessary failures.



A specification based on the better features of existing oilers and designed to remove error producing features is being prepared and is incorporated into EDS GBHE-EDG-MAC-5701'Bearing Lubrication Specification. C AN ENGINEERS CASEBOOK - TYPICAL SHAFT & HOUSING

TOLERANCES - ENG DATA FOR PUMPS WITH ROLLING ELEMENT BEARINGS



D AN ENGINEERS CASEBOOK - THROTTLE BUSHES. The Engineering Specification for centrifugal pumps and API Recommended Practice 610. 'Centrifugal Pumps for General Refinery Purposes’, define the requirements for the majority of centrifugal pumps. Where pumps are fitted with a single mechanical seal, and most are, there is a requirement for non-sparking throttle bush to be fitted into the seal end plate. The diametral clearance between the Inside diameter of this bush and the shaft passing through it is specified to not exceed 0.025 ins. Bushes may be made of brass, sintered bronze, PTFE etc. The purpose of the throttle bush is to restrict the amount of the pumped fluid which is discharged to atmosphere in an uncontrolled way should there be a major leakage from, or total disruption of, the mechanical seal. In a way it acts as a crude auxiliary sealing device and limits the discharge of product to enable the pump to be shut down, prevents and limits the discharge of product to enable the pump to be shut down, prevents the production of a large flammable cloud of vapor if liquefied flammable gas (LFG) or hot flashing liquids are discharged and helps direct spillage along the drain connection from the space outboard of the seal. The throttle bush should not be confused with the throat bush (or neck bush) which is at the inboard end of the seal chamber and serves to limit the clearance between the shaft and pump casing near the back of the Impeller. Under normal circumstances the maximum diametral clearance of 0.025 ins between the throttle bush and shaft should more than cater for shaft displacements arising from impeller imbalance, bearing wear etc. One might suppose that there should be no contact, no wear and therefore no need to check or replace the throttle bush. Last year a check was carried out on a European Plant in which the diametral throttle bush to shaft clearance was measured on eleven pumps on LFG duty. On only two was the clearance within the specified 0.025 ins, some exceeded 0.100 ins the greatest being 0.180 ins. It had not been this plants practice to check throttle bush clearances during pump overhauls. This is now being done and clearances brought into line with the design figure. This source of hazard approach to area classification assumes that the clearance is not greater than 0.025 ins. The maintenance of this is a necessary part of continued safe operation where pumps handle LFG, flashing liquids and other materials which can give rise to significant clouds of flammable vapor.



E AN ENGINEERS CASEBOOK - HIGH SPEED CENTRIFUGAL PUMPS AND COMPRESSORS.

Sundyne pumps and compressors have been in use on some plants for 30 to 35 years now, and are looked upon for what they are· reliable single stage machines giving multi-stage capacity and performance. Maintenance problems still arise with these machines and when they do they are generally quite different from those of traditional centrifugal pumps. Recently twenty such pumps were commissioned on a Plant with varying degrees of success. The general difficulties encountered and solutions found may be of benefit to other pump users. Vibrations High vibration levels were experienced on the larger size of machines. These were the 400 HP and 200 HP units. Vibration levels 1.2 inches/second and 100 g were not uncommon. Sundstrand literature advises that machines should be shutdown if the velocity level reaches 0.31 inches/second. The gearbox vibrations can be greatly improved by balancing the high speed shaft assembly as a unit. Sundstrand balance individual components when manufactured and then rely on the combined out of balance of the built-up unit to be within tolerance. Our experience shows that dynamically balancing the output shaft assembly as a whole can reduce the out of balance by more than half and consequently the vibration levels. Recently we have resorted to balancing the layshaft assemblies to reduce vibration levels even more. The larger machines can be further improved by fitting helical gears to the input shaft and top of the layshaft. These gears and new bearings are now available as a conversion kit. In addition to dramatically reducing acceleration levels, the noise levels from the gearbox are also reduced by 10 dB to about 93 dB. Loss of Head When fitting the diffuser into the pump casing, It is essential to check that the '0' rings are undamaged. The slightest damage will result in a significant reduction in the delivery pressure of the pump.



Oil Pumps Internal oil pumps on Sundyne units are driven by a singly helical spring type Bissel pin which connects the pump shaft to the gearbox input shaft. It is not uncommon for these pins to shear, causing loss of oil and subsequent bearing failure. It IS important to examine and renew these pins following any failure. Do not be tempted to fit a solid pin in place of the spring type. It will almost certainly fail. The manufacturers have recognized the weakness and now supply double helical Bissel pins. Lubricating Oil It is Important to remove all traces of water from the gearbox. A very small amount will cause frothing in the oil. The manufacturers now recommended that Automatic Transmission Fluid (ATF) is used in the gearboxes of all 'Sundyne' pumps and compressors. Bearing Failures Some early failures of units on this group of pumps were found to be caused by failures of the bottom idler bearing. This is an 11 ball bearing With a nylon cage and is different from earlier machines that had fitted a 12 ball riveted cage bearing. While It is hoped that the manufacturer Will revert back to the riveted cage bearing, the 11 ball nylon cage is still being fitted. It is Important to see that this bearing is installed With the open Side of the cage facing the oil Jet. Suction Filters Although the Sundyne units do not require close clearances for operation, it is extremely important to ensure that the inlet suction filters in the flow to the units are properly maintained. Small foreign bodies can stroke the impeller as It rotates at very high speed, cause out of balance and consequent failure.



F AN ENGINEERS CASEBOOK - GLANDLESS PUMPS. Glandless or 'canned" pumps are used when any leakage of process fluid cannot be tolerated because of toxic, flammable or smell hazards. These pumps have no gland or mechanical seal on the rotating shaft to retain the process fluid within the pump casing. but have the complete rotating assembly including the electric motor rotor and stator enclosed within thin walled cans of Nlmonlc 75 material. The electrical energy to rotate the pump passes through the cans from the stator Windings. Several of these pumps were installed, running at 2.900 rpm on duties where the problem of smell from leakage had to be eliminated, and gave very little mechanical trouble for a number of years. Gradually, the frequency of major failures increased, until a fundamental Investigation Into overhaul procedures had to take place. The type of failure was mainly worn journal bearings resulting in the rotor and stator cans touching causing either puncture of the stator can and ingress of liquid into the electric motor or seizure of the motor. Both types of failure resulted in expensive and Time-consuming pump and motor overhauls. Initial attempts were made to predict the point at which failure occurred by using a hand held vibration monitor but no correlation appeared to exist between pump failure and vibration amplitude. Close examination of the wear rings, and shaft spacer sleeves from a failed pump revealed wear on only one side suggesting there were out-of-balance forces acting. These forces would produce much greater loads on the carbon journal bearing bushes, causing excessive wear and premature failure. At the first opportunity, a multi-stage rotor and a full set of impellers were sent for balancing and, as predicted, the out -of -balance forces were substantial. Generally, at pump overhauls, the impellers were re-used, usually after the wear rings had been replaced, and were not re-balanced before re-fitting. The rotor, if undamaged, was refitted to the pump after new journals had been fitted and again this item was generally not re-balanced. If the rotor required a new can, it was sent back to the manufacturers of the fitting and, unless specified in the order, was not re-balanced. It was therefore not surprising that the frequency of pump failures increased as the pumps became older and the rotating elements became worn and out of balance. Since then, all rotors and impellers have been balanced at overhauls and the pump outage has dramatically reduced.



G AN ENGINEERS CASEBOOK - RUNNING POLICY FOR SPARE AND MULTIPLE UNITS.

Most equipment is subject to wear and tear whilst It is in service and if it is left running long enough it will eventually fail. Where more than one Item of equipment is In service. if all have an approximately equal exposure to the service conditions. then they may be expected to fail after roughly the same number of running hours. You might say that this is stating the obvious; quite so, and quality control over manufacture, precise design and reproducibility tend to ensure that there is a minimum scatter in the time to failure. We have the following recent examples. Four axial flow compressor rotors (3 working 1 spare) are regularly rotated round the three working machines. As a result they have similar running hours, in this case about 6 years. Blade cracking in one was rapidly followed by failure in another and examination of the remaining two also revealed cracked blades. In another case a fleet of four dozen rail tank cars, of identical design and construction, all had nominally, the same number of running hours, they all developed cracks in the barrels at about the same time as a result of which there was a major repair requirement and a shortage of cars whilst the repairs were carried out. Situations like this can be avoided by ensuring that one of the operating units is deliberately ahead of the others in terms of running hours, thereby giving it an opportunity to reveal any built-in weakness. In the case of multiple units phased inspection can also help. Instead of inspecting the whole population every say, 5 years a number should be done each year, thereby providing an opportunity of early warning of time-dependant failure by corrosion, fatigue, wear. Etc.



DOCUMENTS REFERRED TO IN THIS GBHE ENGINEERING DESIGN GUIDE This GBHE Engineering Design Guide makes reference to the following documents: GBHE ENGINEERING SPECIFICATIONS GBHE-EDS-MAC-1803 Machinery Guarding.

(referred to in Clause 8) GBHE-EDS-MAC-1806 Bearing Arrangements for Machines.

(referred to in Clause 8) GBHE-EDS MAC 1214 Purchase Specification for Centrifugal Pumps and

Pump Sets. (referred to in 2.1,4.3, and Appendix H) GBHE-EDS-MAC-2102 Limiting Noise Levels of Manufactured Items of

Equipment. (referred to in Clause 8) GBHE ENGINEERING GUIDES GBHE-EDG-MAC-1101 Shaft Couplings for Special Purpose Rotary Machines

(referred to in 4.3.1) GBHE-EDG-MAC-4506 Vibration Monitoring of Machines.

(referred to in Clause 8) GBHE-EDG-MAC-5100 Reliability Analysis - the Weibull Method.

(referred to in Clause 8) GBHE-EDG-MAC-5701 Lubricants.

(referred to in Clause 8) GBHE-EDG-MAC-5702 Mechanical Seals.

(referred to in Clause 8) GBH Enterprises DIRECT DRIVE CENTRIFUGAL PUMP FAULT

CHART (referred to in Clause 9 and Appendix E)



PUMP SCHEDULE (referred to Clause 9 and Appendix B) Data Sheet (referred to in 2.1) API Recommended Practice 610 Centrifugal Pumps for General Refinery Purposes. (referred to in Appendix H)

Code of Practice - Centrifugal Pump Operation and Maintenance

Technology

Transcript of Code of Practice - Centrifugal Pump Operation and Maintenance