Rotating Equipment Switch-Over Frequency - Maintenance.org

Rotating Equipment Switch-Over Frequency

Maximum Reliability Time Interval Study

Abdulrahman Alkhowaiter, Machinery Consultant 2020

of 10 Copyright 2020 Abdulrahman Alkhowaiter

Machinery Switch-Over Frequency Optimization and Methodology for Plant Machinery

Industrial plants utilize spared machinery to provide higher plant availability as machines can fail and standby machinery are critical to plant reliability and safety. This means that plants incorporate a scheduled switchover frequency usually of once monthly period between the operating machine and its standby sister equipment. Some plants use two weeks, others may use two months interval. This report attempts to optimize the switchover period to maximize plant machinery safety, reliability and reduce operator workload. The author has worked with over 200 process plants during the past 25 years, and over time recognized that Industry in general is not applying reliability centered maintenance towards switchover practices of standby equipment. Machinery Switchover Frequency Optimization Benefits

Prolong Machinery life and MTBF including the auxiliaries. Reduce emergency work activities. Increase machinery and plant safety. Minimize operating expenses and total maintenance cost including PM. Minimize machinery component corrosion, crystallization of fluids, and adhesive bonding. Eliminate rotor sagging and vibration problems. Establish true “Hot Standby” capability by actual system testing.





Time Dependent Issues Involved In Setting Switchover Frequency for Standby Machines

1. Machinery shafts and rotors experience some deflection due to gravity weight while the driver and driven machines are stopped. Shaft sag is a time dependent phenomena impacted by rotor weight and stiffness. It imparts a temporary bend in rotors which causes vibration and possible rubbing problems. This is important particularly for larger machines of 500 KW and above which have greater mass and rotor span between bearings. It is not practical to ask plant personnel to perform hand rotation of all shafts to mitigate shaft bending.






2. Bearing Housing Component Corrosion: This occurs to machinery such as pumps and

compressors, gearboxes, motors, and their bearings depending on design, including the shaft sealing areas affected by air humidity while shutdown. Both sleeve and rolling element type bearings must be lubricated once monthly [max interval] or surface corrosion occurs to shaft or bearing. In addition, gearbox teeth are made of alloy steel, and must be lubricated with a film of oil at least once monthly, or corrosion attack from the air will damage tooth surface.

3. Centrifugal Compressor: Dry Gas Seal component materials are chosen from extremely

corrosion resistant materials so no issues arise with these elements. However it is recommended to maintain the nitrogen purge on tertiary DGS seals, functioning all of the time and this will reach the secondary seals at least. The Nitrogen purge pressure can be reduced to a lower pressure such as 2.0 psig during the long shutdown period. This reduces consumption.






4. Pump Mechanical Seals: These liquid seals need to have the sleeve and faces rotated with fluid to reduce fouling buildup, adhesion and “lubricate” contacting parts. The adhesion force during startup will cause face and spring failures upon startup after longer time intervals.

5. Internal Corrosion or Solid Fouling: In some process machinery applications these may have

internal corrosion action due to air or gases trapped in the casing on long shutdowns. This includes reciprocating engines and gearboxes whose gears require lubrication of all surfaces at least once monthly to protect from oxygen and moisture corrosion in air. In liquid applications such as pumps some liquids will be crystalizing or fouling and this is another reason to start them at monthly intervals in order to minimize fouling buildup and unbalance.

6. Rolling Element Bearing Static Load Degradation: It has been claimed that machinery rolling

element bearings undergo microscopic deformations due to the dead weight of rotor when machine is off. The author has not seen evidence for this in 25 years of plant operations. However, it is possible that this was a phenomena of the past when rolling element bearings were under-rated by design meaning undersized compared to modern standards, thus resulting in excessive loading under static and dynamic weight. This may have been true for the 1960’s era machinery and before, but not modern machinery. Another reason why this may have occurred long ago is that the precision of manufacturing balls, rollers, and the races they contact was not as geometrically precise as in modern bearings. Modern bearings actual ball or roller contact area to race is higher, leading to reduced loading per unit contact area. The final difference is that modern steel alloy rolling element bearings utilize Rockwell-C-60 surface hardness and above; while 1960’s bearings were about RC-55.

7. Failure to Start on Demand: The Plant operations personnel need to test the standby

machinery to determine if they have the “Hot” standby capability of being immediately started in case of failure of the primary operating machine. This is another reason for the typical once monthly switchover frequency and is a valid argument. However the confirmation of standby machine readiness and fitness status can be achieved through different methods.

8. Preventive Maintenance: Another reason for scheduled shutdown of the main operating

process machine is to be able to perform detailed preventive maintenance procedures to the driver and driven machines plus associated electrical, lubrication systems, control, and instrumentation. This includes lubrication replacement for machines with self-contained lube oil bearing housings. Maximum recommended oil replacement interval is six months.





Frequent Switchover Introduces the Following Negative Impact to Rotating Equipment

DIPF Curve by ReliabilityWeb.com The machinery lifetime operation chart illustrates the various design, installation, maintenance and operational regimes and their impact on machinery. The failure modes below negatively impact the Proactive and Corrective domains thus reducing total machine life between failures:

Pump or Compressor Mechanical Seal Premature Failure: High startup acceleration torque

occurs with motor drive and causes fatigue failures in mechanical seal components on pumps. This is invisible to the user but it is highly detrimental to seal life, including the thermal cycling failure mode that also occurs at every start-shutdown cycle. Its impact affects liquid seals more than gas seals. For Sealless pumps, these are also impacted by thermal cycling mode, and by the possibility of short dry run periods during startups.

Internal Rotor Deflection at Startup: Frequent starting of larger and high speed machines

introduces the hazard of rubbing at close clearance contact zones as the machines pass through critical speeds. For multistage pumps this would be at the interstage bushings and wear rings, while in centrifugal compressors and steam turbines this can occur at packing glands or interstage seals.





Process Machinery Infant Mortality: Introduction of Infant mortality failures due to operator or auxiliaries errors at startup affects all machinery. This is a common phenomenon in Fluid handling equipment. For example it is possible that poor centrifugal pump priming occurs, so each startup introduces the possibility of dry running hazard.

Increased safety hazards: Fires or destructive failure incidents at faulty startup events. It is

also possible that fires occur during operation of high temperature equipment, but what is meant here is that frequent stopping and starting introduces new chances for such failures.

Thermal cycling from cold to hot and back to cold: Acts to fatigue metal, ceramic, and elastomeric parts and loosen bolted components. Thermal cycling affects M-Seals, motors, gas turbines, reciprocating engines, steam turbines, hot service pumps, and gas compressors.

Motor Starting Electrical Insulation: Excess startups reduce motor electrical insulation life

particularly on larger motors, with high amperage during startup and sudden high winding temperatures occurring at startup. Impacts both Induction and Synchronous motors.

The main detrimental issues from the above are: Sudden acceleration torque at startups, rotor high deflection at startup, thermal stresses at startup and shutdowns, Infant mortality failure at startup, electrical startup stresses in electrical equipment. Practical conclusion from this is that 75% of damage occurs from starting, with about 25% impact from shutting down cycles. For these reasons machinery switchovers should be minimized as they are detrimental to reliability. In other words, the standby machine has increased the system availability; but the installation of the second machine has actually reduced the reliability of the first unit. This is non-intuitive and this interesting conclusion is proven by both theory and actual practice.

Calculation of Start-Stop Damaging Cycles for A, B Configured Equipment A, B Machines Switching On 1-Month Basis in 12 Month Period

A Machine: 6 starts and 6 shutdown yearly B Machine: 6 starts and 6 shutdown yearly Total harmful cycles: 24 cycles per year. A, B Machines Switching On 6-Month Basis in 12 Month Period

A Machine: 1 start and 1 shutdown first six months. During second half of year 6 month shutdown period: 1 short start-stop per month= 5 cycles.





B Machine: 5 start-stops first half of year, and 1 start, 1 shutdown second half of year. Total harmful cycles: 7 per machine= 14 cycles per year. Short Stop is not counted as cycle.

Calculation of Start-Stop Cycles for A, B Configured Equipment A, B Machines Switching On 12-Month Basis in 12 Month Period A Machine: 1 start and 1 shutdown in 12 months. B Machine: 11 short start-stops each month during the year. Short Stop is not counted as cycle. Total harmful cycles: 2 for A-machine, and 11 for B-machine= 13 cycles per year. Optimum Interval Selection: The author considers that the winner in the above calculations is that which provides the highest balance toward achieving PM access to machinery, standby active test ability, and greatest reduction of start-stop cycles on main and standby machinery. This leaves the Six-Month starting-stop model as the ideal. A, B, C Machines Switching On 6-Month Basis in 12 Month Period In this case when 3 x 50% machines are installed and 2oo3 are operating: A Machine: 1 start and 1 shutdown first six months. Then restarted after P.M. B Machine: 1 start and 1 shutdown first six months. Then restarted after P.M. C Machine: Is not designated as Standby, it is designated as temporary service machine to be used whenever A, or B is shutdown during a forced trip or shut down for scheduled maintenance. Conclusion: For A, B, C service machinery: The third machine will not be utilized as standby switchover machine; it is permanently designated as temporary usage machine and will accumulate less hours compared to sister machines. This machine is started once monthly for one hour. This total procedure is not harmful to machine or plant. A, B, C, D Machines Switching On 6-Month Basis in 12 Month Period In this case when 4 x 33% machines are installed and 3oo4 are operating A Machine: 1 start and 1 shutdown first six months. Then restarted after P.M. B Machine: 1 start and 1 shutdown first six months. Then restarted after P.M. C Machine: 1 start and 1 shutdown first six months. Then restarted after P.M. D machine: Is not designated as Standby, it is designated as temporary service machine to be used whenever A, B, C machine is shutdown during a forced trip or for scheduled maintenance. Conclusion: For A, B, C, D service machinery: The fourth machine will not be utilized as standby switchover machine; it is designated as temporary usage machine and will accumulate less hours compared to sister machines. This machine is started once monthly for one hour.





Philosophy of Reliability Centered Switchover Strategy The above calculation attempts to capture the relationship between time interval in

operating and shutdown modes, as compared to frequency of damaging cycles. The author considers that the winner in these calculations is that which provides the highest

balance toward achieving toward achieving PM access to machinery standby active ability, and reducing start-stop cycles on both main and standby machinery. This leaves the Six-Month starting model as the ideal for all types of plants machinery.

This model also incorporates once monthly short start-stopping cycles on standby machines to maintain lubrication of parts and reduction of fouling-corrosion in process machines.

Reduction of Machine Stress Monthly Test Starting; In the 6-Month model the yearly number of starts is same as for 1-Monthly switchover interval, however the operating temperatures of critical parts do not reach the higher temperatures achieved under long term operation. This is why stops are ignored. In addition, during the monthly test startup of reciprocating engine machinery, steam, or gas turbine driven machines, the shaft speed does not have to reach 100%; Even 50% is suitable as long as critical speeds are avoided.

Improved Operational Philosophy: Maintain the operating blowers, compressors or pumps designated as main machine. The standby compressors or pumps are only started once monthly for one hour on recycle mode then shut down without bringing them online. The standby machine is later placed into full operation at six-month intervals. For A,B,C or A,B, C,D configurations: The non-utilized machine is not Standby; it is a temporary usage machine.

There is no benefit to operating centrifugal machines on short interval shared time basis as only positive displacement machines undergo significant wear during operation. Even reciprocating machines can have sharing of wear life in extended intervals of 6 months. Well-designed and maintained modern centrifugal machines do not need to be operated on a frequent switching on-off cycle mode of operation.

Large rotating equipment are equipped with pressure fed circulating lube oil systems therefore it is recommended to start these auxiliary systems every week for a 30 minute time period as a system test. This will also eliminate the possibility of corrosion in lube and seal oil systems, including bearing and seal journal areas on rotor. All non-operating pumps or compressors with lube oil systems should have the lube oil system started once weekly for 30 minutes. This weekly action can be safely automated in the plant central control computer as no startup hazards occur from remote starting of the lubrication pumps.

Preventive Maintenance intervals of self-contained bearing housings can be extended by using high quality synthetic oils which allow longer operation without oil replacement.

This method reduces startup-shutdown cycles per year from 24 down to 14 for the machinery train assuming A, B, or more configuration machines that were normally cycled once monthly. At the same time it confirms that the standby machines are available for action.





Procedure for Motors, Pumps, Blowers, Compressors, Generators, Steam Turbines

1. Maintain the operating blowers, compressors, or pumps designated as main machine. 2. For A, B configuration the standby compressor or pump is only started once monthly for one

hour minimum recycle then shut down without bringing online. The standby machine switches roles with main and enters full operation at six-month intervals.

3. For larger machines with API-614 Special purpose Lube oil systems: The lubrication pump is started for a half-hour weekly or twice monthly which also becomes a lubrication system test.

4. Do not shutdown main operating compressors or pumps except every 6-months for switchover to standby mode. This saves ten startup-shutdown damaging cycles per year.

5. For compressor dry gas seals maintain Nitrogen pressure continuously at all times. 6. Centrifugal Instrument Air Compressors: Apply the same method above of maintaining the

operating air compressors online without switching off, while starting the standby compressors for 30 minutes monthly offline. This increases reliability as frequent starting and shutting down increases machinery failures. Test the standby air compressors at monthly intervals. Start lube oil system weekly.

7. Lubrication of self-contained bearing housing: Apply Synthetic oils to allow extension of oil replacement interval to 6 months. Bearing housing isolation seals further improve reliability.

8. Main Firewater Pumps: These are a special case, only follow Fire Protection NFPA standards.





Procedure for Reciprocating Process Plant Machinery above 200 HP

1. Emergency Diesel Generators: These are not normally in an A, B main and standby

configuration as they are only utilized upon loss of plant power. Typically start tested weekly or twice monthly. It is not recommended to subject them to 52 starts per year. Applying a twice per month period for test starting is sufficient to guarantee availability meaning 26 starts per year. Some plants have extended this to once monthly but that is the maximum possible period for such critical service equipment.

2. Reciprocating Engines or Compressors: These machines are also best not stopped and switched over every month. It is advised to utilize six-month operating intervals for reciprocating engines or gas compressors with standby machine once-monthly startup for 1 hour operation on light load recycle mode, not actually placed into service. The unit, if equipped with electric lube oil motors should have the lubrication system started weekly for 30 minutes. Water cooled cylinders system piping, heat exchangers, pumps, and cooling fluid should be properly selected and modified to resist six month downtime corrosion.

3. Reciprocating Pumps: These pumps can be operated similarly to the reciprocating gas compressors using six-month duty cycles, with standby pump once-monthly startup for 1 hour operation on recycle mode, not actually placed into service. However those with thicker polymerizing, crystalizing, or caking fluids should be handled on a special case by case basis.

Rotating Equipment Switch-Over Frequency - Maintenance.org

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