The most prolific machinery items in any plant are the pumps. They are spared and are not usually considered as critical equipment except, of course, when one of the units is down for maintenance and the other item suffers a component failure.
This chapter address the best practices that will optimize pump reliability, and minimize repair time and maintenance costs.
Best Practice – 1: Use plant, company and industry lesson learned to select the proper type of pump.
First, accurately determine all of the operating condition to be accommodated. Failed to do so account for 80% or more of pump reliability problems. Then use the company guidelines for selection of the pump type if they have any. Whether company guideline available or not, determine the pump type by site and industry lesson learned to avoid plant safety and reliability issues.
Benchmarks: Since 1990s this best practice has been used for duplicate plants in ammonia, ethylene, LNG, and methanol refineries. All past pumps reliability issues were evaluated to produce a best practice pump type selection list for each application, along with other best practice guidelines to be included on the pump data sheet.
Best Practice – 2: Pre-select centrifugal pumps (before the EPC phase) and modify the pump data sheet to include all details.
Select the pump for the noted performance conditions, and determine the pump selections using the acceptable vendor’s pump data book or website information. Select the pump to operate as closely as possible to its efficiency point. Notify the EPC of the selections for each acceptable vendor for the project, nothing specific component requirement and all best practice requirements on the data sheet. Pre- selection of all centrifugal pumps assures that:
- – All pump operating points will be in the Equipment Reliability Operating Envelope (EROE) resulting in optimum pump safety reliability and performance.
- – Specific component best practice requirements will be met (impeller type, journal bearings, Thrust bearings, seal and auxiliary systems).
- – All company best practices will be incorporated into the pump design prior to sending the out the ITB (invitation to bid) to vendors.
Benchmarks: Since 1970s a standard practice has been created to pre- select centrifugal pumps for the stated operating conditions, and note all best practice requirements on the pump data sheets. This practice has resulted in trouble free pump start-up reliability of the highest level.
Best Practice – 3: Accurately define all liquid and hydraulic condition on the centrifugal pump data sheet.
Completely define all possible operating conditions by meeting with process engineering, and site senior operation personnel during the process design phase to accurately determine:
- – Flow rates
- – Head required
- – Liquid properties
- – NPSH available
Define the rated operating point to be as close to the pump best efficiency point as possible, but not greater than 10% or less than 50% from the potential pump best efficiency point for centrifugal pump without and inducer.
Benchmarks: Since 1970s, this practice performed. Reviewing all pump operating condition with a senior process engineer, and plant operator during the process design phase to ensure that conditions are specified as accurately as possible prior to pump selection.
Best Practice – 4: State the required centrifugal pump design (horizontal split or radial and single or double volute) on the pump data sheet.
Review pump vendor experience lists to determine field operating maximum pressures for horizontal split pumps, and confirm acceptable field operation with users above 10,342 kpa (1500 psi) operating pressures. Select a radial split pump case if there are reports of horizontal split pumps joint wear and leakage.
Single volute pumps will produce large radial forces on the rotor at flow rates other. than at the centrifugal pump best efficiency point which will increase pump vibration. and reduce wear ring and throat bushing life.
Double volute pumps will minimize the rotor radial forces, and should be used whenever available. Confirm pump vendor availability of double volute design for all centrifugal pumps prior to vendor selection.
Benchmarks: Since 1970s, all the projects to ensure optimum pump field reliability.
Best Practice – 5: Double suction pump optimum reliability.
Double suction pump reliability is achieved by:
- – Having minimum 3 pipe diameter straight pipe into the suction
- – Suction piping routed perpendicular to the pump centerline
- – Keeping suction specific speed below 10,000 (US costmary unit)
Failure to follow the double suction pump best practices noted above will resulted in the following field reliability issues and MTBFs less than 12 motnhs:
- – Impeller cavitation
- – Pump seizure – at weir rings
- – Shaft breakage
- – Thrust bearing failure
- – Seal failure
Benchmarks: This best practice used since 1980s during pump selection of double suction. Prior to that time, all of the reliability issues were experienced and many pumps were classified as “bad actors”
Best Practice – 6: Pumps with inducers should use a constant flow control scheme to prevent inducer wear and resulting pump cavitation.
The equipment reliability operating envelopes (EROE) for inducers to prevent inducer cavitation and wear out is approximately +/- 10% in flow. Process changes will always be such that a flow change of greater than +/- 10% will be prevent. Most inducer pumps have radial vane pump impellers yielding a very flat performance curve which results in rapid flow changes for small changes in process head required. Design the control scheme to automatically vary the bypass flow for changes in process floe to always maintain a constant pump flow and therefore ensure optimum inducer and impeller MTBF.
With numerous high speed, inducer supplied pump failures that either use manual minimum flow control scheme or automatic minimum flow control system that are not integrated with the process control system. This arrangement permits the high speed inducer pump to operate over a flow range much greater that the acceptable flow range for an inducer, which results in significantly reduced pump MTBF. Observed failure mode have been: inducers, impeller, bearing failure, seal failure, gearbox failure, shaft breakage.
Benchmarks: This best practice used since 1990s, after experience of numerous high speed pump failures in a chemical plant. Since these best practice implemented, MTBFs have exceeded.
Best Practice – 7: Operate centrifugal pumps within “Equipment Reliability Operating Envelope “ (EROE) to achieve maximum mean time between failure (MTBF).
The equipment reliability operating envelope (EROE) also called ‘heart of curve’, assures maximum centrifugal pump MTBF by avoiding all operating areas of hydraulic disturbances.
We define the general EROE range as +10% to -50% in flow from the pump best efficiency point. This range will be reduced for double flow pumps and high speed inducer pumps.
Establishing operator EROE targets for all critical site pumps and all bad actor pumps will ensure optimum centrifugal pump safety and MTBFs.
Benchmark: This best practice used since 1990s in refineries and chemical plants. Once this best practice had been implemented, pumps MTBFs that were less than 12 months (“bad actor”) were improved to greater than 80 months.
Best Practice – 8: Monitor pump EROE in control room for critical centrifugal pumps and define EROE targets.
Pump flow range can be monitored in the control room by inputting the pump shop test curve and dumping the following transmitter signal into spreadsheets to calculate the pump head and flow:
- – Inlet Pressure
- – Discharge Pressure
- – Flow
- – Specific Gravity
EROE targets should be established in flow or the following other methods if flow meters are not installed for each pump:
- – Control valve position
- – Motor amps
- – Pump inlet and discharge piping differential temperature
Majority of centrifugal pump mechanical seal and bearing failures are the result of the pump being forced out of the EROE by changes in process required head without operator being aware of this.
Benchmarks: This best practice has been followed since 2000 in heavy oil field pump applications, where the operating conditions require the use of slurry pumps which are subject to rapid wear. The use of control valve operating point monitoring with EROE alarms has extended pump MTBFs to over 36 months, from less than 12 months before this best practice was implemented.
Best Practice-9: Ensure that centrifugal pumps have a minimum head rise to shutoff (zero flow) of 7%.
The lower the head rise of a centrifugal pump (pump head produced at zero flow divided by head produced at rated point), the greater the change in pump flow for a given change in process head required. The operating flow for centrifugal pumps with head rises that are below 7% will be easily changed by small variances in the process head required. Try to select a centrifugal pump with the highest head rise possible, while taking into consideration the flow range and NPSH available for the required pump.
Benchmarks: This best practice has been used since 1970s to ensure highest centrifugal pump safety and MTBF (greater than 80 months) in all pumps application.
Best Practice – 10: Install discharge flange orifice for centrifugal pumps with less than 5% head rise to ensure stable pump operation.
Centrifugal pumps with flat head vs flow curves (less than 5% head rise from rated to zero flow) produce rapid flow changes for small process changes. Size the impeller for increased head (to compensate for the orifice pressure drop). Installing a discharge orifice in the pump volute or on the discharge flange will produce the desired head rise, and will result in a pump characteristic that produce gradual flow changes for small process changes. This procedure is especially effective for high speed pumps which have a characteristic flat, low head rise performance curve.
Low head rise curve less than 5%, have resulted in the following reliability issues:
- – Rapid inducer and/or impeller wear
- – Pump seizure
- – Bearing failure
- – Seal failure
- – Shaft breakage
- – Gearbox failure
Benchmarks: This best practice has been used since 1970s in all high speed pump applications and where any centrifugal pump had a low head rise characteristic.
Best Practice – 11: Define NPSH margin for the maximum flow point of a centrifugal pump to prevent cavitation for all operating cases.
Ensure that the NPSH margin is acceptable at the maximum possible flow, based on the process control system design. Confirm that the pump is protected, to ensure that there will be a recirculation margin at minimum operating flow. Accurately define all liquid characteristic (vapor pressure, viscosity, SG, and pumping temperature).
Centrifugal pump applications that process liquids with high vapor pressures are susceptible to low NPSH margins when operating at heads less than the rated head. Failure to select the NPSH margin considering the installed control and protection system has resulted in reduced pump safety and reliability.
Benchmark: This best practice has been used since 1990 for pumps with high vapor pressure (SG below 0.7).
Best Practice – 12: Require that suction specific speed for low NPSH required pumps is less than 10,000 (US units) to minimize the possibility of recirculation.
Low NPSH required for centrifugal pumps is attained by having a large impeller suction area and/or by using a double suction impeller (which has twice the suction area of a conventional impeller). Recirculation is a hydraulic disturbance, which is the result of low fluid velocity into the impeller. The increase of impeller suction area to reduce cavitation can produce recirculation if the increase of impeller suction area is too great. Suction specific speed predicts the probability of recirculation.
Recirculation at the rated operating point with a double flow impeller that was not checked for suction specific speed (the suction specific speed exceeded 17,000). The recirculation forces resulted in the following damage to the pump:
- – Impeller damage
- – Shaft breakage
- – Bearing failure
- – Seal failure
Hydraulic disturbance are detected by monitoring the conditions noted below:
- – Loud noise – continuous or varying
- – Suction and discharge pressure pulsations
- – High value of overall vibration
- – Drop in produced head of greater than 3%
- – High values of vane passing frequency in overall spectrum
- – Pressure pulsations in inlet and discharge piping
- – Possible high bearing temperature
Benchmarks: This best practice used since 1080s, to ensure that pumps are not susceptible to recirculation at flows in the operating range of the pump (-50% of the pump best efficiency point).
Best Practice – 13: Obtain references from pump vendors for pump bearings with DN numbers above 350,000 when ring oil lubrication is being offered to determine if oil circulation in the bearing housing is required.
DN is a measure of surface speed of the shaft. It is the product of the shaft speed in revolutions per minute and the bore of the bearing in millimeters. The higher DN number, the greater the frictional heat that will be generated by the bearing. DN values above 350,000 produce a significant amount of heat and require oil circulation through a cooler to maintain the proper oil viscosity in the bearing. Small oil consoles will solve the problem and result in pump with high MTBF. They can be easily retrofitted in the filed if not originally supplied. Many pump bearing low MTBF (less than 12 months) have been resolve by installing small, dedicated, lube oil circulation systems whenever pump vendor experience dictates that the pump will operate at DN above 350,000.
Benchmarks: This best practice has been s=used since 1990s to increase pump high DN bearing MTBFs from less than 12 to greater than 100 months.
Best Practice -14: Install mini-oil circulation flow consoles foe high DN bearings (above 350,000 or when vendor 2 year field experience is not present for the proposed oil lubrication system (typically ring oil)).
Calculate the bearing DN as the product of the shaft speed in revolution per minute and the bearing bore in millimeters. If the calculate value exceeds 350,000, obtain vendor experience lists and contact reference to ensure that the offered, non0continues flow, bearing lubrication arrangement has resulted in MTBFs greater than 40 months. If experience cannot be obtained, require continuous oil circulation.
Benchmarks: This best practice has been used since 1980s to obtain high DN pump bearing MTBFs in excess of 100 months.
Best Practice – 15: Remove temporary pump suction strainer after plant start-up whenever possible. If not possible, confirm proper strainer design and supply with a differential pressure gauge.
Check all temporary pump strainers for evidence of debris after one month, remove them. If strainers show evidence of debris the following action is recommended:
– Ensure that the strainer design is rigid (perforated sheet metal with the appropriate mesh size over the perforated plate facing the direction of flow)
– Install a differential pressure gauge or differential pressure transmitter around the strainers and have the differential checked by operators on their daily rounds.
Many pump failures have resulted either from plugged strainers that have not been monitored, or strainers that were made of only mesh and sheet metal frame, that collapsed and entered the pump. In many cases the entire pump had to be overhauled, which exposed the process unit to shut down during the period of spare pumps repair.
Benchmarks: This best practice has been used since 1980s, resulting in pump applications with optimum safety and MTBF in excess of 80 months.
Best Practice – 16: Use automatic minimum flow bypass valves, for centrifugal pumps, whenever the pumped liquid specific gravity is less than 0.7 and/or hot fluids.
Liquids having a specific gravity below 0.7 are known as high vapor pressure liquids, and they will become a vapor under atmospheric conditions. These liquids also are usually close to their boiling point at field conditions, and therefore have a low NPSH requirement and are susceptible to cavitation and recirculation. Automatic minimum flow bypass valve should be used in all applications involving such fluids, for each pump. The minimum flow set point should be based on suction specific speed circulations and the pump vendor’s recommendation for exact fluid being pumped. As a general rule, the minimum flow point should not be lower than the point at which the NPSH required curve ends at low flow on the pump curve.
Many pumps which handling high vapor pressure (SG less than 0.7) do not have automatic minimum flow bypass valves or manual bypass valves without any indication to operators in the control room of low flow. Pump operating with high vapor pressure liquids at low flow conditions are prone to impeller and wear ring failure and seizure.
Benchmarks: This best practice has been used since 1990s. Which has resulted in pumps of the highest safety and reliability and MTBFs exceeding 80 months.
Best Practice – 17: Install flow switches in manual minimum flow bypass systems to prevent centrifugal pump damage from recirculation and vaporization.
If a pump is supplied with manual minimum flow control valve, it should be equipped with a flow switch set to actuate in the control room at levels which are 10% before the minimum flow value is reached.
It observed that most centrifugal pumps that are subject to recirculation and/or vaporization are only supplied with manual minimum flow control valves which are not instrumented to detect low flow failure to open these valves during low flow conditions subjects the pump to low MTBFs caused by: bearing failure, mechanical seal failure, impeller and case wear ring seizure.
Benchmarks: This best practice has been used since 1990s and has resulted in optimum pump safety and reliability (MTBF exceeding 80 months).
Best Practice – 18: Avoid the use of internal type minimum flow bypass valves whenever there is a possibility of debris or fouling in the pumped liquid.
Avoid the use of internal-type, minimum flow bypass valves with internal orifices and springs for upstream, oil filed application, boiler feed pumps and any other pumps that can come into contact with solid particles. Use external minimum flow bypass valve arrangement that have a flow transmitter which will open a conventional control valve supplied with properly sized orifice(s). External type minimum flow control valve allow operator monitoring of valve position and positively prevent valve sticking and failure to open.
Benchmarks: Since 1990 several instance of complete high pressure boiler feed water pump failure been observed (where the pump was completely destroyed and a new high pressure radial split pump became necessary) in system that used internal type minimum flow bypass. After that incident, it requiring external type minimum flow bypass control systems with multiple orifice chambers for all boiler feed water applications.
Best Practice – 19: Monitor parallel centrifugal pump operation by the use of manual or automatic differential pipe temperature to prevent parallel pump failure.
Pumps that operate in parallel are subject to significant component damage if they are not individually protected against minimum flow and usually they are not. When pumps operate in parallel, the pump with the lowest head produced (worn pump and/or pump operating at the lowest speed) will be forced to reduced flow. Typically, for cost saving reasons, the minimum flow protection for the pumps is installed in the pump discharge header and therefore does not provide individual pump protection. As the flow to any centrifugal pump decreases the pump efficiency reduces and approaches 0% at zero flow. As a result, the pump differential temperature, which is close to zero degrees Celsius in the safe operating region of flow, will increase significantly as pump flow decreases. Monitoring pump differential temperature is therefore a practical way of determining parallel pump internal condition and speed (if the pump driver is a variable speed type).
It has been experienced that most parallel pump systems are not monitored for individual reduced pump flow and are not provided with individual pump minimum flow protection. As a result, significant pump components can and will occur when one or more pumps have internal wear or operate at a reduced speed compared to the other pumps.
Benchmarks: This best practice has been used since 1990s to ensure parallel pump optimum safety and reliability (MTBF greater than 80 months).
Best Practice – 20: Check the pump flange and foundation forces whenever blinding and/or installing pumps to ensure that the external forces on the bearings are minimum.
Most centrifugal pumps use anti friction bearings. The life of any type of anti-friction bearing is inversely proportional to the cube of the forces on the bearing. As an example, if the forces on anti-friction bearing double, the life of the bearing will decreases by eight times. Checking the pipe forces and foundation forces (known as soft foot), whenever piping installed or disturbed will ensure that the forces on the bearings are acceptable and yield optimum bearing MTBF.
Benchmark: This best practice has been used since 1980s to optimize pump safety and reliability. Bearing MTBFs have been increased from less than 6 months to more than 100 months.
Best Practice – 21: Pre- Commissioning – Ensure that every centrifugal pump operates in the equipment reliability operating envelope (EROE) and change impeller diameter if required.
The hydraulic calculations used to determine the pump head required for centrifugal pumps will only approximate the field conditions, and can be conservative, which will result in lower field head required than noted on the pump data sheet. Lower pump head required can force centrifugal pumps to operate at greater flow than the design point. Always confirm that new pumps are operating within the EROE and take corrective action. Most centrifugal pump drivers are sized for +10% power and can be overloaded if the pump flow is greater than design flow. The most cost effective solution to prevent driver overload is to reduce (cut or trim) the pump impeller diameter, to arrive at the desired pump flow at the actual field process head required conditions. Many new plants commissioned do not confirm that centrifugal pumps are operating in the EROE, which results in pump overload and cavitation. This lack of action has resulted in low MTBFs and frequent tripping of motor driven pumps.
Benchmarks: This best practice has been followed during pre-commissioning of all centrifugal pump installation since 1980s to optimize safety and reliability.
Best Practice – 22: Require a plant pump spare changeover philosophy to ensure optimum MTBF.
All pumps are subjected to the highest component forces and changes during transient (start-up and shutdown) operation. Whilst it is true that spare pumps must be periodically checked to ensure safe and reliable operation when required, the periods between checks should be optimized. We have found that the changeover period which results in the highest possible pumps MTBF is between 3 and 6 months in most pump applications. Notable exceptions are firewater pumps (weekly – for safety) and seawater pumps (to eliminate corrosion).
Many plants either do not have a stated change over philosophy, or do not implement the sated policy. In cases where pumps operate on fluids with a high vapor pressure, too frequent changeover (less than 3 months) has resulted in low mechanical seal MTBFs (less than 12 months).
Benchmarks: This best practice has been used since 1990s to optimize pump safety and maximize pump MTBFs (in excess of 80 months).
Best Practice – 23: Consider the drive systems limitation whenever considering impeller diameter increase, to ensure new operating requirements can be met without driver overload.
Driver power requirement increase by the cube of the flow increase. If additional pump flow can be accommodated in the existing process unit, pump impeller diameter or the number of impeller in a multistage pump can be increased by a limited amount, provided that the driver has sufficient power.
Most pump applications are based on industry and end user specifications that only required 10% additional driver power above the rated power of the pump. Since driver power requirement is a cube power function of flow this means that the diameter increase has to be limited to an approximate 2.5% increase if there is only 10% margin excess driver power available. If the electrical system and the pump unit baseplate permit, a larger motor may be able to be used along with a larger coupling for additional pump flow above 2.5% and corresponding power requirements.
Many applications experienced where larger drivers were not originally used and impeller diameter was still increased, resulting in driver overload (Mostly motor drivers). Eventually, the baseplate was modified for a larger driver.
Benchmarks: This best practice has been used since 1970s and always to be inquired during pre- FEED or FEED project phase regarding possible re- investment in a larger motor, in process units where the original design pump capacity will be bottleneck for larger future process unit rates.
Best Practice – 24: The use of variable speed motors in low voltage services is effective in terms of energy and reliability.
Consider a low voltage motor best practice of always using variable frequency driver motors to provide just the amount of power required for process requirement. Install a control system (level, pressure or flow) that will regulate pump motor driver speed to meet set point requirement without the use of a process control valve. This approach will reduce required power in low voltage applications (less than 600 volts) by as much as 25%.
At the present time, the additional installed coast of low voltage VFDs can be paid for in less than three years by the energy savings produced.
Benchmarks: This best practice has been used since 2005 and resulted significant energy savings and increased pump control system safety and reliability.
Best Practice-25: Have every centrifugal pump curve available in the control room and instruct all operators on their use to optimize centrifugal pump safety and MTBF.
Centrifugal pump produce flow inversely proportional to the process head required. The safety and reliability of all centrifugal pumps is optimized if pumps are operated within the equipment reliability operating envelope known as EROE. This flow range is obtained by having operations aware of the centrifugal pump characteristic by having operations aware of the centrifugal pump characteristic, providing process targets and having the pump test curves available for each pump for operator use and understanding. From the experiences, it found that approximately 80% of the root cause of centrifugal low MTBFs (below 36 months) are due to process changes, not operator error, which can be significantly avoided by incorporating this best practice
Benchmarks: This best practice has been used since 1990 in upstream oil and gas refinery and chemical plants to optimize safety and reliability and to produce pump MTBFs in excess of 80 months.