improving the reliability of a decoking jet … · improving the reliability of a decoking jet pump...

Reinaldo Bermudez is a Senior SalesEngineer for Voith Turbo, in Houston,Texas. He is currently responsible for thesales and implementation of hydrodynamicvariable speed technology on a variety ofoil and gas applications with special focusin refining. He has more than 12 years ofexperience in the field of turbomachinery,both in the process refrigeration industryand in power transmission equipment.

Mr. Bermudez has a B.S. degree (Industrial Engineering) fromthe University of Houston, and he is also a member of ASME.

ABSTRACT

Delayed coking is a refining process that consists of the upgradingof petroleum residues into lighter and more valuable hydrocarbonfractions by promoting thermal cracking on the residue stream.Petroleum coke is a by-product of this process and depending on itsmorphology it can be used as fuel for steam generation, cementcalcination or as feed stock for the manufacturing of aluminum andsteel. Delayed cokers are very complex systems and require aconsiderable amount of hardware such as fired heaters, cokedrums, a main fractionator and a hydraulic decoking system. In atypical delayed coker petroleum residues are passed through firedheaters before they are charged, in an alternating fashion, into largedrums as boiling product. In the drums some of this effluentthermally cracks into lighter fractions (vapors) and these areremoved from the drum overhead and returned to the cokerfractionator before they are sent to a gas plant for further refining.The heavier product that stays behind slowly forms into coke andit is allowed to fill the drum over time. The time it takes to do thisis usually referred to as the coking cycle. Once the drum is fulland the feed is switched to start filling an empty drum, a series ofactivities such as sludging, quenching and steam-outs areperformed on the full drum prior to removing the coke. Veryhigh pressure water is used to cut this solid coke out and aftersome preparations this drum is empty and ready for a new cokingcycle to begin again. As stated above, delayed coking is thus acombination of a continuous process through the fractionationtower and furnace and a batch process in the drums. As a result ofthis configuration the high pressure decoking pump only needs tooperate a portion of the total cycle time.

Regardless of its “part-time” operation the water jet coke cuttingpump is still a very important component of the hydraulic decokingsystem because it not only provides the required cutting pressurenecessary to cut the coke but also contributes to the quality of cokeremoved and the duration of the decoking cycle. Recent history showsthat the majority of new hydraulic decoking installations use electricmotor drives with fixed speed operation. The pump generally requiresone operating speed to produce the necessary pressure to cut the cokeand its flow, pressurization, and depressurization duties are controlledby a decoking control valve. During noncutting cycles operators

typically shut off the motor to minimize power consumption of thesystem or they keep the motor running (rarely done), depressurizethe line and operate this valve on bypass to recirculate the waterthrough a holding tank until the coke cutting is resumed. Bothoperating alternatives have their disadvantages with regards toreliability (the first) and energy consumption (the second).

This paper explores the use of a hydrodynamic variable speed drive(HVSD) with an electric motor-driven pump as a way to increase theflexibility and reliability of the hydraulic decoking system. TheHVSD or variable speed coupling (VSC) is able to seamlessly varythe speed of the pump and accomplish the following:

• Keep the motor running and operate the pump at minimum idlespeed during noncutting cycles, avoiding periodic motorshutdowns and start ups

• Eliminate the need for soft starter or special motor designconsiderations to handle periodic start ups

• Reduce work and extend life of the decoking control valve byhelping with depressurization duties during noncutting cycles andcutting tool mode shifting

• Efficiently adjust cutting pressures depending on feed stockquality to avoid coke pulverization.

This paper also describes the function of the variable speedcoupling and how its unique design features are suited for thestart up and intermittent operation of the water jet coke cuttingpump. Field data and operator experience from two existinginstallations are collected and evaluated and the advantages ofvariable speed operation over fixed speed operation of thedecoking pump are highlighted.

INTRODUCTION

Since hydraulic decoking was first introduced to the industry inthe late 1930s many process innovations and design modificationshave been implemented to the system components to improveliquid yields, safety, cycle times, coke handling, reliability andefficiency of the overall operation. Among the most importantadvances is the evolution from manual cutting operations to anautomated process that allows for all cutting activities and modeshifting to be performed by a combination tool that is rotaryactuated and remotely shifted by water pressure changes. The shiftto automation in the early 2000s has resulted in a higher level ofsafety for operators and shorter cycle times. Also advances inpump rotordynamics, metallurgy, and drive technology have beenimplemented to the decoking pump with the ultimate goal ofincreasing pump performance and decreasing the downtime of thiscritical piece of equipment.

The high pressure water jet decoking pump is at the heart ofthe hydraulic decoking system. This is a barrel type multistagecentrifugal pump designed according to API 610. This pumpprovides the necessary pressure and flows at the cutting nozzles toremove the coke so its efficient operation and selection are essentialto the reliability of the hydraulic decoking system. The decokingprocedure starts with drilling or boring a pilot hole gradually from

17

IMPROVING THE RELIABILITY OF A DECOKINGJET PUMP TRAIN WITH A HYDRODYNAMIC VARIABLE SPEED DRIVE

byReinaldo BermudezSenior Sales Engineer

Voith Turbo

Houston, Texas

the top to the bottom of a drum. After the hole is completed the toolis switched to cutting mode and various passes are made up anddown the drum to cut the coke horizontally. Cutting time will varywith the hardness of the coke, the condition of the hydraulic cuttingnozzles, and the available cutting water pressure (Lieberman,1983). One important characteristic about coking processes andespecially hydraulic decoking is that the high pressure water pumponly needs to operate a portion of the total cycle time.

The chart in Figure 1 shows a typical 16 hour coking cycle for asix-drum coker. It can be seen that for the 32 hour total cycle thepump will cut for approximately 18 hours in three hour periods.Assuming good conditions for the jet pump, the cutting tools and acoke that is neither too soft nor too hard, the pump will idle forapproximately 14 hours of the total 32 hour cycle time. Even forshorter 12 hour coking cycles (in a six-drum coker) wheredecoking periods are approximately two hours, the idle time will be12 hours of the total 24 hour cycle time. This significant idle timeis primarily the reason why delayed coker operators with electricmotor driven decoking pumps have tended to favor turning off themotors during idle periods rather than keeping the motor runningand throttling back capacity. This method is certainly in line withenergy consumption and emission reduction initiatives that arebeing implemented by many refineries today. However, this off andon operation has brought about other challenges related to thedesign of motors that can handle multiple starts and electricalsystems that can minimize inrush current during across the linestart ups. Because of their variable speed capability, variablefrequency drives (VFD) and steam turbines have certainly beenconsidered as methods to drive the pump that could reduce powerconsumption at periods of lower or no demand but they areusually not the preferred options. In the case of VFDs, there is alack of experience in the particular application and they requiresometimes up to 40 percent higher capital cost investment than theconventional fixed speed systems, deterring end users fromselecting this option. Steam turbines as mechanical drives for waterjet pumps are more commonly used but their implementationdepends on the steam balance and high pressure steam availabilityof the particular refinery. Although steam turbines are proven to bevery reliable as mechanical drivers and can provide seamless speedvariation to the jet pump to adapt to their intermittent operation,they are less efficient than electric motor drives in transmittingpower. Also, their utilization usually results in higher emissionsand higher capital costs (and cost of ownership) than fixed speedelectric motor systems. Therefore, the use of steam turbines asdrivers for jet pumps depends on the careful economic evaluationof engineers in charge of designing the coker, high pressure steamavailability, and the overall refinery’s processing philosophy.Nevertheless, with regards to drivers for these pumps, the currenttrend points to the selection of electric motor driven jet pumps onalmost all recent hydraulic decoking projects.

Figure 1. Six Coke Drum Cycle—Sixteen Hours Coking Cycle.(Courtesy Foster Wheeler USA Corporation)

It could be argued that most decoking pumps do not require abroad range of operating points that would make them suitable forthe utilization of adjustable speed drives (ASD) to generatesavings in energy consumption. Instead, these pumps usuallyrequire one operating speed or set pressure to cut the coke.Additionally, since most operators shut off the pump duringnoncutting cycles rather than throttling and recirculating the waterto the source tank, the benefit of implementing an ASD, electricalor hydrodynamic, may not be quite evident. Simply put, the powersavings that could be realized by operating the pump at minimumidle speed instead of throttling when the cutting is not requiredwill not be greater than the power consumption savings achievedby turning off the motor.

However, the application of adjustable speed drives in decokingpumps deserves another look. Particularly hydrodynamic variablespeed drives should be examined closely as there are a number ofbenefits they offer to the operation of a decoking pump. Forexample, during noncutting periods an HVSD allows the operatorto seamlessly reduce speed of pump to minimum idle speed andrecirculate water at a lower pressure to the source tank thuseliminating the need for periodic motor shutdowns. Although atreduced speed the power consumption is more than when the motoris off, this consumption is deemed insignificant by the end userbecause the resulting benefits in the long run contribute to a morereliable system and the extension of the maintenance cycle on theequipment. Since the pump and motor are no longer subject torepeated starts, the destructive forces that are usually presentduring across the line starts, known to contribute to bearing wear,water hammer on piping and pump, rotor fatigue and train stresses,can be significantly reduced. Also, since there is no need to startthe motor multiple times during the day and the fact that an HVSDallows for a reduced load startup, a complex soft starter may nolonger be required.

This paper first examines the conventional fixed speedelectric motor-driven pump and presents the special considerationsthat operators have to make to operate the motor-pump trainintermittently. Afterwards, the function of hydrodynamic variablespeed drives is explained in detail. Variable speed couplings (VSC)and variable speed geared couplings (VSGC) are described withfocus on the selection criteria used for the application of water jetdecoking pumps. Supporting field experiences for two installationsare examined with the goal of offering evidence that hydrodynamicvariable speed drives can add flexibility and reliability to thehydraulic decoking system.

BACKGROUND—CONVENTIONALOPERATION OF WATER JET PUMPDRIVEN BY ELECTRIC MOTOR

Although steam turbines have been used as mechanical drivers forwater jet decoking pumps, especially for high speed requirements,most commonly these pumps are driven by electric motors. Inorder to understand the benefits that an HVSD offers to an electricmotor driven coke cutting pump, the operation of the pump underfixed speed configuration should be studied first. Figure 2 shows asimple schematic of a hydraulic decoking system. One can see thedecoking pump trains with motor, gear and pump arrangement,lube oil console, coke drums, decoking control valve and watertank. Depending on the size and number of coke drums, targetedcoking cycle times and required cutting pressures, a decokingpump and appropriate driver are selected. The water pressurerequired typically ranges from 2500 psi for smaller systems to5000 psi for larger ones. Cokers processing large amounts of heavycrude usually require high cutting pressures as the size of the cokedrums become very large in diameter. The pump trains for thesesystems typically incorporate a four-pole motor and step upparallel shaft gear to achieve the high pump speeds required toproduce the necessary pressures at the cutting nozzles. Smallercokers typically have smaller diameter coke drums therefore the

PROCEEDINGS OF THE TWENTY-SIXTH INTERNATIONAL PUMP USERS SYMPOSIUM • 201018

pressures required are less. For these systems two-pole motors arenormally used to direct-drive the pumps as their design speed doesnot exceed that of the motor.

Figure 2. Hydraulic Decoking System.

As stated earlier, due to the intermittent requirement of cutting,most operators make the decision to shut the motor off duringnoncutting cycles to save energy that would be consumed in theoperation of the pump under discharge throttle. Depending on thecoking cycle, number of drums, size of drums and system design,the pump may operate as often as four times per day for two tothree hour periods. Because of this off and on configuration,special considerations have to be made in the selection of drivers.Coker duty motors, which are typically selected, are designed tomeet special starting and load characteristics present during acrossthe line starting. Motor data indicates that coker duty motors aredesigned to consider heat storage and dissipation from rotorelements, heat generation rates, distribution duty cycle analysisand fatigue stress analysis of the rotor elements. The same data alsostates that attention must be given to the heat generation rates andtemperature distribution within the rotor elements, particularly as aresult of the repeated starts and stops associated with this application.This analysis is necessary to determine the allowable locked rotorand stall time limits (Reliance, 1981). In some cases even specialmotor design considerations to handle multiple across the linestarts are not sufficient to prevent motor failures. Therefore,some refineries also implement a form of reduced voltage starter tolimit inrush current such as an electronic controlled starter orconventional autotransformer. For example, an electronic controlledstarter can prevent the creation of mechanical stresses andexcessive heating within the motor by controlling the accelerationand deceleration ramp time and current levels. Gray, et al. (1997),wrote a short paper about the application of an electronic starter fora delayed coker process water jet decoking pump. Described in itis a job site where an electronic starter was installed to addressmany motor and pump reliability problems due to severe startingconditions related to the large number of across the line starts. Thisparticular installation had as many as 1400 motor starts per yearand as many as four starts per day. As a result of this intermittentoperation, engineers concluded that the numerous motor problems,which resulted in maintenance costs, equipment costs andoperational upsets, provided them with an economic incentive toimplement alternative means for controlling the motor.

The main issue with coker duty motors and soft starters is thatalthough they can effectively alleviate motor stresses and protectintegrity of the pump train, they do not completely eliminate allthe problems associated with the periodic motor starts. Thesecomponents are also expensive and the complexity they add to thesystem may result in a less reliable decoking pump train. Someoperators also believe that when a decoking pump is idle for a longperiod of time the fine coke particles present in the water have

enough time to settle and become entrained in the pump, valves,and piping, and this is believed to contribute to the acceleratedwear of these components over time. When liquids containingsolids with a high settling rate are pumped, excessive solidsaccumulation can occur in the pump, causing wear. Therefore, it isparamount that when the speed of the pump is reduced the liquidvelocity be maintained high enough in the pump and in thepumping system to avoid settling out of the solids (HydraulicInstitute, et al., 2004).

What if operators could find a way to keep the motor runningall the time, keep the water flowing and simultaneously reducethrottling duties and power consumption during noncuttingcycles? One way to accomplish this is to implement a variablefrequency drive that controls the rotational speed of an AC motorby controlling the frequency of the electrical power supply to it.During noncutting cycles the VFD could reduce speed of thepump to reduce system water pressure and the decoking controlvalve could be put on bypass to recirculate water to the sourcetank. The VFD would keep the motor running continuously andminimize motor, pump and piping stresses associated withmultiple motor starts. However, for the typical power rating of adecoking pump and the fact that the pump only operates at a setmaximum and minimum speed, the investment for a VFD wouldbe hard to justify. These drives are efficient but they are alsoexpensive. VFDs require significant footprint due to the largenumber of components needed such as a frequency converter, arectifier, a VFD motor, air-conditioning system, fire preventionsystem, and in some cases harmonic filters and isolationtransformers. Additionally, the pump train with a VFD will stillrequire a lube system, and for high speed applications, a speedincreasing gear. Many experts tend to estimate that the lifetimeof power electronic equipment such as a VFD is rather shortsince it may suffer from chemical aging of semiconductorelements and is susceptible to damage by electric shocks(Janknecht, 2008). Because of limited lifetime, number ofcomponents, hazardous and dusty environments, high capitalcost investment, and space required a VFD could be consideredimpractical for this application.

Another solution that is compact, economic and simple is toinstall a variable speed coupling between a standard induction orsynchronous motor and the jet pump. Like a VFD, the VSC allowsoperators to keep the motor running and reduce speed of pump tominimum idle speed during noncutting cycles. The followingsection describes the function of hydrodynamic variable speeddrives, variable speed turbo couplings and variable speed gearedcouplings and their application to decoking pumps.

VARIABLE SPEED OPERATIONIN DECOKING PUMPS

Function

A variable speed coupling is a hydrodynamic transmissiondesigned according to the Fottinger converter to provide seamlessspeed control to a driven machine (Figure 3). It consists of a fluidcircuit that is filled with an operating fluid, typically turbine oil, apump impeller (primary bladed wheel) connected to a driver and aturbine (secondary bladed wheel) connected to the driven machine,all in a closed working chamber where the oil circulates in verysmall space inside a toroidal housing. The small space permits adirect exchange of energy between pump and turbine and reducestransformation and flow losses that would occur in the individualoperation of these components. In a variable speed coupling themechanical energy introduced by the motor is transformed intokinetic energy in the pump and then converted back to mechanicalenergy in the turbine. Since the energy carrier of a variable speedcoupling is oil or fluid, power and torque can be transmittedwithout wear. Figure 4 (A and B) shows the internal components ofa hydrodynamic variable speed coupling.

19IMPROVING THE RELIABILITY OF A DECOKINGJET PUMP TRAIN WITH A HYDRODYNAMIC VARIABLE SPEED DRIVE

Figure 3. Schematic Representation of Fottinger Principle.

Figure 4. Internal Components of a Hydrodynamic VariableSpeed Coupling.

It should be noted that a variable speed turbo coupling does notconvert or multiply torque like a hydrodynamic torque converter,instead it permits input and output shafts to rotate at different speedsdue to slip. The transmittable torque depends on the deflection offluid flow in the turbine wheel and thus on the speed ratio v. Slip isexpressed in percent in accordance to Equation (1). Torque can onlybe transmitted with a difference in speed between pump impellerand turbine wheel, although this difference might be a small one.

where:np = Input or pump impeller speednt = Output or turbine speedv = nt/np = Speed ratios = Slip

Figure 5 shows a simplified longitudinal section of a variablespeed turbo coupling with standard components. Essentially, thetorque and speed a coupling can transmit can be changed byvarying the degree of filling in the working chamber. A scoop tubeactuated by a 4 to 20 mA signal is used to vary the fill level ofturbo couplings and in most cases its operation is integrated into aprocess control circuit of the driven machine. The position of scooptube ranges from 0 percent (close to zero output speed) to 100percent of travel (maximum output speed.) The scoop tube isespecially designed to remove more oil from the working chamberthan can be injected by the shaft driven circulation pump so itsposition controls the amount of oil flow that is transferred fromprimary or pump wheel to turbine wheel. Therefore, the higher thequantity of oil in the chamber, the higher the speeds and torquetransmitted to turbine wheel and to output shaft. A working ortransmission oil cooler is also needed because in the work of powertransmission between pump wheel and turbine wheel there arefriction losses that result in the heating of the oil. The scoop tuberemoves the working oil from the shell, drains into a sump and thisoil is transferred by the shaft driven circulation pump into a heatexchanger or cooler to return the oil to its operating temperature.Larger hydrodynamic turbo couplings may require separate oilcircuits to handle higher power ratings and oil flows. The fill levelof the operating fluid can be varied during operation between fulland drained, thus enabling exact dynamic speed control of thedriven machine across a wide range when the coupling operatesagainst different load characteristics. This operating range dependson the load characteristics (torque relative to speed) and the controlaccuracy required.

Figure 5. Simplified Longitudinal Section of a Variable SpeedTurbo Coupling.

Selection Criteria for Water Jet Decoking Pumps

Depending on the pump and motor design speeds, a variablespeed turbo coupling or variable speed geared coupling, as seen inFigure 6, can be chosen. To select the appropriate frame size for thehydrodynamic variable speed drive that can handle the power andspeed to be transmitted to the decoking pump, the pump powerconsumption and operating speeds at design point (maximum) andidle point (minimum) are needed. If the maximum operating speedof the pump is lower than the motor speed then a VSC can be usedto control speed from 100 percent down to about 20 percent andpeak efficiency can be up to 97 percent. If the water jet pump speedis higher than motor speed then a VSGC, with up to 96 percentpeak efficiency, is selected and this machine incorporates a parallelshaft gear arrangement, designed according to API 613, AGMA orDIN standards. Both types of variable speed couplings incorporatea shaft driven hydraulic oil pump to transmit the hot working oilfrom the oil sump through an oil cooler and back into the fluidcoupling chamber. Both machines can also incorporate as anoption an integrated lube oil system that consists of a shaft-drivenpump, auxiliary oil pump for pre and post lubrication duties, and alarger reservoir. The shaft driven and electric motor drivenauxiliary pumps are designed to provide lubrication to the motor,


fluid drive, jet pump bearings and gears (if applicable), thusthey eliminate the need for an external lube oil console. Both theshaft-driven pump and auxiliary oil pumps for this integrated lubesystem are of positive displacement design.

Figure 6. Variable Speed Coupling and Variable Speed GearedCoupling.

Even though an HVSD can provide seamless speed control fromthe maximum pump design speed percent down to about 20 percentof this speed, for a decoking pump application the maximum(cutting) and minimum (idle) speed would typically be the onlyoperating points. Because of their design, variable speed couplingsoffer a number of advantages to the operation of the water jet pumpwhen compared to the conventional fixed speed operation. Forexample, since driving and driven machines are separated when thecoupling is drained, a load free start up can be realized. Eventhough the motor is usually kept running in jet pumps with ahydrodynamic variable speed drive, having a no load start up is stillbeneficial in the event the operator decides to shut down the motorperiodically, like it will be seen in the case 1 installation describedbelow. Other criteria that make HVSD suitable for decoking pumpapplications are:

• Reduced power and elimination of high wear throttle device.

• High control accuracy and fast reaction times.

• Wear free transmission of power through hydrodynamic energyof a fluid.

• Easy to operate, low maintenance components and easymaintenance.

• Seven to eight years time recommended between scheduledoverhauls.

• Robust design with long service life and high availability.

For large systems requiring water jet pumps with step up gears aVSGC with integrated lube oil system could actually result in alower capital cost investment than a coker duty motor, soft starter,step up gear, and lube oil console

• Provide damping of torsional vibration and shock loads

• Possibility to use VSC integrated oil system to lubricate motorand pump bearings

• Suitable for the demanding environmental conditions of adelayed coker

Although VFDs are known to be more efficient than HVSDs,especially at lower speeds, HVSDs do offer some advantages overVFDs with regards to price, space savings, durability, and reliability.For example, for high speed jet pumps for large crude capacitysystems, a variable speed geared coupling can result in a lowercapital investment when compared to a VFD system with lube oilconsole and step up gear. This difference in price can be as large as40 percent. A VSGC integrates speed control, lube oil console andspeed increasing gear in one housing so it results in a morecompact and often more economical solution. Also, as statedbefore, due to the aging of components and sensitivity to heat,electric shocks, harmonics and environmental conditions, a VFDpractical lifetime can be between 12 to 15 years. In contrast, anHVSD, as long as preservation methods and maintenance cyclesare followed, can last for decades. For example, there are manycases of hydrodynamic variable speed drives that have been inoperation since the early 1960s and are still running today. Withregards to reliability, the mean time between failure (MTBF) of ahigh power rating VFD is on the average much shorter thanHVSDs. The published MTBF of one of the main manufacturers ofhydrodynamic variable speed drives is more than 200,000 hours.The fact that a VFD requires a greater number of accessories thanan HVSD and that each of its components has a reliability andefficiency factor associated with it may explain why the reliabilityover efficiency ratio of the hydrodynamic solution is higher thanthe VFD solution. If loss of production and capital costs areconsidered, HVSDs usually end up being more economical to theirowners over the long run.

APPLICATION OF HYDRODYNAMICVARIABLE SPEED DRIVES INWATER JET DECOKING PUMPS

Hydrodynamic variable speed drives have been successfullyemployed to drive decoking pumps in recent years. Describedbelow are two case installations that use VSDs at two delayedcokers with two different operating philosophies and equipmentconfigurations. Selection criteria and performance data for a VSCand a VSGC are examined.

Case 1—Variable Speed Turbo Coupling

In 1985 a variable speed coupling with a wheel profile diametermeasuring 562 mm was installed to drive a water jet pump at a twodrum, 6600 barrel per day, delayed coker in a refinery in theMidwestern United States. There is not much information availablewith regards to the decision making behind the selection of thehydrodynamic variable speed drive but it can be assumed that itwas chosen to add flexibility to the hydraulic decoking operation,especially during manual cutting tool switching. The VSC wassupplied with an integrated lube oil system consisting of a shaftdriven oil pump, oil sump, piping, and instrumentation to handlelubrication of the motor, VSC, and pump bearings. At this coker,the water jet pump runs for six hours a day and it is shutdown for18 hours of the day to save on power. The hydraulic decokingsystem does not incorporate an automated cutting mode switching(combination) tool therefore the operators manually switch fromboring to cutting and during this period they slow down the pump


•

and divert water through a recirculation valve back to the cuttingwater tank. Due to the small size of this coker and the lower cuttingpressures required a speed increaser was not necessary so a two-poleinduction motor was chosen together with a hydrodynamic variablespeed coupling. The VSC controls speed from 3580 rpm down toabout 30 percent of this speed. For approximately four hours a daythe pump operates at maximum speed during drum cuts. Forabout two hours a day the pump operates at minimum speedduring manual cutting tool switching. For the remainder of time,approximately 18 hours, the operators shut down the motor to saveon power consumption. Table 1 below indicates some generalinformation obtained from this installation.

Table 1.

Service reports and historical data on the decoking pump trainindicate that the hydrodynamic variable speed drive has beenoperating very reliably over the last 24 years. The VSC wascommissioned in 1985. In the year 2000 this coupling wasinspected for the first time. During this first overhaul, 15 yearsafter start up, the rotating element was removed and dismantledand the unit was found to be in an “as new” condition. Parts werecleaned and bearing clearances were found to be within tolerances.It should be noted that the VSC was not operated continuously forover 15 years. Instead, the pump train was operated intermittentlyso it is estimated that the VSC had about 33000 hours of operationand 40 percent of the time was operated at minimum idle speed.This may explain the good condition of the internals of the VSC;however, it is still remarkable to find a machine in such goodcondition after it was subjected to so many startups. Since the year2000, the VSC continued to operate without problems. In 2005 theoperators reported that the VSC was overheating during pumpoperation. When the service technician arrived on site he found themechanism for the control valve actuator was grossly out ofadjustment. This restricted the amount of oil going to the oil cooler.The service technician proceeded to loosen the clamping screwfrom the cam and adjusted the position on the shaft. No one at sitehad any idea how the cam came to this new position. No damagedparts were found. Further inspection found the fusible plugs wereblown in the unit. The fusible plugs are installed on the primarywheel of these drives to protect the machine components and trainin the event of excessively high oil temperatures. The plugs meltand drain the oil into the reservoir of the VSC. These plugs werereplaced and the unit has operated continuously till present day.

At the time this installation was designed, the automatedcombination cutting tool was still not available in the market. Thiscould explain the chosen operating philosophy for this hydraulicdecoking system. The VSC is mainly used to aid in the waterpressure reduction during manual switching of cutting tools and

also during motor start ups. Because of the unloaded start capabilityand lower inertia requirements that the VSC provides, the motorcan be started across the line without any inrush current problemsand the pump can be smoothly accelerated. The operators believethat not only the VSC is a reliable piece of equipment but that it hascontributed to the reliability of the train components by allowingfor unloaded starts of motor, minimizing axial loads on bearings,and permitting the operation of the pump at minimum speed,which minimize the negative effect of high speeds on lube oiltemperatures. Having a VSC also contributed to a faster switchingtime and cutting time and increased safety of the operators. By notturning off the motor during switching, the pump could be broughtback to cutting mode speed much quicker than if it had to be startedfrom the off position. Also, the water pressure of the system can bereduced to levels that are much safer for the operators doing theswitching in the event of a failure of the safety mechanisms tobypass water to the source tank. This VSC also incorporates anintegrated lube oil system so the external lube oil console iseliminated thus making this system more compact and with fewernumber of components subject to maintenance and reliabilityconsiderations. For this installation, having a VSC also permittedthe operators to eliminate a decoking control valve because waterpressure changes are controlled by the drive with speed regulation.This is believed to have added reliability to the system because adecoking control valve typically requires significant maintenancedue to potential water quality issues that affect its operation and thewear and tear caused by throttling duties due to high pressuredifferentials across the valve.

Case 2—Variable Speed Geared Coupling

On August 19, 2002, a variable speed geared coupling was putin operation to drive a 4500 hp water jet pump at a four drum,75,000 barrel per day, delayed coker in a refinery in Mexico.Fixed speed, VFD and HVSD systems were considered in theevaluation of pump drivers. The engineers at the refinery hadexpressed concern with a grid capability that could handlemultiple starts and the power factor of the existing electricaldistribution system, therefore a fixed speed conventionalconfiguration was quickly eliminated as an option. The engineerseventually selected the hydrodynamic solution because itaddressed the grid limitations, improved power factor, and alsoprovided reduced footprint and a lower capital cost investmentthan a VFD. The VSGC was supplied with an integrated lubesystem designed to supply lubrication to motor and pumpbearings, therefore, an external lube oil console was alsoeliminated. This feature contributed to the further reduction offootprint and provided additional capital cost savings. During theselection of the hydrodynamic drive there was one concern aboutits efficiency at part load. The peak efficiency of a VSGC can beup to 96 percent so for the design operating point at the maximumpower consumption this efficiency was acceptable.However, at part load or minimum idle speed, the efficiency ofthe VSGC power transmission drops significantly. Since thehorsepower of the pump varies according to the cube of the speed,based on Affinity Laws, it was determined that at the lower speedrange although the efficiency of the power transmission waslower, the power consumed was also much lower (<300 hp) so thelosses through the drive in transmitting this power were onlyabout 500 hp or 10 percent of the rated bhp power consumptionof pump. The VSGC also incorporated a disk brake on its highspeed output shaft to allow the pump to be slowed down to acomplete halt. Since this installation was originally suppliedwithout an automated cutting tool system, the idea was to use thebrake to reduce pressure to zero during manual cutting toolswitching operations without having to turn off the motor. A diskbrake, which was mounted at the flange of the coupling hub,incorporated an actuator, disk, shoe, and console. Its installationresulted in about 10 percent additional reduction of power


consumption because at 0 rpm, only windage of the couplingneeds to be considered to calculate power at the motor shaft. In theend the EPC and end user felt that losses at minimum idle speed(no load condition) were sufficiently low and not significant overthe life of the equipment when compared to the added capitalcosts of a soft starter, coker duty motor, external lube oil consoleand the potential reliability issues related to the intermittentoperation of the motor and pump.

Figure 7. Water Jet Pump Train with Variable Speed Geared Coupling.

It is important to mention that the hydraulic decoking systemat this refinery was upgraded two to three years after its firstcommissioning to include an automated switching tool. As a resultof this upgrade the brake was no longer used for the tooling change.The decoking control valve was instead used to drop systempressure to allow for the change to automatically occur and permitrecirculation to the water tank until cutting was resumed. However,the brake is still used during idling operations (noncutting periods)and it is also part of the logic as a permissive to start the waterjet pump.

Table 2 shows the operation of the variable speed gearedcoupling at this installation based on available service andmaintenance data. The plant operators feel that the HVSD hascontributed to the reliability of the train because this site has notexperienced any problems related to the motor or any start up andinrush current issues existing at other sister refineries. Due to thefact that a synchronous motor was selected to drive the pump,this provided for a power factor correction when the water jetpump motor was not connected to a load. Also, during these idleperiods this motor could supply reactive power required byinduction motors in the refinery. One problem reported on theVSGC actuator was addressed during the noncutting cycle of thepump resulting in no down time for the system. At press time,operators were trying to find out causes for slightly higher levelsof lube temperature and vibration on the VSGC. Although theselevels are not considered serious and are within the allowablelimits, they may be an indication that lube oil coolers may requireinspection and cleaning and that after almost 10 years fromshipment and eight years of continuous operation the bearingsmay need to be inspected. The operators at the plant indicatedthat the pump and decoking valve problems were associated withwater quality and issues with changes in the process. There wasa report of change in the feedstock quality and quantity to thisrefinery that it is believed to have affected the operation of thesystem and some of the reported problems. At the time this plant

was visited by the author the pump was operating at about 85percent of design speed. Having HVSD allowed the plant tooperate more efficiently at part load than if throttling would havebeen implemented. The possibility of changing the logic andoperating philosophy was discussed with operators with the aimof extending the life of the decoking control valve by reducingthe pressure differential during mode switching. Becausethere were reported problems with the decoking control valve butvery little data available at the refinery on the cause of thoseproblems, the thinking of operators is that by reducing the speedof the pump to reduce the system’s water pressure immediatelybefore switching from boring to cutting could result in a lowerpressure drop across the valve. The expectation is that this wouldalleviate the work performed by the valve and as a result extendsits life.

Table 2.

APPROACHES AND FUTURE CONSIDERATIONS

These two cases showed two different operating configurationsand philosophies. The smaller plant with the VSC turns off motorduring noncutting periods and only uses the fluid drive to reducethe speed of the pump (to reduce nozzle pressure) during manualcutting tool mode switching operations and also as a soft starter.The smaller plant also does not incorporate a decoking controlvalve. The larger plant incorporates a decoking control valve anduses the VSGC to reduce speed of pump during noncutting periods(idle time) and for startup purposes. A new delayed coker project,currently in construction, which is larger in size than the onedescribed in case 2, is incorporating a VSGC coupling to drive thecoke cutting pump. The drive will be used to reduce speed of a5700 hp pump during noncutting cycles (Figure 8). The argumentto select it was to avoid intermittent operation of a 6000 hpmotor-pump train and the reliability issues related to turning offand on a motor of this size in a periodic fashion. Engineersinvolved with the project felt that keeping the motor and pump inhot running conditions all the time was more favorable because, forsuch a large system, subjecting the pump and motor to periodicstarts could contribute to accelerated wear of components andcreate a potential for particle settling if the pump speed wasbrought to zero. For this reason a brake was not included with thissystem. Additionally, engineers indicated that other alternativeswere evaluated such as coker duty motors, soft starters, and VFDsbut these represented a larger capital cost investment than thechosen HVSD and standard motor configuration.


Figure 8. 5700 HP Variable Speed Geared Coupling at Test Stand.

The fact that hydrodynamic variable speed drives were utilizedfor different reasons at these plants can provide proof that thesedrives give the operators more flexibility to operate the hydraulicdecoking equipment in many different ways. The HVSD can beadapted according to the needs of the operators depending on theconfiguration of the system, coking cycle times, and size of waterjet decoking pump. Since new hydraulic systems will most likelyincorporate automated combination cutting tools and the trend is tobuild large capacity systems that require high pressures pumps withhigh power consumption, the next step in the application of HVSDto water jet decoking pumps is to optimize their implementation.One area that deserves more investigation is the implementation ofsynchronous motors in combination with these drives to improvethe power factor of the whole distribution system at a refinery.Since these motors are at times connected to no loads (especiallywhen brakes are used) they could be used for power factorcorrection. Also, more thought needs to be given to using thesedrives to extend the life of the decoking control valve by reducingspeed of the pump during tool switching thus reducing pressuredifferential across the valve or possibly altogether eliminating thedecoking control valve as it was shown in case 1. The drive couldbe used to control water pressure reduction required for toolswitching and a simpler bypass valve used to recirculate to thecutting water tank.

One subject that has not been mentioned but that also deservesfuture investigation is the use of HVSD to seamlessly adjust cuttingpressures to prevent coke pulverization. An HVSD can controlspeed of a pump with 1/10 percent degree of accuracy so pressurescould be adjusted precisely according to the type of coke beingproduced. One condition that operators like to avoid is pulverizingcoke because of the problems this can create with regards tomaintenance and coke handling. Delayed cokers with varying typesof feedstock could benefit from having an HVSD because it allowsthe pump to more efficiently adapt to cutting requirements andthe plant to have more operating flexibility to handle differentconditions. Plant operators and engineers designing these plantswould have to devise a procedure to detect the grade and softnessof coke being produced to automatically set the ideal cuttingpressure to produce a product that is of optimum quality.

HVSDs are typically chosen for reliability and flexibilityadvantages they offer operators of hydraulic decoking systems.However, regardless of the manner in which an operator decides torun the jet pump, speed regulation, as a general rule, can also bebeneficial to the performance and service life of the motor andpump. It is a well-known fact that high speeds can result inpotential pump seal wear because of excessive heat generation

between seal faces through rubbing contact. So the ability tolower the speed of a pump can extend the life of the pump and itscomponents. Operating at high speeds can also result in excessiveheating of the lube oil that can reduce its viscosity over timeand lower the lubrication of the pump bearings. Also, as it wasdemonstrated in case 2, in an unpredictable situation where thecapacity of the pump needed to be reduced, variable speedoperation offered the job site power savings by not having tothrottle this capacity with a decoking control valve.

CONCLUSION

Although electric motor-driven water jet coke cutting pumps arenot commonly known to require or implement speed regulation, theexamples given show that for large systems hydrodynamic variablespeed drives can provide a reliable alternative to the operation ofthese pumps under intermittent off and on configuration. Forexample, in case 2, it could be seen that although some of the jetpump and decoking valve problems could be attributed to waterquality and process changes at this refinery, the motor and VSGCexhibited no downtime. For the smaller system under intermittentoperation described in case 1, a hydrodynamic variable speed drivecan also be adapted to help with unloaded start-ups and waterpressure control. For neither case 1 nor case 2, the refineries wereable to provide respective jet pump bearing life or pump MTBFdata to see if the variable speed drives directly contributed toimprovement in those areas. However, operators believe that byavoiding repeated axial loading on pump bearings and waterhammer conditions that happen during start ups, accelerated wearof these components is being prevented with these drives. AnHVSD can help to eliminate the reliability issues associated withoperating these pumps intermittently and also provide the operatorwith the already enumerated functional advantages like the flexibilityto efficiently operate the pump at different speeds according to thesystem’s pressure requirements.

The capacity and type of coker, the number of drums, size ofdrum, and coking cycle times will certainly influence the overalldesign of the hydraulic decoking systems and the selection of awater jet pump train. It is recommended that the designer of sucha system takes into consideration the jet pump horsepowerrequirements, cutting pressures, and idle times, to determine if anHVSD can be technically and financially justified. A life cycle costanalysis that considers such factors as initial equipment cost,energy cost, operating cost, downtime, maintenance costs, andinstallation and commissioning time, can be used to compare thevarious drive alternatives to determine the best solution for theparticular system.

Finally, more investigation is needed to look at the implementationof an HVSD to extend the life of the decoking control valve orpossibly substitute it for a simpler bypass valve. The nozzlepressure changes can be controlled by varying the speed of thepump so cutting pressures can be adjusted to adapt to the type ofcoke being cut to prevent pulverization.

NOMENCLATURE

ASD = Adjustable speed driveHVSD = Hydrodynamic variable speed driveMTBF = Mean time between failureVFD = Variable frequency driveVSC = Variable speed turbo couplingVSGC = Geared variable speed turbo coupling

REFERENCES

Elliott, J. D., 2004, “Delayed Coker Revamps: Realization ofObjectives AM 04-69,” Foster Wheeler USA Corporation, p. 27.

Gray, P., Bagley, A., and Zacharski, J., 1997, “Application of anElectronic Starter for a Delayed Coker Process Water JetCutting Pump,” IEEE Paper No. PCIC 97-30, p. 281.


Hydraulic Institute, Europump, and the U.S. Department of Energy’sIndustrial Technology Program, 2004, “Variable SpeedPumping—A Guide to Successful Applications-ExecutiveSummary,” p. 7.

Janknecht, P., 2008, “Energy Efficient Pump Systems: FrequencyConverters and Hydrodynamic Couplings as Pump Drives,”Pump Users Forum 2008, Session 14-1, p. 508.

Lieberman, N. P., 1983, “Time for Coking Cycle Can Be RoutinelyHalved,” Oil and Gas Journal, August 29, p. 42.

Reliance Electric A-C Motor News Bulletin SPO-7, 1981, “RotorDesign for Decoker Jet Pump Application,” January 14, p. 1.

BIBLIOGRAPHY

Bloch, H. P. and Soares, C., 1998, Process Plant Machinery.

Voith, J. M., 1988, Hydrodynamics in Power Transmission Engineering.

ACKNOWLEDGEMENT

The author would like to thank the following people for theircontribution to this paper: Kerry Gunn, Daniel Quintana, AldrichRichter, Carlos Chacin, Mike Arena, and Victor Hernandez Brandy.


improving the reliability of a decoking jet … · improving the reliability of a decoking jet pump...

Documents