Gas Turbine Compressor System Design Using Dynamic Process Simulation

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Compressor ESD dynamic simulation


<ul><li><p>Proceedings of GT2007ASME Turbo Expo 2007: Power for Land, Sea and Air</p><p>May 14-17, 2007, Montreal, Canada</p><p>ig</p><p>The system modeling may start with some basic simpleassumptiobecome kmodel andstartup annot readilflow diagwas the sseveral ac</p><p>NOMENCASV Cv </p><p>pressures are usually under 3000 PSIG.</p><p>Downloaded Fromns. This initial model can then grow, as more detailsnown. Additional components may be added to the evaluated as an integral part of the system. Issues of</p><p>d shut down may point to system changes that wouldy be understood from the normal steady state processrams. The DYFLO program (1) by QMC Corporationimulation software package used in the evaluation oftual case studies that this paper is based on.</p><p>LATURE Anti-surge valve Valve flow capacity</p><p> Processing plant applications cover a broad spectrum thatcould include NGL removal, LNG facilities, and refinery&amp; hydrocarbon processing plants. The plant transientsinclude variations in gas composition along withupstream/downstream processing upsets.</p><p>Requirements for a dynamic studyAs mentioned above, the dynamic simulation requires</p><p>some components to be specified or approximated. Thecompressor flow map is needed for estimating the performance.The volumes at the inlet and the discharge of the compressorare critical to evaluate the dynamic response of the system. It isimportant to note that the volumes determine the mass</p><p>Copyright 2007 by ASME1 Gas Turbine Compressor System Des</p><p>Norman A. Samurin, PEDresser-Rand Company</p><p></p><p>ABSTRACTAlmost all petrochemical processes are modeled as steady stateto establish the required sizes of pipe, vessels, compressors,pumps, and valves. A dynamic simulation study of the processcompressor system design can now be done as part of the initialplant design activity. The system piping design and properplacement of the anti-surge valves (ASV) along withpreliminary valve sizing can be evaluated. Additional controlstrategies may be evaluated for emergency shut down (ESD).This will confirm the system design to safely shutdown acompressor without surge. Many of the issues that may beuncovered during study execution can save time and expensebefore the initial commissioning of the process. For a plantstart up, the driver capability, compressor loading, positions ofvalves, and control philosophy may be tested before thecomponents are committed to hardware.</p><p>INTRODUCTIONNew advances in computer computation speed and</p><p>improvements in simulation programs now make it possible toprovide simulation modeling as part of the initial plant/processdesign. The requirements to accurately model the majorcomponents of the simulation are discussed in this paper. Thedynamic analysis of the process can be done up front becausethe programs have become less complex to set up and run. Skilland knowledge are still required to properly create a simulationmodel, interpret the data and recognize valid results. The leadtimes to do such an analysis have been dramatically reduced.: on 11/13/2015 TerGT2007-28270n Using Dynamic Process Simulation</p><p>George. C. TalabiscoDresser-Rand Company</p><p></p><p>ESD Emergency shut downH HeadHAZ-Op Hazardous Operation ReviewLNG Liquefied natural gasN Rotating speedNGL Natural gas liquidsOverload High flow region beyond the compressor mapQ Volume flowSurge A point where the flow within the compressor</p><p> becomes unstable.</p><p>Gas Turbine Compressor Applications</p><p> Gas pipeline applications usually have relatively constantgas composition with variations in slow changing upstreamand downstream pressure variations caused by the largevolume of gas.</p><p> Gas reinjection applications can have variations of gascomposition with potential for rapid changes in flowbecause of process upsets. There also could be slugs ofliquid that cause flow disturbances in the compressionsystem. They usually are high pressure ratio applicationsthat have discharge gas conditions greater than 5000 PSIG.</p><p> Gas gathering applications collect untreated gas to make itsuitable for transfer to transmission pipelines. This type ofcompression system can have variations in gascomposition and system transients caused by dehydration,filtration and sweetening system upsets. They aresomewhat similar to reinjection applications. Dischargems of Use:</p></li><li><p>contained in the process system. The compressor is used tomove mass from inlet to discharge. The time to move this mass</p><p> Rotating inertia of the gas turbine component mechanicallyconnected to the load (i.e., power wheel)</p><p> Control system response time to a transient condition</p><p>Dowis determined by the volumes and the given flow capability ofthe compressor. The time it takes to fill a volume determinesthe rate of increase in discharge pressure (head build up). Acompressor operating at full speed may operate in overloaduntil the pressure in the discharge volume builds up. Duringstart up, the compressor will accelerate and move gas massfrom inlet to discharge. Thus there can be a significant lag inthe build up of discharge pressure. However, the compressorpower will follow its overload curve for flow and headinternally even though the external system head may be muchlower. The calculations for a system are based on massdistribution, pressure and temperature. The compressor isoperating between two volumes at different pressures. Thevolume head is fixed and at a given speed the compressor flowis the resultant and not the driving element. To effect a changein the flow, the pressure and/or compressor speed must bemodified. Therefore, the head and speed determine the flow fora dynamic simulation.</p><p>Piping Geometry and Equipment LayoutThe location of components in a compressor</p><p>installation is critical to a plant design. This should be reviewedduring the initial plant design to help prevent operationalproblems. Some of the important points to consider are:</p><p> Large volumes located between the compressor dischargeflange and the recycle valve should be avoided orminimized.</p><p> Cooling of the recycle gas is required for operation incontinuous recycle.</p><p> Liquid separation vessels should be located upstream ofcompressors and downstream of cooling elements in acompressor system.</p><p> Separate and independent recycle loops should be providedfor multiple sections of compression connected in series.Placement of check valves and recycle take off and returnlines is critical to achieving separate loops of compression.</p><p> Parallel compressor units must have check valves installedto isolate each unit.</p><p>The volume of the piping, vessels and coolers determinesthe system response to a change. The change may be to movefrom one operating point to another or a system trip of thecompression train. The dynamic simulation determines themass inventory of each component at a given time. As the massinventory changes, the specific component pressure andtemperature will change.</p><p>Turbine ModelingFor a process simulation the gas turbine is simply</p><p>modeled as a power producing component along with itstransient response capability to load change. The elements ofthe gas turbine are not modeled in detail as compressor,combustor and turbine.</p><p>The knowledge of gas turbine operational sequencesalong with other physical characteristics of the gas turbinecompression system are the essential requirements fordeveloping an accurate model for dynamic process simulation.These characteristics include the following:nloaded From: on 11/13/2015 Te2 Copyright 2007 by ASME</p><p>including:1. fuel shutoff and control valve inherent time lag</p><p>and response characteristics2. stored energy of the fuel between the last fuel</p><p>shutoff valve and turbine combustor(s)3. Turbine unloading through variable geometry or</p><p>blowoffThese all combine to define the power decay</p><p>characteristics of the rotating equipment.Some turbine manufacturers have test stand and or field</p><p>data to accurately determine response characteristics. Whenthese are not available, assumptions have to be made whichmay affect accuracy of results. The customary assumption ofinstantaneous loss of power production in response to atransient event can lead to an over design of components withassociated commercial impact. Dynamic simulation cantherefore provide a tool to gauge the validity of the design.</p><p>Intentional time delay of the turbine trip has been utilizedsuccessfully to prevent compressor surge in response to therapid deceleration. See discussion below</p><p>Other gas turbine characteristics that need to beconsidered for upset conditions include maximum loadacceptance and rejection rates. These define the accelerationand deceleration rates for safe and continuous equipmentoperation. Sometimes these maximum rates may not besatisfactory for connected equipment or stable processoperation. A dynamic simulation will help determine the bestoperational rate of change characteristics required for a specificplant equipment design.</p><p>Gas turbine start up characteristics and sequences mayalso impose some additional limitations. These include turbinewarm up at minimal speed and power producing capabilities forheat soak requirements. These are outside the normal operatingenvelope of compressors and could have special requirements.</p><p>Gas turbine restart with high compressor settle outpressure conditions may impose excessive load at low speedThis should be considered in equipment and start up controlsequence design to prevent an undesirable operating condition.Dynamic simulation can be used to determine safe operatingparameters for plant and equipment start up.</p><p>Turbine Trip DelaySome gas turbine compressor applications can and have used atrip delay feature to prevent compressor surge during anequipment shutdown event. This delay allows anti-surge valvesand optional hot gas bypass valves time to respond prior toterminating the fuel source to the gas turbine. Compressorsurge on trip can cause equipment damage and should beavoided or minimized as much as possible. The use of thiscontrol strategy requires the categorization of all the turbine-compressor equipment trips into two categories. These trips canbe classified as either critical requiring immediate shutdown ornon-critical where a delay of 1 to 3 seconds can be tolerated.The definition of a critical shutdown is any malfunction thatcould pose a threat to equipment, personnel or environment. AHAZ-OP study should be conducted with participation byvarious parties including the owner/operator, engineeringcontractor, equipment suppliers and possibly consultants. Thisrms of Use:</p></li><li><p>HAZ-Op study could be used to help categorize the trips. Someexamples of critical type shutdowns are over-speed, low oilpressure and fire or gas detection. Some examples of non Compressor Map </p><p>Dowcritical shutdowns are high bearing temperature, high processscrubber level or high process gas temperature. The results ofthis categorization are then incorporated into the control systemlogic. This control strategy would not totally eliminatecompressor surge during trip but rather minimize the number ofshutdown events that could cause a surge on trip. A carefulevaluation should be made to determine if this control strategycould be utilized on a specific application.</p><p>Compressor ModelingThe modeling of a compressor may be as simple as</p><p>using classical fan laws defined by the normalized parametersof head divided by the square of the rotational speed (H/N</p><p>2 ) as</p><p>a function of flow divided by rotational speed (Q/N) andefficiency as a function of flow divided by rotational speed(Q/N). This approach works well for single compressor stagesand some multistage arrangements. However, this can lead tosignificant error in determining surge depending on the numberof compression stages in series in the compressor body and theresulting overall compressor performance map especially belowdesign speed.</p><p>Predicted head and flow data for multiple stages inseries are derived from individual stage performance. Thecombined performance prediction may be accurate over a verylimited range of speed range. Extrapolating this data to a largerspeed range can lead to errors of 25% at overload and surge. Amore accurate compressor map must be generated at multiplespeeds especially for start up and shutdown studies. During anESD the shape of the surge curve is critical. Many systemsmodel the surge limit as a linear relation between flow anddifferential pressure developed across the compressor. Thesurge curve for each compression system is unique and maynot be linear in many applications. Modeling the system musttry to be as accurate as possible. The same is required foroverload operation. Many compressor systems are started inoverload. As multiple sections of compression are part of manyprocesses the accuracy of the curves used modeling becomeeven more important.</p><p>The practice of normalizing the head flow curve fromthe Q/N vs. Head /N^2 relationship to represent the entire curvefrom low to high speed will lead to modeling errors. As shownin Figure 1, the performance prediction by the compressormanufacturers extended performance curves does not matchthe extrapolated Q/N based on the normal design speed curve.As can be seen the Q/N speed curves have a much differentshape as the speed decreases. The change in shape will causeerroneous results for the dynamic simulation. Figure 2 showsthe overlay of the surge and overload lines. The Q/N predictedoverload at the end of the curve will be greater than is actuallyavailable at lower speed based on the manufacturers data. Thesurge points will also not be estimated correctly and could leadto inaccurate conclusions. The conclusions drawn from asystem that is not modeled accurately can lead to either overdesign or under design of the compression system components.Over design could restrict operating range, reduce efficiency,and provide poor control system response. Under design maylead to predicting safe operation in an area of the extrapolatedcompressor performance map that is actually in surge ornloaded From: on 11/13/2015 Te3 Copyright 2007 by ASME</p><p>Figure 1 Comparison of fan laws to actual compressor flowMap</p><p>Figure 2 Comparison of fan law estimation to actualperformance showing the shift of the overall compressorperformance map</p><p>overload. The consequences of this could limit the equipmentlife and process operating range.</p><p>Continuous operation in overload is not recommendedby some compressor manufactures (3). While not asdetrimental as operating in surge, the mismatch betweencomponents can cause damage to the internal parts of thecompressor.</p><p>Anti-Surge Valve RequirementsThe proper protection of a compressor requires</p><p>analysis of steady state online requirements and transientrequirements of the valve capacity. Steady state re...</p></li></ul>


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