Air Coolers – What Is Piping

Process Datasheet Defines the suction and discharge conditions (P, T, Flowrate) and gas composition.The main input to the compressor vendor.Define all possible options of suction and discharge conditions and gas compositions, present, and future.The vendor selects the model which fits these conditions the best. Compressor performance curves Input from the compressor vendorRelationship of Flow vs. Head (polytropic, isentropic)Surge point and choking (stone wall)Variation with RPMInput to HYSYS for the generation of performance at different conditions Performance data Input from compressor vendor.Defines the interstage pressures and temperatures.Gives input for the HYSYS simulation. Process Simulation HYSYS simulation modelBased on the Compressor performance dataForms the basis for material and energy balance.Defines the cooler heat duty.Generates property data for the calculations. Fig. 1: An example of typical process simulation System components Compressor and MotorCoupling (e.g. Voith)Lube oil SystemSeal oil / Seal gas systemScrubbersPiping and InstrumentationAir Coolers Fig. 2: Typical system components Advantages Seal gas system No seal oil system requiredNo need to dispose of/clean up contaminated oilEliminates fouling problems due to oil ingress in process streamsLess gas lossThe dry gas seal advantages significantly outweigh the seal oil benefits Process Calculations Settle out CalculationsBlowdown calculationPipe sizing calculationsHydrate calculations Settle out calculation Equalized pressure during a compressor shutdown.High-pressure trip conditions taken as pressures before settle out.Enthalpy balance of the system.It can be done using a spreadsheet or HYSYS.It can define the design pressure for some of the sections. Blowdown calculation Intent: Reduce the pressure of the equipment to 50% of design pressure within 15 minutes during a fire emergency.Typically done using Dynamic depressurizing Utility in HYSYSRelief valves are not a depressurization device.Ball valve + Orifice combination OR control valve The blowdown calculation takes the following into account: Vaporization of liquid due to pressure reduction,Vaporization due to heat input from the external fire,Pressure after 15 minutes is reduced from design pressure to 50% of design pressure,Start at settle out conditions.Gas compressor system is blocked in and no additional mass is fed into the system during blowdown.Maximum allowable depressurization rate for the compressor O-rings of 20 bar/min,There is no other heat input into the system other than fire.The relief rate calculated is not limited by the flare.Use to find the lowest temperature attained and hydrate formation possibilities.Uncontrolled vs. Staged Blowdown Fig. 3: Uncontrolled vs. Staged Blowdown Pipe sizing calculations Importance of pressure drop and machine performance.Tools used.Cooler header sizing.Avoiding loops in suction.Provision of drain boots. Hydrate calculations Hydrates are ice-like non-stoichiometric crystal structures composed of water molecules engaging natural gas molecules.The solid formation chokes piping.Flow problems.The formation depends on P, T conditions and composition.Predicted by HYSYS. Gas Blow-by calculations Caused by losing liquid level in the scrubbers.High-pressure gas flows into the low-pressure system potentially over pressurizing it.Calculations are done to ensure that the downstream system is adequately protected.The control valve is considered to be fully open during this case.The highest operating pressure of the upstream system is considered for sizing. Scrubbers Vertical Knock out vessels.Limit liquid carries over to the compressors.Internals – SMS / SV / SVS Air Coolers Heat duty based on Process Simulation.Process parameters based on the simulation.The vendor does the sizing with HTRI or other proprietary software.Pressure drop is critical. Flare and Blowdown system The flare system needs to be designed forBlowdown depressurizing load.Flaring due to compressor tripFire case reliefBlocked discharge of the compressorThe flare system may require a KOD based on the quality of gas flared. (Liquid presence) Control philosophy Capacity controlAntisurge controlScrubber level control Safeguarding philosophy Process shutdown.Emergency shutdown.Other shutdowns. Process shutdown Close the discharge ESD valve. The suction ESD valve shall remain in an open position. The blowdown ESD valve shall remain in a closed position. The antisurge valves and capacity control valve goes to open position. The motor stops and the compressor settles out to suction pressure. The auxiliaries keep running.Generally initiated on trips on process parameters.Enables faster start-up compared to ESD Emergency shutdown PSD1 shall Trip the compressor motor & auxiliaries, and Close the ESD valve on the suction and discharge header.The antisurge valves and capacity control valve goes to open position.The external seal gas supply shall be isolated by the ESD valve on the seal gas line.The compressor blowdown valves shall open and depressurize the gas to flare.Initiated on Fire, Station ESD. Other shutdowns The suction, inter-stage(s), and discharge scrubbers low-level close liquid outlet ESD valves.The inter-stage(s) and After-cooler fan high vibration shall trip the respective fan.Low temperature at the aftercooler outlet shall trip the first working fan at 30 deg C and the next at 20 deg C.External seal gas high pressure downstream of external seal gas pressure letdown valve for LP casing shall close the external seal gas supply ESD valve. 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