- Defines the suction and discharge conditions (Pressure, Temperature, 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 vendor
- Relationship of Flow vs. Head (polytropic, isentropic)
- Surge point and choking (stone wall)
- Variation with RPM
- Input to HYSYS for the generation of performance at different conditions
- Input from compressor vendor.
- Defines the interstage pressures and temperatures.
- Gives input for the HYSYS simulation.
- HYSYS simulation model
- Based on the Compressor performance data
- Forms the basis for material and energy balance.
- Defines the cooler heat duty.
- Generates property data for the calculations.
- Compressor and Motor
- Coupling (e.g. Voith)
- Lube oil System
- Seal oil / Seal gas system
- Piping and Instrumentation
- Air Coolers
Advantages Seal gas system
- No seal oil system required
- No need to dispose of/clean up contaminated oil
- Eliminates fouling problems due to oil ingress in process streams
- Less gas loss
- The dry gas seal advantages significantly outweigh the seal oil benefits
- Settle out Calculations
- Blowdown calculation
- Pipe sizing calculations
- Hydrate calculations
Settle out calculation
- Equalized pressure during a compressor shutdown.
- High-pressure trip conditions are taken as pressures before settling 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.
- Intent: Reduce the pressure of the equipment to 50% of the design pressure within 15 minutes during a fire emergency.
- Typically done using Dynamic depressurizing Utility in HYSYS
- Relief valves are not depressurization devices.
- 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.
- The 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
Pipe sizing calculations
- Importance of pressure drop and machine performance.
- Tools used.
- Cooler header sizing.
- Avoiding loops in suction.
- Provision of drain boots.
- 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, and 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 overpressurizing 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.
- Vertical Knock out vessels.
- Limit liquid carries over to the compressors.
- Internals – SMS / SV / SVS
- 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 for
- Blowdown depressurizing load.
- Flaring due to compressor trip
- Fire case relief
- Blocked discharge of the compressor
- The flare system may require a KOD based on the quality of the gas flared. (Liquid presence)
- Capacity control
- Antisurge control
- Scrubber level control
- Process shutdown.
- Emergency shutdown.
- Other shutdowns.
- 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 the 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
- 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 the 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.
- 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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