Process Design of Centrifugal Compressor System

Process Datasheet

• 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

Performance data

• Input from compressor vendor.
• Defines the interstage pressures and temperatures.
• Gives input for the HYSYS simulation.

Process 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.

System components

• Compressor and Motor
• Coupling (e.g. Voith)
• Lube oil System
• Seal oil / Seal gas system
• Scrubbers
• 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

Process Calculations

• 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.

Blowdown calculation

• 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.

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, 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.

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 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)

Control Philosophy

• Capacity control
• Antisurge control
• Scrubber 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 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

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 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.

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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