Droplet cutoff criteria: Often specified by compressor manufacturers, e.g., "remove $99\%$ of all droplets $>\text{10 } \mu m$." This defines the minimum droplet size that must be removed.

Removal efficiency: e.g., "$99.9\%$ liquid removal from gas." This is a percentage-based efficiency.

Liquid control volumes: e.g., "provide 5 minutes of surge volume between NLL and LSH for downstream pump trip." This relates to the dynamic capacity of the liquid section.

Avoid Overspecification: Specifying overly stringent separation requirements (e.g., $99.99\%$ removal of $3 \mu m$ droplets) can lead to significantly larger vessels, more complex internals, higher pressure drops, and increased costs without a justifiable process benefit. Specifications should be consistent with the capabilities of available internals technology and the actual needs of the downstream process.

3.2 Process Design Cases

Separators must be designed to operate effectively across the entire range of anticipated process conditions, not just a single "normal" case.

Operating Envelope: Evaluate all relevant operating scenarios.
- Design Flow Rate: The maximum expected flow rate, typically used for sizing the vessel and internals. This often includes a margin above normal operating flow.
- Normal Operating Flow Rate: The typical, steady-state flow conditions. Used for establishing normal operating levels and performance checks.
- Minimum Flow Rate: The lowest expected flow rate. Important for checking turndown capabilities, ensuring stable level control, and preventing issues like foaming or poor liquid-liquid separation due to insufficient velocity or turbulence.
- Transient Conditions: Start-up, shutdown, slugging from upstream, sudden changes in composition, or emergency depressurization. These can impose severe loads and off-design conditions.
Fluid Property Variations: The design must account for variations in fluid properties over the life of the field or process.
- Liquid density: Changes due to temperature, pressure, or composition (e.g., increasing water cut).
- Watercut: The percentage of water in the total liquid phase, often increasing over field life.
- GVF (Gas Volume Fraction): Can vary significantly, impacting gas and liquid handling capacities.
- Fluid properties: Viscosity, interfacial tension, and foaming tendencies can change with time and operating conditions.
Design Margins: While flow rates and fluid properties should include margins to cover uncertainties and future variations, the sizing criteria (e.g., K-factor values, droplet sizes) themselves should be based on established engineering principles and data without adding arbitrary margins.

3.3 Fluid Properties

Accurate fluid property data is critical for accurate sizing and design.

Required Data: Provide comprehensive data for all phases at relevant operating conditions:
- Densities ($\rho$): For gas, oil, and water. Essential for gravity separation and momentum calculations.
- Viscosities ($\mu$): For gas, oil, and water. Crucial for droplet settling/rising velocities and pressure drop calculations.
- Interfacial Tensions ($\sigma$): Between gas-liquid, oil-water, and gas-oil. Directly affects droplet formation, coalescence, and separation efficiency.
Foaming and Emulsion Tendency:
- Foaming: Presence of stable gas bubbles in liquid. Can significantly reduce gas-liquid disengagement c