A General Guide to Process Design Criteria

1.0 Introduction

This document provides a consistent, general-purpose design philosophy for process systems. It covers the selection of mechanical design conditions, equipment design principles, and instrumentation criteria to ensure safe, reliable, and cost-effective plant design.

2.0 Selection of Mechanical Design Conditions

Equipment and piping must be designed for the most stringent coincidental conditions of pressure and temperature (both maximum and minimum) that can be encountered during all operating modes, including startup, shutdown, and upsets.

2.1 Design Pressure for Pressure Vessels

Internal Design Pressure

The design pressure for a pressurized system is set by the maximum operating pressure (MOP) plus a suitable margin.

• Columns: The design pressure is calculated at the top. The design pressure at the bottom is the top design pressure plus the column's pressure drop (ΔP) and the maximum hydrostatic head of the liquid in the sump.
• Flare-Connected Systems: Equipment "floating" on a flare system should have a design pressure equal to the flare knock-out drum's design pressure (typically 3.5 to 7 kg/cm²g minimum).

• Heat Exchangers (10/13 Rule): To avoid needing a relief device for a tube rupture, the low-pressure (LP) side is often designed so that its corrected hydrotest pressure is not less than the high-pressure (HP) side's design pressure. As a minimum, the LP side DP should be $10/13$ of the HP side DP.

External Design Pressure (Vacuum)

Equipment must be designed for full vacuum (FV) if it can be subjected to vacuum during any operation, including:

• Normal operation (e.g., vacuum columns).
• Startup, shutdown, or regeneration.
• Steam-out procedures followed by cooling.
• Vaporizing or condensing services.
• Loss of heat input to a column.
• Gravity draining of a liquid-full, blocked-in vessel or elevated exchanger.

2.2 Design Temperature

Maximum Design Temperature

The maximum design temperature is the maximum operating temperature plus a margin, typically 15°C.

Exceptions include:

• Solar Radiation: Equipment exposed to the sun shall have a minimum design temperature of 65°C.
• Steam-Out: The design temperature must be at least the temperature of the steam used for steam-out (e.g., LP steam temperature).
• Air Coolers: Downstream equipment should be designed for the air cooler's *inlet* temperature, in case of a fan failure.
• Compressor Discharge: Design temperature should be based on the calculated temperature at surge conditions.

Minimum Design Metal Temperature (MDMT)

The MDMT is the lowest temperature the equipment may experience, minus a margin (e.g., 3-5°C). This is critical for material selection to prevent brittle fracture.

Low temperatures can be caused by:

• Low ambient site conditions.
• Auto-refrigeration during depressurization (a "blowdown" event).
• Process upsets or cryogenic services.

2.3 Corrosion Allowance

A corrosion allowance is added to the calculated material thickness to account for metal loss over the plant's life. Typical values depend on the service and material.

3.0 Equipment Design Philosophy

3.1 Vessels (Columns, Drums)

Vessel Sizing & Levels

Vessel L/D (Length/Diameter) ratios are typically 3 to 5 for horizontal vessels and 1.2 to 1.7 for vertical vessels. Liquid levels are defined as:

• HHLL: High High Liquid Level (Trip)
• HLL: High Liquid Level (Pre-alarm)
• NLL: Normal Liquid Level (Control Set Point)
• LLL: Low Liquid Level (Pre-alarm)
• LLLL: Low Low Liquid Level (Trip)

Timing:

• Surge Time (NLL → HLL): Time with no outflow.
• Hold-up Time (NLL → LLL): Time with no inflow.
• A minimum of 1-2 minutes or 150 mm should be provided between alarm (LLL/HLL) and trip (LLLL/HHLL) levels.

The LLLL is typically set 150-300 mm above the bottom tangent line or pump suction nozzle.

Vessel Internals

• Demisters: Vane packs or mesh pads should be specified to remove 99% of liquid droplets > 10 microns.
• Vortex Breakers: Required on all liquid outlets connected to a pump suction or a control valve.

Vessel Nozzles

• Minimum nozzle size: 50 NB (unclad), 80 NB (clad).
• Vent/Drain sizes: 50 NB to 100 NB, depending on vessel volume.
• Inlet nozzle momentum ($\rho_m v_m^2$) should be limited (e.g., < 1500 kg/m·s² with a distributor, < 3500 kg/m·s² for gas outlets).

Access Openings (Manways)

• Minimum 600 NB manway required for any vessel needing internal access.
• Packed Columns: Manways are needed for each bed, at each distributor, and below the bed support.
• Tray Columns: Manways are needed at the feed tray, top, bottom, and at intermediate points (max 10m spacing).

3.2 Storage Tanks

Tank Types

• Cone Roof Tank: For liquids with low vapor pressure (e.g., < 0.05 kg/cm²a).
• Floating Roof Tank (Internal or External): For liquids with higher vapor pressure (e.g., < 0.78 kg/cm²a) or liquids stored near their flash point. Reduces VOC emissions.
• Dome Roof Tank: For liquids with vapor pressure up to 1.05 kg/cm²g (e.g., NGLs).

Tank Operating Levels

• LLLL (Low Low Liquid Level): Set by pump NPSH requirements or, for floating roof tanks, ~300mm above the landed roof height to prevent the roof from landing during normal operation.
• HLL to HHLL: Sized to provide ~15 minutes of operator response time at maximum inflow.
• MLL (Maximum Liquid Level): The highest possible level, set by the HHLL trip plus thermal expansion and system response time. For floating roof tanks, this must be well below the top (~1800mm) to provide space for the roof.

3.3 Shell & Tube Heat Exchangers

• Design oversurface: 5% minimum.
• Velocities: Minimum 1.0 m/s for process fluids and cooling water to minimize fouling. Maximum 3.0 m/s for cooling water (in carbon steel) to prevent erosion.
• Allowable ΔP: Typically 0.3 - 0.7 kg/cm² for liquids, increasing with viscosity.
• Fouling Factors: Based on service (e.g., 0.0004 h·m²·°C/kcal for cooling water).

3.4 Air Cooled Heat Exchangers

• Draft: Forced draft is preferred. Induced draft is used for low approach temperatures (< 8°C).
• Approach: Lowest process outlet temperature should be 12°C above the dry bulb temperature.
• Control: Outlet temperature is controlled by auto-variable blade angle fans or variable speed motors.

3.5 Fired Heaters

• Design Margin: 10% on heat duty.
• Efficiency: Minimum 90-92% should be targeted.
• Features: Low NOx burners are preferred. Provisions for decoking are required for services subject to coking.

3.6 Pumps

• Design Margins (on Normal Flow):
  • Process Pump: 10%
  • Reflux Pump: 20%
  • Reboiler Feed Pump: 15%
  • Recirculation/Intermittent: 0%
• Sparing: 100% (1 operating + 1 spare) for continuous services. No spare for intermittent services.
• NPSHa: NPSH available (NPSHa) must be at least 1.0 meter greater than NPSH required (NPSHr).

• Mechanical Seals: Dual mechanical seals (or sealless pumps) are required for:
  • LPG and lighter fluids.
  • Fluids above their auto-ignition temperature.
  • Toxic or carcinogenic fluids (e.g., benzene).
  • Sour liquids (> 100 ppm H₂S).

3.7 Compressors

• Design Margin: 10% on maximum operating flow.
• Sparing:
  • Centrifugal/Axial/Screw: No installed spare (spare rotor only).
  • Reciprocating: 100% (1 operating + 1 spare).
• Safety: Centrifugal compressors must be provided with an anti-surge control system.

3.8 Columns (Towers)

• Trays: Standard valve trays are common. Sieve trays may be used in fouling service. Tray spacing is typically 450-600 mm.
• Packing: Structured packing is recommended for vacuum service due to its low pressure drop.

4.0 Instrumentation Criteria

4.1 Control Valves

• Sizing:
  • Maximum Flow: Governing HMB flow + 10% margin (or pump rated capacity).
  • Calculated Cv (Max): Should be 80-90% of the valve's Rated Cv.
  • Calculated Cv (Min): Should be > 10% of the valve's Rated Cv for good control.
• Pressure Drop (ΔP):
  • Liquids (non-pumped): Minimum 0.7 kg/cm².
  • Liquids (pumped): Minimum 30% of the total circuit's frictional pressure loss.
• Safety: Control valves discharging to the flare or atmosphere must be specified as TSO (Tight Shutoff).

4.2 Flow Elements

• General: Orifice plates are the most common type for liquid and vapor flows.
• Low ΔP: Venturi meters are recommended where minimizing permanent pressure loss is critical.