Guidelines for Line Sizing & Hydraulics Analysis 📏

1.0 Scope & Overview

This guideline defines the methodology for performing line sizing and hydraulic analysis. Lines are generally sized based on velocity, pressure drop, and the rho-V-squared (ρV²) parameter. Piping is sized for the controlling operating case, with consideration for start-up, shutdown, and other off-design conditions.

2.0 Design & Flow Rate Margins

Flow Rate Basis (Design Flow)

The "Design Flow Rate" is used for sizing new lines, calculated by adding a margin to the "Normal Flow Rate" (from the PFD/Material Balance).

• Process Lines: 10%
• Process Lines (Intermittent/Start-up): 5%
• Cooling Water System: 20%
• Circulating Fuel Oil Header: 25%
• Other Utility Lines: 15%
• Utility Main Headers: 20%

Pressure Drop Margin

A 20% design margin should be applied to calculated pressure drops for comparison with guidelines (e.g., dP/100m). This accounts for manufacturing tolerances and pipe deterioration. No margin is applied to existing pipes.

3.0 Piping Sizes & Roughness

Standard Piping Sizes

• Minimum Process Lines: 25 NB (1")
• Minimum in Pipe Racks: 40 NB (1.5")
• Minimum on Vessels/Exchangers: 50 NB (2")
• Non-Standard Sizes to Avoid: 32mm (1.25"), 65mm (2.5"), 90mm (3.5"), 125mm (5"), etc.

Pipe Wall Roughness (Recommended Values)

4.0 Pump Suction Lines

Suction lines are critically sized to meet the pump's NPSH requirements. The NPSH Available (NPSHa) must be greater than the NPSH Required (NPSHr) by a margin of 1.0 meter of liquid.

• Pressure Drop: Generally recommended to be between 0.035 and 0.085 kg/cm² per 100m.
• Boiling Fluids: For lines handling boiling fluids, the total frictional pressure drop should not exceed 0.6 meters of head.

Recommended Velocities (Centrifugal Pumps)

5.0 Pump Discharge Lines

Discharge lines are sized based on economics, velocity, and pressure drop.

• Pressure Drop: Generally recommended to be between 0.15 and 0.6 kg/cm² per 100m.
• Velocity Limit: For continuous liquid services, velocity should generally be restricted to 6 m/s.

Recommended Velocities (Centrifugal Pumps)

6.0 Special Liquid Cases

Static Charge Generation

For low conductivity fluids (e.g., Naphtha, Kerosene, <50 pS/m) to prevent static charge buildup:

• Maximum velocity in lines is limited to 7 m/s.
• For initial tank fill (until fill pipe is submerged), max velocity is 1 m/s.
• Alternatively, the formula $V \times D < 0.8 \text{ m²/s}$ should be used (max 6 m/s).
• For loading arms, the formula is $V \times D < 0.5 \text{ m²/s}$.

Water Systems

• Cooling Water: Size for dP of 0.3 kg/cm²/100m. Maintain > 1 m/s to minimize fouling.
• Sea Water (Cu-Ni): Max velocity 3 m/s.
• Sea Water (Duplex SS): Max velocity 7.0 - 7.5 m/s.

Gravity Flow

• Slope: Minimum slope of 1:100 (1:50 for fluids with solids).
• Self-Venting Lines: To ensure the line runs partially full and vents gas, the Froude Number (Fr) must be < 0.3.

7.0 General Vapour Lines

Sized based on pressure drop, velocity, or the ρV² (rho-V-squared) criterion. For pressure drops > 10% of upstream pressure, compressible flow calculations are required.

Recommended Sizing Criteria (Continuous Service)

8.0 Special Vapour Lines

Compressor Lines

• Reciprocating Suction: Max velocity ~10-12 m/s.
• Centrifugal Suction: Max velocity ~20 m/s.
• Centrifugal Discharge: Max velocity ~20-25 m/s.
• Anti-Surge Lines: Size for max velocity V = 20 / ρ⁰.⁸³ or 60 m/s, whichever is lower.

Steam Lines

• General Velocity: 25-65 m/s (Saturated), 25-85 m/s (Superheated).
• Turbine Inlet: Max velocity 53 m/s.
• Turbine Exhaust (Back-pressure): Max velocity 76 m/s.
• Turbine Exhaust (Condensing): Max velocity 137 m/s.

Steam Condensate

Lines downstream of traps are two-phase and sized generously. Total pressure drop should not exceed 50% of the available dP. Max dP is ~1.7 bar/100m.

9.0 Two-Phase Lines

Sizing procedure: 1) Calculate erosional velocity, 2) Check flow pattern to avoid slug flow, 3) Calculate pressure drop using appropriate software (e.g., HYSYS, OLGA, PIPESIM).

Flow Patterns

Flow can be classified into patterns (Bubble, Slug, Annular Mist, Stratified, Wave). Slug flow is highly undesirable as it causes pressure pulsation and vibration.

Erosion Limit (API RP14E)

The velocity at which erosion may occur in solids-free, two-phase fluids is calculated by:

• Vₑ = Erosional velocity (m/s)
• ρm = Mixture density (kg/m³)
• C = 122 (Continuous Service, Carbon Steel)
• C = 152 (Intermittent Service, Carbon Steel)
• C = 183-244 (Continuous, Corrosion Resistant Alloy)

10.0 Slurry Lines

Slurry lines must be sized to keep solids suspended by maintaining velocity *above* the critical settling velocity.

Recommended Slurry Velocities

11.0 Relief Valve (PSV) Lines

PSV Inlet Lines

• Pressure Drop: Total non-recoverable pressure loss MUST be **less than 3%** of the PSV set pressure.
• ρV² Limit: Should be ≤ 50,000 kg/m/s² (for P > 50 barg).

PSV Outlet Lines

Sized based on the *rated capacity* of the valve. The maximum Mach number should be 0.75.

Backpressure Limits

12.0 Flare & Vent Headers

Flare Headers

• Mach Number: Max 0.5 (Main Headers), Max 0.7 - 0.75 (Sub-Headers).
• ρV² Limit: For intermittent flow, ρV² < 100,000 kg/m/s² is typical.
• Liquid Loading: Headers must be designed to carry liquid. For example, headers ≤ 250 NB (10") should be designed assuming they are 100% full of liquid for support calculations.

Atmospheric Vents

• Hydrocarbons: Vent velocity should be as high as possible (min 150 m/s) to ensure good dispersion.
• Non-Hydrocarbons (Steam, Air, N2): Vented to a safe location. Noise may be a concern and require analysis.

13.0 Pressure Drop Allowances (for Estimates)

When vendor data is unavailable, use these typical pressure drops for hydraulic calculations.

Flow Elements

14.0 Control Valve Sizing

The pressure drop allocated for a control valve in a pump discharge line should be the largest of the following:

• 1.0 bar (15 psi)
• 50% of the dynamic pressure drop (or 33% of total friction drop *including* the valve)
• 10% of the pump differential head

15.0 Equivalent Length (Le/D)

Used to calculate pressure drop from fittings and valves. The table below provides standard L/D ratios for turbulent flow.

Figure 5.1: Two Phase Flow Regimes

Illustrations of Bubble, Slug, Annular Mist, Stratified, and Wave flow patterns in a horizontal pipe.

Figure 5.2: Horizontal Flow Regime Map

A log-log graph plotting Superficial Liquid Velocity vs. Superficial Gas Velocity, showing the boundaries for Dispersed, Bubble, Slug, Annular, Stratified, and Wave flows.

Figure 5.3: Vertical Up-Flow Regime Map

A log-log graph plotting Modified Superficial Liquid Velocity vs. Modified Superficial Gas Velocity, showing boundaries for Bubble, Slug, Annular Mist, and Dispersed flow regimes in vertical pipes.

Figure 8.1 & 8.2: Slurry Flow Regimes

Illustrations of Homogeneous, Heterogeneous, Saltation, and Stationary Bed flow for slurries, along with a map plotting particle diameter against specific gravity to determine the flow type.

Appendix 2 & 3: Liquid Connection Capacity Charts

Log-log charts used to determine the liquid flow capacity (in GPM) of side-outlet and bottom-outlet nozzles based on the liquid head above the nozzle and the nozzle's internal diameter.

Appendix 4: Hydraulic Radius of Partially Filled Pipes

Diagrams showing the cross-section of a partially filled pipe, illustrating the variables (h, r, d, K, s, θ) used to calculate the hydraulic radius for gravity flow.