Slug Catcher Sizing

1. Purpose & Function

A slug catcher is a vessel or pipe assembly located at the terminus of a two-phase pipeline that receives and separates the incoming gas-liquid mixture. Its primary purpose is to absorb transient liquid slugs so that downstream processing equipment (separators, compressors, treating systems) can operate at steady-state conditions.

Without a slug catcher, liquid slugs arriving at a gas processing plant would overwhelm inlet separators, flood compressor suction scrubbers, and cause severe operational upsets. The slug catcher serves as a buffer that:

• Absorbs terrain-induced slugs from hilly pipeline profiles
• Captures pigging slugs during line maintenance
• Handles transient slugs from production rate changes (ramp-ups)
• Provides steady liquid feed to downstream separation and stabilization
• Serves as the initial gas-liquid separation vessel

2. Slug Formation Types

2.1 Terrain-Induced Slugs

In pipelines with elevation changes, liquid accumulates in low points (valleys) during normal steady-state operation. When gas velocity is sufficient to sweep liquid over hilltops, it arrives at the downstream facility as terrain slugs. The volume depends on:

• Pipeline profile (number and severity of elevation changes)
• Liquid holdup fraction at operating conditions
• Gas and liquid flow rates
• Pipe diameter

Terrain slugs are typically moderate in volume but arrive repeatedly during normal operation. The slug length is proportional to pipe diameter and terrain severity, commonly ranging from 100 to 800 pipe diameters.

2.2 Pigging Slugs

Pipeline pigs push liquid ahead of them as they travel through the line. The pig sweeps most of the liquid holdup along the entire pipeline into one large slug that arrives at the slug catcher. This is almost always the largest slug volume and frequently governs the slug catcher design.

Pig sweep efficiency depends on pig type: foam pigs achieve ~70-75%, standard bi-directional pigs ~80%, and aggressive cup pigs ~85-90%.

2.3 Ramp-Up (Transient) Slugs

When production rate increases, the equilibrium liquid holdup decreases because higher gas velocities carry more liquid out of the pipe. The excess liquid drains forward as a transient slug. The volume equals the pipeline liquid volume change:

2.4 Comparison of Slug Types

3. Liquid Holdup Fundamentals

Liquid holdup (H_L) is the fraction of pipe cross-section occupied by liquid at any given point. It is the key parameter for slug volume estimation, as it determines how much liquid is present in the pipeline that could be displaced into a slug.

3.1 Beggs-Brill Correlation

The most widely used holdup correlation for gas-liquid pipeline flow. It predicts holdup based on superficial velocities, pipe geometry, and fluid properties.

3.2 Flow Patterns

The Beggs-Brill correlation classifies two-phase flow into four patterns based on the liquid fraction and Froude number:

3.3 Key Velocities

Superficial velocities are the foundation of holdup calculations:

• V_SG = Gas superficial velocity = Q_gas,actual / A_pipe
• V_SL = Liquid superficial velocity = Q_liquid / A_pipe
• V_m = Mixture velocity = V_SG + V_SL

4. Finger-Type Slug Catchers

Finger-type slug catchers consist of multiple large-diameter pipes (24"-48" OD) arranged in parallel, connected by inlet and outlet headers. The incoming two-phase flow enters the header manifold and distributes across the fingers, where liquid settles by gravity into the lower portion while gas flows overhead.

4.1 Configuration

• Inlet header: Large-bore manifold distributing flow to fingers (typically 1-2 sizes larger than individual fingers)
• Fingers: 2-20+ parallel pipes, sloped ~1-3° toward liquid outlet for gravity drainage
• Gas outlet header: Collects gas from top of each finger to downstream processing
• Liquid outlet: Collects liquid draining from bottom of each finger to storage/separation

4.2 Advantages

• Can handle very high pressures (1000+ psig) using standard pipe — no ASME-coded vessel required
• Modular design — add or remove fingers to adjust capacity
• Lower unit cost at high pressures compared to vessels
• Easy to fabricate from readily available pipe sizes

4.3 Sizing Approach

Each finger is assumed to operate at 50% liquid fill for gravity drainage. The number of fingers is determined by the total required volume divided by the usable volume per finger:

Wall thickness is checked per ASME B31.8 (Barlow's formula) using the appropriate design factor for the location class.

5. Vessel-Type Slug Catchers

Vessel-type slug catchers are single large horizontal pressure vessels designed per ASME Section VIII. They function similarly to inlet separators but with additional liquid storage capacity for slug absorption.

5.1 Configuration

• Horizontal vessel with 2:1 ellipsoidal or hemispherical heads
• Inlet diverter/deflector for initial gas-liquid separation
• Mist eliminator (vane pack or mesh pad) near gas outlet
• Level control instruments (HH, H, N, L, LL) for liquid management
• Multiple liquid outlets for normal drain and emergency dump

5.2 Sizing Approach

The vessel is sized to hold the design slug volume at 50% liquid level, with the remaining upper half available for gas flow and separation:

5.3 Shell Thickness (ASME VIII)

5.4 Vessel vs. Finger Comparison

References

• GPSA, Chapters 7 (Separation Equipment) and 9 (Hydrocarbon Recovery)
• API RP 1160 — Managing System Integrity for Hazardous Liquid Pipelines
• ASME B31.8 — Gas Transmission and Distribution Piping Systems
• ASME BPVC Section VIII Div. 1 — Pressure Vessels
• Brill & Mukherjee — Multiphase Flow in Wells (holdup correlations)
• Beggs & Brill — A Study of Two-Phase Flow in Inclined Pipes (holdup correlation)