1. Overview
Production separators, scrubbers, and slug catchers can be designed in either horizontal or vertical orientation. The choice significantly affects vessel size, cost, plot space, transportation, and operability. Neither orientation is universally superior; the optimal selection depends on the specific application, gas-to-oil ratio, operating pressure, plot space constraints, and field conditions.
Horizontal
High gas + liquid capacity
Preferred for high GOR applications, three-phase service, large liquid volumes, slug handling, and offshore platforms.
Vertical
Small footprint
Preferred for gas scrubbers, low liquid loading, high-pressure service, and locations with limited plot space.
Spherical
Rarely used
Occasionally used for very high-pressure applications where wall thickness governs. Difficult to fabricate and inspect.
Key Differences at a Glance
| Feature | Horizontal | Vertical |
|---|---|---|
| Gas handling capacity | Higher (K = 0.36–0.42) | Lower (K = 0.24–0.28) |
| Liquid retention time | 3–10 min (large volume) | 1–5 min (smaller volume) |
| Three-phase separation | Excellent (long settling length) | Possible but less common |
| Slug handling | Excellent | Poor |
| Foam handling | Fair | Good (natural gravity drainage) |
| Sand/solids handling | Easier (sand jets, drain valves) | Harder (sand settles in bottom head) |
| Plot space | Large footprint | Small footprint |
| Height | Low profile | Tall structure |
| Transportation | Easier (lower profile) | May require special transport for tall vessels |
| Mist eliminator drainage | Complex (vertical drainage path) | Simple (gravity drainage downward) |
Industry convention: In midstream gas processing, horizontal separators are standard for production separators and test separators. Vertical vessels are standard for scrubbers, compressor suction vessels, and low-liquid-loading applications. This convention exists because it optimizes cost and performance for each application.
2. Selection Criteria
Gas-to-Oil Ratio (GOR)
The GOR is the single most important factor in separator orientation selection:
| GOR Range (SCF/bbl) | Recommended Orientation | Rationale |
|---|---|---|
| < 3,000 | Horizontal | Liquid-dominant; need liquid retention volume |
| 3,000–10,000 | Either (evaluate both) | Transition zone; cost comparison needed |
| 10,000–50,000 | Vertical preferred | Gas-dominant; liquid section minimal |
| > 50,000 | Vertical (scrubber) | Essentially a gas scrubber with trace liquid |
Operating Pressure
| Pressure Range | Preferred Orientation | Rationale |
|---|---|---|
| < 250 psig | Either | Wall thickness not a major cost driver |
| 250–1,000 psig | Either (cost comparison needed) | Horizontal may be lighter for same capacity |
| > 1,000 psig | Vertical preferred | Smaller diameter = less wall thickness = lower weight |
| > 1,500 psig | Vertical strongly preferred | Wall thickness drives cost; minimize diameter |
Service-Specific Recommendations
| Application | Recommended | Key Factor |
|---|---|---|
| Production separator (two-phase) | Horizontal | Liquid retention, GOR flexibility |
| Production separator (three-phase) | Horizontal | Oil-water settling length |
| Test separator | Horizontal | Accurate liquid measurement |
| Compressor suction scrubber | Vertical | Gas-dominant, small footprint |
| Pipeline inlet scrubber | Vertical | Gas-dominant, limited liquid |
| Fuel gas scrubber | Vertical | Very low liquid, compact |
| Slug catcher | Horizontal | Large liquid surge volume |
| Flare knock-out drum | Horizontal | Liquid surge from relief events |
| Offshore platform | Horizontal | Lower center of gravity, stability |
Rule of thumb: If the GOR is above 10,000 SCF/bbl and the operating pressure is above 500 psig, start with a vertical separator. If the GOR is below 5,000 SCF/bbl or three-phase separation is required, start with a horizontal separator. For the transition zone, size both and compare total installed cost.
3. Capacity Comparison
Gas Capacity
Horizontal separators have higher gas capacity than vertical separators at the same diameter because the gas flows horizontally over a larger cross-sectional area (the area above the liquid level). The effective KSB factor is higher for horizontal vessels:
Capacity Ratio:
Horizontal KSB = 0.36–0.42 (with wire mesh)
Vertical KSB = 0.24–0.28 (with wire mesh)
Gas capacity ratio ≈ KH / KV ≈ 0.40 / 0.26 = 1.54
A horizontal separator handles approximately 50% more gas
than a vertical separator of the same diameter.
Liquid Capacity
Horizontal separators provide significantly more liquid volume than vertical separators at the same diameter due to their greater overall vessel volume (longer length) and the liquid filling the lower portion of the cylindrical cross-section:
| Liquid Level (%) | Horizontal Volume Fraction | Notes |
|---|---|---|
| 50% (half full) | 50% of total vessel volume | Maximum gas and liquid balance |
| 60% | 60% of total vessel volume | Liquid-heavy service |
| 40% | 40% of total vessel volume | Gas-heavy service |
| 25% | 25% of total vessel volume | Near-scrubber operation |
Effective Separation Length
Effective Settling Length:
Horizontal: Leff = Vessel T-T length minus inlet + outlet zones
Typically Leff = LTT - 2 × D (approximate)
Vertical: Leff = Height between inlet device and mist eliminator
Typically Leff = 2–4 ft
Horizontal vessels provide 3–10x the effective settling length,
which is particularly important for liquid-liquid separation
in three-phase service.
Weight Comparison
For the same gas and liquid capacity:
| Pressure (psig) | Lighter Orientation | Approximate Weight Ratio (H/V) |
|---|---|---|
| < 250 | Similar | 0.9–1.1 |
| 250–600 | Horizontal slightly lighter | 0.85–0.95 |
| 600–1,000 | Vertical lighter | 1.1–1.3 |
| > 1,000 | Vertical significantly lighter | 1.3–2.0 |
High-pressure advantage: At pressures above 1,000 psig, the vertical separator advantage becomes significant. A smaller-diameter vertical vessel requires less wall thickness, which translates to substantially lower vessel weight and fabrication cost. At 1,500 psig, a vertical scrubber may weigh 30–50% less than a horizontal separator sized for the same capacity.
4. Cost & Installation Comparison
Vessel Cost Factors
| Cost Component | Horizontal | Vertical |
|---|---|---|
| Shell material | More material (longer shell) | Less material (shorter shell) |
| Head material | 2 heads (same diameter) | 2 heads (same diameter) |
| Wall thickness | Same for same diameter | Same for same diameter |
| Internals | More complex (baffles, weirs) | Simpler (demister pad, inlet device) |
| Nozzles | More nozzles (gas, oil, water, instruments) | Fewer nozzles |
| Fabrication labor | Higher (more internals, nozzles) | Lower |
Installation Cost Factors
| Factor | Horizontal | Vertical |
|---|---|---|
| Foundation | Saddle supports (2 points) | Skirt or legs (continuous ring) |
| Plot area | Large footprint | Small footprint |
| Height | Low (vessel + saddle, typically 4–8 ft) | Tall (may require platforms, ladders) |
| Access platforms | Ground-level access for most items | Elevated platforms for instruments, manways |
| Piping | More complex routing (side entries) | Simpler (top/bottom entries) |
| Transportation | Standard trucking (low profile) | May need special permits for tall vessels |
| Crane capacity | Heavier lifts (longer vessel) | Lighter lifts (shorter vessel) |
Operational Cost Factors
- Sand/solids removal: Horizontal vessels are easier to clean. Sand jets and drain valves work with gravity. Vertical vessels trap solids in the bottom head, which is harder to access.
- Inspection access: Horizontal vessels have larger manways that are easier to access at ground level. Vertical vessel manways may require scaffolding.
- Liquid level measurement: Both orientations use standard level instruments. Horizontal vessels may need multiple level instruments if the vessel is long (due to wave action effects).
- Pigging: If the separator is integrated into a piggable pipeline system, horizontal vessels are easier to pig than vertical vessels.
Total installed cost: For the transition zone (GOR 3,000–10,000), always request quotes for both orientations. The total installed cost includes the vessel, internals, foundation, structural steel, piping, instrumentation, and electrical. It is not uncommon for the total installed costs to differ by 20–40% between orientations.
5. Special Service Considerations
Foaming Service
Foam in separators reduces effective gas capacity, interferes with liquid level measurement, and can carry liquid out the gas outlet:
- Vertical separators: Better for foam handling. Gas flows upward against gravity, allowing foam to drain naturally. Less turbulence at the liquid surface.
- Horizontal separators: More prone to foam carry-over. Consider anti-foam baffles, higher gas section, or defoaming chemicals. May need to de-rate gas capacity by 50–75%.
Sand and Solids
- Horizontal: Preferred. Sand settles along the bottom of the vessel where it can be removed by sand jets, jetting nozzles, and bottom drain valves. Sand pans or troughs can be installed.
- Vertical: Problematic. Sand accumulates in the bottom head and is difficult to remove without removing the bottom head. Cone-bottom designs help but add cost.
Slug Flow
- Horizontal: Strongly preferred. The large liquid volume absorbs slugs without overwhelming the level control system. Slug volume is distributed along the vessel length.
- Vertical: Poor slug handling. Slugs cause rapid liquid level rise in the small-diameter vessel, potentially flooding the mist eliminator and tripping the high-level shutdown.
Wax and Hydrate Formation
| Concern | Horizontal | Vertical |
|---|---|---|
| Wax deposition | Easier to clean, larger access | Wax accumulates in bottom head |
| Hydrate formation | Larger surface area for inhibitor contact | Less surface area; plug risk at inlet |
| Heat tracing | More surface to heat trace | Less surface but more concentrated |
| Insulation cost | Higher (more surface area) | Lower |
Offshore Applications
Offshore platforms and FPSOs have unique constraints that strongly favor horizontal separators:
- Lower center of gravity improves platform stability
- Horizontal orientation tolerates vessel motion (pitch, roll, heave) better
- Deck space is limited but height is even more restricted
- Weight is critical; horizontal vessels can be optimized for minimum weight at moderate pressures
Special service summary: Foaming, slugging, and sand-laden fluids each strongly favor one orientation. If the service involves any of these, the selection is usually clear without detailed cost comparison. Foaming favors vertical; slugging and sand favor horizontal.
6. Worked Example
Compare horizontal and vertical separator sizes for a two-phase production separator at a gas gathering facility.
Given:
Gas flow: 30 MMSCFD (SG = 0.70)
Liquid rate: 500 bbl/day condensate
GOR: 30,000,000 / 500 = 60,000 SCF/bbl
Operating pressure: 800 psig
Operating temperature: 100°F
Gas density: 2.85 lb/ft³
Liquid density: 44 lb/ft³
Liquid retention time: 2 minutes
Mist eliminator: Wire mesh demister
Vertical Separator Sizing
KSB = 0.26 (vertical, wire mesh, corrected for 800 psig)
Pressure correction: Cp = 0.88
Kcorrected = 0.26 × 0.88 = 0.229
Vmax = 0.229 × SQRT[(44 - 2.85) / 2.85]
Vmax = 0.229 × SQRT[14.44]
Vmax = 0.229 × 3.80 = 0.870 ft/s
Vdesign = 0.75 × 0.870 = 0.653 ft/s
Actual gas volume at conditions:
Qa = 30 × 10&sup6; / 1440 × (14.7/814.7) × (560/520) / 0.90
Qa = 20,833 × 0.01804 × 1.077 × 1.111 = 449.5 ACFM = 7.49 ACFS
Arequired = 7.49 / 0.653 = 11.47 ft²
D = SQRT(4 × 11.47 / π) = 3.82 ft = 45.9 in.
Select: 48-inch ID
Vessel T-T height: ~10 ft (L/D = 2.5)
Wall thickness (SA-516-70, 800 psig): ~0.56 in.
Approximate weight: ~8,500 lbs
Horizontal Separator Sizing
KSB = 0.40 (horizontal, wire mesh, corrected for 800 psig)
Pressure correction: Cp = 0.88
Kcorrected = 0.40 × 0.88 = 0.352
Vmax = 0.352 × SQRT[(44 - 2.85) / 2.85]
Vmax = 0.352 × 3.80 = 1.338 ft/s
Vdesign = 0.75 × 1.338 = 1.004 ft/s
Gas area (above 50% liquid level in horizontal vessel):
Agas = Qa / Vdesign = 7.49 / 1.004 = 7.46 ft²
Total vessel area = 7.46 / 0.50 = 14.92 ft² (50% liquid level)
D = SQRT(4 × 14.92 / π) = 4.36 ft = 52.3 in.
Select: 54-inch ID
Liquid volume = 500 / 1440 × 2 = 0.694 bbl = 29.2 gal = 3.90 ft³
54-inch vessel at 50% liquid, per foot of length: 7.95 ft³/ft
Required length for liquid: 3.90 / 7.95 = 0.49 ft (negligible)
Length governed by L/D ratio: L/D = 3.5
L = 3.5 × 54/12 = 15.75 ft (select 16 ft T-T)
Wall thickness (SA-516-70, 800 psig): ~0.64 in.
Approximate weight: ~14,200 lbs
Comparison Summary
| Parameter | Vertical | Horizontal |
|---|---|---|
| Vessel ID | 48 in. | 54 in. |
| Vessel T-T length/height | 10 ft | 16 ft |
| Wall thickness | 0.56 in. | 0.64 in. |
| Approximate weight | 8,500 lbs | 14,200 lbs |
| Plot area | ~25 ft² | ~90 ft² |
| Gas design velocity | 0.65 ft/s | 1.00 ft/s |
Conclusion: At 60,000 SCF/bbl GOR and 800 psig, the vertical separator is clearly preferred. It is 40% lighter, uses 70% less plot space, and costs less to fabricate and install. The liquid handling requirement is minimal and easily met by the vertical configuration. This confirms the GOR-based selection rule.
7. Decision Matrix
Use this decision matrix as a starting point for separator orientation selection. Score each factor and sum for an overall recommendation.
| Factor | Favors Horizontal (+H) | Neutral (0) | Favors Vertical (+V) |
|---|---|---|---|
| GOR | < 5,000 | 5,000–15,000 | > 15,000 |
| Operating pressure | < 300 psig | 300–800 psig | > 800 psig |
| Liquid retention | > 5 min needed | 2–5 min | < 2 min |
| Three-phase? | Yes | N/A | No |
| Slug flow? | Yes | Occasional | No |
| Foaming? | No | Mild | Severe |
| Sand/solids? | Yes | Minor | No |
| Plot space limited? | No | Moderate | Yes |
| Offshore? | Yes | N/A | N/A |
Quick Decision Rules
- 3 or more factors favor horizontal: Use horizontal separator
- 3 or more factors favor vertical: Use vertical separator
- Mixed results: Size both orientations and compare total installed cost
Common Mistakes in Selection
- Ignoring GOR: Selecting horizontal for a high-GOR application wastes cost and plot space
- Ignoring pressure: At high pressure, the wall thickness penalty for a larger-diameter horizontal vessel is severe
- Ignoring slug potential: Vertical separators in slug-prone service lead to frequent shutdowns and liquid carry-over
- Comparing only vessel cost: Total installed cost includes foundation, structural steel, piping, and instrumentation which differ significantly between orientations
- Neglecting future capacity: Consider future production increases when selecting orientation. Horizontal vessels are generally easier to uprate.
Best practice: Document the selection rationale in the basis of design. Include GOR, pressure, service conditions, and the decision matrix results. This prevents revisiting the decision during later project phases and provides justification during design reviews.
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