36 in OD Horizontal Three-Phase Separator

Design Conditions

Loadings Considered (UG-22)

The vessel is analyzed for the loadings listed in UG-22. Governing loads for this configuration are:

• Internal design pressure of 200 psig at 200 °F.
• Weight of the vessel and its contents at design (water plus oil plus gas) and at hydrostatic test (water-full).
• Two-saddle reactions evaluated by the Zick saddle method per nonmandatory Appendix G-1.
• ASCE 7 wind load on the projected area at 80 mph basic wind speed (Exposure C, Category III), giving a horizontal force of approximately 510 lb on the assembled vessel.
• ASCE 7 seismic load on the operating weight at S_DS = 0.8 g (typical western North America moderate-seismic site), with R = 3 for a non-building structure on saddles, giving a horizontal lateral force of approximately 1,150 lb at the centroid.

The wind and seismic loads are vectorially combined with the gravity reactions at the saddles per ASCE 7 load combinations. The Zick procedure is run with the resulting modified saddle reactions in worked example 5.

Toughness Basis

The Minimum Design Metal Temperature is -20 °F at the full design pressure of 200 psig. The shell and heads are SA-516 Grade 70 Normalized, Curve D under Table UCS-66. Required MDMT at the shell thickness (0.500 in nominal) is approximately -55 °F at no impact testing.

Hydrostatic Test Basis

The shop hydrostatic test follows UG-99(b):

The Class 300 RFWN flanges have a cold rating of 770 psig at 70 °F (B16.5 Gp 1.1), well above the test pressure. SA-516 Grade 70 does not derate below 300 °F so the material LSR is 1.00.

Pressure by Component

The shell governs the vessel MAWP at 420 psig, twice the design pressure. The Class 300 flanges sit at 720 psig at 200 °F (B16.5 Group 1.1), comfortably above the shell capacity.

Governing Thickness, UG-16(b)

Per UG-16(b), the minimum required thickness is 0.0625 in. All components meet UG-16(b) by a wide margin.

Hydrostatic Test Summary

Material Schedule

Nozzle Schedule

NPS 6 inlet and NPS 4 gas outlet exceed the UG-36(c)(3)(a) 3.5 in exemption limit and require UG-37 area calculations; both pass without repads on the 0.375 in corroded shell wall.

Internal Components

The vessel includes a partial-height weir baffle at approximately 60% of the vessel length from the inlet end. The weir is sized so that the oil retention zone upstream provides at least 5 minutes of residence time at design oil flow, and the oil-overflow level sets the oil-water interface in the upstream zone. Water exits the bottom upstream of the weir via N4; oil flows over the weir into the downstream zone and exits via N3. The mist extractor at the gas outlet (NPS 4) is a 6 in deep mesh pad sized for K_SB = 0.21 ft/s and 99% removal of droplets above 10 micron.

Radiography

Category A welds (long seam and head-to-shell circumferentials) are fully radiographed per UW-11(a)(1). Nozzle attachment welds are examined by surface NDE per UW-50.

Dimensional Summary

Every clause applied in the analysis, grouped by topic.

Wall Thickness

Nozzle Reinforcement and Standard Fittings

Supports and External Loads

Three-Phase Separation Sizing

The complementary three-phase separator sizing tool at /engineering/separator-3phase/ runs the same correlations interactively.

Welds, Toughness, Test, Marking

Five worked examples. The pressure-containment calcs (shell and head) are similar to the smaller separator on page #02. The three-phase sizing and the Zick analysis with wind and seismic are the new content.

Example 1. Shell Wall Thickness, UG-27(c)(1)

Required thickness of the SA-516 Grade 70 Normalized cylindrical shell at 200 psig and 200 °F.

Required thickness with CA is 0.240 in. Mill-minimum supplied is 0.438 in (0.500 nominal less 12.5%), corroded to 0.375 in. PASS with 0.135 in margin. Shell MAWP at corroded thickness 0.375 in is 420 psig.

Example 2. 2:1 SE Head Thickness, UG-32(d)

Required thickness with CA is 0.239 in. Head supplied nominal 0.500 in, formed-corroded min 0.388 in. PASS with 0.149 in margin. Head MAWP at 0.388 in is 439 psig.

Example 3. Three-Phase Separation Sizing, Gas Capacity and Liquid Retention

The vessel is sized to accept 5 MMscfd of natural gas (S.G. 0.65) with 1,500 BPD oil and 500 BPD water at design conditions. The sizing follows API 12J for horizontal three-phase separators.

Gas capacity (Souders-Brown)

The gas-phase cross section above the high oil liquid level is approximately 35% of the vessel cross-section. Available area is 0.35 × π/4 × (36/12)² = 2.47 ft². Maximum gas flow is 2.47 ft² × 1.70 ft/s × 3,600 s/h = 15,100 ft³/h actual. Convert to standard cubic feet using P_design at the gas density correction: 15,100 ft³/h × (214.7 / 14.7) × (520 / 660) = 174,000 scf/h = 4.2 MMscfd. Below the 5 MMscfd target by 16%, so the gas outlet area is undersized or the K_SB must be reduced. WARN. The fix in a real design is either to raise the operating level so that less liquid volume is held and more gas headroom is available, or to add a vane-pack mist extractor with K_SB = 0.35 ft/s.

Liquid retention (5 min oil, 3 min water rinse)

The vessel's available liquid volume below the 60% level is approximately 60 ft³ (60% of 100 ft³ total internal volume), so the retention requirements are met. The weir is positioned at 60% of vessel length from the inlet to give the oil 5 min of residence in the upstream zone.

Example 4. Stokes Settling for the Water-in-Oil Droplet Cut Size

Per API 12J the design droplet size for water-in-oil separation is 500 micron (0.5 mm). The Stokes settling velocity is computed with a small Ishii-Mishima drag correction for non-spherical droplets.

The water droplet settles at 0.0273 ft/s. For a 36 in (3 ft) vessel with the oil-water interface at midplane, the maximum distance a droplet must traverse is approximately 12 in (1 ft) from top of oil pad to interface. Time to traverse: 1 / 0.0273 = 36.6 s. Available oil retention time is 5 min = 300 s. PASS with very large margin; the oil retention is governed by interfacial coalescence and rag-layer formation, not by Stokes settling of the design droplet.

Example 5. Zick Saddle Analysis With ASCE 7 Wind and Seismic

The horizontal vessel rests on two saddles. Operating gravity weight gives a per-saddle reaction of 3,250 lb (W_op = 6,500 lb / 2 saddles). ASCE 7 wind and seismic lateral loads add a horizontal couple that modifies the vertical saddle reactions.

Wind load (ASCE 7)

Seismic load (ASCE 7)

Seismic governs over wind by a factor of seven. The seismic horizontal force is combined with gravity per ASCE 7 load combinations; the worst-case saddle reaction is approximately 4,400 lb on the windward saddle and 2,100 lb on the leeward saddle, with each saddle also taking 1,100 lb of lateral shear.

Zick stresses at worst-case test condition (Q = 4,250 lb, hydrotest)

All five Zick stresses are well within allowables even at the modified saddle reactions from the ASCE 7 seismic combination. PASS. On this 36 in vessel the saddle dimensions are governed by the pad-and-clip fabrication detail rather than by stress, and the anchor bolts are sized for ASCE 7 seismic uplift (worst-case uplift at the leeward saddle is approximately 1,400 lb, well within the capacity of four 1/2 in anchor bolts).