Slug Catcher Sizing Spreadsheet

Horizontal Two-Phase Separator Using Empirical Correlations Problem: A slug catcher for a new offshore gas processing facility is to be sized. Gas from a subsea well cluster will be transported to the platform in a common flow line. No pigging facilities will be required for the flow line. A horizontal pressure vessel will be used as both the slug catcher and the primary separator. The gas from the separator will go to a gas dehydration system. The following information is based on preliminary well tests:

Gas Flow Rate Condensate Flow Rate Gas Specific Gravity Condensate API Gravity Slug Catcher Operating Pressure Slug Catcher Operating Temperature Gas Viscosity Gas Compressibility (Z) From Flow Assurance: Slug Volume Slug Duration

150 2000 0.65 46 800 115 0.011 1

MMSCFD BPD

50 1

Bbl min

Psig F cP

Goal: Develop preliminary slug catcher size and dimensions. Solution Step 1 - Calculate the slug surge volume The general equation for slug surge volume is: Slug Surge Volume = (Slug Flow Rate - Flow Rate from Slug Catcher) x (Slug Duration) The slug flow rate is calculated based on the pipeline simulation results: Slug Flow Rate = Slug Volume/Slug Duration = to the normal 72,000.00 BPD The flow rate from the slug catcher is assumed to be equal condensate flow rate of 2,000 bbl/day. Therefore, = Slug Surge Volume 48.61 Bbl Since this is so close to the expected slug volume, it was decided to use 50 bbls as the slug surge volume. 50 Bbl Step 2 - Calculate the liquid capacity Three minute retention time was thought to be adequate for gas/liquid separation. An additional two minutes of retention time will be allowed for high level response time. Therefore, Liquid Capacity

=

=

Design Liquid Flow Rate x (Retention Time + Level Response Time) 6.94

Bbl

Step 3 - Calculate the total required liquid volume The minimum total amount of required liquid volume is the sum of the slug surge capacity volume and the liquid capacity volume:

Total Liquid Volume = Slug Surge Volume + Liquid Capacity Volume = 56.94 =

319.70

Bbl ft3

Note that most of the liquid volume requirement is due to the slug. The desired units for the slug catcher d length are feet. The total liquid volume is converted from barrels to cubic feet: Step 4 - Calculate the maximum allowable gas velocity The maximum allowable gas velocity will be based on empirical correlations, using the following equation:

The liquid and gas densities must be converted to the appropriate units: Liquid Density, lb/ft3 = 62.4 lb/ft3 x 141.5/(API + 131.5) =

49.74422535 lb/ft3

Gas Density, lb/ft3 = Gas Molecular Weight x Operating Psia/(10.73 x Operating °R x Z) = 2.49 lb/ft3 Based on the guidelines presented in Section 7.9, a K value of 0.35 is selected Vm 1.53 ft/s Step 5 - Calculate the minimum gas cross-sectional area The minimum gas cross-sectional area is calculated from the maximum allowable gas velocity calculated in Step 4 and the gas flow rate. The gas flow rate is the design volumetric flow rate at actual operating pressure and temperature: Actual Gas Flow Rate = Standard Gas Flow Rate x (14.7 psia/Actual Psia) x (Actual °R/520 °R) x Z = 34.64 ft3/s The minimum gas cross-sectional area can now be calculated: Minimum Gas Cross-sectional Area = Gas Flow Rate/Maximum Allowable Gas Velocity = 23 ft2 Step 6 - Estimate the minimum vessel internal diameter based on gas capacity The minimum internal diameter based on gas capacity is calculated from the minimum gas crosssectional area calculated in Step 5:

where F is the assumed fraction of cross-sectional area occupied by the vapor space. An initial F value of 0.30 is assumed:

Minimum Internal Diameter

=

6.43

ft

Step 7 - Estimate the minimum vessel internal diameter based on liquid capacity The minimum internal diameter based on liquid capacity is calculated from the total required liquid volume calculated in Step 3 and the vapor space cross-sectional area fraction assumed in Step 6:

An initial L/D ratio of 5 was assumed: L/D Minimum Internal Diameter

5 6.47 ft 6.50 Step 8 - Determine minimum vessel internal diameter which satisfies both gas and liquid capacity criteria For each L/D ratio considered, Steps 6 and 7 are repeated in an iterative procedure in which the value of F is adjusted until the internal diameters are equal. For this example, it has been determined that an F of approximately 0.7 results in a diameter that satisfies both the gas and liquid capacity criteria:

Step 9 - Estimate vessel length The vessel length is calculated from the internal diameter estimated in Step 8 and the corresponding L/D ratio: Length = Diameter x (L/D) =

32.50

Step 10 - Calculate the vertical liquid height The vertical liquid height can be calculated from the vessel diameter calculated in Step 8, the vessel length calculated in Step 9, and the total liquid volume calculated in Step 3. In this iterative procedure, the methodology is to guess a value for the angle a between 0 and 180 degrees until the volume calculated by the geometric formula equals the required liquid volume. Then the vertical liquid height can be calculated. The following formulas are used:

For this example, different values of a are guessed until the calculated volume Vc equals the total liquid volume of 319 ft3 from Step 3. It has been determined that an angle of 71.8 degrees will result in the required liquid volume: α

71.00

o

H

2.19

ft

Vc

=

319.70

ft3

Step 11 - Review design feasibility The results should be evaluated to ensure that any design criteria are satisfied. The guidelines presented in Sections 6.4 and 6.6 can be used to establish appropriate liquid levels and dimensions. A sketch such as the one illustrated in Figure 11:6-1 is typically prepared to assist in determining vessel dimensions. Other L/D ratios should be tried. Adjustments in diameter and length should be made as appropriate. The calculations are summarized on the following pages.

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g Duration

sate

as the

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ow Rate x + Level Response

capacity

nits for the slug catcher diameter and

g the

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pace. An initial F value

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d liquid capacity ure in which the value en determined that an capacity criteria:

d the corresponding

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c equals the total liquid ees will result in the

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