Slug Catcher SizingHorizontal Two-Phase Separator Using Empirical CorrelationsProblem: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 Rate150 MMSCFDCondensate Flow Rate2000BPDGas Specific Gravity0.65Condensate API Gravity46Slug Catcher Operating Pressure800 PsigSlug Catcher Operating Temperature115 FGas Viscosity0.011cPGas Compressibility (Z)1

From Flow Assurance:Slug Volume50BblSlug Duration1min

Goal: Develop preliminary slug catcher size and dimensions.Solution

Step 1 - Calculate the slug surge volumeThe 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 = 72,000.00BPDThe flow rate from the slug catcher is assumed to be equal to the normal condensate flow rate of 2,000 bbl/day. Therefore,Slug Surge Volume=48.61BblSince this is so close to the expected slug volume, it was decided to use 50 bbls as the slug surge volume.50BblStep 2 - Calculate the liquid capacityThree minute retention time was thought to be adequate for gas/liquid separation. Anadditional 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)=6.94BblStep 3 - Calculate the total required liquid volumeThe minimum total amount of required liquid volume is the sum of the slug surge capacityvolume and the liquid capacity volume:Total Liquid Volume = Slug Surge Volume + Liquid Capacity Volume=56.94Bbl=319.70ft3Note that most of the liquid volume requirement is due to the slug. The desired units for the slug catcher diameter and length are feet. The total liquid volume is converted from barrels to cubic feet:Step 4 - Calculate the maximum allowable gas velocityThe maximum allowable gas velocity will be based on empirical correlations, using thefollowing 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)=49.7442253521 lb/ft3Gas Density, lb/ft3 = Gas Molecular Weight x Operating Psia/(10.73 x Operating R x Z)=2.49 lb/ft3Based on the guidelines presented in Section 7.9, a K value of 0.35 is selectedVm1.53ft/sStep 5 - Calculate the minimum gas cross-sectional areaThe 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=34.64ft3/sThe minimum gas cross-sectional area can now be calculated:Minimum Gas Cross-sectional Area = Gas Flow Rate/Maximum Allowable Gas Velocity=23ft2Step 6 - Estimate the minimum vessel internal diameter based on gas capacityThe minimum internal diameter based on gas capacity is calculated from the minimum gas cross-sectional 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=6.43ftF=0.7

Step 7 - Estimate the minimum vessel internal diameter based on liquid capacityThe 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:

FD1D2An initial L/D ratio of 5 was assumed:0.117.00482176824.4889648036L/D5Minimum Internal Diameter6.47ft0.212.02422478524.66871153516.500.39.81773842544.8812122226Step 8 - Determine minimum vessel internal diameter which satisfies both gas and liquid capacity criteria0.48.50241088415.138581945For 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:0.57.60478748385.4605581373Step 9 - Estimate vessel length0.66.94218941655.882207939The vessel length is calculated from the internal diameter estimated in Step 8 and the corresponding L/D ratio:0.76.42721849836.4742075592Length = Diameter x (L/D)0.86.01211239267.4111176022=32.500.95.66827392279.3374230703Step 10 - Calculate the vertical liquid height15.3773967993-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:HVc00-71.3721851135120.0710202976-64.6825593695240.2809772627-40.5485116151360.62069476839.2876315157481.075325529388.6394176022601.625196.2168714155722.2456947683325.8495433546For 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:842.9102824944467.55343623971.00o963.5897175056609.2573291235H2.19ft1084.2543052317738.89000106261204.875846.46745487591325.4246744707925.8192409624Vc=319.70ft31445.8793052317975.6553840932Step 11 - Review design feasibility1566.2190227373999.7894318476The 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.1686.42897970241006.47905759161806.51078.3704188482

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