Horizontal Two-Phase Separator Using Empirical Correlations Problem:
Gas Flow Rate 150 MMSCFD
Condensate Flow Rate 2000 BPD
Gas Specific Gravity 0.65
Condensate API Gravity 46
Slug Catcher Operating Pressure 800 Psig
Slug Catcher Operating Temperature 115 F
Gas Viscosity 0.011 cP
Gas Compressibility (Z) 1
From Flow Assurance:
Slug Volume 50 Bbl
Slug Duration 1 min
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
= 72,000.00 BPD
Slug Surge Volume = 48.61 Bbl
50 Bbl
Step 2 - Calculate the liquid capacity
Liquid Capacity =
= 6.94 Bbl
Step 3 - Calculate the total required liquid volume
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:
Goal: Develop preliminary slug catcher size and dimensions.
The flow rate from the slug catcher is assumed to be equal to the normal condensate flow rate of 2,000 bbl/day. Therefore,
Since this is so close to the expected slug volume, it was decided to use 50 bbls as the slug surge volume.
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,
Design Liquid Flow Rate x
(Retention Time + Level Response Time)
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 Bbl
Step 4 - Calculate the maximum allowable gas velocity
The liquid and gas densities must be converted to the appropriate units: = 49.74422535
= 2.49
Based on the guidelines presented in Section 7.9, a K value of 0.35 is selected
1.53 ft/s
Step 5 - Calculate the minimum gas cross-sectional area
= 34.64
The minimum gas cross-sectional area can now be calculated:
Minimum Gas Cross-sectional Area = Gas Flow Rate/Maximum Allowable Gas Velocity
= 23
Step 6 - Estimate the minimum vessel internal diameter based on gas capacity
Note 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:
The maximum allowable gas velocity will be based on empirical correlations, using the following equation:
Liquid Density, lb/ft3 = 62.4 lb/ft3 x 141.5/(API + 131.5)
lb/ft3
Gas Density, lb/ft3 = Gas Molecular Weight x Operating Psia/(10.73 x Operating °R x Z)
lb/ft3
Vm
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
ft3/s
ft2
The 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 = 6.43 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 5
Minimum Internal Diameter 6.47 ft
6.50
Step 9 - Estimate vessel length
Length = Diameter x (L/D)
= 32.50
Step 10 - Calculate the vertical liquid height
α 71.00
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:
The vessel length is calculated from the internal diameter estimated in Step 8 and the corresponding L/D ratio:
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:
H 2.19 ft
= 319.70
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.
Slug Surge Volume = (Slug Flow Rate - Flow Rate from Slug Catcher) x (Slug Duration)
Slug Volume/Slug Duration 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:
The flow rate from the slug catcher is assumed to be equal to the normal condensate
Since this is so close to the expected slug volume, it was decided to use 50 bbls as the
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.
Design Liquid Flow Rate x
(Retention Time + Level Response
The liquid and gas densities must be converted to the appropriate units:
Based on the guidelines presented in Section 7.9, a K value of 0.35 is selected
Note that most of the liquid volume requirement is due to the slug. The desired units for the slug catcher diameter and
The maximum allowable gas velocity will be based on empirical correlations, using the
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
The minimum internal diameter based on gas capacity is calculated from the minimum gas
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:
Step 8 - Determine minimum vessel internal diameter which satisfies both gas and liquid capacity 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:
The vessel length is calculated from the internal diameter estimated in Step 8 and the corresponding
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
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
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