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Preliminary Sizing of Two-phase Vertical Separator


In previous post, I shared about how to size two-phase horizontal separator. In this pos, I want to share to you how to size two-phase vertical separator.

Schematic of Two-Phase Vertical Separator

Figure below is schematic drawing of two-phase vertical separator.

Vertical two phase separator
Vertical two phase separator

Step-by-Step Sizing of Two-Phase Vertical Separator

Figure below shows step-by-step sizing of two-phase vertical separator.

Step-by-step sizing two-phase vertical separator
Step-by-step sizing two-phase vertical separator

I will use an example to explain step-by-step sizing of two-phase vertical separator.

Example of Two-Phase Vertical Separator Sizing

Determine the diameter and height (seam-to-seam length) of a two-phase vertical separator for the following operating condition:

Gas rate:                              10 MMscfd

Gas specific gravity:            0.6

Oil rate:                               2000 BOPD (barrel oil per day)

Oil gravity:                          40o API

Operating pressure:           1000 psia

Operating temperature:     60oF

Let’s start sizing of two-phase vertical separator.

Step 1: Collect the Data

Gas rate, gas specific gravity, oil rate, oil gravity, operating pressure, and operating temperature are already defined above.

Step 2: Find Other Properties (Z factor, gas viscosity)

For 0.6 gas specific gravity, 1000 psia operating pressure, and 60oF operating temperature, we get Z factor 0.83. Use this figure to find Z factor for gas specific gravity 0.6.

Compressibility factor for 0.6 specific gravity of natural gas
Compressibility factor for 0.6 specific gravity of natural gas

For 0.6 gas specific gravity, 1000 psia operating pressure, and 60oF operating temperature, we get gas viscosity 0.013 cP. Use this figure to find gas viscosity.

Hydrocarbon gas viscosity
Hydrocarbon gas viscosity

Step 3: Calculate K (Constant used in gas capacity equation)

K is obtained from figure below.

Constant used in gas capacity equation
Constant used in gas capacity equation

K is a function of S P/T and oil API.

Where:

S = specific gravity of of gas capacity

P = pressure (psia)

T = temperature (oR)

From the case above,

S P/T = 0.6 x 1000 / (60 + 460)

S P/T = 1.154

For S P/T 1.154 and oil gravity of 40o API, we get K value of 0.28.

Step 4: Calculate Gas Capacity Constraint (d2)

Gas capacity constraint is calculated by using the following equation.

Gas capacity constraint
Gas capacity constraint

We get gas capacity constraint (d) is 24.68 in.

Step 5: Calculate liquid capacity constraint

Liquid capacity constraint is calculated by using the following equation.

Liquid capacity constraint
Liquid capacity constraint

Step 6: Calculate combinations of diameter (d) and seam-to-seam length (Lss) for various retention time (tr)

Seam-to-seam length (Lss) for liquid capacity constraint is calculated by using the following equation.

Seam-to-seam length for gas capacity constraint
Seam-to-seam length for gas capacity constraint

Table below shows combination of separator diameter and length for liquid capacity constraint.

Seam-to-seam length for various retention time
Seam-to-seam length for various retention time

Step 7: Calculate slenderness ratio (12 Lss/d). Choose between 3 or 4

See step 6.

Step 8: Graph results and choose reasonable size with a diameter and length combination above both gas capacity and liquid capacity constraint line

Graph below show vessel internal diameter and vessel length for various retention time.

Result graph
Result graph

Free Spreadsheet for Two-Phase Vertical Separator

You can download and learn about the sizing step by downloading this spreadsheet: Sizing two-phase vertical separator.

I hope you find this post useful.

References:

  • Surface Production Operation Vol. 1: Design of Oil-Handling Systems and Facilities

 

 

 

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