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.

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

Figure below shows step-by-step sizing of 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:                          40^o API

Operating pressure:           1000 psia

Operating temperature:     60^oF

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 60^oF operating temperature, we get Z factor 0.83. Use this figure to find Z factor for gas specific gravity 0.6.

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

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

K is obtained from figure below.

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 40^o 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.

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.

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

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

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

Step 7: Calculate slenderness ratio (12 L_ss/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.

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