Slug Catcher PDF
Slug Catcher PDF
Slug Catcher PDF
Table of Contents
1. Purpose..........................................................................................................................3
2. Applicable Documents...................................................................................................3
3. Definitions......................................................................................................................3
4. Overview........................................................................................................................3
5. General Requirements...................................................................................................4
6. Procedure......................................................................................................................4
6.1. Slug Volume and Slug Catcher Capacity...........................................................................4
6.2. Finger Diameter.................................................................................................................5
6.3. Finger Number and Length................................................................................................5
6.4. Separation Part Length.....................................................................................................5
6.5. High High Liquid Level (HHLL)..........................................................................................6
6.6. High Liquid Level (HLL).....................................................................................................6
6.7. Normal Liquid Level (NLL).................................................................................................6
6.8. Low Low Liquid Level........................................................................................................7
6.9. Finger Spacing .................................................................................................................7
6.10. Slug Catcher Boot Sizing.................................................................................................7
6.11. Slug Catcher Risers and Liquid Outlets...........................................................................8
7. Attachments...................................................................................................................8
Engineering Procedure
1. Purpose
The purpose of this document is the process design of Slug Catchers.
2. Applicable Documents
Reference documents for this procedure are as follows:
Slug volume, reported by simulation department
PFD
Heat and Material Balance
Design Basis
GPSA
P&ID
Plot Plan
3. Definitions
g : Gas density, lbs/ft3; (Kg/m3)
f : Liquid density, lbs/ft3; (Kg/m3)
Dc : Main header pipe inside diameter, ft; (mm)
DL : Diameter of spherical liquid particles, ft; (mm)
P : Operating pressure, Psia; (kPa(abs))
Q : Standard Gas flow rate, scf/d; (sm3/d)
Qv : Gas flow rate, ft3/s; (m3/s)
T : Gas temperature, R; (K)
: Gas viscosity, lb/ft-sec; ( Pa-s)
Vh : Gas velocity in pipe, ft/sec; (m/s)
Vt : Terminal settling velocity of particle due to gravity, ft/sec; (m/s)
Z : Compressibility factor
g : Acceleration due to gravity (Assumed to be 32.2 ft/s2; 9.815 m/s2)
t : particle settling time, sec
A : Pipe cross sectional area, ft2;(m2)
4. Overview
Trunk lines transporting natural gas often operate in the two-phase flow mode due to
liquid injection and/or additional liquid formation by retrograde condensation. Slip in
velocity between the gas and the liquid in the pipeline (liquid hold-up). The largest slug
that can ever occur is that caused by sphering.
The holding capacity of the Slug Catcher must therefore be essentially as great as the
volume of the largest slug. Although liquid carryover must be limited, a Slug Catcher is not
meant to replace the high-efficiency separator.
There are two types of Slug Catchers in operation throughout the world: vessel type and
multiple-pipe type (finger type). A major advantage of the finger type Slug Catcher is the
Engineering Procedure
ease of its operation. Apart from level control, no other instrumentation is required. This
procedure follows the finger-type Slug Catcher sizing rather than the vessel type.
A vessel-type Slug Catcher is designed like a horizontal two-phase separator. Moreover,
an additional capacity shall be considered for holding the slug during the separation
process.
A finger-type Slug Catcher is a device normally made from pipe and fittings that is
designed and built to retain large, rapidly accumulated volumes of pipeline liquids (slugs).
The Slug Catcher functions as a reservoir as these liquids are treated and pumped to
storage or sales. The main components of a Slug Catcher are shown in Figure 1.
It consists essentially of two parts: 1) A separation part, separating the liquid from the
mixed stream arriving under normal flow conditions (steady), 2) A storage part, receiving
and storing the incoming liquid slug created by upset conditions (which also include
running a sphere through the pipeline).
An incoming liquid slug flows via the splitter into the inlet manifold and then via the
horizontal separation part into the sloping fingers. The downward flowing slug displaces
the gas present in the fingers up through the risers mounted on the fingers; from the
risers, the gas flows through the outlet header, carrying small liquid droplets (usually
smaller than 150 microns) and then into the gas- treating plant. The liquid/gas exchange
that takes place in the fingers ensures that the gas supply to the downstream facilities
remains uninterrupted during liquid slug arrival provided excessive liquid carryover can be
avoided. Gas that boils off the liquid in the fingers is captured by the boil-off headers
(risers) and fed back into the main gas flow downstream of the Slug Catcher.
Two important factors that must be considered in the design of a Slug Catcher are flow
tee and the sizing of the main header. The main header should be sized so that the
uppermost liquid particle in the upstream end of the main header falls to the bottom of the
main header well in advance of the down comer of the last (most downstream) finger.
Although a properly designed Slug Catcher will remove more than 95 percent of the liquid
from the gas stream, a liquid separator (preferably centrifugal rather than vane or mesh)
should always be installed downstream of any Slug Catcher.
5. General Requirements
None.
6. Procedure
6.1. Slug Volume and Slug Catcher Capacity
The Slug Catcher capacity is based on an estimated maximum slug volume from holdup
predictions. Experience with Slug Catchers has shown that liquid carryout during slug
arrival can occur whenever the slug volume exceeds about 80% of the Slug Catcher
volume. Thus, the effective Slug Catcher capacity is assumed to be 80% of the Slug
Catcher volume.
The liquid holdup is calculated by several methods as BBM, BBME, DE, etc. These data
shall be reported by Simulation Department.
Engineering Procedure
It is recommended to consider the average amount of these methods as slug volume for
sizing the Slug Catcher.
Separation Part
Gas Outlet
(Riser)
HHLL
HLL
NLL
LLL
LLLL
Liquid Outlet
Boot
g DL2 ( f g )
Vt = (for Particle diameter of less than 100 microns)
18
2. The gas occupies its proportionate share of the tube cross-section (about 20%).
For Calculating dg, we can use the tables in Attachment B.
dg
t= dg
Vt
Dc
Lsep.= Vh t
g DL2 ( f g )
Vt = (for Particle diameter of less than 100 microns)
18
Dc
t=
Vt
3. Determine the gas velocity inside the header:
14.7lbf / in 2 T
Q Z
Vh = P 520 R
Dc2 ( 86400 sec/ day )
4
4. Calculate the minimum spacing between the inlet and last Slug Catcher finger:
7. Attachments
Attachment A: A sample Slug Catcher view in P&IDs
Attachment B: Tables used for calculation of dg
Engineering Procedure
Attachment A
(1 Sheet)
Attachment B
(4 Sheets)