Slug Catcher Design Guide

7/22/2019 Slug Catcher Design Guide

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Engineering Procedure

Title: Slug Catcher Process Design Page 2 of 9

Document No.: Rev.: 0

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 ...........................................................................46.2. Finger Diameter .................................................................................................................56.3. Finger Number and Length ................................................................................................56.4. Separation Part Length .....................................................................................................56.5. High High Liquid Level (HHLL) ..........................................................................................66.6. High Liquid Level (HLL) .....................................................................................................66.7. Normal Liquid Level (NLL) .................................................................................................66.8. Low Low Liquid Level........................................................................................................76.9. Finger Spacing .................................................................................................................76.10. Slug Catcher Boot Sizing .................................................................................................7

6.11. Slug Catcher Risers and Liquid Outlets ...........................................................................87. Attachments ...................................................................................................................8


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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 toliquid injection and/or additional liquid formation by retrograde condensation. Slip invelocity between the gas and the liquid in the pipeline (liquid hold-up). The largest slugthat can ever occur is that caused by sphering.

The holding capacity of the Slug Catcher must therefore be essentially as great as thevolume of the largest slug. Although liquid carryover must be limited, a Slug Catcher is notmeant to replace the high-efficiency separator.

There are two types of Slug Catchers in operation throughout the world: vessel type andmultiple-pipe type (finger type). A major advantage of the finger type Slug Catcher is the


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ease of its operation. Apart from level control, no other instrumentation is required. Thisprocedure 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 separationprocess.

A finger-type Slug Catcher is a device normally made from pipe and fittings that isdesigned 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 tostorage 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 themixed stream arriving under normal flow conditions (steady), 2) A storage part, receivingand storing the incoming liquid slug created by upset conditions (which also includerunning a sphere through the pipeline).

An incoming liquid slug flows via the splitter into the inlet manifold and then via thehorizontal separation part into the sloping fingers. The downward flowing slug displacesthe gas present in the fingers up through the risers mounted on the fingers; from therisers, the gas flows through the outlet header, carrying small liquid droplets (usuallysmaller than 150 microns) and then into the gas- treating plant. The liquid/gas exchangethat takes place in the fingers ensures that the gas supply to the downstream facilitiesremains 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 flowtee and the sizing of the main header. The main header should be sized so that theuppermost liquid particle in the upstream end of the main header falls to the bottom of themain 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 liquidfrom the gas stream, a liquid separator (preferably centrifugal rather than vane or mesh)should always be installed downstream of any Slug Catcher.

5. General RequirementsNone.

6. Procedure

6.1. Slug Volume and Slug Catcher Capacity

The Slug Catcher capacity is based on an estimated maximum slug volume from holduppredictions. Experience with Slug Catchers has shown that liquid carryout during slugarrival can occur whenever the slug volume exceeds about 80% of the Slug Catchervolume. Thus, the effective Slug Catcher capacity is assumed to be 80% of the SlugCatcher volume.

The liquid holdup is calculated by several methods as BBM, BBME, DE, etc. These datashall be reported by Simulation Department.


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It is recommended to consider the average amount of these methods as slug volume forsizing the Slug Catcher.

HHLL

HLL

NLLLLL

LLLL

Boot

Separation Part

Gas Outlet

(Riser)

Liquid Outlet

Water OutletFigure 1

6.2. Finger Diameter

It must be considered that the fingers diameter should be at least one and one halftimes the diameter of the inlet line.

6.3. Finger Number and Length

The number and length of the fingers are normally based on economics, although in allinstances the fingers must have sufficient volume to contain the maximum anticipated

slug size.The Slug Catcher area on the Plot Plan shall be checked in order to finalize theavailable length of fingers.

It must be noted that if more than two fingers are required to contain the maximumanticipated slug size, then those in excess of two should be equally spaced between thefirst and last finger.

The fingers must slop toward the liquid header.

6.4. Separation Part Length

This horizontal part of the Slug Catcher shall be sized as a two phase separator as

follows:


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1. Vt =( )43.029.0

71.014.171.054.3

g

gfLDg(for Particle diameter of 1000 microns)

Vt =( )

18

2

gfLDg(for Particle diameter of less than 100 microns)

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.

t =t

g

V

d

3. Calculate the minimum length for separation part:

Lsep.= tVh

Vh is the gas velocity in pipe:

A

QV vh =

6.5. High High Liquid Level (HHLL)

For calculating the HHLL of the Slug Catcher, the Slug Catcher must be considered tobe totally filled with slug (except the separation part).

6.6. High Liquid Level (HLL)

For calculating the HLL of the Slug Catcher, we should calculate the total slug volumeper finger, first. Then, by assuming that the calculated slug volume exists in SlugCatcher, the High Liquid Level can be determined (It should be between HHLL andNLL).

6.7. Normal Liquid Level (NLL)

For calculating the NLL of the Slug Catcher, the liquid volume stored in each finger ofthe Slug Catcher should be calculated using the normal liquid flow rate and the assumedretention time. Then, the Normal Level of the liquid must be calculated considering thatthe calculated volume of the liquid exists in the Slug Catcher.Low Liquid Level (LLL)

First, we assume that the low liquid volume is 20-30% of the normal liquid volume. Then

by assuming that this calculated volume exists in the Slug Catcher, the Low Liquid Levelcan be determined.

Dc

dg


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6.8. Low Low Liquid Level

This level is usually assumed to be between 150 mm-200 mm above the Slug Catcherlowest part (its end point).

6.9. Finger Spacing

1. Calculate the terminal liquid particle velocity:

Vt = ( )43.029.071.014.171.0

54.3

g

gfLDg(for Particle diameter of 1000 microns)

Vt =( )

18

2

gfLDg(for Particle diameter of less than 100 microns)

2. Determine the particle settling time:

t =t

c

V

D

3. Determine the gas velocity inside the header:

Vh =

( )dayD

ZR

T

P

inlbfQ

c sec/864004

520

/7.14

2

2

4. Calculate the minimum spacing between the inlet and last Slug Catcher finger:

Min Spacing (ft) = tVh

6.10. Slug Catcher Boot Sizing

We usually consider a boot to separate the inlet water, which is mixed with the liquidhydrocarbon and may become separated during the liquid retention time in the SlugCatcher.

For boot sizing, we should consider the water flow rate into the Slug Catcher andassume a retention time for the water in the boot. By considering the maximum watervolume, we can choose the appropriate diameter and height for the boot.

Boots shall be sized for a minimum residence time between HLL and LLLof 5 minutes as a guideline: their diameters shall be the same as the

commercial pipe sizes as far as possible. The height diameter ratio shallbe 2:1 5:1, it shall be determined with consideration given to operabilityand the minimum size of level instruments and equipment.


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Boot diameters shall be 12 inches minimum because good operabilitycannot be provided if the boots are smaller than 12 inches.

6.11. Slug Catcher Risers and Liquid Outlets

We should consider the maximum allowable liquid and gas velocities in pipes, which aredefined in the project Design Basis to size the risers and liquid outlets pipes.

7. Attachments

Attachment A: A sample Slug Catcher view in P&IDsAttachment B: Tables used for calculation of dg

Slug Catcher Design Guide

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