SLUG CATCHER – FINGER TYPE DESIGN 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 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. 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. It is recommended to consider the average amount of these methods as slug volume for sizing the Slug Catcher.
Definitions
ρg: Gas density, lbs/ft3; (Kg/m3 ρf: 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) μ: Gas viscosity, lb/ft-sec; (μPa-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)
Finger Diameter It must be considered that the fingers’ diameter should be at least one and one half times the diameter of the inlet line. Finger Number and Length The number and length of the fingers are normally based on economics, although in all instances 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 the available length of fingers. It must be noted that if more than two fingers are required to contain the maximum anticipated slug size, then those in excess of two should be equally spaced between the first and last finger. The fingers must slop toward the liquid header. Separation Part Length This horizontal part of the Slug Catcher shall be sized as a two phase separator as follows:
High Liquid Level (HLL) For calculating the HLL of the Slug Catcher, we should calculate the total slug volume per finger, first. Then, by assuming that the calculated slug volume exists in Slug Catcher, the High Liquid Level can be determined (It should be between HHLL and NLL). Normal Liquid Level (NLL) For calculating the NLL of the Slug Catcher, the liquid volume stored in each finger of the Slug Catcher should be calculated using the normal liquid flow rate and the assumed retention time. Then, the Normal Level of the liquid must be calculated considering that the 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 Level can be determined.
Slug Catcher Boot Sizing We usually consider a boot to separate the inlet water, which is mixed with the liquid hydrocarbon and may become separated during the liquid retention time in the Slug Catcher. For boot sizing, we should consider the water flow rate into the Slug Catcher and assume a retention time for the water in the boot. By considering the maximum water volume, we can choose the appropriate diameter and height for the boot. Boots shall be sized for a minimum residence time between HLL and LLL of 5 minutes as a guideline: their diameters shall be the same as the commercial pipe sizes as far as possible. The height diameter ratio shall be 2:1 – 5:1, it shall be determined with consideration given to operability and the minimum size of level instruments and equipment. Boot diameters shall be 12 inches minimum because good operability cannot be provided if the boots are smaller than 12 inches. Slug Catcher Risers and Liquid Outlets We should consider the maximum allowable liquid and gas velocities in pipes, which are defined in the project Design Basis to size the risers and liquid outlets’ pipes.
