Sizing liquid surge drum

These surge times are based on an experienced operator s response time to a process upset 

Allow 3 minute liquid surge time from LLL to low low liquid level (LLLL), and 3 minute su e time from ULL to high high liquid level (HHLL). It is assumed that at HLL or LLL, liquid level alarm will be sounded to alert the operator, and at HHLL or LL1.L, the feed flow or liquid outlet flow will be stopped by an emergency shutdown valve. HHLL and LLLL is user s option to have them or not. 

TTis liquid surge volume (VL, in ft3) from HHLL to LLLL is assumed to be 80 to 90% of the total vessel volume without considering vessel head volumes for conservative design. Following equations can be used to size a vertical or horizontal surge drum  

QF is liquid foed design volumetric flow rate, in fl3/hr AT is drum cross sectional area, in fl2, L is vessel length, in ft. 

Examples of vapor/liquid separator are (I) amine flash drum, where majority of inlet flow is liquid with very little flashed vapor, (2) compressor suction knock out drum or ftiel gas knock out drum, where the majority of inlet flow is vapor with very little carryover liquid, (3) hot separator in hydrotreater, where considerable amount of vapor and liquid are in the inlet flow. In general, vapor/liquid separator can be either vertical or horizontal. 

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Vessel sized for liquid surge drum and vapor/liquid separation by gravity settling method can be used for final design. Vessel siz for vapor/liquid separation by mist eliminator method should be checked by the mist eliminator vendor to confirm its size. Vessel sized for Hquid/liquid separation by surge volume method or with coalescer pad is a rough preliminary sizing. Its size should be checked by vendor or an expert. 

We must also calculate the size of the reflux drum and the column base. These provide liquid surge capacity, which helps to filter disturbances in both flow and composition to downstream units. They also permit the column to ride through large disturbances without upsetting the column to the point where liquid or vapor hydraulic limitations are encountered (flooding or weeping), which can result in the loss of separation and the production of off-specification products. 

The conventional heuristic used for sizing a decanter is to specify at least 20 min of holdup time. This is larger than for simple surge drums because the small difference in liquid densities between the two liquid phases requires a large settling time. The inventory of both phases must be measured and controlled. 

Typically the sizing of drums is related to a process consideration such as liquid holdup (surge), storage volume, or velocity considerations for separation. Surge volume in process units relates to the response time required for the alarms and operators to respond to upstream or downstream conditions. 

In preparation for exporting the steady-state flowsheet into Aspen Dynamics, aU equipment is sized. Column diameters are calculated by Aspen Tray Sizing. Reflux drums and column bases are sized to provide 5 min of holdup when 50% full, based on the total liquid entering the surge capacity. Pumps and control valves are specified to give adequate dynamic rangeability. Typical valve pressure drops are 2 atm. 

Typically the sizing of drums is related to a process consideration such as liquid holdup (surge), storage... 

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