Surge volume

The vertical position is preferred if the plot space is tight. Horizontal vessels are easier to support and are preferred when large Hquid surge volumes are required. The Hquid level displacement height for a unit volume is much less for a horizontal vessel than for a vertical vessel, which makes the control range shorter than for a vertical vessel. The displacement height per unit volume is only approximately linear on horizontal vessel when the level is near the center line, however. This can be a problem if the normal Hquid level is too low or too high and the instniments are not tuned for quick response. 

A vessel handling large amounts of liquid or a large liquid surge volume will usually be horizontal. Also, where water must be separated from hydrocarbon liquid, the vessel will be horizontal. A vessel with small surge volume such as a compressor knockout drum will usually be vertical. 

Select appropriate full surge volume in seconds. Calculate the required vessel surge volume. 

For small volumes of liquid, it may be necessary to provide more liquid surge than is necessary to satisfy the L/D > 3. Otherwise this criteria should be observed. If the required liquid surge volume is greater than that possible in a vessel having L/D < 5, a horizontal drum must be provided. 

Suction throttle valves are common in gas-lift service to minimi/c the action of the flare valve. Flow from gas-lift wells decreases with increased back-pressure. If there were no suction valve, the flare valve may have to be set at a low pressure to protect the compressor. With a suction valve it may be possible to set the flare valve at a much higher pressure slightly below the working pressure of the low-pressure separator. The difference between the suction valve set pressure and the flare valve set pressure provides a surge volume for gas and helps even the flow to the compressor. 

In very extreme cases, total containment can be provided to prevent any atmospheric emission or to provide a surge volume for controlled flaring, absorption, or other disposal methods. This approach, however, requires use of a very large pressure vessel to provide the required volume, and is usually only a last choice alternative. 

Over the years some heuristics have been developed that work pretty well in most systems. Holdup times (based on total flow in and out of the surge volume) of about 5 to 10 minutes seem to work well. If the column has a fired... 

J1. Two tOO-barrel tanks are available to use as surge volume to filter liquid flow rate disturbances in a petroleum refinery. Average throughput is 14,400 barrels per day. Should these tanks be piped up for parallel operation or for series operation Assume proportional-only level controllers. 

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Then the inventory loops are revisited. The liquid holdups in surge volumes are calculated so that the time constants of the liquid level loops (using proportional-only controllers) are a factor of 10 larger than the product-quality time constants. This separation in time constants permits independent tuning of the material-balance loops and the prod-... 

Once we have fixed a flow in each recycle loop, we then determine what valve should be used to control each inventory variable. This is the material balance step in the Buckley procedure. Inventories include all liquid levels (except for surge volume in certain liquid recycle streams) and gas pressures. An inventory variable should typically be controlled with the manipulated variable that has the largest effect on it within that unit (Richardson rule). Because we have fixed a flow in each recycle loop, our choice of available valves has been reduced for inventory control in some units. Sometimes this actually eliminates the obvious choice for inventory control for that unit. This constraint forces us to look outside the immediate vicinity of the holdup we are considering. 

The pressure of the system was controlled to within 0.5 mm Hg for each series by a Cartesian diver manostat connected to a positive air leak, a vacuum pump, and a surge volume of 12 liters. An absolute mercury-in-glass manometer with a 0.1 mm sliding vernier was used to measure the pressure. Temperatures were monitored via two copper-constantan thermocouples one was located just above the liquid surface and the other just below it. Maximum differences of 0.4°C were detected, but an average of the two readings was reported to 0.2°C. Glacial acetic acid and acetone, both meeting ACS specifications, were used. 

In general, the equalization surge volume required will vary between 30% and 50% of the daily average flow, but this can vary from each apphcation. 

Minimum surge volume needed to damp out disturbances Main focus here ... 

Major issues include surge volume, recycle, and the control system ... 

Surge volume. As the flowsheet design progresses, sufficient (but not excessive) surge volume must be provided. The trade-off is between a steady-state and a safety viewpoint, in which the surge volumes are eliminated to minimize the capital cost and the volumes of hazardous... 

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