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Surge Drum Sizing


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#1 Guest_Mohan_*

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Posted 21 November 2005 - 03:39 PM

how do you calculate the surge drum dimensions for a closed loop liquid system

#2 abhi_agrawa

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Posted 21 November 2005 - 10:23 PM

I believe (though I have never designed myself) that the volume of the surge durm is governed by the amount of hold-up time required.

#3 djack77494

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Posted 22 November 2005 - 09:44 AM

Mohan,
By the very definition of Surge Drum, it is a vessel that is designed to provide a given volume or residence time for the fluid passing through. I have only heard the term used in connection with liquid services, so I'll address it from that point of view. I suppose the term could also be applied to gases, however.

To size a surge drum, you must know the flowrates of materials into and out of the vessel. Typically, the vast majority, if not all of the flow, would be liquid. You must develop a sizing criteria. Perhaps something such as providing for a (say) 5 minute interruption of flow to the downstream unit. So, if your Surge Drum is at its normal operating level and you must interrupt outflow for 5 minutes, you'd like to do so without impacting the upstream unit. That would be typical of the thinking that might occur during the design process.

You next decide on an acceptable minimum, normal, and maximum operating level for the vessel. The minimum might be due to a pump requirement, for example. The maximum could be to avoid overfilling the drum. Now take your design flowrate (let's call it 2 cubic meters per minute), multiply by the 5 minutes of surge you want to provide, and you have the surge volume. That volume must be contained between the normal and the maximum drum levels. Estimate the total volume needed by estimating the normal and maximum levels in this case, and then calculating the total vessel volume.

So you have a flowrate of 2 m^3/minute into the drum and you want to run it at least 1 meter full to provide the needed pump NPSH. You also want to leave 250mm of vapor space at the top for your blanketing gas. You have a surge volume of 2 * 5 = 10 m^3. A vertical cylinder of dimensions 1.62m diameter x 4.86m high would provide a total volume of 10m^3. Add 1m of unused space at the bottom and 0.25m of unused space at the top, and you have a surge drum. Its dimensions would be 1.62m diameter x (4.86+1+0.25=) 6.11m high.

Though this seems like a simple problem, it can actually get quite a bit more complex. First note the dimensions chosen. The original 1.62m diameter x 4.86m high was chosen to have an length/diameter (usually called L/D) ratio of 3. Using a range of 3 to 5 for the L/D ratio generally gives an economical choice. I chose 3 because I knew the final ratio would be higher as I added in low and high level allowances. For pressure vessels, move closer to 5 at higher pressures, and closer to 3 at lower pressures. (Note that this does NOT apply to atmospheric tanks.) If we use a horizontal vessel, which is more typical, then the calculation of the useful or surge volume is substantially more difficult. In this case, you'd need to assume dimensions, calculate the unusable volume at the top and bottom of the drum, and then determine how much useful volume is left. By hand, this would be a trial and error calculation. The situation worsens further as you consider that the drum almost surely will not have flat heads; it will have elliptical or perhaps hemispherical heads. Calculating the volumes of partially filled heads adds to the complexity of the overall procedure. Fortunately we have spreadsheets and other tools which make this job relatively easy.

Happy designing,
Doug

#4 Guest_Mohan_*

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Posted 22 November 2005 - 10:48 AM

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Thanks Doug, That was a great help. Can you tell me if it would make any difference if I were to consider for an combined expansion and surge drum.

Mohan

#5 djack77494

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Posted 23 November 2005 - 08:55 AM

I'm sorry Mohan, but I'm not entirely sure I understand your question. The only way I have heard the term "Expansion Tank" used is in connection with heat transfer fluids. These are closed systems which can experience significant temperature changes in going from a shutdown to a full operating conditions. Since they are closed and can have a substantial liquid volume, the thermal expansion of the contents can be appreciable. For these fluids, you would size an expansion tank that, for example, would initially be filled to some low liquid level mark. As you commission and heat up the system, the liquid will expand. All parts of the system, except for the Expansion Tank, are normally completely filled with the liquid, so the liquid expands into the Expansion Tank. A normal design would have the tank at the high liquid level mark when the temperature is at its design (or maximum) temperature.

Let's say you know the dimensions of your system, the operating conditions, and the density of the fluid at its coldest and hottest temperatures. You would calculate the expansion volume needed by taking the total cold liquid volume, multiplying by the cold density (to convert this to a mass), and then dividing by the hot density to get back to the hot liquid volume. The Expansion Tank is designed to handle the difference between the hot and cold volumes, and it is designed to do so by allowing the level to fluctuate between the high and low levels. Proceed with designing (or determining the dimensions) of the Expansion Tank in a manner similar to that I earlier described for the Surge Drum.

Hope that helps. If you mean something differeent for "Expansion Tank", please explain further.
Doug




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