Hydrocarbon Compression

Archive for the ‘downcomer’ tag

Seal Pans Downcomer

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The downcomer from the bottom tray is submerged in a seal pan (see Fig. 8.3), to preserve its downcomer seal. I always set the horizontal dimension between the over-flow lip of the seal pan, (dimension y) the downcomer at four inches, so I never have to worry about restricting liquid flow from the bottom tray. This horizontal dimension should be equal to or greater than the vertical clearance between the downcomer and the seal pan floor (dimension ? which is typically, two to three inches). If a deformation of the downcomer reduces the horizontal clearance between the seal pan overflow lip, and the downcomer, the resulting restriction can cause the bottom tray to flood due to downcomer back-up. If the bottom tray floods, flooding will progress up the column. With time, the entire column will flood due to the small restriction in the seal pan. That’s why a detailed trayby-tray inspection is important.

seal pan Seal Pans Downcomer

Written by Jack

April 30th, 2011 at 12:44 pm

Posted in Tower Pressure

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Total Height of Liquid in Downcomer

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To summarize, the total height of clear liquid in the downcomer is the sum of four factors:

• Liquid escape velocity from the downcomer onto the tray below.
• Weir height.
• Crest height of liquid overflowing the outlet weir.
• The pressure drop of the vapor flowing through the tray above the downcomer.

Unfortunately, we do not have clear liquid, either in the downcomer, on the tray itself, or overflowing the weir. We actually have a froth or foam called aerated liquid. While the effect of this aeration on the specific gravity of the liquid is largely unknown and is a function of many complex factors (surface tension, dirt, tray design, etc.), an aeration factor of 50 percent is often used for many hydrocarbon services.

This means that if we calculated a clear liquid level of 12 in in our downcomer, then we would actually have a foam level in the downcomer of 12 in/0.50 = 24 in of foam.

If the height of the downcomer plus the height of the weir were 24 in, then a downcomer foam height of 24 in would correspond to downcomer flooding. This is sometimes called liquid flood.

This discussion assumes that the cross-sectional area of the downcomer is adequate for reasonable vapor-liquid separation. If the downcomer loading (GPM/ft2 of downcomer top area) is less than 150, this assumption is okay, at least for most clean services. For dirty, foamy services a downcomer loading of 100 GPM/ft2 would be safer.

Written by Jack

January 24th, 2011 at 8:06 am

Posted in Tray

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Trays Downcomer Backup

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Liquid flows across a tray deck toward the outlet weir. The liquid overflows the weir, and drains through the downcomer, to the tray below.

Vapor bubbles up through the sieve holes, or valve caps, on the tray deck, where the vapor comes into intimate contact with the liquid. More precisely, the fluid on the tray is a froth or foam—that is, a mixture of vapor and liquid. In this sense, the function of a tray is to mix the vapor and liquid together to form a foam. This foam separates back into a vapor and a liquid in the downcomer. If the foam cannot drain quickly from a downcomer onto the tray below, then the foamy liquid or froth will back up onto the tray above. This is called flooding.

Written by Jack

January 24th, 2011 at 7:51 am

Posted in Tray

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