Rating Bubble Cap Tray

1. Purpose

This procedure provides guidelines for rating a bubble cap tray.

2. Features of a Bubble Cap Tray

2.1 Cap Design

Fractionation Research, Inc. (FRI) has designed a bubble cap that is sometimes referred to as a teacup design. It has the following dimensions:

• Cap outside diameter (nominal) 4 inches

• Cap inside diameter 3.875 inches

• Cap skirt clearance 0.25 to 0.625 inches

• Cap reversal corner radius 0.5 inches

• Cap height 3.0 inches

• Cap slots none

• Riser outside diameter 2.75 inches

• Riser inside diameter 2.625 inches

• Riser height 2.0 inches

• Triangular pitch 5.75 inches

Reference Figure 1 for a sketch of this cap and Figure 2 for an additional sketch and some definitions.

2.2 Cap Layout

Per the FRI Design Handbook, "The standard cap is spaced on 5.75 inch triangular centers (pitch) with the base of the triangle normal to liquid flow."

If the caps are on an equilateral triangular pitch, then the spacing between centerlines of cap rows is 0.866 times the pitch.

Redistribution baffles are a common feature on bubble cap trays. These baffles project from the vessel wall perpendicular to the liquid flow. The baffles have a height that is about equal to a bubble cap. This helps force the liquid flow near the vessel wall to contact with the vapor leaving the outer bubble caps.

2.3 Downcomer Design

Some old bubble cap trays have downcomers that use an envelope design. An envelope type downcomer does not use the vessel wall as the outside boundary of the downcomer. Instead, a plate that is supported by the tray support ring serves as the outside boundary. An inlet weir is frequently used with this type of downcomer. The bubbling area is only the area between the inlet and outlet weirs. The downcomer area is only the area enclosed by the envelope. The distance from the back wall of the downcomer to the vessel wall may be 3 to 4.5 inches which should allow for an increase in the allowable downcomer velocity. However, no formal or calculable credit is proposed here.

3. Rating Method

Tray2 is the recommended method to be used for evaluating bubble cap trays. Select KG valve tray for the tray type. Use a foam derating factor of 0.95.

For an existing bubble cap tray calculate the total bubble cap riser area for one tray. In Tray2 set the number of valves to give a floor hole area that is 80% of this value. Each valve has a floor hole area of 1.84 in2.

Set the chordal dimensions to give the correct bubbling area and check downcomer size. Tray2 is the easiest method of checking this limit, provided that a chordal downcomer has been used. If an envelope downcomer has been used, a hand calculation may be the easiest method.

4. Bubble Cap Tray Layouts for Minimum Liquid and Maximum Vapor Rates

Tables 1 and 2 show the drawing number and summary of standard bubble cap layouts. These layouts are for vessels with diameters that range from 1.5 feet (460mm) to 15 feet (4600mm). They have been designed with minimum downcomer areas for low liquid rates and maximum bubbling areas for high vapor rates. These tray layouts are specifically intended for use in vessels that must accommodate high vapor rates with no vapor/liquid contact during normal operation, but must contact high vapor rates with low liquid rates during the regeneration mode (e.g. recycle compressor suction drums).

These tray layouts include details for a baffled outlet weir, which may be required to accommodate low liquid rates. Research indicates that the amount of liquid entrained in the vapor increases significantly at weir rates less than 1.5 gpm per inch of weir. For example, at 1.0 gpm per inch of weir the amount of entrained liquid increases approximately two times. At 0.5 gpm per inch of weir, the entrained liquid increases to more than four times the liquid entrained at 1.5 gpm per inch of weir. In order to provide good liquid distribution across the tray, the dimension for baffle width (G) may not exceed the bubble cap diameter. The baffle height dimension is one-half of the tray spacing or at least the froth height on the tray.

The 1.5 feet and 460mm diameter tray layouts are designed for 2.5 inch diameter bubble caps (Drawing 65189-A). All of the other tray layouts are designed for 4 inch diameter bubble caps (Drawing 63150-A). Use a Type IV cap to ensure the optimum tray performance. The weir height for the Type IV cap shall be 2.5 inches for the 2.5 inch diameter cap, and 3 inches for the 4 inch diameter cap. These weir heights give a 0.5 inch liquid static seal above the top of the cap slots.

The downcomer and bubbling areas listed in Tables 1 and 2 for the 1.5 feet, 450mm, 2.0 feet and 600mm diameter cartridge tray layouts are estimated. The actual downcomer and bubbling areas for these cartridge trays depend upon the type of sealing device between the tray and the vessel shell that the tray supplier designs.

FIGURE 1

FRI BUBBLE CAP

Confidential

For Use Only by Participants of Fractionation Research Inc.

FIGURE 2

SLOTTED BUBBLE CAP

Confidential

For Use Only by Participants of Fractionation Research Inc.

TABLE 1

BUBBLE CAP TRAY DATA

ENGLISH DIMENSIONS

TABLE 2

BUBBLE CAP TRAY DATA

METRIC DIMENSIONS

TABLE 2 (cont’d)

BUBBLE CAP TRAY DATA

METRIC DIMENSIONS