Pressure relief valves are commonly used to protect distillation columns from overpressure in emergency situations like loss of cooling. An alternative safeguarding method, which may eliminate the need for relief altogether, is safety shutdown of the energy inputs to the column. Keeping the holdup enclosed in the pressure system is especially advantageous in hazardous, corrosive or reacting systems. Furthermore, the time required for restarting operations may be shorter if the holdup is not relieved to a flare.Shutting down the heat input leads to a rapid collapse of the pressure profile and fast draining of low boiling liquid from the internals to the hot column bottom. During the dynamic draining process, the mixture's vapor pressure continuously changes and may increase considerably.

1. Interior Design Manual
2. Ballast Tray Design Manual Bulletin No 49009 10
3. Ballast Tray Design Manual Bulletin No 49009 Mi

The maximum pressure rise during the shutdown process must be considered in the mechanical design pressure of all connected equipment.During shutdown, the flow regime leaves the stable hydraulic operating range. Experimental data from an industrial valve tray column is analyzed. A standard hydraulic model is unable to predict the pressure dynamics. A literature sieve tray weeping model is adapted to valve trays and fitted to plant data. Simulation results show good dynamic agreement with experimental data.

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.The Separations Company'Sixth Edition.To those using this manual' @ This manual is a design procedure for Glitsch Ballast Trays. Certain aspectsof tower design are important if maximum capacity is desired and potential bot-tlenecks are to be circumvented. In the initial phase of tower design, the following points should considered when spacings are1. Extra tray spacing usually is required at transition trays, i.e., where a change in the number of passes is contemplated. A spacing of 4'-0' is preferred, particularly in large towers.2. Extra space is required if the feed is vaporized.3.

Extra space should be provided for internal liquid feed pipes if tower loadings are high at the feed point. Internal piping preferably is located at a point just below trusses of the next higher tray.A column may Hood prematurely for reasons other than tray design. The following are examples:1. The liquid line to the reb oiler is too small, becomes plugged with debris, or the reboiler vapor line is too small or otherwise restricted.

Any of these may cause liquid to back up in the bottom of the column above the reboiler vapor line.2. The reboiler vapor jet stream impinges on the seal pan overflow, resulting in excessive entrain-ment to the bottom tray.3. A restriction to liquid flow through a downcomer exists due to incorrect tray installation or the of4. Reboiler or feed vapor improperly introduced.,5. Excessive foaming or vaporization of liquid in downcomer.6. Internal loads are appreciably higher than design loads due to an incorrect latent heat of vapor-ization, a change in operating pressure, or not having made a proper heat and material balance.phase. An occasional slug of methanol has been used to alleviate hydrate problems,8.

The system pressure is too close to the critical pressure.The design procedure given herein is intended to be neither conservative nor optimistic. As design procedures for some valve type trays frequently indicate a 'calculated' capacity which is higher than calculated by this design manual, it should be understood that Glitsch Ballast trays are guaranteed to have a capacity equal to, or greater than, any other conventional trays on the market.-3-.Ballast Tray FeaturesVebster has defined ballast for usc in reference to 'that which needs to be held down because it is too light, too buoyant, or the like; it implies the addition of something heavy or solid enough to insure stability.' Durmg an early phasc in research and development of Ballast trays, it was found that valves that wcre permitted to seat Hush had a tendency to be unstablc; i.e., at low vapor rates, the vapor would channel through a few wide-open valves in a small aerated zone located at some indeterminate posi-tion.

The remaining valves would be completely closed. With flush seatcd units, liquid can bypass around the aerated zone on single pass trays; and on two pass trays, one side of the tray can be completely inactive, or the activity can switch back and forth from one side to the other.

Interior Design Manual

Glitsch has climinated the problem of instability with various types of Ballast trays.The Glitsch A-I Ballast tray has a thrcc-piece unit consisting of an orifice CO'er, Ballast plate and a travcl stop. At extremely low 'apor rates, the orificc cover rise is limited by thc wcight of the Ballast plate.

Vhen only thc orifice co'ers arc open, the slot area is relatively low which causes a larger por-tion of the capped area to hc actiw'. At highcr 'apor rates, thc Ballast plate rises until it contacts the travel stop. The A-I Ballast tray is vcry resistant to leakage and is highly recommended when the liquid rates are extremely low, or if the absolute maximum Hexibility is required.It is a valve type tray somewhat similar to the 'rivet' tray first used in 1922. It differs from other vah'e type trays in three major respects.

First, the V-I unit has a two-stage slot opening rather than the single-stage conventionally used. This permits a flow of vapor through all of the valves at low 'apor rates and results in a wide range of stable operating conditions. Second, theof the valve is sloped d provided with a 5h at the portion of the lip.

Ballast Tray Design Manual Bulletin No 49009 10

The sharp edge accentuates turbulence at the position where vapor enters the liquid and generates additional 'Clpor-liquid interfacial area to give a high tray effiCiency. Third, a heavy weight unit is normally used except in vacuum towers. The heavy unit increases the pressure drop and thereby increases tray efficiency in the operating region where the valves are not fully open.Advantages of Ballast trays may be summarized as follows:1. Maximum efficiency at low loads insures a minimum quantity of off.specification products dur-ing start-up.

Ballast Tray Design Manual Bulletin No 49009 Mi

The high degree of flexibility makes it possible to operate with a minimum utility expense over a vide range of feed rates.2. High efficiency at conditions 5 to 10 per cent below incipient Hooding results in an increase in usable capacity. This permits more effective utilization of the column and auxiliary equipment.3. High efficiency at intermediate load conditions can be utilized to improve product quality; to reduce the reHux ratio, resulting in a savings in utilities; or, to reduce the number of trays.-4-.4. The combination of low pressure drop and high efficiency for vacuum systems means a lower tower pressure drop. The V-4 Ballast tray has been used to separate the ethylbenzene-styrene system in a single column.5.

The mechanical design of the tabs are such as virtually to eliminate sticking problems. No sticking problem.s bave occurred in approximately 4000 process nnits and 30,000 columns of V-type trays.

Shutdown time is decreased, due to rapid draining. Maintenance is simpli-fied and worker comfort is improved because the top of the disc is smooth and flat. There are no sharp projections above the tray deck.At zero to relatively low vapor rates, the V-type unit is seated on three tabs which hold the disc above the deck by a distance of approximately 0.1'. The 0.1' height is an optimum distance. A higher initial rise results in too much slot area for operation at low loads and a lower initial rise results in tray instability. The line of contact of the tab with the deck is a 90 0 edge which is provided in order to help prevent sticking from mst and corrosion. For special conditions, it may be desirable to permit selected Ballast units to seat completely.

The tabs are omitted to accomplish this.At high vapor rates, the unit rises vertically to a maximum clearance above the deck of approxi-mately 0.32'. At intermediate vapor rates, some units will be completely open and the others will be resting on the deck.Ballast trays may be used in any clean service, and have been used in many services subject to severe fouling with excellent success. By experience in commercial columns where cleaning may be necessary, it has been found that Ballast trays stay on stream for much longer periods of time than do other trays in the same service.use services customers prefer stainless for nmy use tray components not touching the valve. Carbon steel or monel must be used in service where HF is present.

Carbon steel decks have been used in approximately 60 per cent of all installations to date.Carbon steel Glitsch Ballast units are only occasionally used for reasons of economy. They are not normally recommended because the sharp edge on the lip will be lost due to rusting prior to the initial start-up or at shutdowns. The sharp edge is worth 5 to 10 per cent in added tray efficiency. ' I five users in the selection of the Glitsch equipment best suited to their needs. All technical data contained herein teere deueloped under carefully controlled conditions tehich may not duplicate the user's actual process conditions. Therefore, nothing in this manual is to be deemed a war-ranty.

Glitsch will be pleased to give appropriate warranties in its quotation and which tcill beincorporated into the user's purchase order.Glitsch reserves the right to modify or improve these products without notice.-5-.Vol, V-4V-lX, V-4XV-2XFIGURE 1 BallastUnit TypesNomenclature used: X: Flushoseating B: Blanked H: Heavy-6-A-2X, A-5X.0.-2,.0.-5(Flat Orifice)V-4 TYPE (Extruded Orifice),.Description of Ballast UnitsThe various types of Ballast units are shown on the facing page. A description of each unit follows:V-o A non-moving unit similar in appearance to the V-I in a fully open position. It is used in services where only moderate flexibility is required and minimum cost is desired.V-I A general purpose standard size unit, used in all services.

The legs are formed integrally with the valve for deck thicknesses up to%'.V-2 The V-2 unit is similar to the V-I unit except the legs are welded-on in order to create a more leak-resistant umt. The welded legs permit fabrication of Ballast units for any deck thickness or size. Large size units are frequently used for replacement of bubble caps.V3 A general purpose unit similar to the V-2 unit except the leg is radial from the cap center.V-4 This signifies a venturi-shaped orifice opening in the tray floor which is designed to reduce sub-stantially the parasitic pressure drop at the entry and reversal areas. A standard Ballast unit is used in this opening normally, although a V-2 or V-3 unit can be used for special services. The maximum deck thickness permissible with this opening is 10 gage.V-5 A combination of v-o and V-I units.

It normally is used where moderate flex.